Stack Venting of Plumbing Fixtures

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UNIT]' D STATI'lS DEPARTMEN'i' OF COMMERCI', . Cl.arles Sawyer, Secretary NATIONAL I3URF.au OF STANDARDS • V.. U. Ccdoii, Director

Stack Venting of Plumbing Fixtures

by John L. French

Building Materials and Structures Report BMS118

Issued January 23, 1950

For sale by the Superintendent of Documents, U. S. Government Printing Office, ^\ashington 25, D. C.

Price IS cents
Foreword

Ever since the formation of the Department of Commerce Building Code

Committee in 1921, investigations of the various fundamental problems relat- ing to the physics of flow in the water-supply and sanitary drainage systems

of buildings have been in progress at the National Bvireau of Standards. These

investigations have resulted in part or in whole in the publication of the following papers

"Recommended Minimum Requirements for Plumbing in Dwellings

and Similar Buildings," Final Report of the Subcommittee on Plumbing of

the Building Code Committee,' BH2, 1924;

"Recommended Minimum Requirements for Plumbing," Report of

Subcommittee on Plumbing of the Building Code Committee, BH13,

1928 (revised 1931) ; "Cross-Connections in Plumbing Systems," Research Paper RP1086,

1938;

"Backflow Prevention in Over-Rim Water Supplies," Building Materials and Structures Report BMS28, 1939;

"Methods of Estimating Loads in Plumbing Systems," Building

Materials and Structures Report BMS65, 1940

;

"Plumbing Manual," Building Materials and Structures Report

BMS66, 1940;

"Water-Distributing Systems for Buildings," Building Materials and

Structures Report BMS79, 1941.

Investigations of plumbing systems were almost completely suspended at

this Bureau during the recent war but have been resumed during the past two

years, largely owing to the sponsorship of research projects by the Housing and

Home Finance Agency as part of a comprehensive research program which is being sponsored by that Agency as a part of their program under their statutory

authority. The specific problems studied in these recent investigations comprise part of this program and include stack venting, wet venting,^ the self- siphonage of fixture traps, and the capacities of vertical building stacks and

horizontal branches.

The present report relates to a particular type of venting; namely, stack

venting, which can be used satisfactorily under certain restricted conditions

and which makes possible appreciable economies in the construction of building drainage systems. The experimental data presented here atforcl a sound

foundation on which code-writing authorities can base plumbing code requirements relating to stack venting.

E. U. Condon, Director.

II
Stack Venting of Plumbing

Fixtures

by John L. French

This report describes tlie methods used and the results oi)tained in an investigation of the adequacy of stack venting a gi'oup of plumbing fixtures on the top floor of a buiUling.

Trap-seal losses of stack-vented fixtures in an experimental installation are reported, a

test loading having a reasonable probability of occurrence is developed, and a criterion of satisfactory trap-seal loss is proposed. The experimental i-esults are interpreted in the lislit of the adopted test loading and the adopted permissible trap-seal loss. And, finally, con- clusions in a form suitable for the use of code-vpriting authorities are made.

CONTENTS

Page

Forevvford II

I. Introduction 1 II. Statement of the problem : 2

III. Previous consideration of the problem 4

IV. Description of test system 4

V. Test procedure 6 VI. Test results 6 VII. Interpretation of Test Results 8

1. Probable loads from a single group of bathroom fixtures and a combination fixture 8

2. Permissible trap-seal losses 9

3. Conclusions 9 VIII. Factors affecting the performance of stack-vented fixtures 10

1. Rate of fixture discharge 10

2. Diameter of stack 10

3. Vertical distance betvi^een bathtub or water closet drain and combinationfixture drain 11

4. Diameter of combination fixture drain 12

5. Length and slope of bathtub or water-closet drains 14

6. Tj'pe of fitting connecting combination fixture to stack 14

7. Trap dimensions 15

8. Submerged house drain 16 IX. Corrosion and fouling 17 X. Conclusions 20

XI. References 21

I. Introduction

There is general agreement that the flow of sewer gas from the plnmbing drainage system into a dAvelling in appreciable amomits is nndesirable

and nnder some circumstances may even be dangerous. To prevent this flow of sewer gas or air

into the dwelling, traps containing a liiiuid seal are almost universally installed on phunbing fix- tures. The flow of water-carried wastes in a plumbing drainage system creates positive and

negative pressures (measured from the prevailing
atmosj)lieric pressure) in various parts of the sys- tem. In order that fixture trap seals may not be

endangered, vent pipes leading to the atmosphere

are installed to prevent excessive pressure fluc- tuations.

Combination fixture

1-1/4 , 1-1/2 , a 2 -Inch drc ins .

l-l/2-inch drain

8-inch street sewer.

3-inch house droin ond sewer^ ^^^-Jl^ /)

FiGiiKE 1. Test system.

The pressure fluctuations acting on the liquid

seal of a trap may for convenience be divided into

two classes; those caused by the discharge of the

fixture to which the trap is connected, and those

caused by the discharge of other fixtures connected

to the drainage system. The type of venting

known as stack venting, in which the drains of the

fixtures on one floor level connect independently

to the stack without any venting other than that

secured through the stack and stack vent, is a method of drainage-system design in which the

detrimental effects of pressure fluctuations due to the second cause mentioned above are reduced or overcome by (1) limiting the load on the stack

above the stack-vented fixtures and (2) connecting

the stack-vented fixtures to the stack at points

where the fluctuations above and below atmospheric pressure are small.

Figure 1 shows a group of stack-vented fixtures on the top floor of a building. The drains from the

lavatory and the combination fixture connect to the

stack above the water-closet and bathtub drains.

The drains from the water closet and bathtub both

connect into the stack at the same level just below

the floor. While all of these fixtures can be called

"stack-vented," it is only the water-closet and the

bathtub traps, as will be seen from the test data

to be presented later, that cause concern in con- nection with stack A^enting. In the case of the lavatory there is no flow in the stack past the lavatory

drain, and this fixture is, in effect, back-vented.

The flow in the stack past the combination-fixture

drain consists only of the discharge from the lavatory, and this flow is so small that no trap-seal

loss occurs. However, in the case of the water

closet and tub, the flow in the stack past their

drains may consist of the discharge of both the

combination fixture and lavatory, and this flow ' may be sufficiently large to cause seal losses in the

traps of the water closet and bathtub.

Stack venting of plumbing fixtures affords,

under certain conditions, a simplification of the

drainage and venting system, which leads to a lowering of construction costs and, according to some

investigators, to better venting. The purpose of

this paper is to present the results of an experimental investigation of the merits of stack

venting.

The experimental procedure used in the investigation was designed to ansAver in a simple and

straightforward manner the practical question as to the permissibility and safety of using stackA'^ented plumbing drainage systems. The tests Avere not designed to investigate the more fundamental

and complicated, but closely allied, problem of

pneumatic stack pressures immediately below the

point of water entrance to the stack.

11. Statement of the Problem

When the upper of tAvo stack-vented fixtures,

such as are shown in figure 2, is discharged, the

loAver fixture will lose part of its seal under certain

conditions. All of the details of the physical

process by which the loss in trap seal occurs have

not been studied. However, it appears certain

that the principal cause of the seal losses in the

lower fixture trap is the negatiA^e pressure created

in the stack by the discharge of the upper fixture.

DaAvson and Kalinske [1] ^ have investigated this problem experimentally, and some of their results are shown in figure 3. Other factors possibly af- fecting to some extent the amount of trap-seal loss of the lower fixture might include the thickness

and the velocity of the sheet of Avater in the stack

floAving past the drain of the lower fixture and the

1 Numbers In brackets indicate references at tlie end of the paper. 2
diameter of the drain of tlie lower fixture. Tlie

eflFect of these latter factors is believed to be rela- tively small, but they may account, at least in part,

for some of the differences in the test results obtained with a 11/2-inch bathtub drain and a 3-inch

water-closet drain connected to the stack at the same level, where the negative pressure in the stack can be assumed to be the same for both fixture

drains.

Other factors, which will be shown in test results

to be presented in this paper to affect the trap-seal

losses of the lower fixture in figure 2 when the

upper fixture is discharged, include the length and

slope of the lower fixture drain and the depth of

the seal and the diameter of the lower trap.

Stock vent

Fixture drain

Stack-vented fixture

traps

Stack

Figure 2. Stack-vented fixture traps.

It will be noted from Dawson and Kalinske's

data in figure 3 that the negative pressure in a stack at a distance z below the point of water en- trance varies with z, and with the volume rate of

inflow, Q. Since the trap-seal loss of a stackvented fixture trap is dependent on the negative

pressure in the stack, among other things, it fol- lows that the trap-seal losses of stack-ventedfixture traps will also vary with z and with Q.

The problem of this investigation is to determine the restrictions that must be placed on Q in

terms of fixture discharge, and the limitations, if any, that must be placed on s, in order that the fix- tures of a single bathroom and kitchen on the top

floor of a system may be connected directly to the

a>

c

o

»

o

<u Ji

(Uo

co

«A o -40-30-20-10 0 +10 + 20+30*40

Pressure in stack in inches

of water

Figure 3. Variation of air pressure in 3-inch staclc helow

point of icater entrance.

vertical stack without any venting other than that

provided by the stack vent. The terms "depth of seal" and "trap-seal loss"

will be used frequently in this paper. These terms

refer to the dimensions shown in figure 4.

Trop weir

Figure 4. Definition sketch of trap.

7i.=: trap-seal loss. D=: depth of trap seal.

3
III. Previous Consideration of the Problem

The economies that are made possible through

the stack venting of fixtures have been recognized

for a number of years, and several experimental

investigations have been made of the merits of this

type of venting. However, these investigations

for the most part have been made as minor parts

of the study of a broader problem, and conse- quently the detailed test data, other than conclusions, have either not been reported, or the data

reported have either been inconclusive or possibly

not entirely applicable to systems subjected to the

higher rates of flow of some modern fixtures. Perhaps the most detailed investigation of stack

venting was made by Hunter [2] , who concluded

from tests on a 3-inch-diameter stack : "With this

layout (a stack-vented water closet, bathtub, lavatory, and kitchen sink on the top floor of a f)lumbing drainage system) no measurable loss of seal was produced in any trap of the group by any combination or order of discharge of the fixtures of

the group itself, or in conjunction with other fix- tures lower on the stack." These tests were made

over 25 years ago, and certain fixtures, particularly lavatories, kitchen sinks, and combination

fixtures, have been modified since that time so as

to increase their rate of flow materially. As the

magnitude of the trap-seal loss of a stack-vented

fixture will depend on, among other variables, the

rate of flow from fixtures on the stack above the

fixture in question, it follows that the conclusions

expressed by Hunter may not be valid for fixtures

in common use today.

