Fuel Gas includes Natural, manufactured or liquefied petroleum. Natural gas (NG) normally will be supplied from the Utility Company’s street main lines, but liquefied petroleum (LP) gas will supply in an installed tank in property or in portable cylinders.
The energy value in 1 cubic foot of natural gas at atmospheric pressure is called “heating value”. The gas flow is measured in a cubic feet per hour (CFH). The gas heating value varies from 900 to 1100 Btu/cubic foot. Natural gas is nominally rated at 1000 Btu/cubic foot (CFH).
Gas service is usually low pressure (0.5 psi or less), or medium pressure (above 0.5 psi and up to 5 psi).
A. Principles of design
- Gas systems shall be designed in accordance with the Utility Company’s requirements, NFPA Standard No. 54 and all the requirements of the applicable codes.
- Loads for equipment shall be taken from the manufacturer’s ratings.
- Gas piping shall pitch back to the meter wherever possible. All risers and trapped piping sections shall be provided with a drip pockets consisting of nipple and cap.
- All gas risers and branches shall be provided with a shut-off valve.
- Wherever possible, gas mains and risers inside building shall run exposed rather than concealed in shafts or hung ceilings.
- No gas piping shall run in or through air plenum ceilings, clothes chutes, air ducts, dumb waiters, stair enclosures or elevator shafts.
- Gas riser in air shaft shall be encased or separated by masonry construction.
- Gas piping passing through the air plenum ceilings or between ducts without connections, shall be enclosed or sleeved for the full length.
- Verify with local codes if venting of gas sleeves or enclosure is required.
B. Gas piping system criteria
- Layout the piping system and indicate the location of gas meter on main line to the building.
- Indicate the location of all the appliances and gas consume equipment in piping layout.
- Indicate on the piping layout the pipe size connection, required gas flow and pressure for each appliance and equipment per manufacturer catalogue and instruction. Appliances gas demand are also indicated in tables in Uniform Plumbing Code (UPC) or State Plumbing Code book.
- Indicate the total flow (CFH) in each pipe section (total users flow that each pipe section need to serve) by adding up the users flow from the farthest user all the way to the meter.
- Measure the longest run (including horizontal & vertical runs) from the gas meter to the farthest gas user. Assume the longest run is “L”.
- Calculate the Developed Length by considering 25% of “L” as an equivalent length for fittings & valves.
DL= 1.25 x L
C. Pipe sizing
Piping from the street main to the meter is usually sized by the Utility Company. Plumbing engineer is responsible for sizing the piping from the meter to the users.
- The gas pipe sizing tables are indicated in tables in Uniform Plumbing Code (UPC) or State Plumbing Code book.
- In pipe sizing table find the next closest length to the calculated developed length “DL”.
- Entire piping system shall be sized based on the selected longest run row in UPC table.
- Find the flow range for each pipe section in table and determine the appropriate pipe size.
