Astm stp 1161 1993

STP 1161 Leak Detection for Underground Storage Tanks Phtlip B.. Young, edztors ASTM Pubhcanon Code Number PCN 04-011610-65 ASTM 1916 Race Street Philadelphia, PA 19103 Copyright b

STP 1161

Leak Detection for Underground Storage Tanks

Phtlip B Durgin and Thomas M Young, edztors

ASTM Pubhcanon Code Number (PCN)

04-011610-65

ASTM

1916 Race Street Philadelphia, PA 19103

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Leak detectlon for underground storage tanks / Philzp B Durgln and Thomas M Young, editors

(STP 9 i161)

"ASTM publlcatlon code number (PCN) 04-011610-65"

Papers presented at the symposium of the same name, held in New Orleans, LA on 29 Jan 1992

Includes bibliographical references and indexes

ISBN 0-8031-1858-9

is 0-8031-1858-9/93 $2 50 + 50

Peer Review Policy

Each paper pubhshed In this volume was evaluated by three peer reviewers The authors addressed all of the reviewers' comments to the satisfaction of both the techmcal editor(s) and the ASTM Committee on Publications

The quality of the papers in this publication reflects not only the obvious efforts of the authors and the technical editor(s), but also the work of these peer reviewers The ASTM Committee on Publications acknowledges with appreciation their dedication and contribution

to time and effort on behalf of ASTM

Printed m Phdadelphla, PA March 1993

Foreword

This pubhcatlon, Leak Detection for Underground Storage Tanks, contains papers pre-

sented at the symposium of the same name, held in New Orleans, LA on 29 Jan 1992 The

symposium was sponsored by ASTM Committee E-50 on Environmental Assessment Philip

B Durgm of Veeder-Root m Simsbury, CT and Thomas Young of A n n Arbor, MI presided

as symposium co-chairmen and are editors of the resulting publication

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Contents

I N T E R N A L M O N I T O R I N G

Volumetric Leak D e t e c t i o n - - A Systems P e r s p e c t i v e - - w F ROGERS

Error Sources in Automatic Tank Gauging S y s t e m s - - D w FLEISCHER

Evaluation of Metal Oxide Semiconductor and Polymer Adsorption Gas Sensors as

Applied to Underground Storage Tank Leak Detection M A PORTNOFF,

R GRACE~ A M G U Z M A N , A N D J H I B N E R

Fiber Optic Chemical Sensors-An O v e r v i e w - - A E GREY AND J K PARTIN

Field Results of Hydrocarbon Vapor Monitoring to Detect Leaking T a n k s - -

REGULATIONS AND STANDARDS

HOW Well Do Leak Detection Methods Work?: Preliminary Results from the EPA

Evaluation of Pipeline Leak Detection S y s t e m s - - w D GLAUZ, J D FLORA,

Expedited Enforcement of UST Regulations in New M e x l c o - - s A SUTTON-MENDOZA 162

Impact of Standards and Certification on Environmental Impairment Liabihty

S I T E A N D R I S K E V A L U A T I O N

Characteristics of Non-Petroleum Underground Storage T a n k s - - a w HILEGER,

J W STARR, M P MA~ARTHUR, A N D J W MARESCA, JR 175

Risk Assessment to an Integrated Planning Model for UST P r o g r a m s - - K w

Use of On-Site Vapor Analysis in UST Site Assessments: A Summary of Results at

Screening Methodology for Selecting Clean-Up Technologies at Leaking

T h e e n v l r o n m e n t a l d e c a d e of the 1 9 8 0 ' s b r o u g h t w l t h it a s t e a d y

g r o w t h in the n u m b e r a n d s c o p e of e n v l r o n m e n t a l r e g u l a t i o n s M u c h of the c o n c e r n w a s d l r e c t e d at the c o n t a m l n a t l o n of g r o u n d w a t e r s u p p l l e s b y

o r g a n l c c h e m i c a l s A n e w l y - e m e r g e d , w l d e s p r e a d c o n c e r n w a s p r o t e c t l o n

of g r o u n d w a t e r s u p p l l e s f r o m u n d e r g r o u n d s t o r a g e t a n k s (UST) that l e a k e d fuel T h e p u b l l c r e a l l z e d that the p r o b l e m m ~ g h t b e as c l o s e as t h e i r

VIII LEAK DETECTION FOR UNDERGROUND STORAGE TANKS

i n c r e a s e the r i s k at a site O n e of t h e s e as the t y p e of c h e m i c a l an

the t a n k a n d H a l l g e r et al p r o v a d e a n a n f o r m a t a v e s u r v e y of the

