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Gas Leak Detection

Published: 18th Dec 2014

Gas leaks are a serious hazard that can cause harm to workers, buildings and the environment. Detecting leaked gas can help to reduce these risks. The different methods used to detect gas leaks are introduced in this article.

Odour

Gas is intentionally odourised so that the average person can perceive it at a concentration well below the explosive range. That odourant concentration is generally between 0.5 to 1% by volume, or as local applicable codes dictate. Gas odour is a common and effective indication of a leak. A report of gas odour should be investigated immediately. If a leak is found, the migration pattern of the gas should be determined. If an immediate hazard is determined to exist, the hazard potential should be eliminated and the leak repaired immediately. The odour of gas may be filtered out as the odourised gas passes through certain types of soil. It may also be modified by passing through soil and into a sewer system containing vapours or fumes from other combustibles, as well as the sewer odour itself. Odour is, therefore, not always totally reliable as an indicator of the presence or absence of gas leaks. For this reason all gas leak reports should be investigated using a leak detection instrument such as a Combustible Gas Indicator (CGI) or a Flame Ionisation Detector (FI). Gas detecting personnel should remember that the primary purpose of the gas odour is to provide a warning to those who do not have gas detection instruments.

Vegetation

Vegetation in an area of gas leakage may improve or deteriorate, depending on the soil, the type of vegetation, the environment, the climate, and the volume and duration of the leak. Changes in vegetation may indicate slow below ground leaks. A vegetation survey by itself is not an acceptable method of complying with the pipeline safety regulations; leak surveys should be conducted with leak detection equipment.

Insects

Insects such as flies, roaches and spiders migrate to points or areas of leakage, due to microbial breakdown of some components of gas. Some insects like the smell of the gas odourant. Heavy insect activity, particularly near the riser, gas meter or regulator, can sometimes indicate a leak.

Fungus-like growth

Fungus-like growth in places such as valve boxes indicate a gas leak. These fungi grow best where there is a lack of oxygen, such as in a gas leak area. The colour of the growth is generally white or grayish-white, similar to a coating of frost. All such indications should be investigated with a leak detection instrument. Personnel should never enter manholes or other confined spaces without first testing oxygen levels and gas concentrations. Check Occupational Safety and Health Administration (OSHA) regulations concerning confined space entry for further details on this subject.

Sound

Listen for leaks. A hissing sound at a bad connection, a fractured pipe, or a corrosion pit is the usual indication of a gas leak. Using a soap solution directly on the pipe or fitting will indicate the presence of a leak. If a strong gas odour is in the air, care should be taken to turn on and zero gas detection equipment away from the area where the odour is present.

Soap solutions

A soap solution can pinpoint the location of a leak on an exposed pipe, on the riser or on the meter. The solution is brushed or sprayed on and the location of bubbling indicates leakage. When excavating underground piping to investigate a suspected leak location, the pipe should be brushed clean and completely exposed at the top, sides and bottom, as many leaks are found on the bottom of the pipes. A mirror may be used to observe the underside of the pipe. It is recommended that a soap solution specifically designed for pipelines be used.

Unaccounted for gas

For a municipal system, an unexpected increase in the amount of natural gas purchased from the transmission company for a given month, as compared to past gas consumption for the same month, may indicate a leak in the system. The operator is cautioned that changes in load factors and weather must be considered when using this method. The operator should be sure to compare similar time periods, as a change in the meter reading cycle will affect this process. Dividing the gas purchased by the number of days in the billing cycle will give an average per day consumption, which would be more accurate to use in comparing bills. Low load periods such as the summer months would provide the best data.

Leak detection instruments

Gas leak indicators are sophisticated instruments that require regular care, maintenance and calibration, and should be used by trained personnel. Two types are commonly used by the gas industry for surveying and pinpointing leaks: combustible gas indicators (CGI) and flame ionisation (FI) detectors.

A third type of instrument, called a bead sensor type detector, is most often used for inspecting exposed piping and appliances to pinpoint leaks on those facilities. Bead sensor instruments are not generally used for leak surveys of underground piping.

Combustible gas indicator

The CGI shown in Figure 1 consists of a meter, a probe and an aspirator bulb. The bulb is pumped by hand to bring a sample of air into the probe and the instrument. The dial on the instrument indicates the percentage of flammable gas in the air (percent gas scale) or percentage of the lower explosive limit (LEL) scale. Figure 2 shows the modern CGI tool.

