J

I' NATIONAL BUREAU OF STANDARDS REPORT

7449

EFFECT OF BOILING OF THE INSULATION IN ‘UNDERGROUND

HEAT DISTRIBUTION SYSTEMS

by

Selden D. Cole and Paul R. Achenbach

Report to

Office of the Chief of Engineers

Bureau of Yards and Docks

Engineering Division, U.S. Air Force

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT REPORT

1000-12-10401 February 23> 1962 7^49

EFFECT OF BOILING OF THE INSULATION IN UNDERGROUND

HEAT DISTRIBUTION SYSTEMS

by

Selden D. Cole and Paul R. Achenbach

Mechanical Systems Section

Building Research Division

to

Office of the Chief of Engineers

Bureau of Yards and Docks

Engineering Division^ IJ.S* Air Force

national bureau of S'' intended for use within the

to additional evaluation and

listing of this Report, elthei the Office of the Director, fl however, by the Governmen

to reproduce additional cop

IMPORTANT NOTICE

progress accounting documents

Approved for public release by the published It is subjected

Director of , he National Institute of

Standards and Technology (NIST) Such permission Is not needed, on October 9, 201 5. ' IF that agency wishes ' <N^>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
EFFECT OF BOILING OF THE INSULATION IN UNDERGROUND

HEAT DISTRIBUTION SYSTEMS

by

Selden D. Cole and Paul R. Achenbach

ABSTRACT

Simulated field tests were made v^ith several' different

types of insulation used for underground heat distribution

systems to study the effect of boiling the insulation in the

conduit on the heat transmission rate and the physical condition of the insulation. This test procedure v/as developed

as a method for determining the deleterious effect of boiling

insulation materials in an underground conduit in terms of the

increase of the heat transmission rate rather than an evaluation of the deterioration in terms of loss of insulation,,

cracking,, sv/elling^ spallings erosion^ or other chemical or

physical changes. The heat transmission rate of the conduit

was determined for new insulation before boiling and at the end

of each of three 72-hour periods of boiling,, with the insulation dry for each determination. The heat transmiission rate of

the uninsulated conduit was also determined. The tests were

conducted with the steam pipe in the conduit maintained at a temperature of approximately 350 ° during both the boiling

periods and the periods when the steady state heat transmission

rate of the dry insulation was being measured. The results

showed that the heat transmission rates of glass fiber and

rigid cellular glass insulations increased significantly with

Increased exposure to boiling^ whereas that for calcium silicate insulation did not change appreciably. The various insulations used on the bare pipe reduced the heat transmission

rate of the system to approximately one-third of the value

obtained with the uninsulated pipe when the insulation was

new and dry. After boiling for three periods of 72 hours each^

the heat transmission rate of the insulated conduit had Increased

to more than one-half the rate for the uninsulated conduit, except

for calcium silicate which remained about the same.
EFFECT OF BOILING OF THE INSULATION IN UNDERGROUND

HEAT DISTRIBUTION SYSTEMS

1. Introduction

As a part of the Tri-Service Engineering Investigations

of Building Construction and Equipment sponsored 'at National

Bureau of Standards by the Corps of Engineers, Department of

the Army; Bureau of Yards and Docks, Department of the Navy,*

and Engineering Division, Department of the Air Force, a study was made to determine the change in heat loss of a simulated underground heat distribution system caused by

prolonged boiling of the Insulation in the conduit.

Experience has revealed that the insulation space of an

underground heat distribution system may be flooded periodically with drainage water caused by rain, or by an internal

leak in the steam or high temperature water piping. If the

heating pipe remains under pressure when the flooding occurs,

the water in the insulation space will boil and subject the

insulation to the effects of high temperature water, superheated steam, and to the mechanical action of the boiling

process which will usually cause progressive deterioration

of the insulation. The deterioration of various insulating

materials has been studied in the laboratory under conditions

simulating such flooded situations. For this study, the insulation inside the conduit was subjected to three boiling

periods of 72 hours each under specified test conditions. The

steady heat loss of the system was determined on the new

specimen before boiling and again after each of the three

boiling periods^ with the insulation dry in each cas^ to reveal

the rate of deterioration of the Insulation in terms of change

in heat transfer rate.