X LEAK DETECTION FOR UNDERGROUND STORAGE TANKS

Symposium Chairman and Edltor

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Internal Monitoring

Warren F Rogers

V O L U M E T R I C L E A K D E T E C T I O N - A SYSTEMS P E R S P E C T I V E

R E F E R E N C E : Rogers, W F , "Volumetric Leak Detection - A Systems

Perspective," Leak Detection for Undermround Storame Tanks, ASTM

STP 1161, Phllzp B Durgzn and Thomas M Young, Eds , American

Society for Testing and Materials, Philadelphia, 1993

ABSTRACT' Volumetric testing of USTs has grown exponentially in

recent years, stimulated by regulation and by increasing business

sensitivity to potential liabilities from leaking tanks Thls paper

attempts to lay an intellectual groundwork for discussion of such

systems and focuses on the objectives of leak detection and the role of

measurement precision The important methodological issues of

identifying and discriminating among sources of apparent volume changes

which are not related to leakage, and the analysis and interpretation of

test results will also be addressed

KEYWORDS volumetric leak detection, leak detection,

measurement accuracy, precision, leak rate, probability of detection,

probability of false alarm, error, statzstlcal inventory analysis,

automatic tank gauge

I N T R O D U C T I O N

Volumetric testing of underground storage tanks has grown

exponentially in recent years, stimulated in part by regulation and in

part by increasing business sensitivity to potential Izabzlztles from

leaking systems Research and engineering development of leak detection

systems has also expanded dramatically but has been characterized by

little evidence of sczentlflc or engineering dzsclpline

Other than the USEPArs 1988 Evaluation of Leak Detection Methods

for Underground Fuel Storage Tanks[l], little has been published which

would bring coherence to the topic The methodologies employed zn the

USEPA evaluation were reviewed and criticized by Baird [2] The

conclusions drawn in this paper, are based on statistical analysis by

the author of underground storage tank manual and automatic tank gauge-

generated data over a period of twelve years involving many thousands of

samples Unfortunately, the published material on this subject is

largely or totally to be found only zn trade literature The author has

been unable to identify anything of relevance in the scholarly

literature, hence the paucity of references

The purpose of thls paper, however, is not to present definitive

results of documented research but rather to suggest the appropriate

questions for research, to identify the variables which are relevant to

Ipresident, Warren Rogers Associates, Inc , 747 Aquidneck Avenue,

Middletown, RI 02840

Copyright 9 1993 by A S T M International

3

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the objectives of leak detection, and to establish a framework for

intelligent debate on the subject

In short, the author attempts to lay an intellectual groundwork

for d i s c u s s i o n of such systems The paper is in three parts The first

part focuses on two issues t~e objectives of leak detection, and the

role of m e a s u r e m e n t p r e c i s i o n The second part addresses the issue of

identification and d i s c r i m i n a t i o n among sources of apparent volume

changes w h i c h are not related to leakage The third part deals with the

analysis and interpretation of test results

The technology for detecting leakage from u n d e r g r o u n d storage tank

systems is and probably will continue to be dominated by various means

of m e a s u r i n g volume changes of the product in the system Thls paper is

confined to such systems

G i v e n the wide Interest in the v o l u m e t r i c approach, there has been

very substantial investment in research and e n g i n e e r i n g design of the

means to accomplish it Thls w o r k has tended, however, to be v e r y

n a r r o w l y focused, and has generally o v e r l o o k e d some fundamental

tradeoffs w h i c h can lead to more reliable results with substantially

lower cost

As so often happens in the course of technological development,

technical goals became confused with system objectives Obviously the

objective of leak detection is to provide a means of identifying and

terminating losses of product from tank systems before environmental

impairment results Engineering goals in developing leak detection

systems have invariably been stated, however, in terms of leak rates to

be detected or m i n i m u m volumes to be detected w i t h i n some arbitrary

time As we will demonstrate below, the e n g i n e e r i n g goal and the

environmental objective are fundamentally different In fact,

a c h i e v e m e n t of the engineering goal has all too frequently defeated the

environmental objective

Regulations pertaining to underground storage tank systems have

also, tended to focus on this limited goal Thus, many bodies of

r e g u l a t i o n focus entlrely on the precision of instruments and the

accuracy of measurements which may be taken over very short time frames

separated by very extended time intervals rather than on the potential

volume of product which may be released into the environment during the

hiatus b e t w e e n tests By mandating extreme short term accuracy, they

impose very hlgh unit costs which political realism dictated could only

be imposed episodically

In this article, we examine the process of v o l u m e t r i c leak

d e t e c t i o n as a parametric system, and suggest criteria for designing

systems in a somewhat more b a l a n c e d way than has hitherto been done

Conceptual System

V o l u m e t r i c leak detection in general consists of three baslc

activities

i M e a s u r i n g volume change over time

2 A c c o u n t i n g for systematic effects w h i c h cause apparent volume

changes u n r e l a t e d to discharge of product from the system

3 Interpreting the measurements

It is necessary, therefore, that the system include

I M e a s u r e m e n t accuracy

2 Ability to identify and discriminate b e t w e e n physical effects

w h i c h cause real or apparent volume changes

3 Sensitive, robust means of analyzing and interpreting the

m e a s u r e m e n t s

2The use of the terms precision and accuracy seems to generate

d i s c u s s i o n of a somewhat theological nature akin to angels dancing on pln

heads In thls paper, the terms are used as follows "Accuracy" is a

quality associated wlth a measurement actually taken, while "precision" is

a quality of the instrument used to make the measurement

ROGERS ON VOLUMETRIC LEAK DETECTION 5

T h e m o r e p r e c i s i o n , the g r e a t e r c o s t and, t h e r e f o r e , the s h o r t e r the

a f f o r d a b l e u s a g e tlme and, therefore, the n e e d for e v e r g r e a t e r

c o s t l y m e a n s o v e r m o r e e x t e n d e d tlme, w h a t h a s b e e n lostV C e r t a i n l y not

any s i g n i f i c a n t p r o t e c t i o n of the e n v i r o n m e n t , p a r t i c u l a r l y If the

