need for regular functional testing of portable gas detection

Need for Regular Functional Testing of

Portable Gas Detection Equipment

- Evaluation of Field Failure Rates and Interventions in Japan-

Master of Public Health Capstone Project

Haruo Hashimoto

Department of Environmental Health Sciences

Bloomberg School of Public Health

Johns Hopkins University

May 1, 2006

2

Executive Summary

While the early detection of leaks using portable gas detection equipment is a vital measure

to protect workers from hazardous work environment, the practice of regular functional testing of

this equipment has not been established in several countries in Asia and Europe; Japan is the

typical example. The objective of this study is to evaluate the failure rate of portable gas

detection equipment in order to demonstrate the need for regular functional testing. Evaluating

gas detector performance is an important public health issue. When operating properly, gas

detectors provide important engineering controls that help to prevent worker exposure to acutely

toxic or hazardous chemicals. Simple pre-use inspection of detectors is commonly practiced by

workers before their use. However, there can be a certain type of failures which can not be

detected by the pre-use inspection, and those failures are identifiable by functional testing with

challenge gases.

The rates of equipment failures were evaluated, using challenge gases, for 269 hydrogen

sulfide alarm detectors and 206 functional components (of 60 multiple-gas detectors) in three

Japanese refineries. , The identified average failure rates for hydrogen sulfide alarm detectors,

gas detector components, and the total devices were 2.5%, 4.7%, and 3.3% per year respectively.

These failure rates are considered to be unacceptably high. The major causes of the failures were

3

identified as inattentive usage by workers, which meant the failures occurred randomly and

externally. It was demonstrated that increasing functional test frequency, from the typical present

practice of a yearly basis to a monthly basis, was effective in mitigating the risk of a worker

selecting and using a malfunctioning device.

The study result suggests the definite need for regular functional testing of portable gas

detection equipment. It is strongly recommended that all equipment users in local industries, the

equipment manufacturers, and Japanese government (the Ministry of Health, Labor and Welfare),

be informed of the potential failure rate problem, and that frequent functional testing program be

implemented. In addition, since the gas detection equipment is used around the world, the

awareness of this issue should be broadly disseminated.

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1. Introduction

The research question of this study is whether regular functional testing (or bump testing)

are necessary for portable gas detection equipment. The need for regular functional testing of the

equipment has not been established in Japanese workplaces. The objective of this study is to

evaluate the failure rate of portable gas detection equipment in field use and to demonstrate the

need for the regular functional testing.

Why this is a public health issue? Early leak detection is an important primary prevention

measure. If malfunctions occur, workers are at risk for potentially dangerous over-exposures. It is

important to determine if gas detector malfunctions are due to their inappropriate handling or due

to the intrinsic failure of the devices. If the major causes of failure rest on workers’ inappropriate

handling of the devices (a workplace management and training issue), then a potential promising

intervention for this problem may be to increase the frequency of functional testing. If the major

cause of the device failure is a mechanical problem intrinsic to the instruments, then the solution

would involve better engineering or maintenance of the devices.

Portable gas detection equipment has been used extensively in manufacturing and

construction workplaces, and is considered a critical safety measure that directly protects

workers’ lives. For example, at refineries and chemical plants, gas detection equipment is used to

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detect oxygen deficient or explosive atmosphere before confined space entry, such as tanks and

reactor vessels. Another purpose is the leak detection function. Workers wearing the toxic gas

leak detector devices are protected if the detectors sound an alarm when elevated concentrations

of toxic gases are detected.

It is common practice for workers to perform pre-use inspection of detectors, that is, he/she

checks the device visually and performs an automated electrical circuit test - so called

“auto-zero-check”. This pre-use practice is of limited utility. Intrusion of water/oils or physical

impact during past use can result in deteriorated response to the actual hazardous atmosphere,

although the device is tested normal during the pre-use inspection. Therefore, it is essential to

test the device against challenge gases every time before use.

However, such functional testing has not been practiced in the five refineries of the

company E in Japan (Table 1). The Japanese manufacturers of gas detection equipment, as well

as the Japanese manufacturers’ professional association, do not recommend their customers

perform functional testing every time before use, and only recommend functional testing or full

calibration once or twice a year1,2,3. The typical practice in most of the major local manufacturing

industries is yearly or biyearly calibration, and the equipment is not tested before its use4,5. It was

even mentioned by a local equipment manufacturer that major proportion of the equipment

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having been sold in the local market and currently used in the field has not had any functional

testing nor calibration since their first use4.

