Standing, working and operating pressure

The same gauge on the same test point can show very different readings depending on what's running. Knowing which pressure you're measuring, and under what conditions, is what makes the reading mean something.

The terms

• Standing pressure: the pressure with no gas flowing, all appliances off.
• Operating pressure (OP): the pressure in the system under normal operating conditions, with gas flowing. IGEM/G/13 notes that operating pressure can also be called working pressure, and different documents use both terms.

Because the words overlap, always say where you measured: at the ECV outlet, the meter outlet, or the appliance inlet.

The meter outlet check

IGEM/G/13 sets one check that is used before installing an appliance, after commissioning it, and whenever there's a complaint of poor pressure:

1. Run all the appliances at high operating load (below).
2. Wait one minute for the pressure to settle.
3. Read the pressure at the meter outlet. It should be no less than 18.5 mbar and no more than 23 mbar.

ACS CCN1 Issue 16 §7 still assesses this check against 19–23 mbar. IGEM/G/13 notes that it differs in detail from current British Standards and company procedures, and that these are due to be updated. Lesson 4.4 gives both figures.

When to do the check

• Before installing a new appliance: if practicable, check the existing installation is working at the right pressure, and confirm the supply has enough capacity (see 3.1).
• Commissioning: follow the manufacturer's instructions, including checking the appliance's inlet pressure at its maximum load. Be aware of any loss between the inlet test point and the isolation valve.
• After commissioning: repeat the meter outlet check with everything at high load, to confirm the new appliance hasn't upset the installation.
• When there's a complaint, or you can't get an appliance to its required pressure or heat input.

If the reading is out of range

1. Phone the National Gas Emergency Service on 0800 111 999 (Northern Ireland: 0800 002 001). Get a reference number, and give your Gas Safe registration number if asked and a contact number.
2. The emergency service should attend within 2 hours. Wait for them if you can. If the appliance is new, you must wait, or arrange to meet them on site, so you can investigate together.
3. If you can't wait, your job report should record: that the internal pipework is correctly sized, the pressures you measured, any recent changes (such as new appliances or a moved meter), and the installation's total load.
4. If an existing appliance's safe operation is affected (for example its combustion or flame stability), make it safe under the Unsafe Situations Procedure. If the appliance is new, disconnect it and seal it as an uncommissioned appliance (GSIUR reg. 26(5)).

The emergency service takes overall responsibility for investigating, but it can't work on or set up appliances, so it relies on you for those checks. It measures at the ECV outlet as well as the meter outlet. Comparing the two shows whether the problem is in the supply, the meter installation or the pipework downstream.

Sources: IGEM/G/13 Appendix 1OP definition, high load IGEM/G/13 4.1–4.3procedures IGEM/G/13 5.1–5.2reporting IGEM/G/13 6.1, Table 1joint investigation GSIUR 1998 reg. 26(5)uncommissioned appliance IGEM/G/11 Ed. 2 Table 1situation 2.2 CCN1 Issue 16 §7assessed

▸Tutor notes

• Have learners set up "high operating load" on a training rig: which appliance goes to maximum, and what the others do.
• Give readings (meter outlet 20 mbar at high load; 17 mbar; 24 mbar) and ask what happens next in each case.
• Point out the difference between IGEM/G/13 (18.5–23 mbar) and CCN1 Issue 16 (19–23 mbar), and explain which the assessor will expect.