To a d d r e s s this issue r e q u i r e s first that we r e - d e f l n e our terms

a n d c l e a r l y s t a t e our o b j e c t l v e P r o p e r l y stated, the o b j e c t i v e is to

l i m i t the a m o u n t of p r o d u c t lost f r o m a tank s y s t e m to some a c c e p t a b l e

v o l u m e b e f o r e the loss is i d e n t i f i e d a n d t e r m i n a t e d T h e r e f o r e , the

some b a s i s m o r e m e a n i n g f u l t h a n leak rate d e t e c t i o n c a p a b i l i t y alone,

one m u s t c o n s i d e r , in a d d i t i o n , test d u r a t i o n a n d the d u r a t i o n of tlme

L e a k d e t e c t i o n s y s t e m s are s u b j e c t to two forms of e r r o r

i E r r o r s w h i c h are i n h e r e n t in the m e a s u r i n g d e v i c e w h i c h are

W i t h p r o b a b i l i t y of d e t e c t i o n of at least 95 a n d p r o b a b i l i t y of

f a l s e l y d e c l a r i n g a leak to e x i s t w h e r e n o n e does e x i s t of no g r e a t e r

t h a n 05, the s y s t e m c a n d e t e c t a loss of i g a l l o n s p e r h o u r ( 3785 liters)

s t a t e d in terms of the e r r o r d i s t r i b u t i o n o b s e r v e d d u r i n g that

t e s t o n l y Hence, the n e e d for i n t e r p r e t i v e a n a l y s i s

The f o r e g o i n g c o n s i d e r a t i o n s l e a d to this f o r m u l a t i o n Let

d e t e c t i o n o f at least 95 S i m p l i f y i n g greatly, we c o u l d say that this

r e q u i r e s that, d u r i n g the d u r a t i o n of a test, for a leak to be detected, the v o l u m e lost m u s t e x c e e d two s t a n d a r d d e v i a t i o n s of the n o i s e

ROGERS ON VOLUMETRIC LEAK DETECTION 7

g a l l o n s p e r h o u r The d u r a t i o n of the t e s t is one h o u r T h e r e f o r e , the

v o l u m e loss d u r i n g the c o u r s e o f the test a n d that w h i c h the test m u s t

d e t e c t is the same as the h o u r l y l e a k r a t e The r e q u i r e d s t a n d a r d

C o n s i d e r n o w a s y s t e m at the e x t r e m e f r o n t i e r of the s t a t e of the art o f p r e c i s i o n , c a p a b l e of d e t e c t i n g 005 g a l l o n s p e r h o u r (19 ml/hr),

in a test l a s t i n g one h o u r To do so, its e r r o r d i s t r i b u t i o n m u s t h a v e

a s t a n d a r d d e v i a t i o n no g r e a t e r t h a n 0025 g a l l o n s (9 5 m l / h r ) A less

s o p h i s t i c a t e d system, to a c h i e v e the same level of e n v i r o n m e n t a l

p r o t e c t i o n , t e s t i n g monthly, w i t h test d u r a t i o n of 30 days, n e e d h a v e an

e r r o r s t a n d a r d d e v i a t i o n of no b e t t e r t h a n 21 9 g a l l o n s (82 9 liters)

A n o t p a r t i c u l a r l y w e l l t r a i n e d o p e r a t o r s t i c k i n g h l s t a n k w i t h o n l y

m o d e r a t e care c a n do this w e l l

T h e p o i n t h e r e is n o t to e n d o r s e e i t h e r a p p r o a c h b u t to p l a c e the issue o f m e a s u r e m e n t p r e c i s i o n in p e r s p e c t i v e M u c h of the d i s c u s s i o n

of l e a k d e t e c t i o n s y s t e m s treats p r e c i s i o n as an e n d u n t o i t s e l f As the p r e v i o u s d i s c u s s i o n s h o u l d m a k e clear, it is l a r g e l y m e a n l n g l e s s

u n l e s s p l a c e d in the c o n t e x t of test duration, c o s t a n d test f r e q u e n c y

In short, as in any e n g i n e e r i n g a p p l i c a t i o n , the c o s t s a n d b e n e f i t s n e e d

to b e a n a l y z e d a n d e v e n a s u p e r f l c i a l e x a m i n a t i o n s h o w s that the

i m p l i c a t i o n s for s y s t e m s a n d p o l i c y w o u l d not be t r i v i a l