Babbitt [3] and Dawson and Kalinske [4] have

also concluded that under certain conditions the

stack venting of top-floor fixtures is satisfactory.

The most recent investigation of the conditions

under which the stack-venting of fixtures is per- missible is that reported by the Plumbing Committee of the Building Research Board of the

Department of Scientific and Industrial Research

[5]. In this investigation, both one- and twostory installations were tested with all fixtures

stack-vented. The Committee concluded that:

"a. Simple one-pipe systems for one- and twostory housing can be designed to operate under

practical conditions of use without siphonage of

traps in spite of the absence of special trap ventilation" (i. e., back vents).

"b. The size of stacks and branches has a controlling influence on the liability to siphonage,

the use of 4-inch stacks with 2-inch branches giving safer conditions than obtain with smaller

stacks and branches."

Unfortunately, the report of the Committee does

not describe in detail the fixtures used and, in

particular, does not give the rate of discharge

from the fixtures. In addition to these direct studies of the merits

of the stack venting of top-floor fixtures, Dawson

and Kalinske [1] have made a rather extensive

investigation of the pressure below a point of

water entrance in a stack without vents of any

kind except a stack vent. These tests were made

by introducing different rates of flow into a stack

through a horizontal branch or fixture drain con- nected to the stack by a sanitary T fitting, and

then observing the pressures in the stack at various

distances below the point of water entrance. As the trap-seal losses experienced by a stackvented fixture, owing to the discharge of a fixture

higher on the stack, must be a function, among

other things, of the negative pressure in the stack

at the point where the drain of the stack-vented

fixture connects to the stack, it is obvious that

Dawson's and Kalinske's data are significant as regards the conditions under which stack venting

of fixtures is permissible. For example, these data

show that, if the rate of flow from the higher

fixtures on the stack and if the vertical distance

between the upper and lower stack-vented fixtures are sufficiently limited, the pressure reductions in

the stack can be made as low as desired ; and hence

it is apparent, with suitable restrictions regarding

the location and rates of fixture discharge, that

adequate protection of fixture traps can be secured

by stack venting. Although the data of Dawson

and Kalinske are significant in a qualitative manner as regards the problem of stack venting and

will be found useful in analyzing some of the test results of the present investigation, they cannot

be used to predict quantitatively the trap-seal

losses that will occur with stack-vented fixtures be- cause (1) the particular rates of flow down a stack

to which stack-vented fixtures are subjected were

not ti'eated in detail, and (2) the diameter of the

drain introducing water to the stack was in general of the same diameter as the stack, while in the case of stack-vented fixtures the upper fixtures

have drains of smaller diameter than the stack. As will be shown later, these factors greatly affect the trap-seal losses of stack-vented fixtures, and

also, presumably, the pressures in the stack near

the point of water entrance.

IV. Description of Test System

The test system is shown in figure 1. The stack The building sewer was constructed of 3-inch-diand building drain were made of transparent ameter cast-iron soil pipe and was connected to methacrylate plastic tubing 3 inches in diameter. an 8-inch-diameter vitrified-clay street sewer.

4
The rate of flow in the street sewer could be varied

at will up to 300 gallons per minute.

The traps and drainage fittings used were, in

genei'al, made of transparent plastic material.

However, many of the tests were made with a con- ventional metal trap and metal drain connected to the combination fixture and with a metal drainage

fitting connecting this fixture drain to the stack. The tests made with the transj)arent drains and

stacks were helpful in visualizing the physical

phenomena involved in the flow. The fixtures used were of current manufacture

and were selected to give a loading on the test system that would be representative of those to be

found on similar systems in service. The water closet used was of the tank-operated,

siphon-jet type. Its average rate of discharge was 26.1 gallons per minute. The volume of dis- charge was 8.0 gallons, which is greater than that

of most water closets in common use. The bathtub used was of standard design, 5 feet long, equipped with a 11/2 -inch trap and drain. The average

rate of flow from the tub when filled to a depth of

6 inches was 12.4 gallons per minute.

A 20 by 24-inch lavatory with a 1%-inch outlet

orifice was used on the system. The lavatory was

connected to the stack by a li^-inch trap and

drain. The average rate of flow from the lavatory was 11.1 gallons per minute.

The combination fixture used on the system was

the typical fixture with a separate sink and laundry-tray compartment. The trap was located di- rectly under the outlet orifice of the sink compartment. The tray compartment was 17% inches

wide by I814 inches long by 13 inches deep. The

sink compartment was of the same dimensions,

except that the depth was 8 inches. The dimensions given are for the top of the combination

fixture. The bottom areas of both the sink and the

tray compartments were slightly less. The tray

compartment was equipped with a metal drain

plug with a cross-bar strainer and rubber stopper.

The diameter of the tray outlet orifice was li%2

inches. The sink compartment was equipped with

a removable basket-type strainer.

The average rate of flow from the sink compartment varied from 30 to 41 gallons per minute,

depending on the type of fixture trap, the length,

slope, and diameter of the w^aste, and on the vertical distance between the trap and the sink. The

average rate of flow from the fixture when the

sink and tray compartment were discharged si- multaneously varied from 30 to 45 gallons per

minute.

The rates of flow given are average rates obtained by measuring the volume of water in the

Figure 5. Transparent iilastic staclc fitting.

fixture and then observing the time required for

the fixture to empty. In the case of the combined

rate of discharge from the sink and tray compartments of the combination fixture, the problem was slightly more complicated, inasmuch as the

sink compartment empties more quickly than the

tray compartment. In this case both compartments were filled with water, and the time required

for the sink compartment to empty was noted.

At the end of the period of discharge from the

sink, the height of the water level in the tray com- partment was observed. In this manner the total volume of water discharged from the combination

fixture during the period of flow from the sink was obtained, and the average rate of flow was, of

course, this volume divided by the time required

for the sink compartment to empty.

The diameters of the lavatory drain, bathtub

drain, and watercloset drain were, respectively,

1%, ^nd 3 inches for all the tests. On most

of the tests the diameter of the combination fixture

drain was IY2 inches, but a few tests were also made

with this drain I14 and 2 inches in diameter.

The water-closet and bathtub drains were con- nected to the 3-inch-diameter stack by means of a

plastic sanitarj^ fitting with a 1%-inch side inlet. This fitting is shown in figure 5. The lavatory

connected to the stack through a sanitary fitting.

Most of the tests were made with the combinationfixture drain connecting to the stack through a sanitary fitting, but some of the tests were also

made wdth a long-turn T-Y at this point.

5
V. Test Procedure

The tests were made by discharging certain of

the fixtures attaclied to the stack and then observing the trap-seal losses or other pertinent behavior

of all the traps.

The lavatory was filled completely to the over- flow level and was then discharged by pulling the

rubber plug from the outlet orifice. The sink and

tray compartments of the combination fixture were filled to a point approximately 1 inch below

their overflow rims. The sink compartment was

discharged by pulling the basket-type strainer

VI. Test

There are three ways in which a plumbing drainage system, subjected to a given test loading, can

fail to function properly. First, the system may

be designed in such a manner, or the test loading

imposed on the system may be such, that sluggish

drainage of some of the fixtures may result. A

failure of this nature implies a i^elatively high

pressure in the fixture drain, and, as would be ex- pected, none of the tests on the single-story stackvented system indicated failure of the system from

this cause. Second, and closely related to the first method, some of the fixtures may flood and overflow, owing to the passage of water from the stack

through the fixture drain and trap back into the

fixture. A failure of this kind implies a heavily

overloaded stack or building drain, and again this

would not be expected to occur on a single-story

stack-vented system. The third method by which

a plumbing system may fail is through the failure

of the fixture traps to prevent the passage of sewer

gas in objectionable amounts from the drainage

system into the dwelling.

A fixture trap may fail in this manner by two

methods. First, if the pressure reduction in the

stack is sufficiently great, the water seal in the trap

may be lowered excessively, so that adequate protection against the passage of sewer gas into the

dwelling no longer exists. Or, secondly, the positive pneumatic pressures created in the drainage

system by the discharge of other fixtures may be

sufficiently high and of sufficient duration, even though the seal of the trap in question has not been

reduced, to force sewer gas through the fixture

trap in objectionable amounts, or the flow of sewer

gas through the fixture traps may be sufficiently

violent to throw a portion of the trap contents out

of the fixture. It became readily apparent as the tests progressed that a single-story stack-vented system

could fail only through failure of the fixture traps

to perform their function properly, and the tests were therefore designed to investigate those conditions which might cause maximum pressure reductions and maximum positive pressures in the stack

completely out of the sink. The tray compart- | ment was discharged by pulling the rubber stop- i per out of the outlet orifice. i For reasons to be given shortly, it was not necj essary to discharge either the water closet or bath- ! tub in most of the tests. In the few tests in which i these fixtures were discharged, the water closet

j was discharged in the usual manner, and the tub was filled to a depth of 6 inches and was discharged

by pulling the rubber stopper from the outlet

orifice.

Results ,

at the points where the fixture drains connected to j. the stack. I The test results in which the water seals in the | fixture traps were reduced by pressure reductions

in the stack will be presentee! first. In preliminary tests it was observed that the

only fixtures subject to trap-seal losses were the tub

and the water closet. The lavatory is subject to trap-seal losses only through self-siphonage effects, and, if the drain is short enough and if its slope is low enough, self-siphonage of the trap can

be made negligible. While the combination fix- ture was connected in these tests to the stack below

the lavatory and hence was stack-vented, the rate and volume of flow from the lavatory were so small

that no trap-seal losses of the combination fixture

trap were observed. Hence it may be concluded

that the combination fixture, like the lavatory, will

be subjected to trap-seal losses only if its drain is so long or is laid on so steep a slope that self-siphonage effects will become apparent. Under these

circumstances, although the whole group of bath- room fixtures and the combination fixture is stackvented, only the water closet and bathtub are subject to trap-seal losses attributable to that fact. In many installations the stack-vented water

closet and bathtub are subjected to the discharge

of only the lavatory. In none of the tests, when

the lavatory was discharged alone or in any combination with the stack-vented bathtub or water

closet, was any trap-seal loss observed. Hence it may be stated without any fui'ther presentation

of data that the stack-venting of a bathtub or a

Avater closet on the top floor of a building is entirely permissible, provided only a lavatory dis- charges into the stack above the stack-vented bathtub or water closet in question. The problem as

to the permissibility of stack venting the top-floor

fixtures in general then becomes one of determining whether the connection of a sink or combination fixture to the stack above the stack-vented

water closet and tub will result in failure of the

water closet or tub traps to perform their function

satisfactorily. i

6
Preliminary tests indicated that the dischai'f>-e of the water closet or tnb, in addition to tlie sink,

tray, or lavoratory had no appreciable effect on

either the tub or water-closet trap seals. These

data are given in table 1.

