NBSIR 73-414

Building and Evaluation of a Polluted

Air Delivery System

G. P. Baumgarten and F. W. Ruegg

Fluid Meters Section

Mechanics Division

National Bureau of Standards

Wasliington, D. C. 20234

April 1974

Final Report on PADS 1

Prepared for Environmental Protection Agency

National Environmental Research Center

Quality Control Branch

^ Chemstrand BIdg

Research Triangle Park, N Carolina 27711

NBSiR 73-414

BUILDING AND EVALUATION OF A POLLUTED

AIR DELIVERY SYSTEM

G. P. Baumgarten and F. W. Ruegg

Fluid Meters Section

Mechanics Division

National Bureau of Standards

Washington, D. C. 20234

April 1974

Final Report on PADS 1

Prepared for Environmental Protection Agency

National Environmental Research Center

Quality Control Branch

Chemstrand BIdg.

Research Triangle Park, N. Carolina 27711

/ 'if % o M. ^

' T A.:.', f

'

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roterts. Diractor

Building and Evaluation

of a Polluted Air

Delivery System

ABSTRACT

The building and evaluation of a prototype SO2 polluted air delivery

system (PADS) is discussed. The delivery system was built to delivery sulfur

dioxide (SO2) in air at a rate of 5 liters per minute with design

concentrations by volume of 1.0, 0.1 and 0.01 parts per million. It

consists of a diluent air delivery system utilizing a critical flow

sonic nozzle and three separate concentrated SO2 in air flow systems

utilizing laminar flow porous plugs, one plug for each desired output

concentration. The delivery system is contained in a dispatch case

and the two gases are delivered to it from pressurized containers

through detachable supply lines. Prospective use by unskilled technicians dictated simplicity and durability and compactness.

By maintaining specific upstream pressures on the critical flow nozzle

and the laminar flow porous plugs of 45 and 12 psig respectivily , the

prototype PADS produced average output concentrations of 0.76, 0.100

and 0.003 parts per million of SO2 in air based on concentration measurements with an NBS calibrated analyzer. The expected output concentrations

were 0.98, 0.105 and 0. 010 respectively , based on flow calibrations of

the individual components. The uncertainty of the output concentration

is estimated to be about ± 10 percent.

List of Symbols Used

A,B and C are the valves selecting the flow at Ci, through the porous plugs

for nominal concentration 1.0, 0.1 and 0.01 ppm respectively.

A is the cross sectional area, in^

B is the barometric pressure in psia.

Bs is the standard barometric pressure, 14.696 psia.

C is the output concentration of the system, ppm

Cl is the

porous

known inlet concentration of SO2 in air supplied to the

plugs, ppm

Cd is the discharge coefficient of the nozzle

c* is a critical flow factor for real gas effects

Pi is the inlet gage pressure for the critical nozzle in psig.

P2

Ap2

Ap3*

is

is

is

the

the

the

inlet gage pressure for the porous plugs in psig.

pressure drop existing across the porous plug,

pressure drop existing across the piping downstream of

the porous plug to the discharge, psi, assumed to be zero for

the evaluation of this unit.

(Qo)n volume flow rate of the critical nozzle at outlet conditions

where the density is Pq, ft^sec"^

(Qo)p is the volume flow rate of the porous plug, ft sec ^at outlet

conditions, where the density is Qq.

R is the gas constant, 0.3705 (Ibf ) (Ibm 1 )f t3in-2°R-l , for air

R* is a constant = 0.007677 in'*atm2sec2lbm~2°R~^ for air.

S is the estimate of the standard deviation, percent

T is the absolute temperature of the flowing gas "R

Tn is the absolute temperature of the inlet gas to the nozzle, °R

y is the viscosity of the gas flow at Ci, Ibm ft~^sec~^

po is the density at the outlet conditions, Ibm ft~^

* Inadvertant or intentional changes of the plumbing downstream from

the porous plugs would require a non-zero value of Ap3 to be used.
I
- 1 -

Polluted Air Delivery System

1. Introduction

This report covers the design, building and evaluation of a prototype

polluted air delivery system, PADS, for use in calibration of

atmospheric pollutant analyzers. The PADS produces known concentrations of sulfur dioxide, SO2 , in air streams to be used in the

field by unskilled personnel to check the performance of SO2 analyzers.

This program was established by the Environmental Protection Agency

at the National Bureau of Standards in order to apply its capabilities

to the needed research, development, evaluation and calibration of

a gas delivery and calibration system for quality assurance of EPA

air programs. The output of this program will be used in quality

assurance activities, specifically interlaboratory performance

evaluations

2 . The Polluted Air Delivery System

The PADS was designed to produce SO2 pollutant concentrations of

0.01, 0.1 and 1.0 parts per million by volume (ppm) in air. Mixtures

of these concentrations were produced by mixing air containing 2000

ppm of SO2 with pure air flowing at a rate of 5 liters per minute.

The 2000 ppm mixture was selected because the flow of pure SO2 would

be too small to be conveniently and accurately metered. A less

concentrated mixture would be more convenient; however, mixtures

containing much less than 2000 ppm are reportedly not stable for as long a period of time as the 2000 ppm mixture.