The flame ionisation (FI) detector

Figure 3 shows an FI detector. It uses hydrogen fuel to power a small flame in a detector cell. A pump or venturi system is used to pass continuous air samples through the detector cell. If the air contains hydrocarbons such as natural gas, they will be burned or ionised in the hydrogen flame. This is detected electronically and displayed on a meter readout. FI units are equipped with meters that indicate gas concentrations from one part per million (PPM) to 10,000 PPM, which is the same as 1% gas in air. They are also equipped with audible alarms to alert the operator when there is a meter deflection.

A leak survey can be conducted more rapidly with an FI unit than with a CGI using the bar hole method. FI units can be carried by hand for a walking survey or mounted on a vehicle for a mobile survey. Any gas indications detected by the FI should be confirmed using a CGI. Leak pinpointing is also done with a CGI.

Leak survey technicians must be trained and qualified in the operation of the FI and CGI. Additional training is required on leak survey procedures, leak classification, recognition of hazards and pinpointing. All gas personnel should also receive training on ‘make safe’ actions. Gas operators are not required to own FI equipment or to conduct their own leak surveys. They may choose to hire a leak survey consultant to conduct inspections. Consultants should also be trained in these topics and in some countries and companies are required to be in a drug and alcohol testing programme, the same as gas personnel.

The ability of the gas to vent at the ground surface is critical for the success of a surface survey with an FI unit. For this reason FI is not to be used when the ground is frozen. A continuous sampling of the atmosphere at buried main and services should be made at ground level or at no more than 2 inches above the ground surface. For exposed piping, sampling should be adjacent to the piping.

In wet, frozen, or windy conditions, the gas may be restricted from venting or be rapidly diluted below FI detection levels. One method to determine if conditions are appropriate for leak survey is to re-survey a known small leak on the system. If it can be detected at normal walking or driving speeds, the survey can continue.

Any indication of gas in a confined space or in a building should be considered a hazardous situation. Persons should be removed from the area, and ignition sources eliminated. Once this is done, the leak investigation should begin, and the leak repaired when found. The facilities affected should be monitored, and the gas migration pattern determined. Gas should be vented from the soil and structure before allowing persons to return to the area.

Odourisation equipment

This section provides information for natural gas system operators who odourise their own gas. Odouriser types include single unit wick odourisers and bypass odourisers. The former odourises the gas by having natural gas flow across a wick saturated with odourant. These are generally used for individually odourised lines such as farm taps. The odourant-saturated portion of the bypass gas is then returned to the main stream. These are generally used for low, more uniform flows.

Bypass type odourisers, on the other hand, divert a portion of the main gas stream using an orifice plate or partially closed valve in the line, through a tank provided with baffles or wicking. Odourisation equipment may need seasonal adjustments. Valves that regulate the amount of gas diverted into the odouriser need to be adjusted between seasons of high flow and low flow of gas (winter versus summer). Based on the equipment manufacturer's recommendation, operational instructions for specific systems can be developed. These instructions should be included in the operations and maintenance plan.

Selection of the appropriate odourant and odourising equipment may require professional advice. Operators should consult with an odourant and/or equipment supplier who can help guide odourant selection. These persons can evaluate an operator's unique pipeline situation and advise on the best odourant and the best equipment. Operators are encouraged to consult with more than one of these organisations to obtain the best odourant and equipment for the best price.

Types of odourants

Most odourants used in the United States are mercaptans or mercaptan sulfide blends, both of which contain tertiary butyl mercaptans as their main component. The only other odourant in wide use is cyclic sulfide (thiophane), which is mainly used in gas systems containing natural odourants.

The human sense of smell is very discerning and can detect mercaptans at a concentration of only one part per billion (ppb), which is currently beyond the capabilities of most instruments. The sulfur content of the odourants in the gas stream can be measured by gas analysis, which may then be used to determine if there is sufficient odourant.

The main types of detectors are odour concentration meters, titrators and gas chromatographs dedicated to sulphur analysis. Of these instruments, the most common is the odour concentration meter. The pipeline safety regulations require operators to odourise a combustible gas in a distribution pipeline. The operator is referred to 49 CFR §192.625 for the existing requirements regarding odourisation of gas.