Table 1. Effect on the trap-seal loss of a stack-vented fix- ture of discharging another fixtvre the drain from which

connects to the stack at the same level

[The combination fixture was installed with a conventional metal trap and

drain. The stack and piping were made of plastic material. Each value

represents the ma.\imum or average reading of five tests made under identical conditions]

Fixtures discharged

Trap-seal reduction

Tub Water closet

Maximum Average Maximum Average

in. in. in. in. Sink and lavatory 0.75 0. 62 1.00 0.90

Sink, lavatory, and bathtub.. 0.0 0.0 1.12 1.07

Sink, lavatory, and water

closet .88 .65 0.0 0.0

Sink, lavatory, and tray.

.

1.00 .97 1.50 1.42

It is apparent from table 1 that the addition of

the water closet or bathtub to the test loading

consisting of the simultaneous discharge of the

sink and lavatory causes only minor and inconsequential increases in the trap-seal losses of the

stack-vented bathtub and water closet. It is also

clear from table 1 that the addition of the tray

to the test loading of sink and lavatory causes

a substantially greater increase in trap-seal loss than does the addition of either the water closet or tub. As will be shown presently, the probability of the simultaneous discharge of more than

two, or at most, three, of the fixtures under consideration is so I'ciiiolc as (o be negligible. Tlciice

it a])p(>ai'S ol)\ ioiis I liat I lie test loading or loadings

adopic*] for del erinini ng the ade(ju;icy of slack

venting should consist of some combination of

discliarg(>, of the lixinics connecting to the stack

above the stack-AcnIcd bathtub and water closet. For this reason, all sub.sequent tests were made

with various coinliinations of the discharge from

the sink, I ray, and lavatory.

It was found in the tests on the single-story

stack-vented system that, as was to have been ex- pected, trap-seal losses of the tub and 'water closet varied greatly with the rate of discharge from

the fixtures above the tub and water closet, and

the discharges from these fixtures, in turn, varied

greatly with the dimensions of the trap and drain

attached to these fixtures and Avith other dimensions of the system. For this reason, the tests were repeated with different trap sizes and with

other variations in the set-up.

In order that the physical phenomena involved

in stack venting might be observed visually, many

of the tests were made with the fixture drains,

traps, and vent fittings, as well as the stack, made

of plastic tubing. These tests, as well as those

made with conventional metal fixture traps, drains,

and vent fittings, will be reported.

Tables 2 and 3 give test results for the li/2-inchdiameter combination-fixture drain with a shortturn fitting connecting the fixture drain to the 3- inch-diameter stack. All of the data of tables 2

and 3 were obtained with conventional metal traps

and drains on the combination fixtures with a metal fitting connecting the drain to the trans- parent plastic stack. Many other tests were made

Avith the combination-fixture trap and drain made

of the plastic tubing. In general, the trap-seal

losses of the water closet and tub were greater when

Table 2. Trap-seal losses of a stack-vented water closet

[Figures in italics represent those trap-seal losses in excess of the standard of satisfactory trap performance adopted for these tests. Each of the trap-seal losses given below represents the maximum or average of 10 tests made imder identical conditions. A sanitary tee was used in all cases to coimect the drain from

the combination fixture to the stack. The piping, other than that used with the combination fixture and budding sewer, was of plastic material.]

Fixtures discharged.

Sink.

Sink and tray '

Sink and lavatory

Sink, tray, and lavatory...

Height of combination fixture drain above

water closet drain... in

(Flow rate qpm

{Trap-seal ioss...{^:;;:::;:;:;:J«::;:

Flow rate gpm....

Trap-seal ioss...{^-;:;::::::;:;:j«:;-

Flow rate -.-qpm

Trap-seal ioss...{^;:;::::::::;:j-:::

{Flow rate gpm

Trap-seal ioss...{^;:;::::::::::j-;::

Trap used with combination fixture

IH-in. wrought

iron

IK'-in. cast brass VA-m. streamlined copper

1 l-i-m. brass tubmg

18 18 18 18 18 18 18 18 20.5 20.5

1st ser. 2d ser. 1st ser. 2d ser. 1st ser. 2d ser. 1st ser. 2d ser. 1st ser. 2d ser.

34.6

0.25

.25

35.2

0. 25

.25

30.3

0. 38

.33

30.8

0. 12

.12

33.8 0.50

.40

35.8

0. 50 .50

37.5

0.38

.38

37.2

0.38

.33

35.4

0. 25

.25

35.5

0.38

.30

35.3

0.38

, .38

35.2 0.38

.38

30.4

0. 38 .38

31.4

0.38

.27

36.5

0.50

.48

37.0

0. 62

.58

38.7

0. 62

.57

38.9

0. 75

.67

30.7

0.50

.50

37.3

0. 50

.50

46.0

1.00

0. 85

45.9

0.88

.76

41.7

0. 50

.47

41.8

0. 50 .50

46.8

1.12

1.05

46.0

1.12

1.00

49.5

0.88

.86

48.4

1.00 .98

46.7

0.88

.77

47.8

0.88

.88

46.8

1.12

0.90

47.

1

1.00

1.00

41.4

0. 75

.70

41.4

0.C2

.52

48.9

l.SO

1.32

47.5 1.50

l.SO

49.4

l.Sa l.SS

50.2

1.50 1.4$

48.6

1.S5 1.10

48.0

1.00

1.00

1 Adopted test I oadings.

843065—50 2 7
a smooth plastic trap and drain were used with

the combination fixture than when these items were

of metal. This result could have been expected,

since the rate of discharge of the combination fix- ture is somewhat greater with the plastic trap

and drain than with the conventional metal ones. It will be noted that tables 2 and 3 do not list trap-seal losses of the water closet or the bathtub

when the tray is discharged alone or in combination with the lavatory. Preliminary tests showed

that trap-seal losses for these loadings were negligible compared with those which included the dis- charge of the sink.

For the conditions under which the data in

tables 2 and 3 were obtained, these tables may be

used to determine the permissibility of stackventing a bathtub and water closet when a lavatory

and combination fixture connect to the stack above

the water closet and bathtub in question.

Table 3. Trap-seal losses of a stack-vented bathtub with a drain 38 in. long laid on a slope of in. /ft

Figures in italics represent those trap-seal losses in excess of the standard of satisfactory trap performance adopted for these tests. The data represent the maximum and average of 10 tests under identical conditions. A sanitary tee was used in all cases to connect the combination-fixture drain to the stack. When the plastic traps were tested, the combination-fixture drain and the sanitary tee connecting it to the stack were also of plastic material. When the metal traps were used, the drain and the sanitary tee were also of metal]

Fixtures discharged

Sink

Sink and tray i_

Sink and lavatory >

Sink, tray, and lavatory

Height of combination-fixture drain above

water-closet drain _ in.

(Flow rate gpm.

iTrap-seal loss {^---v::-::::;:::-:::^^

[Flow rate -gpm.

iTrap-seaj loss JFlow rate.. gpm.

iTrap-seal loss {^-----------t

(Flow rate -._ _ gpm

ixrap-seal loss {^----——"—^

Trap used with combination fixture

VA-in. wrought

iron

18

35.2

0. 25 .23 35.2

.50

.42 45.9

0. 50 .42

47.1

0. 62

.62

IK'-in. cast brass

30.8

0. 38

.38 30.4 0.50

.43 41.7

0..50 .45 41.4

0.62

.57

l!6-in. streamlined copper

35.8

0.25

.12 37.0

0. 75 .56

46.0

0. 62 .50 47.5

1.00

0. 72

VA-ia. brass tubing

37.2

0. 75

.42 38.9

1. ^5

0.99

48.7

0.88

.75 49.6

1.00 0.98

48.4

0.88

.43 50.2 2.00

1.2S

20.5

35.4

0. 38

..30 36.7

0.50

.50

45.5 0.62

.60 48.2

1.12

0. 77

46.4

0. 50 .50 48.6

0. 75 .75

1 Adopted test loading.

VII. Interpretation of Test Results

In determining the adequacy of a drainage sys- tem with a particular type of venting, it is neces- sary first to determine by experiment the trap-seal

losses that occur in the system under various loading conditions. This has been done for the 3-inch

stack in tables 2 and 3. Secondly, it is necessary

to establish a criterion of satisfactory trap performance, that is, to establish a dividing line between trap-seal losses that may be considered sat- isfactory and those that may be considered suffi- ciently large to impair the ability of the trap to prevent the entrance into the dwelling of sewer gas

in objectionable amounts. And third it is neces- sary to establish a criterion of what constitutes a reasonable test loading. That is, it is necessary to

select a portion of those fixtures on the system, the use of which can be assumed to occur simultaneously with reasonable frequency, to serve as a guide in determining whether a trap-seal loss caused by the discharge of a particular group of

fixtures is a sound basis for rejecting or accepting

stack venting of plumbing fixtures as an adequate

method of venting.

To summarize, tables 2 and 3 present trap-seal

losses of stack-vented fixtures under certain conditions. In order to apply these data to the problem

of determining if stack-venting of plumbing fix- tures is admissible, it is necessary (1) to determine

what trap-seal losses are permissible and (2) what

combination of simultaneous fixture discharges is likely to occur. The latter problem will be con- sidered first.

1. Probable Loads From a Single Group of

Bathroom Fixtures and a Combination

Fixture

This problem, insofar as the question of the adequacy of stack venting of plumbing fixtui'es is con- cerned, is simplified somewhat by the data in table

1. These dat a indicate that the discharge of either

the tub or the water closet will have little or no

effect on trap-seal losses. Consequently, attention

may be confined to the lavatory and the sink and

tray compartments of the combination fixture. There are no data available on the frequency

of use of the lavatory, sink, and laundry tray in

single-family dwellings, but a consideration of the

usual habits of family living would indicate that

the simultaneous drainage of any two of the fix- tures would occur only infrequently and that the

8
use of all three at the same time would occur very

rarely. For example, assume that the sink com- partment of the combination fixture is filled and

discharged at random once every 5 minutes, the

tray compartment once every 10 minutes, and the

lavatory once every 3 miiiutes. The duration of

flow from these fixtures will be approximately 15

seconds for the sink, 40 seconds for the tray, and 9

seconds for the lavatory.