The three dilute mixtures are produced as follows. The diluent

air flow of actually 5.7 liters per minute, used for all three

mixtures, was metered with a critical flow nozzle having a throat

diameter of 0.0156 inches and operated with an upstream pressure, , of 45 psig. The 2000 ppm pollutant, actually 2230 ppm, was metered

through one of three porous plug laminar flow restrictors at a time,

operated at an upstream pressure of 12 psig. These flows are nominally 0.025, 0.25 and 2.5 cc/min for the nominal 0.01, 0.1 and

1.0 ppm concentrations respectively.

A schematic diagram of the PADS is shown in figure 1. This specific

design was chosen to satisfy the EPA's controlling considerations.

Namely that the unit use conventional off the shelf items, be small

and light, simple to set up and operate, have a minimum of dials to

be read and reported or things to be set, be inexpensive, and have

an output uncertainty of less than 25 percent of concentration. The

critical nozzle was chosen because it is a metering device in which

only the upstream temperature Tjj and absolute pressure (Pi+B) , which

must be greater than twice the value of the barametric pressure, need

to be known to accurately determine the flow rate through it. The

porous plug laminar flow restrictor was chosen to meter the low flows

of the concentrated pollutant because of its low flow characteristics

and easy replacement. For accurate flow determination the midpoint
- 2 -

temperature and absolute pressure P2+B-Ap2/2 are needed as well as the differential pressure across the device. Both these flow devices will

meter flow with an uncertainty of less than 1/2 percent when sufficient care is taken in making these measurements. However this

degree of accuracy is not needed in this application. Since that

is the case and a minimum number of items to be recorded is desired,

it was decided to assume an atmospheric discharge pressure B for both

devices and only measure the upstream static pressures, P^and P2« Considering

total cost, size and ruggedness, gages with 1 percent full scale

uncertainty were chosen to measure these pressures.

The critical nozzle was made at NBS and the porous plug restrictors

were purchased from a vendor. With the exception of the outlet

manifold, figures 4, 5 and 7, the entire flow system was constructed

of stainless steel parts. The manifold block was of mild steel

and the two outlet nozzles were chrome plated brass. The insert, seen in figure 7, in the analyzer supply nozzle of 1/4 inch O.D.

stainless steel tubing was added in an effort to help stabilize

the output concentration. Bellows-sealed stainless steel valves

were used in the system to eliminate the possibility of leakage

that might occur in a valve with stem packing. The input connections for the pollutant and pure air to the apparatus are of different

sizes to eliminate the possibility of interchanging these connections.

A dispatch case was chosen to house the system so that it would be

small, easily carried and shipped. All piping, valves, etc. were

imbedded and fastened between styrofoam to secure the system in the case and to provide shock absorption.

The PADS was evaluated by measuring the concentration C of the output

gas with an analyzer for SO2 calibrated by the Air Pollution Analysis

Section of NBS. See the appendix, page 19 and 20, for the analyzer

calibration data. The analyzer was calibrated using its analog

output voltage and not the visual dial readings. The analog output

of the calibrated analyzer was read with a calibrated recording

voltmeter. The visual dial readings were up to 20 percent higher

than the analog output and were not used for evaluation of the PADS.

3. Evaluation of the PADS

3.1 Requirements . The experiments performed in the evaluation of

the PADS were designed to provide the necessary information to

determine the following PADS characteristics as mutually agreed

upon by EPA and NBS. The various desired performance characteristics

are listed below:

1. Kinetic behavior of output concentration for each restrictor.

2. Effect of order of pollutant flow restrictor use on behavior

of output concentration.
- 3 -

3. Effect of ambient temperature (60 to 90°F) on output concentration

and output kinetics.

4. Reproducibility of output concentration.

5. Noise in steady state.

6. Effect of poor packing and shipping on output concentration,

noise and reproducibility.

7. Effect of fraction bled off on output concentration, noise

and reproducibility.

8. Sensitivity of output concentration to gage settings.

9. Estimate of ease of repeating a given gage setting.

10. Estimate of total time from opening of shipping box to full

operation based on experience of novice.

3.2 Derived System Performance . An evaluation of the experimental

data can be assisted by a development of expected performance as based on flow equations for the meters involved. As stated before,

two meters are used in the PADS. Mass flowrate in a critical nozzle

when operated over a narrow range of conditions is practically

proportional to the inlet absolute pressure divided by the square

root of the temperature. Volume rate of flow through a porous plug

can be based on Poxseuille's law for incompressible flow through

a capillary tube modified for gas compressibility. As derived in

the appendix, equation H, the output concentration C can be expressed

by the relation

P2 Ap3

C = Ci

(Qo)p

(Qo)n

= Ci

(P2-Ap3)(l+

Pl+B

2B 2B B (constant)

(1)

The constant in (1) should depend only on the portion of the syotem

used as Gclectcd by valves A, B or C. Evaluation of the constant

can be approached (a) through measurement of C and the various

physical variables in the right side of (1) , or (b) through measurement of the ratio (Qo)p/(Qo)N volume rate provers and the

variables on the right side of (1). Ci is the concentration of the

inlet SO2 mixture, y its viscosity and Ap3 the pressure drop of the

downstream piping from the valves to the outlet. Ap3 is assumed to

be zero for the evaluation of this unit.

