Gas pipe sizing
Pipework in the home is used to deliver gas from the gas meter to each of the your appliances. The manufacturer of each appliance will have specify the maximum rate of gas consumption for the appliance. The pipework must be large enough able to supply this rate of consumption to all the appliances at the same time, without excess pressure drop, for the appliance to work correctly and safely.
This article explain how you can calculate the sizes required, and for example, assess if the existing pipework will be adequate for a new or upgraded appliance.
Your gas meter and the main gas rate governor, should be set to deliver a gas at dynamic pressure of 21 mbar (note the dynamic pressure is the pressure measured when gas is flowing - the static pressure may be a few mbar higher).
Gas appliances in the UK are typically designed to expect an input pressure of 20 mbar. So the pipework can drop no more than 1 mbar between the supply and the point of use.
The pressure drop of a fluid flowing in a pipe will be dictated by:
- Bore of the pipe (usually specified in mm diameter)
- The flow rate of the gas in cubic meters / hour
- The effective length of the pipe
The flow resistance of a pipe will be higher at the point it changes direction. The easiest way to make an assessment of the higher pressure drops caused by direction changes, is to adjust the effective length of the pipe. The effective length is the actual length of the pipe, with and additional 0.5m added for each 90 degree elbow, and 0.3m added for each bend or swept elbow.
When checking a pipe design, you will need to assess the total gas rate that needs to be delivered though each section of pipe. Some sections of pipe will likely branch to more than one appliance, so the section of the pipe before the branch will potentially carry the sum total maximum gas rates of all the appliances that follow the branch.
This table (taken from BS 6891, "Installation of low pressure gas pipework of up to 35 mm (R1¼) in domestic premises (2nd family gas) — Specification"), contains a typical gas discharge rates for various pipe types and sizes: