Chemical Hazards in the Workplace - ACS Publications - American

tions require careful attention to the sample collection and ... Figure 1 shows the battery discharge effect on flow rate(_3) . ... 10. 15. 20. 25. 30...
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30 Industrial Hygiene Air Sampling with Constant Flow Pumps W. B. BAKER, D. G. CLARK, and W. J. LAUTENBERGER

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Applied Technology Center, Ε. I. du Pont de Nemours and Company, Incorporated, Wilmington, DE 19898

In air monitoring program accurate concentration determina­ tions require careful attention to the sample collection and subsequent analysis of the collected sample. The analysis is the most c r i t i c a l step, especially i f very low levels (ppb) of contaminants are being determined. However, in many cases, sampling may be the least accurate step since this job is performed in the f i e l d with portable equipment under conditions far less favorable than those that can be created and controlled in the lab(1). NIOSH states that acceptable a i r monitoring methods must come within ±25% of the true concentration for 95% of the samples taken. The error factor attributed to sampling pumps is a coefficient of variation of 5% (2). If there is no bias in the sampling pump, the accuracy i s : Accuracy = 2 CVp (Assumes no bias) =+10% where:

CV = 5% p

Errors in sampling (sample volume determination) are due to erroneous measurement of time or flow rate. Time can be measured so a c c u r a t e l y that f l o w r a t e e r r o r s make up the m a j o r i t y of the -10% v a r i a t i o n a t t r i b u t e d to the average sampling pump. Sampling E r r o r s f o r Flow V a r i a t i o n s Flow r a t e v a r i a t i o n s cause three types of sampling e r r o r s : 1) d i r e c t sample s i z e e r r o r ; 2) time-of-day b i a s ; and 3) r e s p i r a b l e sampling e r r o r s . A d i r e c t e r r o r occurs when the

0097-6156/81/0149-0491 $05.00/0 © 1981 American Chemical Society

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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492

C H E M I C A L HAZARDS IN T H E W O R K P L A C E

sampler i s c a l i b r a t e d to operate a t a given flow r a t e but changes f l o w r a t e . I f a sampler i s c a l i b r a t e d a t 1 l i t e r / m i n . but operated a t an average f l o w r a t e of 0.9 l i t e r s / m i n . , t h i s 10 percent f l o w r a t e e r r o r would cause a 10 percent e r r o r i n sample s i z e . D i r e c t e r r o r s can occur i n simple p o s i t i v e - d i s p l a c e m e n t pumps as the r e s u l t of b a t t e r y discharge and other e f f e c t s . F i g u r e 1 shows the b a t t e r y discharge e f f e c t on f l o w rate(_3) . The i n i t i a l f l o w r a t e decrease i s caused by the NiCd b a t t e r y discharge c h a r a c t e r i s t i c . D i r e c t e r r o r can a l s o r e s u l t from the e f f e c t of i n l e t pressure changes on flow as shown i n F i g u r e 2(3). Since the pump i s working on a compressable f l u i d , a i r , any i n c r e a s e i n the pressure drop reduces the flow r a t e . In the f i e l d , the c o l l e c t i o n device w i l l accumulate d i r t and dust i n a d d i t i o n to the d e s i r e d sample. This w i l l cause the pressure drop t o i n c r e a s e w i t h time. I f the sampling pump does not c o n t r o l the f l o w , i t w i l l decrease as pressure drop i n c r e a s e s . Some samplers i n c o r p o r a t e an accumulated volume i n d i c a t o r such as a stroke-counter t o permit c o r r e c t i o n f o r the above e f f e c t s . However, time-of-day b i a s can occur i f the exposure i s not constant. A h i g h contaminant l e v e l would be erroneously i n d i c a t e d i f a h i g h exposure occurred during the s t a r t of the sampling p e r i o d r a t h e r than a t the end. When performing r e s p i r a b l e sampling, the t o t a l e r r o r w i l l be g r e a t e r than the f l o w r a t e e r r o r . Cyclones used i n r e s p i r a b l e sampling a r e designed t o operate a t a s p e c i f i c f l o w r a t e . A d e v i a t i o n from t h i s s p e c i f i c f l o w r a t e w i l l cause g r e a t e r c o l l e c t i o n e f f i c i e n c y v a r i a t i o n s depending on the type of cyclone and f l o w r a t e ( 4 ) . I t becomes evident that accurate sampling r e q u i r e s a constant f l o w r a t e . Many methods have been used i n the a t t e p t to provide a constant flow sampler, i n c l u d i n g : rotometers, motor-voltage r e g u l a t o r s , r e s t r i c t i n g o r i f i c e s , pump s t r o k e counters and motor-current compensation. These methods attempt to provide compensation i n d i r e c t l y f o r the e r r o r e f f e c t s ( 5 , 6 ) . The f l o w c o n t r o l method described i n t h i s d i s c u s s i o n provides d i r e c t monitoring and c o n t r o l of sampler flow r a t e . Theory o f Operation A simple p o s i t i v e - d i s p l a c e m e n t sampler system i s shown i n F i g u r e 3. The b a s i c system contains a b a t t e r y and motor connected to a p o s i t i v e - d i s p l a c e m e n t pump mechanism and provides an e f f i c i e n t means f o r moving a i r through the sampler. I n order to provide feedback f l o w c o n t r o l the system must be expanded t o i n c l u d e the means t o monitor a i r f l o w through the pump t o compensate f o r f l o w v a r i a t i o n s . F i g u r e 4 shows a simple v e r s i o n of a feedback f l o w c o n t r o l sampler system. An o r i f i c e and pressure switch a r e used t o monitor a i r f l o w .