The use of functional testing seems to be variable depending of each region/country. In the

United States, the practice of frequent functional testing has been widely established; three US

refineries of the company E perform the test every time before use (Table 1). On the other hand,

European refineries perform the test with frequencies varying from one month to 1.5 year, and

one refinery in Asia, outside Japan, holds the frequency of one to three months similarly.

The US OSHA recommends that the accuracy of gas detection equipment be verified

before each day’s use, and ISEA (International Safety Equipment Association, a trade association

for manufacturers of protective equipment and instruments) also provides similar

recommendation6,7. Interviewing ten manufactures of personal gas detection equipment in the

U.S. revealed that all of them recommend at least monthly functional testing, and many of them

recommending daily testing (Table 2)8. The reason of the observed good test practices in the U.

S. may be ascribed to either high failure rates identified in actual workplaces or historical

incidents caused by gas detection equipment failures. However, no report or published paper on

the failure rates of gas detection equipment was identified through an extensive literature search.

The studies on such failure rates might tend not to be published because of perceived low

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generalizability, because such studies are usually aimed at limited equipment model(s), and also

because failure rates are influenced by the specific handling practices or work environment of the

equipment users. Similarly, no information on past incidents cases caused by gas detection

equipment failures were identified through literature searches, interviews of Japanese

manufacturers, and record search within the company E.

This paper presents an evaluation of the failure rates of the portable gas detection

equipment used in the Japanese workplaces. Based on the failure rate analysis, an evaluation of

the test frequency for appropriately mitigating the risk of potential incidents by equipment

failures is also presented.

2. Methods

The failure rates of portable hydrogen sulfide alarm detectors and portable gas detectors

were evaluated in three manufacturing sites (refinery A, B and C) of the company E in Japan.

2-1. Portable hydrogen sulfide alarm detectors

A portable hydrogen sulfide alarm detector detects hydrogen sulfide gases in the

atmosphere and produces an alarm sound and flashing light when the airborne concentration

exceeds a preset gas level that is usually 10 ppm (Figure 1). 269 hydrogen sulfide alarm

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detectors made by a Japanese manufacturer (Shin-Cosmos electric Co.) that had been in use in

two manufacturing sites A and B were evaluated. The manufacturer of the alarm detector

guarantees its use for two years without the need for functional testing or calibration, and thus,

the alarm detectors used at Sites A and B had not been tested with standard gas after their start of

use. An initial screening of about 290 alarm detectors by visual inspection and electric functional

check (auto-zero-check) was performed. The visibly-broken or clearly malfunctioning alarm

detectors were excluded, and only the remaining 269 alarm detectors were included in this study.

As a result, this study identified failure rates in a population of apparently normal alarm detectors.

The response of the 269 alarm detectors was tested against the standard gas which was composed

of 25 ppm H2S in nitrogen. The tested personal alarm detectors have a two-step alarm system -

low and high - which beeps as well as flashes at 10 and 15 ppm of hydrogen sulfide levels

respectively. When exposed to the standard gas, the alarm detector typically responds in a

two-step manner at 10 and 15 ppm levels, since the standard gas gradually diffuse into the sensor

part of the alarm detector. Therefore, the alarm detectors that responded to the challenge gas

properly in this two-step manner were defined as having “passed”; other alarm detectors were

classified as “failure”.

2-2. Portable gas detectors

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A gas detector indicates the atmospheric gas concentration of oxygen, combustibles, or

toxic gases (such as hydrogen sulfide and carbon monoxide), and alarms by sound and flashing

light above preset gas levels. “Multiple gas detectors” that have multiple gas sensors are

commonly used (Figure 2). Sixty gas detectors made of a Japanese manufacturer (Riken

Instruments Co.) that had been in use in two manufacturing sites B and C in Japan were also

evaluated as a part of this investigation. The time duration since the last calibration of the

detectors was one year for site B, and one year or one month, depending on sensors, for site C.

None of the detectors had not been tested with standard calibration gas since their last calibration.

Similar to the hydrogen sulfide alarm detectors, the detectors were first examined by visual

inspection and electric auto-zero-check, and the apparently-abnormal detectors were excluded.