2. Description of Test Apparatus

A 20-foot section of conduit was used for the tests, consisting of a nominal 4-inch steam pipe enclosed in the insulation to be tested and in a welded metal conduit sufficiently

large to provide a 1-lnch annular air space between the condui

and the exterior of the Insulation. The 4-inch steam pipe'

extended about 3 inches beyond the end of the metal conduit at

each end. Two different sizes of conduit were used for the

study because of the difference in the thickness of one of the

insulating materials.
- 2 -

The apparatus used for the boiling test is illustrated

diagrammatlcally in Figure 1. The apparatus consisted of a reinforced plywood box for supporting and enclosing the conduit specimen, immersion electric resistance heaters for

determining the heat transfer rate, a steam boiler, and the

necessary piping, gages, and controls for conducting the

test. The plywood box measured 4 ft x 4 ft x 19 f't 4 in.

in size, with an open top. During the test the top was

covered with a sheet of plastic to control random' circulation of air around the specimen. The conduit specimen was

supported in a level position along the longer axis of the

box and projected through holes cut in each end of the box

to fit the exterior of the conduit . The conduit extended

about 3 Inches beyond either end of the box. The conduit was

wrapped with a blanket of glass fiber Insulation 1/4 in. thick

and stapled together at the lapped joints to simulate the

thermal resistance of approximately 2 ft of earth cover.

The ends of the conduit were closed with metal plates, one

of which was welded to the conduit and the other was flanged

and secured with bolts. The Joints between the end plates and

the exterior of the 4-inch steam pipe were fitted with gland

seals to prevent ’leakage. Holes for 1-inch pipe were provided

at the top and bottom of each of the end plates and one of the

end plates was provided with two additional holes for l/4~inch

pipe for attachment of a sight glass. The 1-lnch pipe connections at the top and bottom of the end plates were used for

filling and draining, for venting of steam, and for attachment

of a pressure gage, as required. The details of the end plates

and the openings in them are illustrated in Figure 1.

The 4-inch steam pipe was operated as a closed boiler with

immersion electric heating elements during the periods when the

heat transmission rate of the specimen was being measured. Conventional pipe flanges were attached to the threaded ends of the

4-lnch pipe. Steel plates 3/4 inch thick were bolted to the

flanges at each end. Each of these plates was drilled and

tapped to receive a 2-element immersion electric heater located

along the center line of the pipe. In addition, holes for 1/4-

inch pipe were drilled and tapped at the top and bottom of each

of the steel plates such that they were approximately tangent

to the interior surface of the 4-inch pipe. A third hole was

provided near the top of the pipe in either end to establish

the water level when the pipe was being used as an electric

boiler.

The immersion electric heaters consisted of two hairpin

elements each with a maximum capacity of 3,000 watts. The four

elements were wired to provide individual control and each ele-

‘ment was connected to a separate watthour meter through a disconnect switch. One heater element in each end was connected
-3-

dlrectly to the power supply and the -other element in each end

was actuated by a pressure control connected to the 4-lnchpLpe.

The power supply was arranged so that either 230 volts or II5

volts could be applied to the heating elements. A second

pressure control served as a safety device and was used to disconnect all of the electric heaters by means of a magnetic

switch if the pressure reached some selected value a little

above the operating pressure level.

Additional plywood boxes were built around the ends of the

conduit that projected through the main box. These boxes were

filled with granular perlite insulation during the electric heat

transmission test to reduce the heat transfer from the end flanges

and other fittings to a very small value.

A steel boiler of about 1,000,000 Btu/hr heating capacity

was connected to the 4-lnch steam pipe during the boiling cycles

since the electric heaters were of inadequate capacity for this

purpose

.

For the boiling test, the steel plates containing the electric immersion heaters were replaced by two other steel plates.

One of these two plates was drilled at its center and fitted

with a 1 l/2-inch pipe. This pipe extended into the 4- inch

steam pipe about I8 inches and projected about 6 inches beyond

the steel plate on the outside. The inner end of the 1 1/2-inch

pipe was capped, and enough l/4-inch holes were drilled through

the sides of this l8-inch projection to comprise a cross section

area which exceeded the cross section area of the 1 1/2-inch pipe.

The other end of the 1 l/2-inch pipe was connected to the steam

boiler. The steel plate on the other end of the 4-inch pipe was

provided with a hole for a 1/4-inch pipe at the top, and a second

hole for a 3/4-inch pipe at the bottom. The upper connection was

used as an air vent and as a connection for a pressure gage. The

3/4-inch pipe connection was used to remove condensate. This

pipe was connected to a steam trap and to a water-cooled heat

exchanger to cool the condensate to room temperature. The ends

of the conduit were not insulated during the boiling cycles.