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

30.

BAKER E T A L .

493

Constant Flow Pump Sampling

2.2 j -

2.03

Ε

·—•—· -

--

1.{

1.6

1.4

J

1.2 2

L

3

4

J 5

L

J

I

6

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Time (hours)

Figure 1.

Flow rate vs. time, simple positive-displacement pump

2.0

Ε =•

1.5

1.0

10

Figure 2.

in,et

15 20 25 30 Inlet Pressure (in. of H2O)

35

40

Flow rate vs. inlet pressure, simple positive-displacement pump

vlypump-

Exhaust

Positive Displacement

Battery

Figure 3.

Simple positive-displacement sampler system

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

494

CHEMICAL

HAZARDS

IN

THE WORKPLACE

The pressure developed across an o r i f i c e v a r i e s w i t h flow as shown i n the equation(7): F2 Ρ = Constant χ

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Where: F = flow r a t e Ρ = f l u i d density As f l o w i n c r e a s e s , pressure drop i n c r e a s e s . When the pressure exceeds the s w i t c h set p o i n t , the contacts w i l l c l o s e . The f l o w r a t e a t which t h i s occurs i s determined by the o r i f i c e s i z e and the pressure s w i t c h set p o i n t . A v a r i a b l e o r i f i c e i s used so that flow r a t e can be adjusted. The pressure s w i t c h i s connected to an i n t e g r a t o r c i r c u i t . The i n t e g r a t o r c i r c u i t i s c h a r a c t e r i z e d by an output v o l t a g e which changes s l o w l y . T h i s v o l t a g e increases when the pressure s w i t c h contacts are open i n d i c a t i n g that the f l o w r a t e i s below the set p o i n t . When the f l o w r a t e i s above the set p o i n t , the switch contacts a r e closed and i n t e g r a t o r output decreases. The output v o l t a g e of the i n t e g r a t o r i s a m p l i f i e d and a p p l i e d to the motor. In t h i s system the motor s t a r t s t o speed up when the f l o w r a t e i s low and slows down when the f l o w r a t e i s h i g h . Thus the c o n t r o l system w i l l always c o r r e c t i n the proper d i r e c t i o n f o r any f l o w r a t e e r r o r . F i g u r e 5 f u r t h e r i l l u s t r a t e s i n t e g r a t o r o p e r a t i o n and a s s o c i a t e d ramping e f f e c t s . Again i t can be seen that w i t h the pressure s w i t c h open, the i n t e g r a t o r ramps up w i t h time; and conversely when the switch i s c l o s e d , the i n t e g r a t o r ramps down. I n a pump w i t h p u l s i n g f l o w , the s w i t c h opens and c l o s e s w i t h each pump s t r o k e because i t responds to the momentary increases i n f l o w and pressure. During each r e v o l u t i o n the pump b r i e f l y speeds up and slows down f o r equal periods of time when operating a t constant c o n d i t i o n s . F i g u r e 6 shows the o p e r a t i o n of the c o n t r o l system when a f l o w i n c r e a s e i s r e q u i r e d . Some e x t e r n a l i n f l u e n c e reduce the f l o w r a t e below the set p o i n t . This caused the pressure switch t o stay open longer during each c y c l e and the output v o l t a g e g r a d u a l l y increased u n t i l the f l o w r a t e was r e s t o r e d to the set p o i n t . Thus i t i s shown that the feedback f l o w c o n t r o l system w i l l a c t t o c o r r e c t f o r any c o n d i t i o n that would cause f l o w variation. With a p u l s i n g f l o w , there i s an e r r o r e f f e c t which may cause overcompensation. The equation f o r t h i s e r r o r e f f e c t is(7): (Error) Where:

e = Constant χ *f χ Δ Ρ ~{~ i s frequency of p u l s a t i o n i s the pressure drop

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

30.