3.3 Experimental Results . The majority of the evaluation tests were

performed in a laboratory where the temperature ranged between 23

and 26°C. The barometer varied between 75 and 76 centimeters of

mercury. The SO2 in air, C, and the pure air were purchased from a vendor and the SO2 was analyzed by the Air Pollution Analysis
- 4 -

section of NBS to be 2230 ± 22 parts per million. The pure air was checked with the calibrated analyzer for background sulfur

dioxide content which was found to be effectively zero. The

expected output concentrations were calculated from the relation

of the critical nozzle and the porous plug flows, as follows:

(Qo)p

C = Ci

(Qo)N

•

The volume flow through the porous plug and the volume flow

through the critical nozzle, were determined by calibration with

small positive displacement provers. The gages measuring the

pressure on the flow elements were calibrated on an air lubricated

dead weight tester. The gage calibrations are in the appendix,

page 16 . In order to assess many of the performance characteristics

of the PADS, a set of tests was designed whereby various flow

pressures at the nozzle and pollutant flow restrictor were chosen

and random combinations of the two required pressures were used

to obtain output concentrations, table 2. The results are discussed

and numbered below in the same order as previously listed for the

desired performance characteristics. The results are also listed

in table 1 and comparison plotted in figure 10. Because of the

lack of calibration, resolution and accuracy of the analyzer at

and below 0.01 ppm, the results at this concentration will not

be discussed but are tabulated and plotted for interest.

1. It was agreed, between EPA and NBS, that the magnitude and

repeatability of the time periods to reach 90 and 95 percent of

the equilibrated output concentration, as determined from the

recorder tracings, would satisfy the question of kinetic behavior

of the output concentration. Even though the final equilibration

time periods occasionally were as long as one hour, especially

for the 0.01 ppm concentration, the periods required to attain

95 percent of the final output concentration value were 10 minutes

or less for nominal concentrations of 1.0 and 0.1 ppm. To obtain

one test analysis for the SO2 at each of the three nominal design

concentrations, at the 95 percent level of equilibration time,

requires less than the two hour maximum time period as desired

by EPA.

2. There didn't appear to be any effect due to the order in which

the pollutant flow restrictor selector valves were used, but if

the pollutant flow is started before the diluent air is turned

on the equilibration time increases significantly.

3. To determine the effect of ambient temperature on the PADS,

it was moved to a room where its temperature could be maintained

at 60 and 90°F. There was no change in output concentration with

the 30° temperature change at the nominal 1.0 ppm concentration
- 5 -

and about a lA percent increase in output concentration with a 30°F increase in temperature at the nominal 0.1 ppm level. A

calculation of the expected output concentration change due to

a 30° increase in operating temperature using equation (J) in the

appendix, indicates a decrease in output concentration of approximately

7 percent should be expected for any one of the desired outputs.

The possible increase in output concentration due to the temperature

increase is unexplained. Possibly the higher temperature caused

desorption of the SO2 that had saturated the inner wall surfaces.

These observations were performed over a period of 4 hours, possibly

insufficient to reach equilibrium.

4. The reproducibility of the output concentration was also

obtained from the characterization test sets as well as other

data at the design conditions. Equation (1) is derived to describe

the system performance. A test of its validity can be made by

inserting experimental values from Table 2, using corrected values

of Pi and P2, to determine if the quantity

P2 Ap3

Ci (P2-AP3)(1+ ^ + 2ir^ (—) = K (3)

C (Pi + B)

is a constant for these tests in which B, T and y were essentially

constant. For valves A, B and C, the quantity K had average values

of 803, 6121 and 202,470 respectively, with estimates of standard

deviation S of 4.4, 4.6 and 61 percent respectively. The relatively

small values of S for valve A and valve B indicate (a) that the

equation (1) is valid and (b) that the anlayzer repeatability and

accuracy are poor near 0.01 ppm considering the large value of S when valve C is used. These values for the constant K apply only at

the conditions which existed during evaluation at ^fBS. Any other evaluation

of the unit at different conditions would need different values of

K derived by application of equation (1).

5. Noise in the steady state condition was measured from the width

of the recorder trace of the recording voltmeter. An indication of

the magnitude of the noise can be seen on the sample recorder output,

figure 2, and listed in table 1. Its magnitude is of the order of

2 percent for the nominal concentration of 1.0 and 0.1 ppm.

6. To evaluate possible shipping effects, the PADS was shipped via

Air Freight to EPA in North Carolina in a cardboard box with little

padding between box and dispatch case. After being inspected there,

it was repacked in the same box and returned in the same manner. There

was no visible damage to the case or its components upon its return.