At any arbitrarily chosen instant of observation the probability, P, that all three of these fix- tures will be found discharffine is p_15 40 . 300 '^600'^ 180' 0.000167,

or approximately 1 in 6,000.

Obviously, the simultaneous discharge of the sink

and tray compartments of the combination fixture

and the lavatory will occur very infrequently. For

this reason the simultaneous discharge of these

fixtures cannot be considered a reasonable test load on which to base the acceptance or rejection of any

venting or drainage system. In this connection

it may be observed that Hunter [6] has recom- mended that plumbing systems be designed to carry only those loads whose probability of occur- rence is gi"eater than 0.01, and loading tables based on this probability of occurrence have been used

for a number of years by some of the Federal departments, apparently with satisfactory results.

• The values for the frequency of discharge of the

sink, tray, and lavatory used above correspond

in general to the frequency of use of identical or

similar fixtures in public washrooms given by

Hunter [6, 7] and do not represent an estimate of use of these fixtures in private dwellings, such as we are here considering. The frequencies of use of

these fixtures, used in the computation of the

probability of their coincident discharge, are obviously higher than would be found in the usual

private dwelling and have been used merely to demonstrate that, under the most severe loading

conditions, it is not reasonable or logical to assume

that the proper test loading should consist of the

combined simultaneous discharge of the sink and

tray compartments of the combination fixture and

the lavatory.

The probability, of the simultaneous dis- charge of either the sink and tray or the sink and

lavatory, with the third fixture not discharging is

300^180^600^300^600-^180

or aioproximately 1 in 180. The probability that

the sink and tray or the sink and lavatory will be

in use at any instant of observation is thus approximately 30 times as great as for the simulta- neous discharge of all three fixtures and is suffi- ciently large to make the selection of these combinations of fixture discharge as suitable test loads

both reasonable, and not overly conservative, in

determining the adequacy of stack venting.

2. Permissible Trap- Seal Losses

As has been stated previously, the functi(m of a

fixture ti'aj) is to prevent sewer gas in objectionable

amounts from entering the dwelling. A trai>-seal loss which does not prevent a ti'ap from pei'forming this function under the pressure conditums

prevailing in the di'ain or stack at the point where

it is vented cannot be considered objectionable.

If a pressure reduction of sufficient magnitude exists in a stack at a point where tlie drain of a stackvented fixture connects to the stack, air will be

drawn from the dwelling through the trap and

into the drainage system. Obviously this phenomenon can in no way endanger tlie occupants of

the building nor impair in any manner the o])eration of the drainage system. It is apparent that

the only manner in which a reduction in seal of a stack-vented fixture trap might be objectionable

would be through the reduction in its ability to prevent air from being forced under positive pres- sure from the stack through the trap and into

the dwelling. In a single-story system or on the

top floor of any system it is clear from Dawson's

and Kalinske's data [1] that positive pressures, immediately below the point of water entrance,

do not occur under normal conditions. It might

be concluded, therefore, insofar as stack-vented

fixtures on the top floor of the usual system are concerned, that any trap-seal loss which does not

completely and permanently destroy the seal would be satisfactory.

However, as will be seen presently, it is sufficient for the purpose of this investigation to assume the more conservative requirement first stated in "Recommended Minimum Requirements for Plumbing

in Dwellings and Similar Buildings" [2] that,

for traps having a 2-inch depth of seal, the seal

loss shall not exceed 1 inch. In applying this cri- terion of satisfactory trap performance, we shall use the maximum trap-seal loss observed in 10

consecutive tests made under identical conditions.

3. Conclusions

Knowing the permissible trap-seal loss and the

assumed maximum loading of the system derived

above, we can use the data in tables 2 and 3 to determine directly the adequacy of single-story stackvented systems in which the combination-fixture

drain is II/2 inches in diameter and the drain is connected to the stack by a short-turn fitting.

Reference to table 2 will show that, for the loadings of sink and tray or sink and lavatory, a trapseal loss of 1 inch, for the stack-vented water

closet was exceeded only 3 times by the maximum

seal losses listed, and then by only i g inch. In

view of the fact that the basket strainer was completely pulled out of the sink on these tests, and

the fixtures were filled to a greater depth than

will be found convenient or will occur frenquently

in practice, it is concluded that the three minor

deviations in table 2 for the loadings of sink and

tray, and sink and lavatory, from the adopted cri9
terioii of satisfactory trap-seal reduction, are negligible and inconsequential.

It is ap]:)arent, for combination-fixture drains of

fi/^-inch diameter connectino- to a 3-incli-diameter

stack through a sanitary fitting, that the stack

venting of a water closet in a single story installation on the top floor of any system is a satisfactory

method of venting.

From table 3, for bathtubs with drains 38 inches

long on a i/2-inch-per-foot slope, it will be noted

that for the adopted test loading of sink and lavatory or sink and tray, there was only one instance

in which the adopted criterion of satisfactory trapseal loss was exceeded. Inasmuch as this criterion was exceeded only once, and then by only ^/^ inch,

and in view of the safety factors introduced by the

methods of the tests and by the adopted criterion

of satisfactor}^ trap-seal loss, it is concluded that

this single deviation from the adopted standard of

satisfactory trap-seal loss is of minor importance

and can be neglected, and hence the stack-venting

of bathtubs with drains 38 inches long on %-inchper-foot slope may be considered a satisfactory

method of venting.

In practice, of course, other lengths and slopes

of bathtub drains are used. In like manner, the

above conclusions are based on tests with

inch-diameter combination-fixture drains, and in

practice larger diameters are sometimes used.

The effect of change in these dimensions and the

elfect of other variables on the above conclusions

will be investigated.

VIIL Factors Affecting the Performance of Stack-Vented Fixtures

1. Rate of Fixture Discharge

As Avould be expected, it was found that the tra^D- seal losses of stack-vented fixtures increase with

increase in the rate of flow from the fixtures con- nected to the stack above the stack-vented fixture

in question. The data of tables 2 and 3, together

with results of tests with transparent plastic traps

and drains, have been plotted in figures 6 and 7. It will be observed from these figures that combined rates of flow of 40 gallons per minute from

the sink and tray or 48 gallons per minute from the

sink and lavatory will cause an average trap-seal

loss of approximately 1 inch in either the stackvented water closet or bathtub.

Reference to tables 2 and 3 will show that with

fixtures in common use, these rates of flow are

closely approached or slightly exceeded when the

2,0

15

1.0

.5

Fixtures*, discharged Combination

fixture droin

Plastic Metal

Sink e

Sink and trov 0

Sink ond lavatory a 0

Sink. tray ft (av. 0 1 n

- <J

']

1

'

I

c

c

c

-

t

> Sink

I 0

-a

-9

Sink a troy P r- n

a

> -t >^ u a

n A>a

9 —i 4 /Sink a lavotory /Sink.trnv ft Invalory _

if> - -« —

8

•

Z5 30 35 40 45 50 55

Rate of flow into stock obove water closet —Qpm. Figure 6. Trap-seal losses of a stack vented water closet.

10

basket-type strainer is completely withdrawn from

the sink compartment of the combination fixture. It is obvious that any future change in fixture design that increases appreciably the rates of flow

from the lavatory or the sink and tray compartments of the combination fixture m^iy cause excessive trap-seal losses of stack-vented fixtures con- nected to a 3-inch-diameter stack.

2. Diameter of Stack

All of the tests in this investigation were made

on a 3-incli-diameter stack. However, 4-inchdiameter stacks are sometimes required by plumbing codes, even for a single-story system; and

on multistory systems, where it may be desirable

1.5

0.5

0

Fi xtures dischorged Com bination

fixture drain

Plastic Metal

Sink •

Sink ond troy 0 o

Sink ond lovotory a o

Sink, troy, 6 lov. 0 o

nches Tub drain 7ft. long on

. 1 _ Sink ond lavatory.

4A

- O

_ O

Sink, tray, a lov. n s

o Sink -] 0

1 " a

. a

- «a

' o

- >

Sink ond troy ) B

o- o

-e a

d

•-

~*

1 25 30 35 40 45 50 55

Rote of flow into stack obove bofhtub 9pni Figure 7. Trap-seal losses of a _ stack-vented iatJitul).
to stack-vent the top-floor fixtures, the stack may

be even larger. The question therefore arises as

to Avhether conchisions based on tests made with

3-inch-diameter stacks woukl also be valid for

stacks of larger diameter.

In this connection Dawson and Kalinske [2]

have demonstrated that, for a given volume rate of flow into a stack, the pressure reduction below

the point of water entrance will decrease as the

diameter of the stack increases. Since the trapseal loss observed in a stack-vented fixture trap,

when fixtures connected to the stack above it are discharged, is caused by the pressure reduction

created in the stack by the discharge of the upper

fixtures, it follows from Dawson and Kalinske's

data that the seal losses of stack-vented fixtures

will decrease, for a given fixture discharge above

the fixture in question, as the diameter of the stack

is increased. Therefore, it may be stated that

the conclusions expressed previously as to the

adequacy of stack venting of fixtures comiected to a 3-inch-diameter stack, will be at least equally

valid for stacks of greater diameter.

3. Vertical Distance Between Bathtub or

Water-CIoset Drain and CombinationFixture Drain

Fixtures are connected to a stack at various

heights above floor level, depending on custom and

on convenience. For tliis reason tests were made

with the combination-fixture drain connected to the stack at various distances above the watercloset and tub drains. This dimension was varied

by using different lengths of tail pieces on the

combination fixture. The height of the rim of

the fixture above floor level was held constant at 39 inches.

Variation in the vertical distance between the

water-closet or tub drain and the combination

fixture drain can affect trap -seal losses of the

stack-vented tub or water closet in two ways.

First, an increase in this dimension will cause a decrease in the rate of flow from the higher fix- tures, since it involves a deci-ease in the head tending to produce flow from these fixtures; and, sec- ond, from Dawson and Kalinske's data (Ij, it is api^arent that the pressure reduction in the stack

varies with the distance Ijelow the point of water

entrance, and it would therefore Ije expected that

trap-seal losses of stack-vented fixtures would

also vary with this dimension.

Test results for the combination fixture drain

18 and 20.5 inches above the water-closet and bathtub drains have been given in tables 2 and 3. In

table 4 data for the combination fixture drain 15.5 inches above the water closet and tub drains are compared with similar data obtained with this

dimension equal to 18.0 inches.