BAKER E T A L .

495

Constant Flow Pump Sampling

Orifice l n , ? t

Exhaust

V-L/Pump I I ' ό ) Motor >

Feedback Signal

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Battery

Switch Position

Integrator/ Amplifier

Figure 4.

Feedbackflowcontrol sampler system

J—I

Γ

Integrator Output Voltage

Time-

Switch Position

Figure 5.

Integrator operation

u — L _ T

Integrator Output Voltage

Time-

Figure 6.

Integrator operation duringflowincrease

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

496

CHEMICAL

H A Z A R D S IN

THE WORKPLACE

As pump load i s i n c r e a s e d , t h i s e r r o r e f f e c t causes the f l o w r a t e t o i n c r e a s e more than necessary. This problem i s e l i m i n a t e d by u s i n g an accumulator to f i l t e r the f l o w through the o r i f i c e . F i g u r e 7 shows the completed feedback c o n t r o l system which i n c o r p o r a t e s an accumulator between the pump and o r i f i c e .

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P r a c t i c a l Systems F i g u r e 8 shows the a i r passages through a t y p i c a l feedback c o n t r o l constant f l o w sampler. The a i r enters on the l e f t , proceeds through the v a l v e s and diaphragm of the pumping mechanism i n t o an accumulator. This accumulator i s a s m a l l space w i t h a l a r g e s u r f a c e covered by an elastomer sheet to provide flow-smoothing a c t i o n . A pressure switch i s connected to taps l a b e l e d P.S. High and P.S. Low. The pump mechanism i s operated by a s m a l l e l e c t r i c motor and gearhead. A d d i t i o n ôf the c o n t r o l c i r c u i t and b a t t e r y complete t h i s constant f l o w system. Other sampling pump a p p l i c a t i o n s r e q u i r e m o d i f i c a t i o n s of t h i s c o n t r o l system f o r proper performance. When the previous system was a p p l i e d t o a h i g h f l o w m u l t i - c y l i n d e r pump, the i n h e r e n t l y smooth f l o w e l i m i n a t e d the need f o r an accumulator. The pressure switch experienced r a p i d wear due to the high r a t e of o p e r a t i o n , about 100 c y c l e s per second (6000 per minute). This problem was e l i m i n a t e d by adding a pneumatic f i l t e r as shown i n F i g u r e 9. The pneumatic f i l t e r c o n s i s t s of a d i f f e r e n t i a l accumulator and f o u r o r i f i c e s and f i l t e r s the pressure s i g n a l before i t reaches the pressure s w i t c h . This f i l t e r had a time delay i n passing the s i g n a l t o the pressure s w i t c h , but by v a r y i n g the s i z e of the o r i f i c e s , i t was p o s s i b l e to a d j u s t the time delay so that the pump and c o n t r o l system c y c l e d a t a r a t e of 8 c y c l e s per second, extending switch l i f e f o r s e v e r a l years of normal o p e r a t i o n ; y e t , the c y c l i n g remained r a p i d enough t o e l i m i n a t e any n o t i c e a b l e v a r i a t i o n i n flow. Another v a r i a t i o n of the system was r e q u i r e d f o r a s i n g l e c y l i n d e r (diaphragm) pump that operated a t a high speed and produced p u l s a t i n g flow. As shown i n F i g u r e 10, t h i s pump uses a combination of the two c o n t r o l systems p r e v i o u s l y described. Because t h i s p a r t i c u l a r sampler was not r e q u i r e d to f i l l bags, the pneumatic f i l t e r was not designed w i t h d i f f e r e n t i a l accumulators. A l a r g e r accumulator i n the a i r f l o w passage preceding the o r i f i c e provided enough f l o w smoothing f o r proper o p e r a t i o n of the needle v a l v e . An accumulator w i t h two diaphragms was used to smooth the i n l e t f l o w of t h i s sampler. Thus, i t has been demonstrated that p r e c i s e f l o w c o n t r o l i s p o s s i b l e w i t h many d i f f e r e n t pump mechanisms.