PART T W O

S y s t e m a t i c E r r o r s

In the p r e v i o u s section, we d i s c u s s e d the i n h e r e n t m e a s u r e m e n t

a c c u r a c y o f l e a k d e t e c t i o n systems, t h e i r i n h e r e n t e r r o r rates a n d the

i m p l i c a t i o n s of these for s y s t e m a c c u r a c y a n d e n v i r o n m e n t a l p o l i c y In this section, w e d i s c u s s e r r o r s w h l c h are e x t r a n e o u s to the m e a s u r i n g

d e v i c e b u t i n h e r e n t in the q u a n t i t y b e i n g m e a s u r e d T h a t Is, the a c t u a l

object b e i n g m e a s u r e d are assumed known to a level of accuracy

commensurate w i t h the measurement to be attempted

Neither of these criteria are met when attempting praccical field

m e a s u r e m e n t of product volumes in functioning u n d e r g r o u n d storage tank

systems The words "practical" and "functioning" here were carefully

chosen The objective is to t e s t real w o r l d systems in their operating

environments Many of the difficulties this imposes can and have been

offset under controlled laboratory conditions The artificialities

introduced by such condltlons, in attempts to date, have generally led

to conclusions w h i c h were not supportable or capable of b e i n g replicated

u n d e r field conditions

It is useful to separate the various e r r o r - i n d u c i n g phenomena into

two categories

i Those w h i c h tend to distort and detract from the usefulness of

simple, manual Inventory record-keeping as a leak detection

m e c h a n i s m

2 Those w h i c h became evident when, on-site, p r e c i s i o n testing

methods were introduced, presumably to offset those in the first

group

Historically, this was h o w the subject evolved Manual inventory

was found to be inadequate as a leak detection system and means were

sought to improve on it Unfortunately, the assumed sources of

inventory control deficiencies were not subjected to empirical

evaluation Many of the deficiencies deemed crucial turned out, on

subsequent analysis, to be of little consequence Others, w h i c h have

s u b s e q u e n t l y b e e n identified as crucial, were o v e r l o o k e d

Thus, it was accepted as a given that a dominant contributor to

error was the inaccuracy of manual stick m e a s u r e m e n t as a means of

m e a s u r i n g volume Whence came the overriding concern wlth precise

m e a s u r e m e n t addressed in the first section But, wlth the disruption of operations required for precise measurement came the imperative for

severely constrained time duration of measurement Thls, in turn,

amplified the distorting influence of such variables as temperature

fluctuation during the course of the test, thereby introducing a wholly

new source of extraneous error whi(h h a d had llttle or no impact on

inventory control accuracy

It is one of the ironies of thls development sequence that as It

became apparent that temperature v a r i a t i o n could have a crucial impact

o n short term precision measurement, it became accepted, again

uncritically, that it should have a similar overriding influence on

inventory accuracy Our analyses of inventory records generated from

ATG data w h i c h incorporated temperature measurements have shown that

this is not typically the case

Now, well after the fact, extensive statistical analysis of many

thousands of inventory records, correlated wlth physical evaluation of

the tank systems involved, has deflnltlvely established the following

1 Manual inventory recordkeeplng alone is an inadequate means of

leak detection in an active tank system for leaks of less t h a n 20

gallons per day

2 The sources of manual inventory deficiency are correctable, but

either testing or In-tank gauging devices, were designed to

correct

3 M e a s u r e m e n t (sticking) accuracy is in fact the least critical and

most easily corrected Source of error in this system A well

trained, conscientious operator can c o n s i s t e n t l y m a i n t a i n error

rates of less t h a n I0 gallons (37 85 liters) on average As

discussed in the previous section, such measurements taken

c o n s i s t e n t l y over a sufficient tlme span can yield measurement

preclsions as good as, or in excess of, those achievable by a

p r e c i s i o n measuring device employed over a more llmlted time

The inaccuracies and errors which limit manual inventory control

as a leak detection system are unrelated to measurement They are

entirely due to the dynamics of tank system operations during the test

ROGERS ON VOLUMETRIC LEAK DETECTION 9

It is, of course, i m p r a c t i c a l to take a c t i v e r e t a i l sales or

v e h i c l e s e r v i c e tanks out of o p e r a t i o n for s u c h e x t e n d e d p e r l o d s

In that sense, therefore, w l t h the e x c e p t i o n of the issues

a d d r e s s e d below, t h e i r i n t r o d u c t i o n adds n o t h i n g n e w to the d i s c u s s i o n

o t h e r t h a n the fact that b o t h of the t e c h n i c a l a p p r o a c h e s a d d r e s s e d are

c o m b i n e d in one d e v i c e The a p p r o p r i a t e p a r t s of the a n a l y s i s can,