The PADS was then retested against the analyzer and the output concentration values were found to be within the range of values obtained before

shipping. It is hoped that normally the PADS will be packed, possibly

with the pressure regulators and
- 6 -

maybe the pollutant bottle, in a reuseable container that satisfies

"Air Transport Association" (A.T.A.) specifications number 300,

category 1.

7. An attempt was made to determine the effect of increasing

the mixture flow to the analyzer. This flow was varied by bleeding

off additional mixture through an auxiliary tube, figure 3. This

procedure was followed because it was not possible to vary the

flow extracted by the analyzer. The additional flows were bled

off through a calibrated laminar flow meter at rates of 1.5, 3

and 4.2 liters per minute. As can be seen in figure 10, there

was no measureable effect at the nominal 1.0 ppm concentration.

The output values were all within the spread of output concentrations

without any bleed. The output values, with additional bleed, for

the nominal 0.1 ppm concentration were within the previous spread

of output values without bleed except at one rate, 1.5 1pm, where

the concentration was lower than the previous data.

8. Information on the sensitivity of the output concentration

to the setting of the gages can be obtained from the results of

the characterization test sets, table 2, as expressed by equation

(3), or as expressed by differential equation (J) in the appendix.

The effect of pressure change on concentration is -1.7 percent

for a one psi increase in Pj , and 11 percent increase in output

concentration for a one psi increase in P2. These are listed in

table 1 in terms of concentration, ppm/psi for the two highest

output concentrations.

9. Repeating a given pressure setting to within 1 percent of

the design pressure is not difficult. The pressure adjustment

with the regulators should be made while lightly tapping the PADS's

gages and the line of sight should be normal to the face of the gage.

10. The last item on the list of characteristics to be investigated

is a request for an estimate of the time required for a novice to

put the PADS in operation. In our opinion it should not take any

longer than one hour.

The evaluation of the PADS was performed with the flow configuration

as shown in figure 4. Refer ing to figure 10, the data at test one

was taken with the piping arranged similar to figure 5 but with the

porous plugs to the left or downstream of the selector valves. For

test number two, figure 10, the valves and plugs were switched, figur"&

5, to see if the arrangement would decrease the equilibration time.

This arrangement was a marginal improvement. For the remainder of

the tests the piping was arranged as in figure 4. The downstream

piping from the selector valves to the mixing tee was replaced with

0.010 inch inside diameter capillary tubing. The installation of

the capillary tubing made a marked improvement in the equilibration

times. The porous plugs were left upstream of the selector valves

so that the plugs could be kept saturated at the supply pressure of

the pollutant.
- 7 -

It could be assumed that changing the position of the valves and

plugs, and the addition of the tubing, caused the decrease in output

concentration, shown in figure 10, by increasing the back pressure

on the porous plugs which would have decreased the pollutant flow

and thereby the output concentration. A theoretical calculation

of the pressure drop, using equation A, indicated that the pressure

drop across the downstream capillaries with inside diameters of 0.01

inch would decrease the flow and the concentration 2.5 percent for

the largest pollutant flow, .2 percent for the middle flow, and .015

percent for the lowest. These calculated decreases in pollutant flows

and concentrations are smaller than the observed concentration decreases

of from 10 to 50 percent. A calculation, based on equations 2, A and

J, indicated that the diameter of the capillary tubing would have to

be .006 and .004 inch rather than the presumed .010 inch to cause the

decrease in output concentration exhibited for the nominal 1.0 and

0.1 ppni portions of the PADS system. It is felt that the decrease can

not be attributed solely to a smaller than expected capillary, but to

unknown effects.

Accuracy of the results should be estimated through comparison of

results obtained by two or moore independent methods. In this case

a second method and its results are available from use of equation

2 and flow calibrations of the porous plugs and critical nozzle as listed in the Appendix, page 17. The output bias is defined as the

difference between the average value of the output concentration (as

calculated from the average value of K, equation 3) and the concentration calculated from equation 2. This difference is -0.22 and

-0.005 ppm, respectively, at the nominal concentrations of 1.0 and

0. 1 ppm. An estimate of the overall uncertainty of the flow calibrations

of about 3 percent cannot account for the difference of 0.22 ppm

which on a percentage basis is about 22 percent. The difference of 0.005

ppm is reconcilable considering the estimated uncertainty of 3 percent

for the flow calibrations and about 2 percent for 3 times the standard

error of the reported values of the concentration by the analytical

procedure. Without further experimentation and intercomparison of

methods, and considering both results, it appears that the accuracy

cannot be estimated as better than about 10 percent of output concentration.

4 . Operating Instructions

The following steps are necessary to install the PADS in the field

and make it ready for operation. (Fig 6)

1. Assemble the gas bottles, regulators, connecting tubing and PADS

at the site of the analyzer to be checked.

2, Connect the regulators to the gas bottles.

3. Attach the tubing between the regulators and the PADS inlet

connections and between the PADS outlet manifold nipples and

analyzer and exhaust.
- 8 -

A. Set the diluent air and SO2 gas flow pressures, 45 and 12 psi

respectively, on the PADS gages with the pressure regulators.