The remaining sixty detectors were included in test pool for functional testing using challenge

gases. Four kinds of standard calibration gases – oxygen, n-pentane, hydrogen sulfide and carbon

monoxide were used. The gas concentrations are shown in Table 3. The gas detectors, when

exposed to the challenge gas, that both displayed a read-out exceeding the preset alarming levels

and generated alarm sound/flashing properly were defined as having “passed”; others were

classified as “failure”. For oxygen sensors, those that failed to display a read-out of less than

20.0% were classified as “failure”.

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A typical gas detector has one pump and one (or more) sensor(s), and each of these

functional “components” can be the cause of failure. There is other type of models, “passive”

sampling devices, that does not equip a built-in pump. A “pump failure” was defined as

insufficient pump suctioning if the sucked air barely reached to the sensor(s) while the pump

motor itself was diagnosed as normal, based on the rotating sound. A “sensor failure” was

defined as deteriorated response of a sensor. The failure rates were analyzed in two ways; per

detector basis, and per functional “component (pump or sensor)” basis. The total number of the

components was 206, comprising 54 pumps and 152 sensors, among 60 detectors tested in total.

3. Result

3-1. Portable hydrogen sulfide alarm detectors

The total number of alarm detectors evaluated was 269; 174 and 95 for Site A and B

respectively. The time duration of use was variable depending on each alarm detector, as shown

in Figure 3. There were total seven failures identified, five for Site A and two for Site B (shown

as “x” in Figure 3). The crude failure rates for Site A, Site B, and combined Sites were 2.9%,

2.1%, and 2.6% respectively (Table 4).

Assuming, based on the reasoning discussed later, that the malfunctioning of the alarm

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detectors originated randomly during their past field use, the observed results were analyzed by

applying the person-time concept from epidemiology. The cumulative time of use for a group of

alarm detectors, “piece-months”, was defined in the same manner as “person-time” in

epidemiology, as follows:

[In epidemiology]

(Incidence rate) = (Number of episodes) / (Total person-years of exposure)

[In this study]

(Failure rate) = (Number of failures) / (Total piece-months of field use)

The result of functional testing for alarm detectors is summarized in Table 4. For Site A, the

cumulative time of use was 2211 piece-months. This number, when divided by 5 (the number of

failures), gives the average “lifetime” of an alarm detector which represents the average time for

unit failure; this number is 442.2 months. The reciprocal number of lifetime, which represents

the “failure rate”, is therefore 0.226% per month. Similarly, the average failure rate of alarm

detectors for Site B is 0.174% per month. There is no statistically significant difference between

the results for Site A and B; the Poisson test was performed using STATA statistics software. The

combined average failure rate for Site A and B is 0.208% per month, which translates into

probabilities for a new alarm detector experiencing a failure before 12 month-age and 24

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month-age of 2.5% and 5.0% respectively. This means that 5 out of 100 alarm detectors would

experience failures at 24 months time - current end-of-life period, if no functional testing has

been performed during its time of use. This failure rate seems to be very significant and

problematic, in view of the purpose of this equipment that should secure workers’ safety.

3-2. Portable gas detectors

The raw result of functional testing of the portable gas detectors is shown in Table 5. In

Table 6, the analysis result of the testing, per detector basis, is given. The total number of

detectors evaluated was 60; 43 and 17 for Site B and C respectively. There were total 8 failed

detectors identified; each detector had one failed component within it respectively. The numbers

of failure were 7 for Site B and 1 for Site C, which gives crude failure rates of 16.3% for Site B,

and 5.9% for Site C. The combined crude failure rate is 13.3%. The cumulative time after last

test for all detectors at Site B was 516 piece-months, which gives, with 7 failures in total, an

average failure rate of 1.36% per month. Similarly, the average failure rate is 2.00% per month

for Site C. There is no statistically significant difference between the results for Site B and C.

The combined average failure rate for Site B and C is 1.41% per month.