It will be observed from tables 2 and 3 that the

effect of decreasing the dimension in question

from 20.5 to 18 inches is, consistently, to increase

both the rate of discharge from the combination

fixture and the seal losses in the traps of the water

closet and bathtub. From table 4 it is apparent

that the further decrease of this dimension from

18 to 15.5 inches, in the case of bathtubs, will cause

a substantial decrease in seal losses, and in the

case of water closets only a minor and inconsequential increase. Since the seal losses of stackvented bathtubs are a maximum when the combination-fixture drain is approximately 18 inches

above the tub drain, it is clear that the conclusions

based on table 3 will hold for all bathtub installations in which the stack-vented bathtub is subjected to the flow from a lavatory and combination

fixture located on the same floor level. In like manner, since the tests have shown, (1)

that the trap-seal losses of a stack-vented Avater

closet will be decreased by increasing the dimension in question above 18 inches, and (2) that trapTabI/E 4. Trap-seal losses of stack-vented fixtures at various distances below point of water entrance to stack

[Figures in italics represent those trap-seal losses in excess of the standard of satisfactory trap performance adopted for these tests]

Fixtures discharged

Sink.

Sink and tray

Sink and lavatory

Sink, tray, and lavatory.

(Flow rate ---ffpm. lTrap-seaiioss{^;-----"";"""::::::::::::::;J^^;

(Flow rate - ffpm- iTrap-seal loss {^;--;----;--;-----;;;-:-: J^J; jFlow rate ffpm lTrap-sealloss{^""-"---;--";--;----:;;;;j;j^

JFlow rate -- - spm

lTrap.seaiioss{^-:;;;;-::";:;;:::::::;::::;:;::::i;t

Trap-seal losses of Water closet Trap-seal losses of Bathtub

Distance of -.vater-closet and bathtub drain below combination-fixture drain (in.)

15.5 18.0 15.5 IS.O

39.fi 37.4 39.8 37.2

0.38 0.38 0.12 0. 75 .37 .36 .07 .42

41.6 38.8 42.

1

38.

9

0.81 0.68 0. 75 l.SS

.68 .62 .58 0.99

50.5 49.0 49.4 48.7

l.OR 0. 94 0.62 0.88

1.01 .91 .45 .75

52.4 49.8 52.1 49.6 1.50 l.SS 1.00 1.00 l.SS l.SS 0.62 0.98

11
seal losses are substantially constant for a change

in this dimension from 18 to 15.5 inches, and. since

Dawson and Kalinske's work [1] has shown that

decrease in stack pressure becomes less as this dis- tance is decreased below approximately 18 inches,

it may also be stated that the conclusions based on the data of table 2 will hold for all water-closet

installations in which the stack-vented water closet

is subjected to the flow from a lavatory and combination fixture located on the same floor level.

4. Diameter of Combination Fixture Drain

The data in tables 2 and 3 were obtained with a 1%-incli-diameter fixture drain. However, in

practice, combination-fixture or sink drains 2

inches in diameter are sometimes installed. For

this reason, the effect of change in diameter of the

combination-fixture drain on trap-seal losses of

the stack-vented water closet and bathtub was in- vestigated, and the test results are given in tables

5, 6, and 7. It will be observed that with the fixtures and

traps tested, an increase in diameter of the combination-fixture drain caused, insofar as the II/2- and 2-inch-diameter drains are concerned, a de- crease in trap-seal losses. It is apparent, therefore,

that the data of tables 2 and 3 for li/2-inch-diameter combination-fixture drain, and the conclusions

based on them will apply with at least equal safety

to an installation with a 2-inch-diameter combination-fixture drain.

It is interesting to note in tables 5, 6, and 7 that,

while an increase in the diameter of the combination-fixture drain from II/2 to 2 inches caused decreased trap-seal losses, it also caused an increased

rate of discharge from the fixture, and we have the

unexpected result that an increased rate of flow

into the stack is accompanied by a decrease in trapseal losses. In like manner, the decrease in diameter of the combmation-fixture drain from I14 to

11/4 inches resulted in a decreased rate of discharge,

for the test loads adopted, accompanied by an in- crease in the trap-seal losses of the bathtub and

substantially the same seal losses for the water

closet. Obviously, the pressure reduction in a stack

immediately below the point of water entrance is not only a function of the volume rate of inflow,

but it is also a function of some other variable,

the diameter of the drain admitting water to the

stack, or, what amounts to the same thing, the

velocity of the inflowing water. In this connection

it appears reasonable to assume that the trap-seal

losses, A, in stack-vented systems which are otherwise geometrically similar will depend on the

volume rate of flow past the level at which the

drain of the stack-vented fixture in question connects to the stack, d^^ the diameter of the stack,

^^2, the diameter of the horizontal or fixture branch

through which water enters the stack, s, the verti- cal distance between the point at which the drain

from the stack-vented fixture enters the stack and

the point of water entrance, y^, the specific weight

of the water, ya, the specific weight of the air, and

^, the acceleration of gravity.

We can express this mathematically by writing

A=function {Q, di, di, z, y^, ja, g)- (1)

Using the customary methods of dimensional

analysis, we may write eq 1 in the form

A— 1 Q %£\ /r,N d, ^\di d,' d,'^~^^ yaj ^

^

Table 5. Effect of change in diameter of combination-fixture drain on trap-seal losses of stack-vented water closets

[Figures in italics represent those trap-seal losses in excess of the standard of satisfactory trap performance adopted for these tests. The combination-fixture

drain, which was 16 in. long, was connected to the 3-in. stack through a sanitary tee fitting at a point IS in. above the water-closet drain. The data given below are based on 10 tests made under identical conditions.]

Fixtures discharged

Sink.

Sink and tray '

Sink and lavatory

Sink, tray, and lavatory.

[Flow rate__. _.^gpm.

iTrap-seai loss {^::::::::::::;:::::::::.^. [Flow rate --gpm lTrap.seaiioss{^-::;;;:;:--::::-:;:>

(Flow rate gpm. iTrap-seaiioss{^;_;::-::-;;::;::::;:::j-

[Flow rate gpm. lTrap-seaIloss{^-;:;;;;---:;-::;;J»:

IJ^-in.-diameter plastic trap on combination

fixture

lV4-in. -diameter plastic drain on combination fixture

1st ser.

34.9 0.75

35.7

1.12

1.02

45.5

\.S8 1.19

47.0 l.BO

1.60

2d ser.

34.8

0. 75 .67

36.0

1.12 1.05

45.6

1.S8

1.19

47.0

1. eo

l.BO

lJ4-in. -diameter plastic drain on combination fixture

1st ser.

38.0

0. 75 .70

40. 4

1. 12 1.07

49.6

1.12

l.OIt

51.5

2d ser.

39.8

0.62

.62

41.0

1.12 1.02

49.5

l.Si

1.07

52.1 2.0 1.92

2-in.-diameter metal

trap on combinationfixture and 2-in. -dia- meter metal drain on

combination fixture

1st ser.

41.2

0.62

.57

44.6

1.0.90

51.1

56.7 1.60 H7

1 Adopted test loadings.

12
As far as the method of dimensional analysis,

unassisted by any physical considerations, is concerned, other forms of eq 2, particularly as regards

the variables hjdi and Q/di^^gdi, can be obtauied.

The form given above will be found convenient in one of the later applications in the paper, while in another application it will be found preferable to replace Qldi-4gd\ by Qldx-y[gd2.

For all practical purposes, ju> and 7„ will be

constant in the tests described here, and hence the

term ya can be treated as a constant. For a given value of zjdi, then, eq 2 becomes

di^-yJgdiJ

(3)

This grouping of the variables has been used

in figure 8 to plot the data given in tables 2 and 5 for water closets for the simultaneous discharge

of the sink and tray compartments of the combination fixture or the sink compartment alone.

Similar data obtained with smooth plastic fixturetraps and drains have also been plotted in figure 8. In figTire 9 the data for bathtubs from tables 3

and 7 have been plotted, together with test results obtained with smooth plastic fixture traps and

drains.

From these figures it is evident that, for a given

size of stack and a given volume rate of inflow, an increase in da, which, of course, corresponds to

Table 6. Effect of changes in diameter of combination-fixture drain on trap-seal losses of stack-vented hathtub

[Figures in italics represent those trap-seal losses in excess of the standard of satisfactory trap performance adopted for these tests. The combination

fixture drain, which was 16 in. long, was connected to the 3-in. stack through

a sanitary tee fitting at a point 18 in. above the water-closet drain. The

data given below are based on 10 tests made under identical conditions.]

Bathtub drain 38 in. long on J^-in.-per-foot slope

Fixtures dis- charged

Sink.

Sink and tray

Sink and lavaFlow rate. ...ffpra. Trap-seal lossj^/g'-;--;:'^;

(Flow rate gpm.

{Trap-seal ioss{--:::::;:-

(Flow rate.-- -..gpm. {Trap-seaJloss{---_;::J-

(Flow rate gpm.

|Trap-sealloss{^--_:::J15^-in.-diameter

plastic trap on

combination fix- ture

IM-in.- diameter

plastic drain on

combination fixtme

34.8

0. 75 .67

36.0 2.00

1.90

45.6

1.62 0.92

47.0 2.25

2.17

iH-in.- diameter

plastic drain on

combination

fixture

39.8

0..38 .28

41.0 0.0

.0

49.5

0.38

.28

52.1 0.0

.0

2-in.- diameter metal

trap on

combinationfixture and 2-in.- diameter metal

drain on

combination

fixture

40.3 0.0

.0

45.4 0.0

.0

50.4

0. 25 .17

56.8

0.0

.0

Tablk 7. Effect of changes in diameter of combination-fixture drain on trap-seal losses of stack-vented tjalhlub

[Kigincs ill itiilics rciiri sciit lliosc tr:ip-scal lo.sses in excess of the standard of salisfactory pi i InrMiiiiicc iHlii|ilcrl for these tests. The combination

fixlurc drain, which was 1(1 in. loriK, was connected to the 3-in. stack

throiigli a sanitary lee fitlitig at a point 18 in. above the bathtub drain. The data given below are based on 10 tests made under identical condition.s.]

Bathtub dr ain 38 in. long on H-in.-Per-foot slope

Fixtures dis- charged

Sink.

Sink and

tray.i

Sink and lav- atory. 1

Sink, tray, and lavatory.

(Flow rate.- gpm.