Take care to lightly tap the PADS gages while setting the pressures and to view the gages normally to their faces. Diluent

air will be flowing at this point.

5. Before any data is taken it should be determined that nothing is

restricting the flow from the outlet manifold nozzles. Otherwise

the output concentrations would be erronous due to the higher

back pressure created by the restriction.

6. Open valve A and allow 30 minutes to achieve an equilibrium state.

7. Read and record the PADS gages gas pressures and the concentration

indication of the analyzer being checked.

8. Close valve A and allow about 5 minutes before opening valve B.

9. Repeat steps 6, 7, and 8 for valve B and then C.

10. If available, a barometer reading, and area temperature reading

should also be recorded at the same time as the data in step 7.

11. In order to compare the output concentrations of the PADS, as indicated by field analyzers, with the calibrated output concentration of the PADS, it is necessary to make corrections to the

calibration output concentration, for the conditions that existed

at the field analyzer. This can be done by using the corrections

in equation J of the appendix in the following manner. Let a"o"

subscript designate the output concentration at the time of PADS

calibration and"n"designate the output concentration at the time

of comparison with a field analyzer. Cc will designate the PADS

calibration output concentration corrected to field analyzer

conditions.

Cln 1.3(P2n-P2o) Bn-Bo Tn-To Pin'P]

1 + r + .46 ( -—- ) -1.3 ^= .75

'^2n ^ ^in

The differential change of Cj , inlet concentration of SO2 , is not

used above in the event that varies widely from one supply bottle

to the next. As stated in item 3 of 3.3, the measured effect of

temperature change on the output concentration is not fully understood and does not agree with that in the above equation.

12. During routine inspection of the PADS, between its uses in the

field, the PADS should be recompared to a calibrated analyzer.

During this test the pressure set on the SO2 regulator pressure

gage, to attain the necessary pressure on the PADS SO2 gage.
- 9 -

should be recorded. If and when in subsequent inspections an

increase in the difference between the two pressures is noted

it is likely that the filter element needs to be replaced, barring

any leaks in the connections between the regulator and filter.

13. Inspection for leaks from the PADS can be accomplished by removing

the system from the case and using a commercial leak detection

liquid, or a soap and water solution, on the connections while

the system is pressurized to its normal pressures.

5. PADS Specifications

1. All parts that would come in contact with the SO2 mixture were

purchased in stainless steel.

2. All parts that would come in contact with SO2 except the gauges,

valves, porous plugs, filters, quick disconnects and capillary

tubing, were cleaned in 30 percent nitric acid solution and rinsed

in distilled water.

3. The valves are of the bellows sealed type, installed so that the

bellows is on the upstream side of the valve or the higher pressure

side of the valve.

4. The gages have stainless steel bourdon tubes and pressure scales of

15 and 60 psi respectively. They have manufacturers stated

accuracies of 1 percent full scale and were calibrated at NBS on

an air lubricated dead weight tester.

5. The stainless steel porous plugs which restrict the pollutant

flow are assembled into 1 3/8 inch long, 1/4 inch O.D. stainless

steel tubing. They were ordered with nominal flowrates of 0.025,

0.25 and 2.5 cc/min when the differential pressure across them is

12 psig with atmospheric discharge.

6. The critical flow nozzle, figure 8, was made from 1/4 inch O.D.

stainless steel rod 1 3/8 inch long. It was drilled out to within

1/32 inch from the opposite end. The nozzle throat was drilled

1/64 inch I.D. in the blank end. The nozzle was shaped by hand.

The flow through the nozzle was designed for a nominal 5 liters

per minute of air with an upstream pressure of 45 psig and

atmospheric discharge, but calibration proved it to be 5.7 liters

per minute.

7. All the tube fittings used in the PADS use compressed ferrules as

the method of sealing the tubing to the fittings, and fit 1/4 inch

O.D. stainless steel tubing.
- 10 -

8. The tubing used between the regulators and the PADS was 1/4 inch

CD. stainless steel for the SO2 mixture and 1/4 inch O.D., 1/16

inch wall plastic tubing for the air.

9. The regulator used for the SO2 concentrated pollutant was all

stainless steel, sized for the flow range designed for, .025 to

2.5 cc/min. The regulator used for the air was conventional 2 stage air regulator.

10. The assembled flow system of the PADS was sandwiched between

styrofoam for shock absorption and fitted into a dispatch

case.

11. After final assembly the PADS was evaluated against a calibrated

SO2 analyzer at normal room temperatures and barometric pressure,

and at room temperatures of 60° and 90°F.

Recommendat ions

1. It is felt that stainless steel (316) or Teflon should be used

wherever SO2 will be in the system.

2. All stainless steel parts of the system, except those which have

other components that would be damaged, should be cleaned in a

97 percent nitric acid and 3 percent hydroflouric acid solution

for 10 minutes at room temperature, rinsed in distilled water

and dried.

3. The valves should be of the bellows sealed type, constructed of

316 stainless steel and installed with the bellows on the upstream

side of the valve.