Table 7 shows the analysis result of the testing per component basis. As for 206 components

of 60 detectors tested, there were 3 pump failures and 4 sensor failures (2 for combustible gas

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sensors and 2 for hydrogen sulfide sensors) for Site B, and 1 sensor failure (combustible gas

sensor) for Site C. The crude failure rates for Site B, Site C, and the combined Sites are 4.5%,

2.0%, and 3.9% respectively. The cumulative time after last test for all components at Site B was

1860 piece-months, which gives, with 7 failures in total, an average failure rate of 0.376% per

month. Similarly, the average failure rate is 0.546% per month for Site C. There is no statistically

significant difference between the results for Site B and C. The combined average failure rate for

Site B and C is 0.392% per month, which translates into the 4.70% probability for a detector

component of having failure after one year’s use from the last test. This failure rate seems to be

very significant.

The integrated result of functional testing for the total devices (hydrogen sulfide alarm

detectors and detector components) is shown in Table 8. There is no statistically significant

difference between the results for alarm detectors and detector components. The number of the

total devices evaluated was 475, and there were total 15 failures were identified, which gives the

crude failure rate of 3.2%. The overall average failure rate for the total devices is 0.278% per

month (3.33% per year).

4. Discussion

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The manufacturer examined all of the malfunctioning hydrogen sulfide alarm detectors.

They found that the causes of the failure were either partial corrosion of the circuit board, filter

clogging, or inner mechanical damage9. This suggests that those failures were caused by

rainwater intrusion or incidental physical impact (falling-off etc.) during field use. Therefore, it is

reasonable to interpret that the identified failure rates were not primarily related to the intrinsic

breakdown characteristics of the alarm detector, but to the extrinsic and random failures due to

inattentive usage by workers of the Site A and B.

The gas detector manufacturer reported that the cause of the sensor failures was

deterioration of catalytic combustion or electrochemical sensors10. The manufacturer generally

guarantees that the potential deterioration of the sensors within one year will be insignificant; the

sensor sensitivity typically goes down only gradually. This suggests that the identified rapid

sensor deterioration was most probably caused by sucking overly dense gases of combustibles or

toxics. Also, it was reported that the most probable root cause of the pump failures was the

malfunction of rubber diaphragms of the pumps; the diaphragms were deteriorated by steam or

oil mist sucked erroneously during use10. Thus, it is reasonably interpreted that the identified

failure rates of the gas detectors were also primarily ascribed to the random breakdowns due to

inattentive usage by workers.

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For workers’ safety, it is essential to reduce the risk of a worker selecting a malfunctioning

device when randomly picking up an alarm detector or a detector from a pool of devices. The

following measures are potentially effective in mitigating this risk, based on the failure causes

mentioned above:

(A) Frequent functional testing by workers,

(B) Communication and training for workers on appropriate/attentive handling of

equipment,

(C) Improvement of equipment durability by manufacturers against inappropriate handling.

Among these, option (A) seems to be the most dependable and effective. However, it is

important to pick the proper functional testing frequency.

Let us define a variable, “selection risk (p)”, which represents the probability of a worker

selecting a malfunctioning device among a pool of devices; those devices have been

function-tested with specified single frequency, but the time duration after the last test for

respective device is randomly distributed. Also, suppose the test frequency being every “m”

months and the common failure rate being “a” % per month. Considering an equipment failure as

analogous to a disease event, the selection risk (the proportion of malfunctioning devices at a

time) represents the point prevalence. There is a following relationship among prevalence,

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incidence rate, and disease duration.

(Prevalence) = (Incidence Rate) x (Duration)

In the case of equipment failure, the “failure rate” represents the “incidence rate”, and “the

average time duration of a device being malfunctioning”, which is “m/2 (month)” represents the

“duration”. Thus, the above equation can be translated into the following relationship. This

equation is also used in the filed of mechanical reliability engineering11.

p = a*m/2

The relationship between “m” and “p” is proportional, and the actual figures are given in

Table 9 for a hydrogen sulfide alarm detector and typical gas detectors. It is demonstrated that

the selection risk, “p”, is significantly reduced in proportion to “m”. While the risk was originally

2.5% for testing in every two years, the selection risk becomes 0.1% with increasing the

functional test frequency (i.e., reducing “m”) to a monthly basis, based on the obtained monthly

failure rate of 0.226% for hydrogen sulfide alarm detectors. The selection risk further goes down

to essentially 0% with daily testing before use.