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

30.

BAKER E T A L .

Constant Flow Pump Sampling

/"—\

l n l e t

Collection Device

Accumulator " — , w

497 Adjustable Orifice Exhaust

Vj^/Pump

Motor (

ô

Pressure Switch Feedback Signal

Battery

Feedback Control

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Figure 7. Feedback flow control system for low-speed pump

Adjustable Orifice

Inlet

Collection Device

V

Motor!

y

Battery

Figure 9.

!

yVariable Speed P u m p Differential Accumulator

Exhaust

^Orifice (Typ.) Feedback Signal

J Pressure Switch

Feedback Control

Feedback flow control system for high-speed pump with smooth flow

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

498

CHEMICAL

HAZARDS

IN

THE

WORKPLACE

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Performance The f o l l o w i n g data i l l u s t r a t e s the performance of feedback flow c o n t r o l samplers under t e s t c o n d i t i o n s which represent f i e l d c o n d i t i o n s . F i g u r e 11 shows the f l o w r a t e s t a b i l i t y versus time f o r a sampler operated on i t s b a t t e r y . The flow c o n t r o l system maintained a constant f l o w r a t e even though the b a t t e r y was d i s c h a r g i n g ( 3 ) . F i g u r e 12 shows the f l o w r a t e s t a b i l i t y versus l o a d (input pressure drop). A v a r i a b l e o r i f i c e connected to the sampler was used to a d j u s t the pressure drop over a range of 0-16 inches water column pressure drop. Flow r a t e was monitored by a rotameter. The flow r a t e was constant to w i t h i n * 2 percent Q ) . F i g u r e 13 shows the f l o w r a t e s t a b i l i t y versus ambient a i r temperature. The samplers were operated i n an environmental chamber over a temperature range of 0°C to 70°C. A heat exchanger was connected to the sampler i n l e t to ensure that the temperature of the i n l e t a i r was the same as the ambient a i r temperature. The sampler f l o w r a t e was monitored w i t h a bubble tube o u t s i d e the chamber. F i g u r e 14 shows the a l t i t u d e e f f e c t on the sampler f l o w r a t e . T h i s t e s t was performed i n a vacuum chamber. The f l o w r a t e was monitored w i t h a bubble tube which was mounted i n the chamber and operated by remote c o n t r o l . Each data p o i n t i s an average of 18 p i e c e s of data: three samplers and three f l o w r a t e s which were monitored w h i l e both i n c r e a s i n g and decreasing the vacuum. I f sampling must be performed under p r e s s u r e , temperature and a l t i t u d e c o n d i t i o n s which d i f f e r from the c a l i b r a t i o n c o n d i t i o n s , the above e f f e c t s must be known and used as c o r r e c t i o n s i n sample volume c a l c u l a t i o n s . Calibration Any sampler, even one which w i l l m a i n t a i n constant f l o w , must be c a l i b r a t e d to ensure accuracy. D a i l y c a l i b r a t i o n ensures r e p e a t a b i l i t y . D i f f e r e n t methods used f o r c a l i b r a t i n g samplers i n c l u d e : rotometers, w e t - t e s t meters, pressure gauges across f i x e d o r i f i c e s , mass f l o w meters, hot w i r e f l o w meters and bubble tubes. Each of these c a l i b r a t i o n devices r e q u i r e s an a p p r o p r i a t e c o r r e c t i o n f a c t o r . Some of the devices measure mass f l o w r a t h e r than v o l u m e t r i c flow. Sampling r e q u i r e s volumetric flow c a l i b r a t i o n . Most of the c a l i b r a t i o n devices r e q u i r e o r i g i n a l c a l i b r a t i o n by a primary standard. A bubble tube i s the only d e v i c e mentioned that i s a primary standard(8). I t can be checked w i t h simple l a b o r a t o r y t o o l s .

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

BAKER E T A L .

499

Constant Flow Pump Sampling Main Accumulator

Inlet Collection Inlet Device Accumulator

Exhaust

Pump

, ·

Motor(o)

Orifice ( T y p . ) — •

Feedback Signal Battery

Kl Ha-

I Pressure Switch

Feedback flow control system for high-speed pump with pulsating flow

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Figure 10.