{Trap-seal loss...{^/g^-_TM-

(Flow rate gpm.

{Trap-seal loss...{°;^--j:-

{Flow rate. gpm.

Trap-seal loss.._{--:-J-

(Flow rate -gpm.

{Trap-seal Ioss...{^/g=';_-J>-

154-in. -diameter

plastic trap on combination fix- ture

IK-in.- diameter

plastic drain on

combination

fixture

34.9

0. 75

.60

35.7

2.12

1.97

45.5

1.00 0.90

47.0 2.12

1.92

m-in.-

diameter

plastic drain on

combination

fixture

38.0 0.62

.45

40.4

1.00

0. 87

49.6

0. 62

.49

51.5

1.62

1.32

2-in.- diamcter metal

trap on

combinationflixture and 2-in.- diamcter metal

drain on combination

fixture

41.2

0.25

.17

44.6

0.88

.45

51.1

0. 50

.28

56.7

1.00

0. 60

1 Adopted test loadings.

a decrease in the velocity of inflow, will cause a decrease in trap-seal losses of stack-vented fix- tures. Although the data plotted in figures 8 and 9 are for stacks 3 inches in diameter, it is to be expected

from eq 3 that the curves would hold for stacks

of any diameter.

0.4

0.3

0.2

1 1 d, /d, = 0.53,

p

o

h ).45—

.

f

i d, = c .68 op

o /

-Or/—

o

o

til \/3r9

1 Adopted test loadings. FiGTOiE 8. Relative trap-seal losses for a stack-vented water closet.

13
0.6

0.5

0.4

0.3

0.2

0.1

>

= 0.4 5

h

= 0.5

I— %

/

o

o

)

—o— /Ao

dz/d| = 6.68 ^

1 1 1 0.2 0.3 0.4 0.5 0.6 0.7 Q

Figure 9. Relative trap-seal losses for a stack-vented

hathtul).

5. Length and Slope of Bathtub or WaterCloset Drains

In a group of stack-vented fixtures, as has been

stated prevously, the flow from the lavatory and

combination fixture causes a negative pressure in

the stack which produces seal losses in the stackA^ented bathtub and water-closet traps. However,

this is only one facet of the phenomenon that oc- curs when a stack-vented bathtub loses its seal. In

the tests described here, the bathtub drain entered

the stack through a single 3 -inch sanitary tee with

a 1%-inch side inlet. That is, the water-closet and

tub drains entered the stack at the same elevation

through a standard side-inlet fitting. This fitting,

shown in figure 5, was made of transparent plastic

material and was identical in internal dimensions

with the usual threaded-type 3-inch sanitary tee with a iy2-inch side inlet. The radius of curvature of this tyj)e of fitting where the li/^-inch side

inlet joins the 3-inch stack is small, being approximately inch. With this type of fitting, a small

portion of the water floAving down the stack enters the 11/2-inch side inlet and forms a shallow pool

in the tub drain, and, if the tub drain is short and

on a sufficiently low slope, this pool will extend to the trap weir (see fig. 4) in which case the result- ing flow of water from the stack though the tub

drain and into the trap will reduce any trap-seal

loss that may have been caused by a negative pres- sure in the stack. As long as the water surface of

this pool extending from the stack back through

the tub drain is higher than the level of the trap

weir, there will be at least partial refill of the trap

after the pressure in the stack has returned to at- mospheric pressure. If the drain is sufficiently

long and on a sufficiently high slope, so that the

surface of the pool is lower than the level of the

trap weir, no refill will take place, and maximum

trap-seal losses will result. The details of this phase of the problem were

not investigated systematically, but enough data were obtained to verify the general description of

the phenomenon given above and to indicate quantitatively the effect of variation in length and slope

of the tub drain on trap-seal losses. In table 8 are given seal losses for tub drains

38 inches long on a 14-inch-per-foot slope. Comparison of these data with those given in table 3

for tub drains of the same length but on a i^^-inch- per-foot slope indicates that, for the two slopes

tested, the effect in general of reducing the slope

for this length of drain is consistently to reduce

tub trap-seal losses. In table 9 are given trap-seal losses under certain

loadings for bathtub drains 7 feet long on a slope

of % inch per foot. These data have been plotted

in figure 7, and it will be observed that the results agree well with those obtained with the tub drain

38 inches long on a i/2-inch-per-foot slope. In

view of this fact, and in view of the fact that it was

observed in the tests on the tub drain 38 inches long on a %-inch-per-foot slope that refill of the tub

trap did not occur, it is concluded that the trapseal reductions observed on tests with a bathtub

drain 38 inches or more in length on a %-inch-perfoot slope or greater will be as great or greater than

will occur for a bathtub drain of any length or any

slope. Hence, the conclusions previously given as

to the adequacy of stack venting, which were based on the data of table 3 will be valid for bathtubs

with drains of any length or slope.

In the case of stack-vented water closets there

is no possibility of refill of the trap from the fix- ture drain owing to the conventional type of in- stallation which places the weir of the water-closet

trap several inches above the horizontal portion of

the water-closet drain. For this reason it appears

obvious that the trap-seal reductions of stackvented water closets will be independent of slope

and length of water-closet drain, provided these

factors are not such as to cause self-siphonage.

6. Type of Fitting Connecting Combination

Fixture to Stack

All the experimental data given previously in

this paper have been for a sanitary tee fitting con- necting the combination-fixture drain to the stack. However, a long-turn fitting is also often used for

this purpose in practice, and in like manner double

fittings connecting the combination-fixture and

lavatory drains to the stack are also frequently

14
Table 8. Trap-seal losses of a slack-vented hathliih loilh a drain 38 inches long on a slope of in. per ft.

[Tlio (lata represent the nuuiniuni and averaso ol 10 tests niaile imUer idi'iilical ('(mdilioiis. A saiiilar.v Ice was used in all oases to connect the cfiriihirinllon- fixture drain to the stack. When the plastic traps were tesled, llie eondjiiiatioii-llxtiire drain and I lie saiiilary lee connecting the draiii to the stack were also made of plastic material. When the metal traps were used, the drain and sanitary tee were also made of meial.]

Fixtures discharged

Sink,

Sink and tray ' _

Sink and lavatory i_

Sink, tray, and lavatory..

Height of combination-fi.xturo drain above

water-closet drain in.. Flow

Trap-seal loss.-.{^-i«;

Flow rate gvTM-

Trap-seal loss. /max. ill,.. "la Flow rate -.gpm. Trap-seanoss„{^-

Flow rate .gpm..

Trap-seal loss.

'TM- "\avg. i?j

Trap used with combination fixture

IH-in. wrought

iron

34. G 0.0

.0

35.3 0.0

.0

46.0 0.50

.22

46.8

0. 12 .02

cast brass

30.8

0. 25

. 10

31.4

0.38

.27

41.8

0. 38

.33

41.4

0. 38

streamlined copper

IVi-m. tirass tubing

15.5

39.5

0.0

.0

41.

1

0.0

.0

51.6 0.0

.0

52.7 0.0

.0

18.0

37.5

0. 38

.25

38.7

.62

.47

48.1 0.50

.15

49.9

0. 12 .02

18.0

49.6

0. 75

.00

49.4

0. 62

.38

20.5

35.5 0.0

.0

37.3 0.0

.0

44.9

0. .50 .42

47.9 0.0

.0

20.5

47.8

.0.0

.0

48. 0 0.0

.0

Adopted test loadings.

used. For these reasons tests were made with

these fittings. The results of these tests, for stackvented water closets, are shown in table 10 with

comparable test results with the single sanitary

or short-turn fitting. It will be observed that the

long-turn fitting gave the smallest trap-seal losses and that the single short-turn fitting gave the

ii'reatest.

7. Trap Dimensions

There are two methods by which a tub trap-seal

loss, due to negative pressures in the stack or fix- ture drain, may occur. First, if the negative

pressure, expressed in inches of water, in the tub

drain created by the flow down the stack is less than the depth of trap seal; then, assuming that

the tub drain is sufficiently long and on a suffi- ciently high slope to prevent refill of the trap from

Table 9. Tub trap-seal losses for stack-vented bathtub drains 7 feet long on a slope of % in. per ft. Figures in italics represent those trap-seal losses in excess of the standard of satisfactory trap performance adopted for these tests. The data represent the maximum and average of 10 tests rmder identical conditions. A sani- tary tee was used to connect the combination-fixture drain to the stack. The combination-fixture trap was 15i in. in internal diameter and was made

of transparent plastic material. The remainder of the drainage system was

also made of plastic material, with the exception of the building .sewer, which was cast iron.]

Fixtures discharged

Sink and traySink, tray, and lavatory.

Lavatory.

Lavatory and water closet.

Flow rate gpm..

Trap-seal loss

Flow rate 9P>n-- Trap-seal loss

{Flow rate gpm_.

Trap-seal loss

[Flow rate gpm.. jTrap.sealloss{^;;;;;;:;:;J;-

39.9

1.7B

1.28

50.5

1.75

0. 92

10.5

0.0

.0

0.0.0

the drain, water will be drawn from the trap until

equilibrium occurs, and, when the flow in the stack

ceases, the seal reduction in the tub trap will be

one-half the magnitude of the negative pressure

head in the tub drain, and is independent of the

depth of the trap seal and of any other dimension

of the trap, such as its diameter. However, if the

flow down the stack is large enough to cause a pressure reduction in the tub drain greater than

the depth of the tub trap-seal, air will be bubbled

from the atmosphere through the trap into the tub

drain. This process pumps water out of the tub

trap and into the drain and hence results in rela- tively large trap-seal losses. Consequently, it is apparent, for a given pres- sure reduction in the tub drain, that if the depth

Table 10. Effect of type of fitting connecting combination

fixture to .stack on trap-seal reductions of stack-vented water closet

[Figures in italics represent those trap-seal losses in excess of the standard

of satisfactory trap performance adopted tor these tests. The data were

obtained from tests on the complete plastic drainage system shown in figure 2. The vertical distance between the combination-lixtme and

water-closet drain was 15.5 in. for the single short- and long-turn fittings and 22 in. for the double short-turn fitting.]

Fixtui-es discharged

Sink.

Sink and tray.

Sink and lav- atory

Sink, tray, and lavatory.

Rate of flow gpm..

Trap-seal loss

(Rate of flow gpm..

(Trap-seal loss {^;:::::;:;:|-:

(Rate of flow gpm..

(Trap-seal loss

(Rate of flow gpm..

|Trap-seal loss /max.

^ \avg.