Similar reasoning holds for gas detectors. For a detector having one functional component, a

sensor, while the risk was originally high - 2.35% - for the current yearly test frequency, the

selection risk becomes 0.2% with monthly testing, based on the obtained failure rate of 0.392%

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per month. Since a typical gas detector has multiple components within it, it might be

worthwhile to consider the selection risk per each detector which represents the probability of

one (or more) of the detector components being malfunctioning. The following equation holds

for a typical detector having one pump and three sensors (four components altogether) within it:

p = (a*m/2) x 4 = 2a*m

As shown in Table 9, while the selection risk is very high – 9.41% - for yearly test frequency, the

risk becomes reasonably low - 0.78% - with monthly frequency. Based on the above

consideration, it is demonstrated that the selection risk can be significantly reduced by only

reducing the testing frequency to monthly from the current 1-2 year interval. It is further

desirable to reduce the frequency to daily, to make the risk minimal.

In order to mitigate the risk for a worker of using malfunctioning devices, it would be

additionally important to educate workers on appropriate usage of the equipment, especially to

avoid physical impact or suction of water/oil/concentrated gases. In addition, the necessity of

immediate ad hoc functional testing should be emphasized in cases where a device has

experienced abnormal conditions or a destructive environment.

It may not be appropriate to generalize the identified failure rates to other gas detection

equipment in field use because of two reasons; one is that this study included a limited selection

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of equipment models, and the other is that the field environment that had been experienced by

the tested devices was also limited to a relatively severe environment in a few refineries.

Many of the overseas manufacturers have developed and are selling gas-supplying adaptors,

and even automated calibration equipment for gas detectors, usually called docking stations12.

On the other hand, Japanese manufacturers do not sell such tools locally, although one of them

actually sells their adaptors only in overseas market4. Japanese manufactures should be

encouraged to provide state of the art functional testing equipment.

5. Conclusion

It has been demonstrated that there are cases where the failure rates of portable gas alarm

detectors and detectors are significantly high. The identified average failure rates for hydrogen

sulfide alarm detectors, gas detector components, and the total devices were 2.5%, 4.7%, and

3.3% per year respectively. This suggests the strong necessity for equipment users to regularly

perform functional testing of these devices. It is strongly recommended for all the local gas

detection equipment users to acknowledge this risk and to initiate regular functional testing, with

at least monthly frequency. It is recommended that the Japanese government, the Ministry of

Health, Labor and Welfare, widely disseminate information on the failure risk of the gas

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detection equipment as a potentially major occupational safety/health management issues. The

agency should also be encouraged to establish a guideline for implementation of the frequent

functional testing. Currently, the government has no guideline for functional testing for portable

gas detection equipment, while it merely requires functional testing once or twice a year only for

stationary (non-portable) gas detectors attached to the specified high-pressure gas manufacturing

facilities. The manufacturers of gas detection equipment should actively notify their customers of

the necessity for functional testing. It is also recommended that these manufacturers develop and

provide adaptors or equipment for functional testing in the local market. In addition, the

necessity of frequent functional testing should be shared on global basis, to relevant countries in

Asia and Europe.

This study result had been communicated and advocated by the author within the company

E in Japan. As a result, a new program has started to initiate functional testing for hydrogen

sulfide alarm detectors and gas detectors at all operational sites in Japan, with monthly frequency

at first, and with daily frequency within a very near future. Also, the author has communicated

the study finding to the overseas operational sites of the company E, in order to encourage

implementing the similar program. The author also have orally presented this study results at the

annual meetings of the Japan Society for Occupational Hygiene and Engineering in November

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2004 (Tokyo, Japan) and in November 2005 (Takamatsu, Japan)13, 14.

Acknowledgement

The author acknowledges Messrs. Toshiaki Gotoh (ExxonMobil Japan Co.), Kazuo

Sasaki (Kyokuto Petroleum Industries Co.) and Kazuyoshi Katoh (ditto) for their assistance in

testing the failure rates of gas detection equipment.

