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Flaring Small Volume Lpg Pipework

Barrie Church of Global Energy Associates Ltd makes the case for relaxation of the requirements in UP/1 for flaring all lp gas purge operations.

For some time now, there has been concern amongst lpg gas installers that the requirements in UP/1 for flaring all lpg purge operations may be too excessive. The argument goes further in that, for small volumes of natural gas up to 0.02m3, it is permissible to purge in some internal areas.

There have also been some serious incidences of installers venting off lp gas in an unsafe manner. So, could there be a case for some relaxation for lpg?

Firstly, we need to understand that flaring gas is a preferable and ‘greener’ solution to venting. Secondly, flaring does not rely on achieving a velocity, as with a normal purge, since it is simply a means of burning off the gases at their contained pressure, especially during decommissioning.

When the burn off is complete, the normal purge procedure can take place after depressurisation. Thus, the size of the flare burner is related to the time available to perform the flare process and the maximum size of flames acceptable on the particular site.

The flare unit from Global Energy Associates Ltd (GEA) is designed for natural gas and lpg gas systems and the burner can burn up to 350kW of gas (35m3/h natural gas or 15m3/h lp gas) at up to 1 bar gas pressure. These are large flow rates, but the flame is relatively small and not a hazard when positioned at least five metres away from buildings. It is also possible to work beneath the burner head when it’s alight.

However, most lpg gas purge jobs involve pipe sizes up to 25mm and less than 30m in length, giving a volume of some 0.02m3 and less than 0.5kW of energy. So, if the lp gas was vented through a 10kW burner, it would take only a few seconds to depressurise and burn off. But this is only relevant for depressurising the system, and to empty the gas out of the system would then require air or nitrogen to be blown into the pipe to push the remaining lpg through. Totally emptying the pipe would take about three minutes of burn off [(0.02/0.40) x 60 = 0.05 hours x 60 = 3 minutes].

10kW	Flow rate (m3/h)	Jet bore (mm)	Area (mm²)	Velocity (cm/s)	Flame speed (cm/s)
Natural gas	1	2.4	4.5	61	36
Lpg	0.4	1.8	2.5	44	38

Approximate figures for comparison only. Low-pressure gas flow.

During commissioning to gas, the problem is nowhere near so big, as the pipe is full of air to start with. Taking UP/1, if you purged at 0.6m/s, a 60m run would be completed in 60/0.6 or 100 seconds.

For these small flaring jobs, a suitable burner could be used and, if an ignition source (e.g. an lpg gas flame torch) was permanently kept running at the burner, any gas coming through would immediately be ignited and burnt off.

The table also shows that the speed of the gas passing through the injector is always above the flame speed with natural gas and lpg, so there should not be a worry of the flame going back down into the pipe. Indeed, when the tests were being performed many years ago by British Gas to prove the forward velocity figures of 0.6m/s, etc, they actually ignited the natural gas air mixture within the pipe to see what would happen during the purge. Very little is the answer, as the small flame front was soon extinguished by the CO2 from the combustion process and the nitrogen in the air.

What does this all mean? Firstly, it shows that it is possible, at little cost, to buy a burner and lp gas hose that would be suitable to burn off small volumes of gas internally or externally, providing the burner is safely located and there is adequate ventilation. Secondly, there should not be a concern over the need to use an inline flame arrestor, as the velocities should be such as to ensure safety. However, GEA has one available (weighing only about 600gm) at reasonable cost if needed and in compliance with UP/1.

Lastly, what it may mean is that, with most burners/appliances, it would be necessary to break into the supply near to the boiler to fit the 15mm flare hose. This may also require performing a let-by test before disconnection of the burner at the union to get to a suitable valve location.

Contact Global Energy Associates Ltd on 01780 481987 www.globalenergy.co.uk

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