4. The gages should have 316 stainless steel bourdon tubes and be

accurate to within 1 percent of full scale. The pollutant gage

should be 15 psi full scale and the pure air gage 60 psi full scale.

5. The porous plug flow restrictors should be made of 316 stainless

steel mounted into 13/8 inch long 1/4 inch CD. 316 stainless

steel tubing. They should have output flows of 0.025, 0.25,

and 2.5 cc/min, when the differential pressure across them is

12 psi and the discharge is at atmospheric pressure, for nominal

output concentrations of 0.01, 0.1 and 1.0 parts per million.

6. The critical flow nozzle should be made of 316 stainless steel

as in figure 8. A mandrel should be made to shape the inlet

radius of curvature. The nozzle could also be made by the electroforming process and fastened to the end of a 1 3/8 inch long,

1/4 inch CD. piece of stainless steel tubing.
- 11 -

7. The gages should be checked against a transfer standard to verify

their accuracy.

8. The tubing delivering the SO2 to the PADS from the pressure

regulator should be either stainless steel or Teflon. Stainless

steel may be more awkward to use because of its rigidity.

9. The completely assembled PADS should be evaluated with a calibrated

SO2 analyzer. The evaluation should include data taken at various

pressure levels in order to determine the effect of changing the

inlet air and SO2 pressure on the output concentration. If possible,

the evaluation should include data taken at various surrounding

temperatures and barometric pressures. Where this is not possible

a theoretical correction must be made for data taken at temperatures

and barometric pressures different than those taken at the time

of initial evaluation, as outlined under "Operating Instructions".

Parts List**

Case - dispatch - NBS storeroom FSC 8460-782-6726*

Critical nozzle - NBS shop made - 0.0156 inch ID, 1/4" CD

Porous plugs - Purchased from Mott Metallurgical Corp.: 2.5 cc/mln,

0.25 cc/min, 0.025 cc/min; 0.025 cc/min plug replaced

with 0.10 cc/min before unit was returned to EPA.

Gages - Purchased from Marsh Instrument Co.

Part No. 1-BJC-03160-ADL-15 psi

Part No. W 0372 60 PSI

Valves - Hoke 4100 series. Part no. 4172 G4Y

Filter - Hoke series 6300, Part no. 6321G4Y

Quick Disconnects - TOMCO Part no. 3452-R6 and 3472-R2, 1/4" SS. : Part no. 3650-R6 and 3670-R2, 3/8" brass

Capillary tubing - Small Parts Inc. Part no. CTX-6210 .010 inch I.D.

316 S.S.

All tube tees - Hoke part no. 4TTT - S, 1/4" S.S.

Female elbows - Hoke part no. 4LF4-S, 1/4" S.S.

Male connector - Hoke part 4CM4-S, 1/4" S.S.

Tubing - 1/4" S.S. NBS storeroom FSC 4710-914-5137*
- 12 -

Capillary tube end fittings - NBS shop made from S.S. rod FSC 9510-950-3671*

Outlet manifold block - NBS shop made of mild steel

Outlet nozzles - From surplus lab sinks.

* General Services Administration Federal Stock Control number.

** In no case does the identification of the manufacturers in the above

listing imply recommendation or endorsement by the National Bureau of

Standards, nor does it imply that the item identified is necessarily

the best available for the purpose.
- 13 -

Appendix

In order to determine the effect various physical variables have on the

output concentration of the PADS, it is necessary to look at the equations

of flow of the critical flow nozzle and the laminar flow porous plug.

Laminar flow of an incompressible fluid through a capillary tube is given

by the well known Poiseuille relationship

^ Ap TrD*^ ^ 128 ML

Therefore the volume flow is proportional to the pressure drop Ap through

the porous plug and the "effective" inside diameter D of the plug, to

the 4th power, and inversely proportional to the "effective" length L

of the plug and the viscosity y of the flowing fluid.

When a compressible fluid is used, the results for a laminar device are more consistant and linear if the volume rate is defined at the conditions

presumed to exsist at the midpoint of the laminar device. P2 , as measured

on the PADS, is the pressure drop across the porous plug laminar flow

restrictor and all piping downstream from the plug to the outlet manifold.

P2 can be said to consist of two parts, Ap2 the drop across the porous

plug, and Ap3 the drop across the piping downstream of the porous plug

to the discharge, and

P2 = Ap2 + Ap3. (B)

Ap3 is assumed to be zero for the evaluation of this unit.

The midpoint absolute pressure in the porous plugs is given by

P'^2m P? + B

-

2 (c)

where B is the barometric pressure. Rearranging (B) and combining with

(C) produces

:

P2 Ap-^

Let (Qo)p equal the flow rate at outlet conditions where the density is Po,

and the pressure is B. If we assume that the temperature at the outlet of

the porous plug is equal to the temperature at the midpoint of the porous

plug the relationship between (Qo)p and the volume rate Qp defined at the

midpoint pressure in the plug is given by

B+l^+^3

(Qo)p = ( ^—- ) Qp- (E)
- 14 -

Upon substitution of Qp for 0 and Ap2 for Ap in equation (A),

it becomes

(Qo)p = 144 (P2 - Ap3) ttD** 128m L

P2 Ap3

(1 + ^ +

2B 2B

(F)

where the factor 144 converts (P2 - Ap3) in psi to psf

.