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Table 1. Functional testing frequencies of portable gas detection equipment in refineries of company E

Region/Country Site Frequency A 1-2 y B 1-2 y C 1 m -1 y D 3 m -2 y

Japan

E 3 m -2 y F (Malaysia) 1-3 m G (Australia) Every time before use H (Thailand) Every time before use

Asia Pacific

I (Singapore) Every time before use J (Italy) 1 m K (UK) 6 m

L (France) 1 y M (Germany) 1 y

Europe

N (Netherlands) 3 m - 1.5 y O Every time before use P Every time before use USA Q Every time before use

Table 2. Recommended functional testing frequency by US-based gas detector manufacturers8

Name of Manufacturer Recommended Frequency B. D. daily ~ monthly B. W. daily D. I. daily E weekly ~ biweekly I. S. daily I. A. C. daily (for confined space work) M daily R. S. daily (rechargeable), weekly (non- rechargeable) S monthly T daily ~ weekly

Site A Site B Total Number of alarm detectors tested 174 95 269 Number of failures 5 2 7 Crude failure rate (%) 2.9 2.1 2.6

Cumulative time of use (piece-month) 2211 1150 3361

Average time of use (month) 12.7 12.1 12.5

Average time per unit failure (month) 442.2 575 480.14

Failure rate (% /month) 0.226 (*1) 0.174 (*1) 0.208 95% CI (*2) 0.073-0.528 0.021-0.628 0.084-0.429 Failure rate (% / 12 month) 2.71 2.09 2.50 Failure rate (% / 24 month) 5.43 4.17 5.00

*1: No statistically significant difference between Site A and B (p=0.80, Poisson method)

*2: Poisson method

O2

(ppm) Combustible gas

(%LEL*) H2S

(ppm) CO

(ppm)

Gas concentration 19.3 25

(n-pentane, 0.35vol%) 25 103

Pre-set alarming level

18.0 10 10 25

Table 4. Summary result of functional testing for hydrogen sulfide alarm detectors

Table 3. Challenge gas concentrations and the pre-set alarming levels of gas detectors

*LEL: Lowest explosion limit

22

23

Table 5. Result of functional testing for gas detectors

# of Components failed

Site # of

Detectors tested

# of pump per detector

# of Sensors

per detector

# of Components per detector

Duration after last test (month)

Cumulative time after last

test (piece-month) Pump O2

LEL

(*2) H2S CO

Total # of

failures

2 1 1 2 12 48 0 0 0 0 0 0

14 1 2 3 12 04 0 0 1 0 0 15

26 1 3 4 12 48 3 0 1 2 0 612B

1 1 4 5 12 60 0 0 0 0 0 0

(subtotal) 43 60 3 0 2 2 0 718

6 1 1 2 1 12 0 0 0 0 0 0

5 1 2 3 1 15 0 0 0 0 0 0

0 0 3 3 0 0 0 0 0 0 0 0C

6 0 4 4 *1) 56 0 0 1 0 0 16.5 ( 1

(subtotal) 17 83 0 0 1 0 0 11

Total 43 3 0 3 2 0 860 20

*1: Averaged for all components; 1 month for oxygen and combustibles sensors, 1 year for hydrogen sulfide and CO sensors *2: Combustibles sensors which indicate rates (%) against the lower explosion limit (LEL) concentration

24

Table 6. Summary result of functional testing for gas detectors: per detector basis Site B Site C Total

Number of detectors tested 43 17 60 Number of failures 7 1 8 Crude failure rate (%) 16.3 5.88 13.3

Cumulative time after last test (piece-month) 516 50 566

Average time after last test (month) 12.0 2.9 9.4

Average time per unit failure (month) 73.7 50.0 70.8

Failure rate (% /month) 1.36 (*1) 2.00 (*1) 1.41 95% CI (*2) 0.054-2.80 0.051-11.1 0.610-2.79 Failure rate (% / 12 month) 16.3 24.0 17.0

*1: No statistically significant difference between Site B and C (p=0.68, Poisson method)

*2: Poisson method Table 7. Summary result of functional testing for gas detectors: per component basis

Site B Site C Total Number of components tested 155 51 206 Number of failures 7 1 8 Crude failure rate (%) 4.52 1.96 3.88


Average time after last test (month) 12.0 3.6 9.9

Average time per unit failure (month) 265.7 183.0 255.4


*1: No statistically significant difference between Site B and C (p=0.68, Poisson method)

*2: Poisson method

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Table 8. Integrated results of functional testing for the total devices (hydrogen sulfide alarm detectors and gas detector components)

Hydrogen sulfide alarm

detectors Gas detector components Total devices

Number of devices tested 269 206 475 Number of failures 7 8 15 Crude failure rate (%) 2.6 3.9 3.2



