Gas Safe ACS - Accredited Certification Scheme Tightness Testing and Purging Questions and Answers — Questions and Answers
Question 1: An engineer is performing a tightness test on an existing domestic Natural Gas installation with a U6/G4 meter and 22mm pipework. If there is no smell of gas, what is the maximum permissible pressure drop over the two-minute test period according to IGEM/UP/1B?
- 0 mbar, as no pressure drop is ever acceptable.
- 2 mbar, provided all appliances are disconnected first.
- 8 mbar, but only if the meter is an E6 type.
- 4 mbar. (Correct answer)
Correct answer: 4 mbar.
According to the guidance in IGEM/UP/1B, for an existing domestic Natural Gas installation with a U6/G4 meter and pipework up to 28mm, a maximum pressure drop of 4 mbar is permissible over the two-minute tightness test, provided there is no smell of gas.
Question 2: What is the primary purpose of purging a new gas installation pipework before commissioning any appliances?
- To displace air with fuel gas, ensuring a safe, non-explosive mixture is present at the appliance burners. (Correct answer)
- To clean the pipework of any water or debris left over from the installation process.
- To conduct a high-pressure strength test on all the pipework joints and fittings.
- To verify the correct operation of the gas meter's measurement index.
Correct answer: To displace air with fuel gas, ensuring a safe, non-explosive mixture is present at the appliance burners.
Purging is the process of replacing the air or inert gas inside a new or modified pipework installation with fuel gas. This is a critical safety step to ensure that a flammable mixture of gas and air is not present when an appliance is first ignited, preventing potential explosions.
Question 3: Which of the following describes the correct sequence of tests when recommissioning a domestic gas installation at the meter, according to IGEM/UP/1B?
- Tightness test, Purge, Let-by test.
- Let-by test, Tightness test, Purge. (Correct answer)
- Purge, Let-by test, Tightness test.
- Strength test, Let-by test, Tightness test.
Correct answer: Let-by test, Tightness test, Purge.
The correct procedure is to first conduct a let-by test on the Emergency Control Valve (ECV). If that passes, a full tightness test is performed on the installation. Only after the installation is confirmed to be tight is it then purged of air to introduce fuel gas.
Question 4: An engineer performs a tightness test on an existing installation and records a pressure drop of 7 mbar over 2 minutes, which is outside the permissible limit. The leak cannot be immediately located or repaired. What is the correct immediate action required under the Gas Industry Unsafe Situations Procedure (GIUSP)?
- Advise the customer to call the National Gas Emergency Service and then leave the property.
- Leave the gas supply on but issue an 'At Risk' warning notice to the homeowner.
- Turn off the gas supply at the Emergency Control Valve, attach an 'Immediately Dangerous' warning notice, and inform the responsible person that it must not be used. (Correct answer)
- Re-test the installation at a higher pressure to make the leak more obvious.
Correct answer: Turn off the gas supply at the Emergency Control Valve, attach an 'Immediately Dangerous' warning notice, and inform the responsible person that it must not be used.
A failed tightness test indicates a gas leak, which is classified as 'Immediately Dangerous' (ID) under GIUSP. The engineer has a duty to make the situation safe by isolating the gas supply at the ECV, clearly labelling the installation as unsafe with a 'Do Not Use' warning, and informing the responsible person.
Question 5: According to IGEM/UP/1B, when must the gas from a direct purge be ignited at an open burner rather than being vented to the atmosphere?
- When the installation volume (IV) is 0.02m³ or greater. (Correct answer)
- Only when purging commercial or industrial installations.
- Whenever the pipework diameter exceeds 22mm.
- In all circumstances to ensure complete removal of air.
Correct answer: When the installation volume (IV) is 0.02m³ or greater.
The guidance in IGEM/UP/1B specifies that for Natural Gas installations with a volume of 0.02m³ or more, the purge gas must be ignited at an open burner. This is because venting such a volume of gas directly into a property would be dangerous and could create an explosive atmosphere.
Question 6: During a tightness test on a domestic Natural Gas installation, what are the specified test pressure and duration for the main tightness test phase, following the initial 1-minute stabilisation?
- 10 mbar for a duration of 5 minutes.
- 50 mbar for a duration of 1 minute.
- 20 mbar for a duration of 1 minute.
- 20 mbar for a duration of 2 minutes. (Correct answer)
Correct answer: 20 mbar for a duration of 2 minutes.
As per IGEM/UP/1B, after the initial let-by test and a 1-minute stabilisation period at the test pressure, the main tightness test for a domestic Natural Gas installation is conducted at approximately 20 mbar for a duration of 2 minutes.
Gas Safe ACS - Accredited Certification Scheme Tightness Testing and Purging Questions and Answers
An engineer is performing a tightness test on an existing domestic Natural Gas installation with a U6/G4 meter and 22mm pipework.
If there is no smell of gas, what is the maximum permissible pressure drop over the two-minute test period according to IGEM/UP/1B?