Single

short- tuin

fitting

Single

long- turn

fitting

Double

short- turn 1 fitting

0.88

.75

1.S8

2.00

1.57

2 12 1.97

42.6

0.38

.24

50.5 0.G2

.54

53.8 0.8S

.66

28.3 0.25

.19

30.0

0. 75

. 52

39.4 0.88

.76

41.1

1.00

0. 95

1 The lavatory was also connected to this fitting

15
of the tub trap-seal is greater than the pressure

reduction in the drain, a decrease in depth of trap

seal will have no effect on tub trap-seal losses until

a point is reached at which the depth of trap seal

is equal to the pressure reduction in the drain.

Any further reduction in depth of trap seal will cause a rapid increase in trap-seal losses. Hence

the eEect of increasing the depth of tub trap-seal

from a value less than the pressure reduction in

the drain to a value greater will be in general to decrease trajD-seal losses substantially. And the

effect of increasing the depth of tub trap-seal,

when it is at all times greater than the negative

pressure in the tub drain, will be nil. In the tests reported here, the depth of tub trapseal was 2 inches or more, and the adoi)ted crite- rion of satisfactory performance was a trap-seal

reduction of 1 inch. A trap-seal reduction of 1 inch corresponds to a negative pressure in the tub

drain of 2 inches. Hence, the particular tests on which the conclusions as to the adex^uacy of stack

venting have been based were made with loadings

which produced negative pressures in the tub

drain equal to or less than the depth of trap-seal. It is apparent, therefore, that the conclusions ex- pressed in this report for traps with depths of

seal of 2 inches would not be changed by increasing the depth of trap -seal. The diameter of the bathtub traps used in these

tests w^as 1% inch, corresponding to some of the

nominal li/j-inch-diameter cast traps. However,

in practice, traps such as f i/^ -inch-nominal diameter brass-tubing traps, which may have an actual

diameter of as little as 1% inch, are sometimes

used. The question arises as to what effect this

decrease in tub-trap diameter might have on the

trap-seal losses observed in these tests. From the

discussion above on the effect of change in tub

trap-seal depths on tub trap-seal losses it is evi- dent that a change in tub-trap diameter could have

no effect on tub-trap seal losses, as long as these

losses were less than 1 inch, which, as has been

stated before, has been adopted in this report as the

criterion of satisfactory trap performance. For

this reason it is apparent that the conclusions ex- pressed regarding stack venting apply with equal

force to tub-trap diameters of any value.

For seal losses greater than 1 inch in a trap

with a depth of seal of 2 inches, air will be drawn

through the trap, as noted previously, and, when

this condition arises, the diameter of the tub trap

undoubtedly has some effect on the amount of trapseal reduction. This problem was not investigated, but it would be expected that a decrease in

tub-trap diameter would cause greater trap-seal

losses. From the above discussion of the effect of tub

trap-seal depth and tub-trap diameter on the trapseal losses of stack-vented bathtubs it would be

expected that the lower portion of the curves in

figures 6 to 9, corresponding to trap-seal losses of 1 inch or less would be identical for all traps.

while the upper portion of the curves, corresponding to trap-seal losses of more than 1 inch would

vary according to the depth of trap seal and possibly the diameter of the trap.

While the above discussion has been confined

to bathtub traps, it is evident that it woidd also

apply to a stack-vented water closet or other

fixture.

8. Submerged House Drain

The increased positive pressures in a plumbing

drainage system caused by submerging the outlet

end of the house sewer have been observed by

Hunter [2], Babbsitt [3], and Dawson and Kalinske [4]. These increased positive pressures occur

in stack-vented systems, as well as in systems with

other types of venting.

^'VTien the two upper fixtures in the system shown

in figure 1 are discharged, a negative pressure nor- mally occurs in the stack at the point where the

stack-vented bathtub and water closet connect to the stack, and trap-seal losses of these fixtures

result. However, if the house or building sewer

is sufficiently submerged by the water flowing in

the street sewer, it is possible, under certain conditions, to obtain a momentary positive pressure

in the stack at the point where the stack-vented

water closet and bathtub connect to the stack. It

is obvious that, if these positive pressures ai'e suffi- ciently great, sewer gas or air will be forced out of

the system through the water-closet and bathtub

traps and into the dwelling.

There are three types of stack-vented installations to be considered in this connection. They

are (1) a stack-vented group of bathroom and

kitchen fixtures on the top floor of a multistory

building in which vented fixtures connect to the

stack at lower levels, (2) a single-story installation

in which only a single bathroom group of fixtures

is stack-vented, and (3) a single-story installation

in which a kitchen sink or combination fixture, as well as a single bathroom group of fixtures, are

stack-vented.

In a multistory system, the top floor of which is stack-vented, the positive pressures in the stack

due to submergence of the building sewer will be

confined to the lower portion of the stack, and

will be relieved by the vents to the fixtures on the

lower floors. It is apparent, therefore, that in

multistory systems positive pressures due to submergence of the building or house sewer will not

occur in the stack at the top floor and hence can- not cause stack-vented fixtures on this floor to operate improperly.

With regard to the second type of stack-vented

installation, in which only a single group of bath- room fixtures is stack-vented on a single-story sys- tem, tests made on the system shoAvn in figure 1 with the street sewer flowing full, and tests made

on the laboratory arrangement shown in figure 10

demonstrated that the discharge of the lavatory,

tub, and water closet, in coincidence or in any

16
combination, will not cause positive pressures suffi- ciently large to break the seal of any trap. Hence,

it is apparent that the performance of a stackvented single group of bathroom fixtures on a single-story stack will not be adversely affected by

submergence of the house sewer. With regard to the third type of installation

noted above where a stack-vented sink or combination fixture, as well as a stack-vented group of

bathroom fixtures, are connected on one floor level to a single-story stack, without vents of any kind

connecting to the stack at a lower level, tests made

on the system of figure 1 with the street sewer

flowing full so as to submerge the house sewer, and tests on the system of figure 10 with various

amounts of submergence of the house sewer have

shown that it is possible to submerge the end of the

house sewer sufficiently to cause bubbling of air

through the water-closet trap and into the dwelling when either the sink or tray compartments is discharged. When the house sewer of the system

of figure 10 is submerged 3 inches, no bubbling of

either the water-closet or tub traps was observed

for any combination of fixture discharge, while

with a submergence of 4 inches the results noted

occur. The bubbling observed occurred immediately

after the discharge into the stack began, and con- tinued for approximately 1 second. The bubbling of air from the system, through the trap and

into tlie dwelling was not sufficiently violent, with

any combination of fixture discharge, to throw

water out of the fixture. Althougli the deptli of

seal of tlie water-closet trap was % inch greater

thaiT for tlie bathtub trap, bubbling occurred only

through the water-closet trap.

Although an occasional bubbling of sewer gas

through the water closet in the amounts and for

the durations observed in these tests cannot be

considered a health hazard, it is, nevertheless, undesirable that such bubbling occur frequently, and

under such conditions it may be considered objectionable on esthetic grounds alone. The fre- quency with which this phenomenon will occur in practice depends on the frequency and degree of

submei'geiice of the house sewer by flow in the

street sewer. If the street sewer is not overloaded

and has been otherwise properly designed, the

submergence of the house sewer will occur only

infrequently, if at all, and a stack-vented singlestory system consisting of both bathroom and

kitchen fixtures will operate satisfactorily. On

the other hand, if the street sewer is sufficiently

overloaded to cause frequent and severe submergence of the house sewer, objectionable bubbling

of sewer gas or air through the stack-vented

water closet or tub will result if a sink or combination fixture is connected to a single-stoi\y stack

above a stack-vented water closet or bathtub.

IX. Corrosion and Fouling

Corrosion and fouling of drains and traps may

increase the hydraulic roughness and change the

effective diameter of these parts of the drainage

system.. The drains and traps in a stack-vented

system, the corrosion and fouling of which might

adversely affect the operation of the system, consist of the bathtub and water-closet traps and

drains, the portion of the stack between the stackvented bathtub or water closet and the combination

fixture and lavatory, and the traps and drains of

the combination fixture and lavatory.

As indicated in section VIII-7 of this paper,

any reduction in diameter of a bathtub or watercloset trap, such as might be caused by corrosion

and fouling, could not cause increased trap-seal

losses to occur with the test loads adopted here,

which gave trap-seal reductions of 1 inch or less with clean and noncorroded traps. Tests were

not made to determine the effect of diameter of

either the bathtub or water-closet drain. However, comparison of the data in figure 6 for the

3-inch water-closet drain with those in figure 7

for the 1%-inch-diameter bathtub drain will show

that no appreciable effect on trap-seal losses should

be expected from a decrease in diameter of these

fixture drains caused by corrosion and fouling.

As indicated in section VIII-2, any decrease in

diameter of the portion of the stack between the

water closet and tub drain and the combinationfixture and lavatory drains, owing to corrosion and

Lovofory

1-1/4 inch diometer

3 Inch diometer,

Bathtub

Figure 10. Test setup for simulating submerged house

sewer.

17
fouling, will cause increased trap-seal losses for

the stack-vented water closet and bathtub. However, this portion of the stack is washed by the

relatively clean and nonfouling water of the lavatory, and hence it appears reasonable to assume

that whatever fouling occurs in this portion of

the stack will not be sufficient to affect trap-seal

losses materially.

Corrosion and fouling of the combination-fixture drain or trap presents a more complicated

problem. The effect of corrosion and fouling of

this drain or trap to any degree will result in a

decrease in rate of discharge from the fixture. However, the effect of folding or caking in the

combination-fixture drain at the point where it connects to the stack will also result in a decrease

in cross-sectional area at this point, which may

cause, under certain circmnstances, an increase in

velocity of the water entering the stack. The test results previously discussed have shown that a decrease in volume rate of flow into the stack will cause a decrease in trap-seal losses of stack-vented

fixtures, while an increase in entrance velocity

will cause an increase in trap-seal losses. It is possible, therefore, that while the corrosion and

fouling of the combination-fixture drain will cause a decrease in rate of discharge from the fix- ture, it will also cause an increase in trap-seal

losses of the stack-vented water closet and tub.

Consider the combination fixture, its trap, and

its drain up to the point where the latter joins the

stack. Let H be the total head acting on the

system; that is, assuming atmospheric pressure

in the stack at the point where the drain connects

to the stack, let H be the difference in elevation

between the water level in the fixture and the outlet end of its drain to the stack. The head, /7, must

overcome the pressure drop through the trap, and

the pressure drop through the drain and must

create the velocity imparted to the water. Therefore, we have the relation

/^=Ao +A2 +

where

(6)

Ao=the sum of the frictional and curvature

losses in the trap, the entrance loss occasioned by flow from the fixture

into the trap, and the loss due to the

change in cross-sectional area, if any,

at the point where the trap joms the

horizontal drain,

A2=the pressure head lost through frictional

resistance in the drain.