For the critical nozzle the mass flow in Ibm sec~^ is represented

by the equation

Mn = Cd AC* ( )//R*Tn

If we let (Qo)n ^"l^^l the volume flow rate at the outlet condition where

the density is Poin Ibm ft~^, then

Cd AC* ; (Qo)n = T— (Pl+B) /Bs /R*Tn

Po

(G)

Previously it was stated in equation (2) that C = Ci (Qq)p/ (Qq)n and

inserting equation F and G into it produces:

C = Ci

(Qo)p

(Qo)n

= Ci

(P2-Ap3)(l -f f|4-|f3)

(Pi + B)/Bs

ttD'+(144)B/R*Tn

RTq128ijL Cd A C*

(H)

where B/RTq from the gas law has been used in place of Pq.

Equation (H) can be used as is to evaluate effect of changes of the

physical variables on C, or its differential can be used for the same purpose. Taking the differential of the log of C produces

dC

C~

dC- dP: +

Ci P2-AP3

1 +

P2-AP3

P2+2B+Ap3

d(Ap3)

P2-AP3

r 1 - P2+2B+AP3

dB

1 - P2+Ap3 B dP^

P2+2B+AP3 ~ Pi+B ~ Pi+B

1.3dT

dL dA dD d(C*Cd)

_ __ + 4 __ _ _____ (I)
- 15 -

1 3ciT

where du/u Is replaced by 0.8 dT/T, and added to d (log /t~ ), giving —

It can be shown that the sum of the terms (~^~^"*"'^^) is equal

to cxdT, assuming that the gas temperature for the porous plug and nozzle

are the same and, where a is the linear coefficient of expansion for the

nozzle and porous plug material. Since stainless steel is used in the

PADS, adT is very small for ordinary temperature variations, and can be ignored

In the term ^i^^^^'^ , C*C^ varies very little, less than ±0.1 percent,

L'^Ld

within the pressure and temperature range which it would normally encounter,

and is also ignored.

Equation (I) can be reduced to;

dC dCi dP2 d3 dPi dT d(Apo) ^-^+1.3 ^+.46^-75-5^ -1.3 ^-0.7^^ (J)

for operation near the design pressures and temperature for the PADS. The

term containing the effect of change of Ap3 is retained as a reminder that

the output concentration can be changed by inadvertant or intentional

changes of plumbing downstream from the porous plugs. Equation (J) is

a differential equation and is strictly accurate only in the limit of

all differential quantities approaching zero.

As an example of the use of equation (J) , consider what effect changes

in barometric pressure would have on the output concentration. If the

barometer pressure increased by 1 percent, the change in output concentration at 1 ppm would be,

dC = 0.46 ^C = 0.46 (.01)(lppm) = + .0046 ppm.
- 16 -

Gage Calibrations

Marsh 60 psi gage

Type 11-S

Actual pressure Average**

psi Gage Reading

15.21 14.98

20.21 19.82

25.20 24.95

30.20 30.0

35.20 34.9

AO. 20 39.82

A5.19 44.95

50.19 50.1

55.19 55.28

60.19 60.5*

Marsh 15 psi gage

. Type 11-S

Actual pressure Average**

psi Gage Reading

5.22

10.21

15.21

5.4

10.45

15.57*

* Estimated overscale readings.

** Averages of 5 readings
- 17 -

Calibration of Porous Plugs *

Nominal

Flow Rate

cc/mln

0.025

0.25

2.5

Porous Pluj

Pressure

psl

11.71

11.71

11.71

Calibrator

Volume-cc

0.1044

0.2090

1.2789

Average

Calibrator

Time-sec

251.6

45.86

30.42

Average

Calibrator

Flow Rate-cc/min

0.025

0.27

2.52

Calibration of Critical Nozzle *

Nominal Flow 5 liters/min.

Calibrator volume = 1.4999 liters

Nozzle Calib. Calib. Barom. Nozzle Calib. Calib. Flow

Temp. °C Temp. °C Press, psig psia Press, psig Time-sec (314.7 psia &

21 .8 21.95 .059 14.42 42.65 16.091 5.475

21.8 21.85 .059 14.41 39.65 16.961 5.195

21.85 21.95 .059 14.41 37.65 17.578 5.011

22.05 22.25 .059 14.41 35.65 18.274 4.815

22.35 22.05 .059 14.61 34.65 18.848 4.736

22.35 22.05 .059 14.61 29.88 - 21.027 4.246

* Five determinations of flow were made at each rate.
FORM NBt-34« - 18 -

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

WASHINGTON, D.C. 20234

REPORT OF ANALYSIS

of

Tank of 0.2 Percent SO.

Submitted by G. P. Baumgarten, 213.06

Project NOo 2130161 Div. Req. No. 213-6003-74

Titration with standardized sodium hydroxide shows 2230 ppm

SO2 accurate within ± 22 ppm.