3 • Description of Insulating Materials

In this investigation, the heat transfer rate of the conduit

specimen was studied with the 4-inch pipe bare and also covered

with each of four different types of pipe insulation, as follov/ss

(1) glass fiber blanket of 7 1/2 lbs/ft3 density formed into annular sections 2 feet long- with walls 1 1/2 inches thick. The annur. lar sections of insulation were wrapped with a plastic-coated

Plberglas netting resembling window screen- as fabricated by the

Rlc-Wil Corporation: (2) split sections of calcium silicate insulation of 11 lbs/ft3 density, 36 inches long and with a wall
-4-

thickness of 1 1/2 Inches as manufactured by the’ Johns-Manvllle

Company and marketed as Thermobestos j ( 3 J split sections of cellular glass Insulation of about 8.5 Ibs/ft^ density^ I8 Inches long

with a wall thickness of 2 1/2 Inches as manufactured by the

Pittsburgh Corning Corporation and marketed as FoamGlas Pipe InsU'

lation; (4) split sections of premolded glass fiber insulation of

about 3 1/2 Ibs/ft^ density, 36 inches long with a wall thickness

of 1 1/2 inches as manufactured by the Gustin-Bacon Company and

marketed as Molded Glasfiber Pipe Insulation. These materials

were obtained from the manufacturers and were materials being

sold for insulation of underground heating pipes.

4. Test Procedure

At the beginning of a test the 4-inch pipe was operated as

a closed steam boiler, using the energy consumption of the electric Immersion heaters to determine the heat transmission rate

of the conduit specimen when new and before any boiling of the

Insulation occurred. To prepare for this test, all openings in

the conduit flanges except one vent were closed, the 4-inch pipe

was completely filled with water, leaving one of the openings

in the end flange at the top of the pipe open for escape of air.

When the pipe had been filled, the vent was closed and the electric heaters were energized at a low rate of heat input. One of

the openings, about a half inch from the top of the pipe, was

opened slightly through a valve and water was allowed to escape

slowly as expansion occurred and as the steam pressure built up.

When steam only began to escape from this opening, it vras assumed

that the water level had reached the level of the- opening and

the valve was closed completely. The steam pressure was then

raised to approximately 125 Ibs/in.^ gage to attain a steam pipe

temperature of 350° F. A thermocouple peened into the 4-lnch

pipe at one end was used as an indicator for setting the pressure

control for the electric heating elements. Thereafter, the pressure in the steam pipe was kept constant and the energy input to

the electric heaters was observed at regular Intervals. The heat

transmission test was terminated when the heat transfer rate, as

measured by the electric energy used, had been steady for 24 hours

.

After the steady state heat loss of the specimen was determined, the electric heaters were removed and the alternate steel

plates were bolted to the flanges on the 4-inch steam pipe thus

permitting the boiler to provide steam for the system.

The steam trap and heat exchanger were connected to the outlet end of the 4-lnch pipe. The insulation space of the conduit,

was filled with water to saturate the insulation, after which
-5-

enough water was drained out to lower the level to the halffull mark, as Indicated by the sight glass.

Steam from the boiler was admitted to the 4-inch pipe to

raise the gage pressure to approximately 125 Ib/in.^ as rapidly

as possible. The steam pressure was maintained essentially

constant for a period of 72 hours during which the water in the

insulation space boiled vigorously. The steam produced in the

insulation space was allowed to escape from one of the vents in

the end plate, and cold water was fed into the insulation space

to maintain a constant water level.

At the end of 72 hours of boiling, the water was drained

from the insulation space, the steam boiler and steam trap were

disconnected from the 4-inch pipe, and the alternate steel plates

containing the electric resistance heaters were again attached

to the pipe. The 4-inch pipe was operated again as a closed*

steam boiler at a pressure of 125 Ib/in.^ to gradually dry out

the insulation. The drying test was continued until a new value

for steady state heat transfer rate was obtained.

Three such cycles of boiling and drying were carried out

following this procedure. The effect of the boiling on the insulating quality of the materials could then be expressed in term.s

of the change in heat transfer rate related to the length of time

that it had been boiled. The Insulated steam pipe was removed

from the conduit for inspection and photographing at the end of

the third boiling and drying cycle.

In separate tests the heat transfer rate of the two sizes

of conduit without insulation on the 4-lnch steam pipe was determined by operating the steam pipe as a closed steam boiler.

