September 2008: Natural Gas and Gas Piping

The use of natural gas is very common in most industrial and commercial buildings. The gas can be used for process and/or heating.

Natural gas is highly combustible, and various precautions are needed to ensure safe usage. While there are numerous factors to consider for a safe installation, major concerns are listed below.

Construction and Location

If possible, do not install piping under buildings. Piping should be protected from corrosion by location or coating.

Bury gas service piping in a separate trench located as far as practical from other underground services such as steam, water, sewer or electric.

To prevent condensate from freezing, piping should be pitched uniformly back to mains or to drip pots.

Piping

Materials such as cast iron, ductile cast iron, copper, brass, aluminum alloy, stainless steel and plastic are all acceptable under certain conditions. There are several ANSI/ASME standards governing the type and diameter of pipe allowable based on operating pressure and other factors. Before selecting gas piping, review the local codes in your area.

Equipment and Processes

If your building is in an earthquake zone, the guidelines set forth in the local codes should be strictly adhered to. Items to consider if in an earthquake zone include:

· Shock operated shutoff valves.

· The use of welded fittings instead of threaded.

· Clearances where pipe passes through foundations, walls and/or floors.

· Hangers provided with shock absorbing connectors.

Buried pipe should be covered with a least 6 in. of compacted sand or non-corrosive earth followed by at least 2 ft. of backfill having no large stones.

High-pressure pipelines (250 to 1,100 psi) should be at least 500 ft. from important buildings.

Where pipe may be exposed to highly corrosive conditions (wet concrete floor, crawlspace) proper corrosion protection is needed for that environment. Corrosion protection include:

· Shop or field wrapping or coating.

· Cathodic protection.

Buried outdoor piping should rise above grade before entering a building. If a below grade entrance is necessary, review the local codes for clear space, pipe sleeves and the sealing of openings. Buried pipe joints, such as welds should be at least 10 ft. from wall entrances or any other points where pipe may be rigidly held against ground movement.

Indoor piping should be above grade and located to avoid physical damage. Areas to avoid include elevator shafts, chutes or ventilation ducts. If it is necessary to install piping in a crawl space, it should be encased in a larger pipe.

Valves/Meters

There are numerous valves that can be used with gas feeds. Specifications should be based on the service intended. For gas-fired process equipment, safety shutoff valves (SSOVs) in the fuel supply lines to both the pilots and main burners are recommended. For small pilots or fuel burners having less than 400,000 Btuh input, one SSOV is adequate. For larger units, two are recommended.

Meters and pressure regulators should be protected against overpressure. Regulators should be vented. Review the ANSI/ASME standards to ensure the valve and/or regulator selection is suitable for the intended use.

Meters should not be located in the following areas:

· Crawlspaces.

· Small unventilated areas.

· Where may be exposed to corrosion or physical damage.

The following are basic recommendations that apply to gas fired boilers:

· Provide permanent and ready means for making periodic tightness checks of the main burner safety shutoff valve. The need for tightness checks can be eliminated if an FM Global Approved automatic leak testing system is installed. This type of system pressurizes the space in between the SSOVs, and checks for pressure drop each time a boiler is started. Perform a manual leak test annually if equipped with the automatic system.

· Provide high and low gas pressure interlocks for boilers greater than 2.5 MBtuh by means of FM Approved pressure switches arranged to shut off and lock out all fuel and electric ignition in the event of dangerously low or high pressures. Locate the switches between the pressure regulator and safety shutoff valves.

· The fuel train should be designed to withstand the maximum pressure that can develop because of a pressure regulator failure. The high-pressure switch should be set lower than this pressure.

Operation and Maintenance

All piping/valves should be cleaned, inspected and tested before being put in service. Pressure testing is the most critical test. The piping should be able to hold the required test pressure for at least ½ hour per 500 ft3. Other tests will be necessary based on service application and local code requirements.

Summary

There are numerous FM Global Data Sheets and National Fire Protection Standards governing the installation, operation, testing and maintenance of natural gas and gas piping. Risk Logic, Inc. can assist in the proper installation to help ensure that all systems are installed and used in a fire safe manner.

Risk Logic can help in determining the safety of your building's gas piping.