E. R. Deardorff

Chemist

.^^'John K. Taylor

Chief

Air Pollution Analysis Section

/s I. Shultz

ice Analysis Coordinator

Analytical Chemistry Division

August 1, 1973

74-lla
FORM NBS-S4t - 19 -

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

WASHINGTON, D.C. 20234

REPORT OF ANALYSIS

Calibration of Total Sulfur Analyzer

Submitted by G. P. Baumgarten, 213.06

Project No. 2130161, Div. Req. No. 213-6003-74

Two total sulfur analyzers, a Meloy SA-185-2 and a Bendix

8300, were calibrated with air containing known amounts of

sulfur dioxide. The concentration of sulfur dioxide was fixed

using permeation tubes (NBS Standard Reference Materials 1625

and 1626) and measured flows of air.

The instruments were calibrated by the procedure specified by

the manufacturers. The attached table summarizes the analytical

results

.

John K. Taylor, C4iief

Air Pollution Analysis Section

September 24, 1973

Enclosure

74-11
- 20 -

MliLOY SA-185-2*** BENniX 8300

Concentration of Sulfur Dioxide

in Parts Per Million

Calculated* Observed**

0.953

0. 795

Q.68 3

0. 533

0.475

0.437

0.370

0.324

0.269

0. 198

0.112

0.0775

0.067

0. 056

0.954

0.805, 0.810

0.687, 0.696

0.540, 0.550

0.492, 0.498

0.441, 0.446

0.372, 0.374

0.321, 0.328

0. 264

0.186, 0.188

0.098, 0.096

0.065, 0.063

0.056, 0.054

0.046, 0.046

Calculated* Observed**

0 .953

0.795

0.683

0. 533

0.437

0.370

0.269

0.941

0.935

0.803

0. 800

0. 695

0.686

0.54 3

0. 536

0.947, 0.937,

0.948

0.798, 0.798

0. 801

0.694, 0.688,

0.688

0.545, 0.540,

0.536

0.444, 0.443, 0.438,

0.434

0.372, 0.373, 0.372,

0.370, 0.365

0.262, 0.263, 0.262,

0.255

* The calculated value is based on the known rate of the permeation tube

and the measured flow of air. Estimated inaccuracies in the rates of

permeation and of air flow are ±0.5 percent and ±2 percent, respectively.

** The observed values were recorded from the analyzers analog output on a calibrated recording volt meter.

*** Used in the evaluation of the PADS

Note: The Bendix unit was borrowed from EPA but not used for this evaluation.

It will be used in subsequent work.
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Polluted Air Delivery System

Critical

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Pure VI

1 V. 5 LPM

r MIXER

I

^0 2.5X10"^ LPM

/OS 25X10-^ LPM

01 PPM SO2

1 PPM SO2

I PPM SO2

POROUS PLUGS

2000 PPM SO'

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NBS.114A IREV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. i^i;HI.IC A l lON OR RHPOK T NO. NBSIR 73-414

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLIBuilding and Evaluation of a Polluted Air Delivery System

5. Publication Date April 1974

6. Performing Organization Code

7. AUTHOR(S)

G. P. Baumgarten and F. W. Ruegg

8. Performing Organ. Report No. NBSIR 73-414

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract /Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Environmental Protection Agency

National Environmental Research Center

Quality Control Branch

Chemstrand Bldg., Research Triangle Park, N- Carolina 27711

13. Type of Report & Period Covered

Final Report on PADS 1

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less [actual summary o[ most signilicant information. If document includes a significant

bibliography or literature survey, mention it here.) The building and evaluation of a prototype SO2 polluted air delivery system (PADS)

is discussed. The delivery system was built to delivery sulfur dioxide (SO2) in

air at a rate of 5 liters per minute with design concentrations by volume of

1.0, 0.1 and 0.01 parts per million. It consists of a diluent air delivery

system utilizing a critical flow sonic nozzle and three separate concentrated

SO2 in air flow systems utilizing laminar flow porous plugs ^ one plug for each

desired output concentration. The delivery system is contained in a dispatch

case and the two gases are delivered to it from pressurized containers through

detachable supply lines. Prospective use by unskilled technicians dictated

simplicity and durability and compactness.

By maintaining specific upstream pressures on the critical flow nozzle and the

laminar flow porous plugs of 45 and 12 psig respectively, the prototype PADS

produced average output concentrations of 0,76, 0,100 and 0.003 parts per million

of SO2 in air based on concentration measurements with an NBS calibrated analyzer.

The expected output concentrations were 0,98, 0,105 and 0.010 respectively, based on flow calibrations of the individual components. The uncertainty of the output

concentration is estimated to be about ± 10 percent.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Air pollution; critical flow; laminar flow; nozzle; porous plug; sulfur dioxide

concentration

18. AVAILABILITY [X Unlimited

1 For Official Distribution. Do Not Release to NTIS

1 Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C13

1

' Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF PAGES

32

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

USCOMM-DC 29042-P74