T h e l e a k d e t e c t mode, w h e n a p p l i e d for a s i n g l e e x t e n d e d time

period, is in all w a y s e q u i v a l e n t to a p r e c i s i o n test S o m e w h a t

d i f f e r e n t is the case w h e n the d o r m a n t mode Is e x t e n d e d to i n c l u d e non-

o v e r l a p p i n g I n t e r v a l s w h e n the s y s t e m is not in o p e r a t i o n a n d the

In this, the A T G test d e p a r t s v e r y s i g n i f i c a n t l y f r o m p r e c i s i o n

ROGERS ON VOLUMETRIC LEAK DETECTION 11

T o a d d r e s s the m e a n s for o v e r c o m i n g the c o m p l e x l t i e s i n t r o d u c e d

The n e x t b e s t a l t e r n a t i v e is to a s s u m e that the n o m i n a l g e o m e t r y

o f the t a n k r e f l e c t s its real g e o m e t r y a n d m e a s u r e the d l m e n s l o n s of the

t a n k a n d its o r i e n t a t i o n in its e x c a v a t i o n For example, if we are

W h i l e the f o r e g o i n g w o u l d s e e m o b v i o u s a n d s e n s i b l e steps to take

w h e n i n s t a l l i n g n e w tanks, they are v e r y r a r e l y t a k e n F u r t h e r m o r e ,

W e w o u l d e m p h a s i z e that in an ideal world, the p r e f e r r e d a p p r o a c h

is the f i r s t one d e s c r i b e d In fact, there are c i r c u m s t a n c e s w h e r e it

is the o n l y one w h i c h is p r a c t i c a l For example, zf the t a n k s h a p e is

d i s p e n s e r m e t e r s w o u l d be e q u i v a l e n t This, of course, a s s u m e s that b o t h

the A T G a n d d i s p e n s e r are p e r l o r m l n g a c c u r a t e l y But a m a j o r f u n c t i o n of

s t a t i s t i c a l a n a l y s i s is to e s t a b l i s h w h e t h e r t h e y are, in fact,

f u n c t i o n i n g p r o p e r l y , a n d that r e a s o n i n g Is, therefore, c i r c u l a r

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dynamic complexities Either suitable measurements are made in routine

operations to identify and quantify such effects, w h i c h Is rare in

practice, or their potential effects are defined m a t h e m a t i c a l l y and

their presence and magnitude is derived from the inventory d a t a itself

Thus, it is well k n o w n that deliveries of product are rarely

accurate This could be overcome during a test p e r i o d by careful

m e t e r i n g of d e l i v e r e d product, but this is rarely done If it is not,

we acknowledge that the inaccuracy exists and estimate its magnltude

from the data Fortunately, the appearance of such an event is

distinctive Product volume reported delivered, w h i c h is in excess of

or less than actual, leaves a permanent imprint of fixed magnitude on

all subsequent stick measurements

Removal discrepancies can be treated similarly Meters can be

c a l i b r a t e d prior to test but, again, rarely are Instead, the k n o w n

characteristics of the m e t e r i n g system are defined m a t h e m a t i c a l l y

Typically, though not always, metering errors are p r o p o r t i o n a l to

volumes dispensed In this way, they, too, can be identified and

removed from the data

Temperature Effects

As m e n t i o n e d earller, temperature v a r i a t i o n is typically of less

consequence in this context than in p r e c i s i o n testing There are

exceptions, however, and these are addressed b e l o w

In p r e c i s i o n testing, temperature v a r i a t i o n is of consequence

because of the extremely short duration of the test Minor fluctuations

w h i c h w o u l d have little discernable effect on an Inventory analysis

produce volume variations w h i c h can be significantly large relative to

the small total volume change the tester needs to identify during the

short p e r i o d of the test

On the other hand, the system is dormant during the test, so that

only short term effects, Internal to the tank system, need to be

identified

Inventory analysis on the other hand must deal with a dynamic

system over an extended time frame Three possible sources of

temperature v a r i a t i o n must, therefore, be addressed

First, there is diurnal change If measurements are taken at the

same time each day, thls is of no consequence Our analysis of data

where multiple readings were taken each day, however, has shown that

diurnal effects are significant So much so that, absent temperature

readings, it is not possible to achieve satisfactory results w h e n all

readings are m e r g e d into a single data set Data collected at different

times must be analyzed separately

Thls creates a problem at very active sites where deliveries occur

daily Multiple dally measurements are necessary at such sites to

distinguish b e t w e e n delivery discrepancies and other effects For such

applications, temperature measurements generated from an ATG or other

source are essential

The second source is seasonal v a r i a t i o n The A g r i c u l t u r e

Department has, for many years, recorded u n d e r g r o u n d temperatures

throughout the U n i t e d States The data shows that there is typically

v e r y little variation, certainly insufficient to affect inventory

results over short periods of tlme on the order of thirty days There

can be, however, significant effects over more extended periods

Therefore, if inventory analysis is attempted over periods In excess of

30 days, u n d e r g r o u n d temperatures should be m e a s u r e d and accounted for

Third is the effect of introduclng product into a tank at a

s i g n i f i c a n t l y different temperature than the product stored in the tank

