Gas installation design and pipework sizing
What this domain tests
This domain assesses whether you can design consumer pipework that delivers the required flow at an acceptable pressure, with correct materials, joints, isolation, protection and accessibility. The exam rewards a clear sizing method and awareness of the applicable GU code and utility requirements.
Core rules
- Size from the meter or supply point to each appliance, taking the worst-case route. The appliance inlet pressure must remain within the manufacturer's tolerance and the code's permitted loss.
- Use equivalent length: actual pipe length plus allowances for elbows, tees, valves and reducers. Fitting allowances are geometry- and size-dependent, so use the code or manufacturer's table.
- Check velocity as well as pressure drop; excessive velocity causes noise and erosion, while oversized pipe wastes material and space.
- Copper, stainless steel corrugated tube and approved PE are common. Joints must be appropriate to the material and location; concealed joints are restricted.
- Provide isolation at the meter and at each appliance, sleeving through walls, protection against corrosion and electrical bonding as required.
- Keep pipes away from heat, electrical equipment and spaces where leakage could accumulate.
Worked example
A domestic town gas run has 12 m of straight pipe, six elbows and two tees. Using the utility's equivalent-length table, each 22 mm elbow is 0.5 m and each tee 1.0 m, so equivalent length = 12 + (6 x 0.5) + (2 x 1.0) = 17 m. The appliance needs 2.4 m3/h and the permitted loss is 0.10 kPa. In the utility's low-pressure table, 15 mm copper at 17 m equivalent length carries about 1.8 m3/h, while 22 mm carries about 4.6 m3/h. Select 22 mm. Check velocity: 2.4/3600 = 0.000667 m3/s; 22 mm copper with about 20.2 mm bore has area 0.000320 m2, giving 2.1 m/s, which is acceptable.
Traps that cost marks
- Using actual length only and ignoring fittings, which underestimates pressure loss.
- Sizing on connected load rather than the actual simultaneous appliance demand.
- Forgetting the meter outlet pressure or the pressure loss through the meter and regulator.
- Concealing joints or failing to sleeve a pipe through a wall or floor.
- Selecting a material or jointing method not approved for the location, gas type or pressure.