Odour testing

instruments

Odour testing instruments are used to determine the lowest concentration at which gas can be detected through odour. The unit is actually a measuring device that indicates the percent of gas by volume in a sample when an odour is detected. To determine the odour threshold level, the unit is used in an area where it can draw in fresh air and be connected to a gas source. A blower in the unit draws ambient air through the analyser and out the exhaust chamber.

The procedure for running a test is as follows:

1.  After the instrument is connected to gas and running, quickly open the gas valve fully to make certain that gas is entering the instrument and then shut the valve quickly.

2.  Wait for any odour to dissipate and then proceed with the test.

3.  With just air coming from the exhaust port, inhale at the opening.

There are two important points to remember when running a test. First, put the nose as close to the opening as possible - this is the location at which the instrument is calibrated. If the operator does not put his or her nose close to the opening, the sample will be diluted by surrounding air and not representative of the percent gas shown by the odour testing instrument. Second, after sniffing at the opening, immediately withdraw the nose to fresh air and take a breath. If the nose is left at the opening, the mercaptan odour will temporarily deaden the operator's sense of smell.

To repeat, take a sniff or two of only air at the opening, getting a breath of fresh air between sniffs, and become familiar with the smell of the air coming from the instrument. The operator does this because each instrument has its own unique background odour. After a couple of sniffs of air at the opening, slowly open the gas valve and sniff. Continue to open and sniff, getting a breath of fresh air between sniffs. Repeat this procedure until a change in odour can be detected. At that point, record the percentage of gas and mark it ‘threshold’. Threshold, as used here, is the minimum concentration of a gas in air at which one can detect a change in odour. The odour cannot be readily identified at that concentration, but a change in odour is barely detectable. Do not shut the valve, but continue to open and sniff (getting a fresh breath in between and remembering to put the nose as close as possible) until there is a sufficient odour for the operator to decide that it is a readily detectable gas odour. Record that reading and mark it ‘readily detectable’. Turn off the gas valve at this point.

Do not skip the threshold part of the test. This part of the test is very important, because it slows the operator's decision process and prevents him from making a premature decision. Manufacturers recommend that odour testing instruments be recalibrated on an annual basis, and are currently offering to update old instruments.

Monitoring techniques

Operators of master meters or small natural gas systems should periodically verify the odour level with the gas company or have a consultant run an odour test using some type of odour testing instrument on the gas in the system to determine if it is properly odourised. The best time would be when there is a low usage of gas by customers. Operators should check with their respective regulatory agencies to see whether there are additional requirements.

Operators should include as an operating procedure the requirement that sniff tests be made whenever a meter set, repair to system or leak check is made. A sniff test is when one or more observers smell gas from an open valve or gas burner. The name of the person, the date, and location of the test must be kept on file. Operators should make a sniff test at extremities of the system at least once a month.

Follow up inspection

The adequacy of leak repairs should be checked before backfilling. The perimeter of the leak area should be checked with a CGI. Where there is residual gas in the ground after the repair of a Class 1 leak, a follow-up inspection should be made as soon as practicable, after allowing the atmosphere to vent and stabilise. OPS suggests follow-up inspection within 24 to 48 hours, but in no case later than one month following the repair. In the case of other leak repairs, qualified personnel should determine the need for a follow-up inspection.

A method to remember when investigating gas leaks and determining the classification is to ask where the gas is, as follows:

•  Where is the gas? Use a detector to confirm gas is present

•  How much is there? Take readings on the CGI

•  What is the extent of the spread? Determine the migration pattern

•  What is its relation to other structures? Is gas detected in or near buildings or in manholes?

•  Evaluate or evacuate? Classify the leak and take appropriate action

Conclusion

Gas leak detection is the process to be conducted to avoid catastrophic consequences for humans, infrastructure and the environment. Detection methods and equipment have to be applied and used for this purpose. Odourants, sound, vegetation, fungus-like growth and soap are methods used to detect gas leaks. Adding odourants requires special tools named odourisers, which can be either single-unit wick odourisers or bypass type odourisers. Tests have to be conducted to monitor odourisers and the follow-up odourisation techniques.

Competent gas detection can be the difference between life and death. Make sure everyone in your business is aware of the dangers of gas leaks to ensure the safety and longevity of your workforce and business.

Published: 18th Dec 2014 in Health and Safety Middle East

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Dr Abdel-Alim Hahsem El-Sayed