This can be of signlflcance w h e n the differences are extreme However,

the effects are less extreme than might be expected due to the dynamics

of the system

For example, suppose that i0,000 gallons of product is delivered

into an empty tank where the u n d e r g r o u n d temperature is twenty degrees

ROGERS ON VOLUMETRIC LEAK DETECTION

fahrenheit lower than that of the delivered product Then it is

conceivable that

13

lO,OOOx2Ox OOO7=14Ogal

(6)

of shrinkage could occur

In order for thls to happen, however, it w o u l d be necessary for

the tank to remain dormant wlth no additions or removals of product

until the temperature decayed to the u n d e r g r o u n d ambient In this case,

the Inventory analysis is trivial Merely record the temperature of the

delivered product, the u n d e r g r o u n d temperature, make the appropriate

adjustment and measure the product level over time

In the typical application, of course, this Is not realistic, and

several dynamic effects enter w h i c h greatly mitigate the temperature

effect

First, product is rarely introduced into an empty tank The

initial shrinkage of the delivered product is, therefore, limited to the

effects induced b y the temperature of the combined products

Second, product is typically b e i n g removed through sales, and,

therefore, smaller and smaller volumes are exposed to shrinkage

Finally, repeated dellverles typically take place before residual

product decays to underground ambient With each addition under these

conditions, the temperature of the residual product is higher than it

was at the time of the prior delivery resulting in lesser temperature

differentials

Extraneous Errors in Precision Testing

The errors induced by the tank system in the course of precision

testing differ substantially and fundamentally from the dynamic effects

on inventory of continued system operation The tank system during

p r e c i s i o n testing is dormant The distorting effects are largely

functions of the short term duration of the test

Large volumes of new product are usually introduced into the tank

system prior to the test Consequently, the tanks must be allowed to

stabilize to prevent tank deflection, extreme temperature fluctuation,

or the formation and dissipation of vapor pockets during the course of

the test All of these effects are known and are routinely referred to

in the descriptive materials provided by m a n u f a c t u r e r s of test

equipment They are large effects p a r t i c u l a r l y when compared to the

magnitude of the volume change due to leakage that the system is

attempting to detect

While all of these effects are known, there does not seem to be a

consensus as to h o w they should be measured Thus, for example, end

plate d e f l e c t i o n or vapor pockets are a c k n o w l e d g e d to exist as

distorting influences, but there does not appear to have been any

rigorous mathematical treatment of their defining c h a r a c t e r i s t i c s or, in

the case of vapor pockets, their rate of decay and its influences on

test results

Temperature effects are the most frequently discussed, but the

discussions have yielded no generally accepted means of measuring them

or d o c u m e n t a t i o n as to the precision with which they are b e i n g measured

There are at least three schools of thought on thls

i C i r c u l a t i o n of product to produce uniformity of temperature

2 Single thermistor measurement at the mid-point of the tank

3 Multiple in-tank thermistors with simple temperature averaging

The V i s t a research paper[l] addressed this issue, but Baird[2]

provides some cogent arguments that the methodologies used were flawed

Absent a more definitive treatment, sufficient to w i t h s t a n d an objective

and technically qualified peer review, I believe the issue must remain

In v l e w of the clear differences in a p p r o a c h and the lack of

consensus on this issue, it w o u l d appear that, at the v e r y least,

different effects are b e i n g m e a s u r e d with v e r y different volume Implica-

tions All however, are b e i n g interpreted Identically They all assume

that the temperature varlatlons they measure, or average, have equal

impact on the total volume of product and produce volume changes

strictly p r o p o r t i o n a l to it

For these, if for no other reasons, one has to q u e s t i o n w h e t h e r

the c l a i m e d a n d u s u a l l y d e m o n s t r a t e d inherent p r e c i s i o n of the m e a s u r l n g

devices used, is at all relevant It is at least arguable and certainly

needs to be addressed, w h e t h e r the p r e c i s i o n of such systems is, in

fact, determined by the extraneous influences induced by the tank system

u n d e r test The m e a s u r e m e n t of the effects of such influences on the

overall p r e c i s i o n of the testing systems, has not b e e n well documented

and their magnitude is p o t e n t i a l l y m a n y orders of magnitude larger than

errors which are inherent to the m e a s u r i n g system alone and on w h i c h its

claim to p r e c i s i o n Is b a s e d Such problems are clearly solvable just as

the dynamic Influences of system operations have b e e n m a t h e m a t i c a l l y

a n a l y z e d and accounted for The physics of these various p h e n o m e n a are

s t r a i g h t f o r w a r d and their m a t h e m a t i c a l treatment should be no less so