Feedback Control

π Signal d Accumulator U(Typ.)

140 Γ

120 -

100

80

60 40 3

4

5

Time (Hours)

Figure 11.

I

200

r

150

-

100

-

Flow rate vs. time, pump model P-125

50

6 Δ Ρ (in. of

Figure 12.

8 H 0)

10

14

16

2

Flow rate vs. load (pressure differential), pump model P-125

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

500

CHEMICAL

10

20

30

40

HAZARDS

60

50

IN T H E W O R K P L A C E

70

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Temperature (°C)

Figure 13.

Temperature effect, pump model P-2500: (O) Pump #1, Ο #2

Pump

20 ι -

Ο

1

2

3

4

5

6

7

8

Altitude (χ 1000 Ft.)

Figure 14.

Altitude effect, pump model P-2500

Flow

LABORATORY

SAMPLER

Bubble Solution C u p

FlowBubble Solution Bulb

Figure 15.

Bubble tubes

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

30.

BAKER E T A L .

Constant Flow Pump Sampling

501

I f the bubble tube i s connected t o the i n l e t s i d e o f the pump, no c o r r e c t i o n s are r e q u i r e d . T h i s r e q u i r e s the sampler bubble tube shown i n F i g u r e 15. I f a standard l a b o r a t o r y type bubble tube i s connected to the exhaust s i d e of the pump, the volume of a i r which has gone through the sampler pump becomes a d d i t i o n a l l y h u m i d i f i e d by the soap s o l u t i o n i n the bubble tube causing an i n c r e a s e i n the volume. Connecting a bubble tube t o the exhaust s i d e of the pump can cause e r r o r s i n the range of 1 t o 3% i f the proper c o r r e c t i o n i s o m i t t e d ( 9 ) .

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Conclusion A sampling method i s c u r r e n t l y a v a i l a b l e i n the f i e l d w i t h accuracy that exceeds NIOSH c r i t e r i a . T h i s accuracy r e s u l t s from a sampler which maintains true constant f l o w throughout the sampling p e r i o d . Acknowledgements The authors wish t o express t h e i r a p p r e c i a t i o n t o the many persons who have provided i n v a l u a b l e a s s i s t a n c e i n c l u d i n g : D. 0. Conn, I I I , f o r e a r l y guidance; V. W. Keedy, D. N. Mount and C. J . Lundgren f o r recent help i n design and t e s t i n g ; and A. S. P o l l a c k f o r e d i t o r i a l a s s i s t a n c e . Literature Cited 1. 2.

3.

4.

5. 6.

7.

The Industrial Environment - Its Evaluation and Control, NIOSH, 1973, p. 101. Taylor, D. G . ; Kupel, R. E.; Bryant, J. M. "Documentation of the NIOSH Validation Tests," NIOSH, Cincinnati, Publication No. 77-185, A p r i l 1977. Parker, C. D.; Lee, M. B . ; Sharpe, J. C. "An Evaluation of Personal Sampling Pumps in Sub-Zero Temperatures;" Research Triangle Institute Report for NIOSH. Contract No. 210-76-0124, September 1977. "Air Sampling Instruments for Evaluation of Atmospheric Contaminants;" American Conference of Governmental Industrial Hygienists, F i f t h E d . , Cincinnati, 1978. Moore, G . ; Steinle, S.; LeFebre, H. Am. Ind. Hyg. Assoc. J., 1977, 38, 195. Almich, B. P . ; Rubenzahl, Μ. Α . ; Carson, G. A. "Electronic Refinements for Improved Operation of Portable Industrial Hygiene for A i r Sampling Systems," NIOSH, Cincinnati, Publication No. 75-169; May 1975. Stearns, R. F.; Johnson, R. R.; Jackson, R. M . ; Larson, C. A. "Flow Measurements with Orifice Meters," D. Van Nostrand Company, Inc.; New York, 1951.

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

502

CHEMICAL HAZARDS IN THE WORKPLACE

8.

Nelson, G. O. "Controlled Test Atmospheres - Principles and Techniques," Ann Arbor Science Publishers, Inc.; Ann Arbor,

9.

Baker, W. C.; Pouchot, J. F. Flowmeter Survey for Measuring the Flow of Air and Other Gases; Teledyne Hastings - Raydist; Hampton, Va., March 15, 1979, 24-25.

1976.

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RECEIVED October 17, 1980.

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.