5 . Test Results and Discussion

A summary of the heat transfer of the 20-foot section of

conduit with each of the four types of insulation and without

any insulation is presented in Table 1 and the results are

shown graphically in Figure 2. The graph shows a progressive

increase in heat transfer rate with longer exposure to boiling

conditions for the two types of glass fiber and the cellular

glass Insulations, but very little Increase in heat transfer

rate for the calcium silicate insulation throughout the 216

hours of boiling. Table 1 shows that the increase in heat

transfer rate as a result of boiling the insulation in the

conduit for 216 hours ranged from less than 10 percent for

the calcium silicate material up to 90 percent for the preformed

glass fiber blanket wrapped in the glass fiber screen.
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TRANSFER

OPA

SECTION

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UNDERGROUND

CONDUIT

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BEFORE

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AFTER

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THE

INSULATION

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-7-

Table 1 shows that the four types of insulation, in the

thicknesses used for these studies, reduced the heat transfer rate of the conduit section to a value ranging from 31*8

to 35.5 percent of the heat transfer rate of the conduit

without Insulation. Thus the insulating effect was approximately equivalent in each case. At the end of the 216 hours

of boiling the heat transfer rate of the conduit section

ranged from 37*6 percent for the calcium silicate- material

to 60.4 percent for the preformed glass fiber blanket of the

corresponding value for the uninsulated conduit

.

The measured heat loss shown in Table 1 was assumed to

represent the heat loss of a 20- foot section of conduit. However, it more nearly represents the heat loss of 19 1/2 feet

of conduit because of the thick layer of perlite insulation

that surrounded the ends of the conduit which projected throu^

the supporting box. The insulating effect at the ends was

kept constant for all tests so the unit values of heat loss

for the conduit as reported in Table 1 are comparable values.

The physical condition of the four insulating materials

at the end of the three cycles of boiling and drying is indicated in Figure 3-10 Inclusive.

Figures 3 and 4 are photographs of the preformed fiber

glass blanket Insulation. Both figures show significant portions of the steam pipe devoid of insulation even though the

glass fiber wrapper remained intact. The leaching action of

the boiling water and superheated steam embrittled the fibers

and pulverized them into a powdery dust which sifted through

the wrapper and was deposited on the lower inside surface of

the conduit

.

Figures 5 and 6 are photographs of the molded glass fiber

insulation. It was not enclosed in a wrapper but was secured

to the pipe with two bands per section. Figure 5 is a full

length view of the test section which indicates that the maximum damage (Occurred approximately at the water line and that

portions of insulation of limited size were completely dislodged from the steam pipe. Figure 6 is a close-up view of the

damage to the left of the center spacing ring in Figure 5 j which

shows that the loss of insulation occurred principally at the

longitudinal and circumferential Joints. Some embrittlement and

powdering of the glass fibers also occurred in this insulation

near the surface of the steam pipe, but not to the same degree

as for the blanket glass fiber material.
- 8 -

Flgures 7 and 8 are photographs of the Thermobestos calcium silicate insulation. Figure 7 is a full length view of

the specimen revealing only a limited amount of surface spalling of the material. Figure 8 is a close-up view showing

limited erosion and loss of material at the longitudinal and

circumferential joints. In no case did the erosion extend

the full thickness of the insulation.

Figures 9 and 10 are photographs of the cellular glass

insulation. Both figures show extensive cracking of the cellular glass in every section of insulation. The cracking of the

Insulation could be heard during the heating up period at the

beginning of each of the boiling cycles. The broken pieces of

insulation probably did not fall clear of the steam pipe while

it was still inside the conduit because there was only 1-inch

clearance between the bottom o’f the insulation and the bottom

of the conduit. However^ if the broken pieces dropped down to

the bottom of the conduit the water would have increased access

to the steam pipe causing a greater heat loss.

This test procedure was developed as a method for measuring

the deleterious effect of boiling Insulating materials in an

underground conduit in terms of the increase in the heat transmission rate rather than a less quantitative evaluation of the

deterioration in tems of loss of Insulation, cracking, swelling,

spalling, erosion, or other chemical or physical changes. In

addition, it provides a measure of the heat transfer rate of a conduit with the insulation in new condition. Using a l/4-inch

wrapper of glass fiber blanket on the exterior of the conduit

to simulate the Insulating effect of an earth cover reduced'

greatly the time required to attain a steady state heat loss.

It evaluates the effectiveness of an insulated conduit in terms

of its heat transmission rate which is the most significant

characteristic associated with the purpose of insulation.

USCOM-NBS-DC


HEAT

TRANSFER

RATE

,

KW

HEAT LOSS OF SYSTEM VS HOURS OF BOILING

0 72

ACCUMULATED BOILING HOURS

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!I. S. DKPAHTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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