P A R T T H R E E - A N A L Y S I S A N D I N T E R P R E T A T I O N O F R E S U L T S

In the previous sections of this paper, we dealt with the problems

associated w i t h m e a s u r i n g volume changes in a tank system We

identified two distinct and independent sources of error zn m a k i n g such

m e a s u r e m e n t s First are those which are inherent in the m e a s u r i n g

device Secondly, there are those Induced by the changing dynamics of

the system b e i n g m e a s u r e d

We now address the final step in volumetric testlng, the analysis

of the m e a s u r e m e n t s derived from the test to determine If the tank

system is leaking

That such post-analysls is not only desirable but essential would

seem to be obvious Leakage is but one of many potential sources of

volume change w h i c h may take place in a system under test Frequently,

such n o n - l e a k related fluctuations wlll exceed by orders of magnitude

the volume loss from leakage which the detection system seeks to

identify

In addition, the inherent random component of error zn the

m e a s u r i n g system used, whether a mechanical device or manual stick

readings will v a r y from test to test and operator to operator If

specific test results are not subjected to analysis, the magnitude of

the random component of measurement is u n k n o w n and c o n s e q u e n t l y the

m a g n i t u d e of a leak which could have gone undetected during that test is

also u n k n o w n

C e r t i f i c a t i o n procedures as m a n d a t e d by the U S E P A serve the useful

purpose of e s t a b l z s h l n g that a particular leak detection system, method

or apparatus, dld in fact function as specified under one set of

circumstances at one point In time They provide no guarantee, nor do I

b e l i e v e they were intended to, that in all subsequent uses, the same

standards will be achieved

Statistical Inventory Analysis, llke all other v o l u m e t r i c methods,

is b a s e d on measurements of volume recorded over time It overtly and

explicitly acknowledges that such measurements Include errors inherent

in the m e a s u r i n g device, and volume changes which are u n r e l a t e d to

leakage induced by the dynamics of the system being tested

The accuracy and the limitations of the results a c h i e v e d on each

individual test are calculated and reported in terms of the m i n i m u m leak

rate w h i c h could have b e e n detected in the light of the quality and

accuracy of the measurements p r o v i d e d for analysis

The reason that thls issue has b e e n consciously and e~plzcztly

a d d r e s s e d from the outset of this methodology was the r e c o g n i t i o n that

in this approach, Instrument p r e c i s i o n was being traded off for

m e a s u r e m e n t d u r a t i o n

ROGERS ON VOLUMETRIC LEAK DETECTION 15

1 I d e n t i f y a n d c o r r e c t for v o l u m e changes, b o t h real a n d a p p a r e n t ,

w h i c h are i n d u c e d b y the d y n a m i c s of the s y s t e m u n d e r test b u t are

m o r e c o n f i d e n c e t h a n that t h e y are a c c u r a t e a n d e f f e c t i v e In the

a b s e n c e o f a n a l y s i s of the test results, t h e i r q u a l i t y is s i m p l y u n k n o w n

a n d u n k n o w a b l e T h e y are u n k n o w n to the o p e r a t o r w h o p e r f o r m e d the

test, the t a n k o w n e r w h o p a i d for it, a n d the r e g u l a t o r w h o r e q u i r e d

R E F E R E N C E S

Robert D Roach, James W Starr, Joseph W Maresca, Jr Evaluatlon

of Volumetric Leak Detection Methods for Underground Fuel Storage Tanks, Risk Reduction Engineering Laboratory, US Envlronmental Protectlon Agency, Edison, New Jersey, November 7, 1988

2 William E Baird,"Critical evaluation of EPA's UST testing apparatus" Pollution Engineering, pp 86-89, July, 1988

E R R O R S O U R C E S I N A U T O M A T I C T A N K G A U G I N G S Y S T E M S

Gauging Systems," Leak Detection for Under~round Storage Tanks,

ASTM STP 1161, Philip B Durgln and Thomas M Young, Eds ,

Amerlcan Society for Testing and Materials, Phlladelphla, 1993

FLEISCHER/ERROR SOURCES IN AUTOMATIC TANK GAUGING SYSTEMS 19

M o s t A T G S s a t t e m p t to d e t e r m i n e the a v e r a g e liquid

t e m p e r a t u r e c h a n g e from a s i n g l e v e r t i c a l a r r a y of

t e m p e r a t u r e sensors T h e r e is u s u a l l y no a t t e m p t to

d e t e r m i n e h o r i z o n t a l t e m p e r a t u r e d i f f e r e n c e s D u e to the

fact t h a t h o r i z o n t a l layers of l i q u i d w i l l t e n d to h a v e the

same density, t h e y will also t e n d to h a v e the same

t e m p e r a t u r e and therefore, h o r i z o n t a l t e m p e r a t u r e

d i f f e r e n c e s are u s u a l l y q u i t e small However, h o r i z o n t a l

d i f f e r e n c e s in the t e m p e r a t u r e of the fill s u r r o u n d i n g the

t a n k can c a u s e larger than n o r m a l h o r i z o n t a l l i q u i d

t e m p e r a t u r e d i f f e r e n c e s This can o c c u r if, for instance,

t h e r e is g r o u n d w a t e r flow past one p a r t of the t a n k or if a

p o r t i o n of the tank is located u n d e r a b l a c k top s u r f a c e

e x p o s e d to the sun

S u b s t a n t i a l s t a b l e liquid t e m p e r a t u r e d i f f e r e n c e s can