2^2= the velocity of the water in the drain.

The pressure drops, Ao and A2, can be related to the velocities and dimensions of the trap and drain

through the equations

and

2g

(7)

(8) where / is the Darcy-Weisbach friction factor, I and do are, respectively, the length and diameter

of the drain, Vo is the velocity of flow through the

trap, and g is the acceleration due to gravity. The

quantity k is commonly assumed for simplicity

to be a constant as applied to elbows, return bends,

and similar pipe fittings, although it actually

varies somewhat with the hydraulic roughness of

the fitting, the Reynold's number, and the ratio of

the bend radius to the diameter of the fitting, and

as used in eq 7 it will also vary with the ratio do/d^^

where do is the diameter of the trap.

The loss due to change in cross section between

the trap and the horizontal drain will be small, for

the range of values of do and dz in which we are

interested, compared to the losses in the trap due

to curvature and friction, and it will be assumed

for the present that ^ is a constant as applied to traps.

Substituting the above expressions for Aq and

hi in eq 6 and noting that dlvz^d^Vo, we have

and

V2-- 2gH

d* / dt and it follows that the volume rate of flow Q from

the fixture is 2gH

(9)

Equation 9 makes 'it possible to compute for the conditions assumed the rate of discharge from

a fixture installation in which the trap and fixture

drain have been fouled and corroded to various

degrees.

It has been shown by the data in figures 8 and

9 that trap-seal losses of stack-vented fixtures in- crease, for a given stack diameter, as the rate of

flow of the combination fixture increases and also, for the range of variables tested here, as the di- ameter o?2 of the drain of the combination fixture

decreases. The data in these figures can be made

to fall, approximately at least, on a single curve by plotting h/di against Q/idi^fd^), as shown in figures 11 and 12. Consequently, it follows, for the range of the test results reported here, that a decrease in the factor N=Q/{d'i^d2g) will be ac- companied, for a given stack diameter, by a

decrease in trap-seal losses of stack-vented fixtures. Substituting for Q, in the expression for A^, its value from eq 9, we have

2H

Ad

(10)

18
0

1 1 1 1 11 M [ 1 d^/d, Fixtures discnarged 0.45 9.53 0.68 —

—

B Sink —T

/-

9 o • Sink a tray

ib- "9 —

1

—

-Jr 1

n

c3 -o %

ii

-n f 0.4 0.5 0.6 0.7 Q

d|^/d7T

FiGUKE 11. Relative trap-seal losses for a stack-vented water closet.

Obviously any decrease in do or increase in /

due to corrosion or fouling M-ill cause a decrease

in N' and hence decreased trap-seal losses of stackvented fixtures. With regard to reduction in the

diameter d2 of the combination-fixture drain, it is apparent that the effect on will depend on the

relative magnitudes of k, do, f, I, and c?2, and it is obvious from eq 10 that a decrease in c?2 will more

readily cause an increase in N when k is large,

and / and I are small.

In practice, the minimum possible length of I is approximately one foot. There are no data

available on the value of k for traps, but it ap- pears reasonable to assume that it would not in

general exceed three times the value of k for a single 90-degree elbow. The experimental data on the value of k for 90-degi'ee commercial pipe

elbows are fragmentary and contradictory. Commonlj' used values for that x^ortion of k that is due

to curvature alone range from 0.75 to 0.90 for el- bows. Inasmuch as these values are for screw -pipe

elbows in which the diameter of the elbow is greater than that of the pipe, whereas in the case of the bend in a trap there is no such change in cross section, it appears reasonable to assume that

the value of k for ordinary plumbing P-traps,

which consist essentiall}' of three short 90-degree

bends, will not be larger than 3.0 when the traps

are clean and uncorroded.

Under the assumption of a trap diameter do of

1% inches, a value of k of 3.0, and a length Z of 1

foot, the value of the term Y=k(d2/do*) +f{l/d2*)

+ l/d-,'' has been plotted against d. in figure 13

for various values of the friction factor /. It is apparent from this figure that a decrease in di- ameter of a nominal li/2-inch-diameter drain

owing to corro.sion and fouling will result, for a given /, in a decrease in Y and hence in an increase

in trap-seal losses only for relatively small reductions in diameter, and that severe reductions in di- ameter will cause an increase in Y and hence a decrease in trap-seal losses of stack-vented fixtures.

It is also apparent that the increase in the friction

factor /, which will normally accompany corro- sion and fouling, will materially increase Y and

hence decrease trap-seal losses. It has been uniformly found by investigators

that corrosion and caking cause an increase in the

friction factor /, and relatively minor amounts of

corrosion and caking cause a material increase in /. In this connection. Hunter [7, p. 42] estimates that

a reduction in new pipe diameter of 0.05 inch will

be accompanied by an increase in / from approximately 0.025 to approximately 0.035 ; and a further

decrease in pipe diameter of 0.1 inch will be accom-

]>anied with an increase in / to approximately

0.054.

0.5

0.4

0.3

0.2

0.1

d2./d, Fixtures

discharged

0.46 0.53 0.68

a o Sink

Sink nnd

« < •

tr ay

h — Tub waste 7 feet long

on 3/4 inch slope

>l1

1

0

0 m —

1

-a —

i

>—

p-

« w

-o

0.4 0.5 0.6 0.7 Q

Figure 12. Relative trap-seal losses for a stack-vcntcd

bath till).

19
2.0

A 1.6

\ \ \

\

\

/V*

•8 1.0 12 1.4 1.6 1.8 2.0 dg— inches Figure 13. Effect of f and di on Y. An increase in Y represents a decrease in trap-seal losses. The broken curve in figure 13 is based on

Hunter's estimate of the relation between the friction factor, /, and the decrease in diameter, dz,

and, if we assume these estimates to be correct, it is apparent that the net effect of corrosion and

caking of the drain will be to increase the quantity

Y, and hence to decrease the trap-seal loss of stackvented fixtures. As has been stated previously, it has been as- sumed that k is a constant. It has been shown

by Beij [8] that Z?, for 90° bends, increases with an

increase in the relative hydraulic roughness of the

wall material. As the ordinary plumbing trap is a succession of three such bends, it would be ex- pected that k for traps would also increase with

the hydraulic roughness of the trap material. It

is clear, therefore, from eq 10 that any corrosion or fouling of the trap which results in an increase

in its hydraulic roughness will cause a decrease

in N and hence will result in a decrease in the

trap-seal losses of stack-vented fixtures connected

to the stack below the fixture trap in question.

For the above reasons it would not be expected

that tlie trap-seal losses observed in these tests of

stack-vented systems with clean and uncorroded

pipes would increase with age.

X. Conclusions

1. A group of stack-vented bathroom fixtures

consisting of a water closet, lavatory, and shower

stall or bathtub, with or without shower head, will

operate satisfactorily under the pressure conditions occurring in a plumbing drainage system,

provided that

:

(a) The water-closet and tub or shower drains

connect to the stack at the same level,

(b) The stack is 3 or more inches in diameter,

(c) No other fixtures connect to the stack at a higher level,

(d) The lengths and slopes of the fixture drains

are such that self-siphonage of the fixture traps

does not occur.

2. A stack-vented group of fixtures consisting

of the fixtures of a single bathroom and a kitchen

sink or combination fixture located on the top

floor of a multistory system will operate satisfactorily under the pressure conditions occurring in

a plumbing drainage system, provided the conditions of conclusion 1 are observed and vented fix- tures connect to the stack below the top floor or that the stack or building drain is otherwise

vented below that point.

3. A stack-vented group of fixtures, consisting

of the fixtures of a single bathroom and a kitchen

sink or combination fixture located on the top

floor of a system in which no vented fixtures connect to the stack below the top floor, or the building drain is not otherwise vented, will operate

satisfactorily under all normal vacuums and positive pressures occurring in the stack, but will be

| subject to objectionable bubbling of air from the ; drainage system through the water-closet and

bathtub traps when the street sewer is sufficiently

overloaded to cause frequent and severe submergence of the outlet end of the house or building

sewer. :

Acknowledgment is made to the Housing and

Home Finance Agency for its support of the in- vestigation reported in this paper. Acknowledgment is also made to the Uniform Plumbing Code

Committee, sponsored by the Housing and Home

Finance Agency, and especially to its Chairman,

Vincent T. Manas, for the formulation of the problems investigated in this paper and for many helpful suggestions on the conduct of the tests; to Robert S. Wyly of the National Hydraulic Laboratory Staff, under whose direction the tests were made, for his assistance in designing experimental

apparatus and planning the testing procedure;

and to Herbert N. Eaton, Chief of the Hydraulic

Laboratory Section, National Bureau of Standards, for many valuable criticisms and suggestions

regarding the paper.

20
XL References

[1] F. M. Dawsou and A. A. Kalinske, Report on hydranlics and pneumatics of plumbing drainage systems

I, Univ. Iowa Studies in Engineering, Bui. No. 10 (1937).

[2] Recommended minimum requirements for plumbing

in dwellings and similar buildings, NBS BH2

(1924).

[3] Harold E. Babbitt, Tests on the hydraulics and pneu- matics of bouse plumbing, Univ. 111. Eng. Exp. Stat. Bui. No. 143 (1924).

[4] F. M. Dawson and A. A. Kalinske, Report on hydraulics and pneumatics of the plumbing drainage sys- tem, Natl. Assn. Master Plumbers Bui. No. 2 (1939)

.

[.")] Plumbing Committee on the Building Research Board

of the Department of Scientific and Industrial Research, Ministry of Works (Great Britain ) , "Plumbing", Post-war Building Study No. 4 (1044 i. [G] R. B. Hunter, Methods of estimating loads in pluiiit)ing systems, NBS Building Materials and Structures Report BMS 65 (1940).

[7] Plumbing Manual, Report of the Subcommittee on Plumbing, Central Housing Committee on Research,

Design, and Construction, NBS Building Materials

and Structures Report BMS 66 (1940).

[8] K. Hilding Beij, Pressure losses for fluid flow in 90°

pipe bends, J. Research NBS 21, 1 (1938) RPlllO.

U. S. GOVERNMENT PRINTING OFFICE; I9S0 21

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