Sizing excess flow valves for gas installations

MPAQ receives many enquiries about the requirements for installing excess flow valves in multilayer piping used for gas installations. Multilayer pipes have less durability and mechanical strength than metallic pipes when subject to fire. This has resulted in the inclusion for provision for fire emergency isolation for multilayer pipe in AS/NZS 5601.1:2022. The requirements were further updated with amendment 2:2024. 

Clause 5.2.11 specifies the requirements for fire emergency isolation where multilayer pipe is used as part of consumer piping. For Class 1a buildings in Australia it provides the option to install a device, e.g., excess flow valve, that automatically shuts off the gas supply in the event the gas tightness is adversely affected. 
An Excess Flow Valve must be fitted in an ‘accessible’ location prior to any multilayer piping it is intended to protect. They must be of the correctly approved type and sized to manufacturer’s instructions to ensure safe and correct operation. It is not a case of one valve with suit all applications. 

Excess flow valves should be selected to suit the direction of gas flow. Excess Flow Valves are designed to be installed in either vertically up or horizontal directions or an alternatively a different valve may be required for the vertically down direction. While one manufacturer may identify the direction of flow in the part number labelling e.g., a valve with a ‘Z’ included in the labelling would indicate it is to be installed in the vertically up or horizontal flow direction while a ‘D’ would be a downward position.

Another manufacture may identify the direction of installation with a solid arrowhead on the labelling.

Below are examples of the GST Excess Flow Valve Selection Charts for Natural and LPG Gases

For this exercise we will use the Natural Gas Selection Table 1.1. The first column is the thread sized of the valve in mm. Column 2 is indicates the flow capacity of each valve expressed in cubic metres per hour (m³/hr). The Third and Fourth Columns are the nominal flow capacity through each size valve depending on the application, e.g., a DN 20mm valve with a nominal flow rate of 2.5 m³/hr has recommended flow rate of 98.1 MJ/hr. This calculated using the higher heating value for Natural Gas as provided in AS/NZS 5601.1, Appendix F, 38MJ/m³for Natural gas. 2.5 m³ x 38 = 95MJ total. NOTE: The physical thread size of an excess flow valve is not an indication of the flow capacity. A 20mm valve could have a nominal flow of 2.5 m³/hr or 4 m³/ hr. It is important to select the correct one for the installation. 

The last column is the maximum flow through the valve at the point at which will cause the valve to shut off. This is calculated by multiplying the nominal flow rate through the valve by a factor of 1.45, e.g., 98.1 x 1.45 = 142.245 (142.3 rounded).

Pipe Sizing Example

Before selecting an excess flow valve for a particular installation, it is vital that any multilayer piping gas installation is accurately sized. Caution should be exercised at this time to use the correct manufacture’s sizing chart for the brand of pipe chosen as not all multilayer pipe sizing charts are the same. For this exercise I have selected the Iplex K1 Composite Gas Pipe System Sizing table 1.1 for Natural Gas with an available meter pressure of 1.25 kPa and an allowable pressure drop of 0.012 kPa

The total length of pipe from A to C = 17m (13+4). Referring to Table 1.1 shown below, the next pipe length up from 17m is 18m. In accordance with AS/NZS 5601.1:2022 Clause F.5.2.7 (a) the larger pipe run shall be selected.

Using Table 1.1 Pressure Drop 0.012 kPa (Meter Pressure 1.25 kPa), we see that a 32mm pipe with a length of 18 m will deliver 259 MJ.
 

Having selected 32mm as the required pipe size we now need to choose the correct size excess flow valve from Table 1. A 32mm valve can provide a flow of 10 m³/hr as a main run or up to 392.5 MJ/hr. The shut off energy required is given as 569.1 MJ/hr. The shut of energy required to fully close the excess flow valve is calculated by multiplying the piping capacity by a factor of 1.45 (392.5 X 1.45 = 569.12)

This now presents us with the problem where in the event of a catastrophic pipe failure, such as a fire, using a 32mm multilayer pipe could not supply sufficient gas to completely shut off flow of gas. From Table 3.1 we see that we would need a pipe size of 50mm to ensure that sufficient gas volume is available to satisfactorily completely close the 32mm excess flow valve.

In this scenario the Gas Work Licence holder may consider alternative measures to limit installation costs by reducing pipe sizes for branch lines. However, while this would mean smaller branch sizes it would also require an excess flow valve of the correct size to be installed on each branch. In addition, 5601.1 requires reversion fittings (Figure 5.2.12) to be incorporated upstream of any branch take off fitting when to length of the main run exceeds 10m and the installation of a minimum of 1m of metallic pipe from a joint with non-metallic components prior to exiting a wall (Clause 5.2.12). 

These additional requirements may prove uneconomical, and the Gas Work Licence holder may elect an alternative piping material that does not require the installation of an excess flow valve or multiple valves.

MPAQ members who have questions about this can call the MPAQ technical team on 07 3273 0800.