*1: No statistically significant difference between hydrogen sulfide alarm detectors and gas detector components (p=0.23, Poisson method)

*2: Poisson method

Table 9. Relationship between “selection risk” and functional test frequency

Test Frequency Selection risk (p, %)

month Hydrogen sulfide

alarm detector Gas detector

(*1) Gas detector

(*2) 2 yr 24 2.50 (*3) - - 1yr 12 1.25 2.35 (*3) 9.41 (*3) 6 month 6 0.63 1.18 4.70 3 month 3 0.31 0.56 2.35 1 month 1 0.10 0.20 0.78 Weekly 0.23 0.02 0.05 0.18 Daily 0 0 0 0 p = a*m/2 p: Selection risk (Probability of selecting a malfunctioning device ) a: Failure rate (hydrogen sulfide alarm detectors; 0.226%/m, components of gas

detectors; 0.392%/m) m: Frequency of functional testing (month) *1: In case of a detector with one component (sensor) *2: In case of a typical detector with four components, one pump and three sensors *3: Typical current test frequencies

26

Figure 1. Portable hydrogen sulfide alarm detector.

Figure 2. Typical portable gas detector. (This detector has one pump and three sensors built-in.)

Numb0 20 40

0-12-34-56-78-9

10-1112-1314-1516-1718-1920-2122-23D

urat

ion

of U

se [m

onth

]

Figure 3. Result of functional testing of hyd

xx

x x

x

er of Alarms60 80

A Site B Site

rogen sulfide alarm detectors.

X F il

xx

27

References

1. Riken Keiki Instruments Co. Instruction manual of combination gas detector GX-85 (in Japanese).

2. Shin-Cosmos Electric Co. Instruction manual of hydrogen sulfide gas alarm detector XS-200N (in Japanese). 2002.

3. Industrial Gas Detector & Alarm detector Manufacturing Association of Japan. Instruction manual for maintenance and inspection (in Japanese).

4. Ishikawa Y (Riken Keiki Instruments Co.). Personal communication. Interview result at ExxonMobil Tokyo office. 2005 Jul 27.

5. Kawaguchi K (Shin-Cosmos Electric Co.). Personal communication. Interview result at ExxonMobil Tokyo office. 2005 Oct.

6. US Department of Labor, OSHA. Verification of Calibration for Direct-Reading Portable Gas Monitors. Safety and Health Information Bulletins, SHIB 05-04-2004. 2004 May 5.

7. International Safety Equipment Association. ISEA statement on verification of calibration for direct read portable gas monitors used in confined spaces. 2002 Nov 20. Available from: http://www.safetyequipment.org/calibration.PDF

8. Thayer E. Personal communication. Interview result of gas detection equipment manufacturers at the American Industrial Hygiene Conference & Exposition. 2003 May.

9. Shin-cosmos Electric Co. Personal communication to Kyokuto Petroleum Industries Co. Investigation report for functional failures of hydrogen sulfide alarm detectors. No. 303289. 2004 Feb 26.

10. Ishikawa Y, Saito E (Riken Keiki Instruments Co.). Personal communication. Interview result at ExxonMobil Tokyo office. 2005 Aug 3.

11. Tiezema RJ. (Yokogawa System Center Europe B.V.). Risk reduction in the process industry, eliminating the unexpected – part 10. 2003 Mar. Available from: http://www.sipi61508.com/ciks/tiezemar1.pdf

12. Industrial Scientific Co. Technical bulletin of DS2 Docking Station. Available from: http://www.indsci.com/prod_ds2.asp

13. Hashimoto H, Gotoh T, Suzuki H, Kato K. Evaluation of failure risk for hydrogen sulfide alarm detectors and its mitigation measures. Proceedings of the 44th Japan Society for Occupational Hygiene and Engineering, BK-17. 2004 Nov.

http://www.safetyequipment.org/calibration.PDF

http://www.sipi61508.com/ciks/tiezemar1.pdf

http://www.indsci.com/prod_ds2.asp

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14. Hashimoto H, Gotoh T, Suzuki H, Sasaki K, Miyano T. Evaluation of failure risk for gas detectors and its mitigation measures. Proceedings of the 45th Japan Society for Occupational Hygiene and Engineering, BK-16. 2005 Nov.

need for regular functional testing of portable gas detection

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