Domestic Natural Gas Installations Marc Billington

How is Gas delivered? Gas is delivered to the seven reception points (called beach terminals) by gas producers operating Offshore Facilities from over 100 fields beneath the sea around the British Isles. In addition a newly commissioned terminal at the Isle of Grain allows Liquefied Natural Gas to be delivered to the terminal by sea. After treatment, which includes checking the quality meets the safety requirements and measuring the calorific value (the amount of energy contained in the gas), it is transported through 275,000km of mains pipelines.

Simplified National Gas Transmission System Diagram of UK

Origins of Gas Pressure The National Transmission System (NTS) is the high pressure part of National Grid’s transmission system and consists of more than 6600km of pipe operating at a pressure of up to 85bar (85 times normal atmospheric pressure). The gas is pushed through the system using 26 strategically positioned compressor stations.

Origins of Gas Pressure From over 140 offtake points, the NTS supplies gas to 40 power stations, large industrial consumers and 12 Local Distribution Zones (LDZ’s) that contain pipes operating at lower pressure which eventually supply the consumer. Gas arrives in the customer’s meter at a pressure of 21millibars. The pressure in the service pipe leading in from the road is about 30–50 mbars, and pressures in the street could be between 30 mbar to 7 bar.

Regional Gas Network

Low Pressure Gas Installations Scope Low pressure meter installations are installed in accordance with BS6400 Part 1:2006 Specification for installation, exchange, relocation and removal of meters with a maximum capacity not exceeding 6m³/h – Low pressure 2 nd family gases. The gas supplied to any property on a low pressure main must not exceed 75millibars. It must also be fitted with a primary regulator downstream of an Emergency Control Valve.

Typical Gas Meter Boxes

Example of Semi-Concealed Meter Box Meter Regulator Primary Gas Meter Emergency Control Valve (ECV) Low Pressure Gas Service Pipe

Typical Domestic Gas Meters

Line Diagram of a LP Meter Installation

Typical Gas Meter Installation

Simple Constant Pressure Regulator

Regulations The Gas Safety (Installation & Use) Regulations 1998 are those that we must adhere to when dealing with gas. We have already looked at gas meter boxes, primary gas meters and regulators. These fall under regulations 9 to 16 of the above regulations.

Determining the Size of a Meter The capacity of the gas meter must be such that it can pass a sufficient quantity of gas to meet the demands of the gas installation with minimal pressure loss across the installation. To determine the size of the gas meter we need to know the maximum gas flow rate of the installation. To do this we use the manufacturers stated heat input for each appliance.

Determining the Size of a Meter Since it is unlikely all appliances will be on at the same time, a diversity factor is applied to each appliance heat input. These figures can be taken from the following table.

Example of Meter Sizing A house has the following appliances installed, what size gas meter is required? 40kW Combination Boiler 6.43kW Gas Fire 19.4kW Oven 12.6kW 6 burner hob First we need to look at the diversity factor table and apply our findings.

Combination Boiler40x0.8=32.0 Gas Fire6.43x0.6=3.858 Oven19.4x0.6= Burner hob12.6x0.6=7.56 Total Gas Load = We know need to use the following formula. Total gas rate (m³/h) = Gas Load (kW) x 3.6 Calorific value (MJ/m³)

Total gas rate (m³/h) = Gas Load (kW) x 3.6 Calorific value (MJ/m³) =55.058kW x MJ/m³ =5.11m³/h We now need to the at the meter sizing table on the next slide to determine the size of meter rquired.

Gas Meter Sizing Table The table shows that an installation that requires 5.11m³/h of gas would need a U6 gas meter which is capable of supplying a maximum of 6m³/h.

Example of a Meter being tested for Tightness

Gas Installation Pipework Regulations 18 to 24 of the Gas Safety (Installation & Use) Regulations 1998 must be followed when working with gas installation pipework. In addition we must also follow British Standard 6891:2005 +A2:2008 Installation of low pressure gas pipework of up to 35mm (R1¼) in domestic premises.

Pipe Sizing and Pressure Loss A gas pipe system must be sized so that it can supply all the appliances simultaneously with a maximum pressure absorption of 1mbar from the gas meter outlet to all the appliance inlet connections. Assuming the working pressure at the gas meter outlet is 21mbar this will ensure that the appliances are supplied with their design pressure of 20mbar.

Pipe Sizing Procedure Determine the minimum size of copper pipe required for the installation shown.

Answer The total heat input is 58kW. The maximum gas rate is (58 x 3.6) ÷ = 5.39 m³/h (not taking the diversity factors in to consideration). The gas meter in the diagram is therefore big enough to supply the installation. Complete the table you will be given.

Answer Continued Step 1 - The pipework must be treated as separate sections determined by the gas flow. Enter these in column 1. Step 2 – The heat input required by each section of pipework is then entered e.g. A – B must supply sufficient gas flow rate for all the appliances, which is 5.39 m³/h. Enter these in column 2.

Answer Continued Step 3 – The actual length of each section is then enter. Enter these in column 3. Step 4 – Enter the allowance for fittings or bend. Tee pieces are only relevant to the section of pipe containing the branch. Enter these in column 4.

Answer Continued Step 5 – Add up columns 3 and 4 to give you an equivalent length in metres. Enter this in column 5. Step 6 – An arbitrary pipe size is then selected, although if it results in a pressure loss in that section close to the maximum of 1mbar then it will probably require increasing in size. Place this pipe size in column 6.

Answer Continued Step 7 – Using the Discharge table on the next slide determine the maximum length for the selected pipe size/type. Exercise common sense, e.g. for 28mm copper, 20m, the heat flow is 5.9m³/h when 5.92m³/h is required, enter 20m, not 15m.

Discharge Table

Answer Continued Step 8 – Determine the pressure absorption from the meter outlet to each appliance and ensure it does not exceed 1 mbar. If it does, the pipe section sizes will require to be increased. From the meter outlet to the combi boiler, the gas must flow through three pipe sections which are A-B, B-C and C-D. What is the pressure absorption between the meter outlet and the appliance inlet?

Pipework Pipework must be adequately supported in accordance with Table 2 – Maximum intervals between pipe supports, from BS6891.

Example of Typical Exterior pipework

Pipework in Cavity Wall

Main Equipotential Bonding

Bonding of pipework is an essential aspect of electrical systems, most systems that are installed require earth bonding and you need to be aware if the bonding has been installed correctly. Copper pipework can provide a route for stray electrical currents and can cause corrosion of the pipework.

Main Equipotential Bonding The size of the bonding cable and it’s location are essential for the gas installation. The cable must be 10mm² and must be connected to the gas pipework no more than 600mm from the gas meter outlet. The above are requirements of BS6891:2005 but must also conform to BS7671.