e x i s t in v a r i o u s p a r t s of the tank w i t h o u t a f f e c t i n g leak

t e s t results, p r o v i d i n g the a v e r a g e t e m p e r a t u r e c h a n g e of

the liquid is a c c u r a t e l y m e a s u r e d by the A T G S d u r i n g the

gal (38,000L) tank of g a s o l i n e to p r o d u c e a t h e r m a l v o l u m e

c h a n g e of 0.2 g a l / h (0.76 L/h), the leak rate an A T G S m u s t

however, c a n n o t be tolerated, since a 0.i g a l / h (0.38 L/h)

t h r e s h o l d is t y p i c a l l y u s e d for d e c l a r i n g a 0.2 g a l / h (0.76

o r d e r to h a v e a low p r o b a b i l i t y that the t o t a l of all leak

m e a s u r e m e n t e r r o r s will e x c e e d this limit, it is a s s u m e d

h e r e that any s i n g l e e r r o r s h o u l d be k e p t to less t h a n 0.03

a v e r a g e t e m p e r a t u r e c h a n g e error of the g a s o l i n e in the

i0,000 gal t a n k s h o u l d be k e p t to less t h a n 0 0 0 4 5 ~

h o u r s b e t w e e n any p r o d u c t a d d i t i o n s to the t a n k and the

one h o u r s h o u l d also be o b s e r v e d b e t w e e n the end of

d i s p e n s i n g and the start of a leak t e s t to r e d u c e the error

c a u s i n g e f f e c t s of t e m p e r a t u r e c h a n g e s t h a t o c c u r t h r o u g h o u t the tank w h i c h are not m e a s u r e d due to t h e i r d i s t a n c e from

the t e m p e r a t u r e sensors E x t r e m e l y a c t i v e t a n k s r e q u i r e

l o n g e r w a i t i n g times

K n o w i n g the exact v o l u m e of liquid s t o r e d in the tank is

FLEISCHER/ERROR SOURCES IN AUTOMATIC TANK GAUGING SYSTEMS 21

FLEISCHER/ERROR SOURCES IN AUTOMATIC TANK GAUGING SYSTEMS 23

FLEISCHER/ERROR SOURCES IN AUTOMATIC TANK GAUGING SYSTEMS 25

FLEISCHER/ERROR SOURCES IN AUTOMATIC TANK GAUGING SYSTEMS 27

FLEISCHER/ERROR SOURCES IN AUTOMATIC TANK GAUGING SYSTEMS 29

C O N C L U S I O N S

A n A T G S can be a v e r s a t i l e and c o n v e n i e n t d e v i c e for

a u t o m a t i c a l l y k e e p i n g track of liquid i n v e n t o r y in USTs and

for d e t e c t i n g losses of this i n v e n t o r y t h r o u g h leaks This

can p r o t e c t the e n v i r o n m e n t and p r e v e n t the n e c e s s i t y of

c o s t l y cleanups However, in order to a c c u r a t e l y m e a s u r e

and d e t e c t any leaks w h i c h develop, the A T G S m u s t o v e r c o m e

the e f f e c t s of several sources of error w h i c h can i n f l u e n c e

its results To r e d u c e these e f f e c t s the A T G S u s e r s h o u l d

u n d e r s t a n d h o w each of these errors can occur and h o w t h e y

can be minimized The u s e r should also ask the A T G S

m a n u f a c t u r e r w h a t steps they h a v e taken to c o n t r o l the

e f f e c t s of t h e s e errors

Sources of error include liquid t e m p e r a t u r e c h a n g e

inaccuracies, a b s o l u t e and d i f f e r e n t i a l liquid level

m e a s u r e m e n t inaccuracies, tank chart errors, liquid

t e m p e r a t u r e c o e f f i c i e n t of e x p a n s i o n inaccuracies, liquid

evaporation, t a n k t h e r m a l expansion, p i p e l i n e liquid t h e r m a l

expansion, vapor pockets, tank deformation, and g r o u n d w a t e r

pressure These errors can either m a s k or f a l s e l y indicate

a leak by i n d i c a t i n g changes in the h e i g h t or v o l u m e of

liquid in the tank

R E F E R E N C E S

[1] U.S E n v i r o n m e n t a l P r o t e c t i o n Agency, "Part 280 -

T e c h n i c a l S t a n d a r d s and C o r r e c t i v e A c t i o n R e q u i r e m e n t s

for Owners and O p e r a t o r s of U n d e r g r o u n d S t o r a g e Tanks,"

Washington, D.C.: Federal R e g i s t e r / V o l 53, No 185,

C3] Schwendeman, T.G and Wilcox, H.K., " U n d e r g r o u n d

S t o r a g e Systems," Lewis Publishers, Chelsea, Michigan,

P r o c e d u r e s for E v a l u a t i n g Leak D e t e c t i o n Methods:

A u t o m a t i c T a n k G a u g i n g Systems," Washington, D.C.: U.S

30

FLORA ET AL ON AIRPORT HYDRANT SYSTEMS 31