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Distributing a Safer Lp Gas Supply

Distributing a Safer Lp Gas Supply By Richard Smith, lpg business development manager, LPG Provida

For years now, the lpg business has been distributing lp gas to metered estates at a medium pressure of around 750 mbar.

Just recently, the Health & Safety Executive (HSE) has taken an interest in the way we distribute our lpg gas to our metered estates. They are questioning why we don’t fall inline with the way that natural gas is distributed and, to be honest, I think they are probably right.

We normally distribute lpg from a typical tank farm at 1 bar. In the event of the pipeline being damaged, the customer would lose a lot of lp gas quickly and, more importantly, there may be a significant safety issue regarding the release of medium pressure lpg gas into the atmosphere.

If distribution is at 75 mbar, lp gas is transmitted at 10 times less pressure and, hence, the system is safer. If a leak occurs in the system, it is estimated that the amount of lpg gas lost at 750 mbar is 0.75kg/hr. At 75mbar, this would be at least halved.

There is a greater risk of re-liquefaction of lpg when medium pressures are used from multi vessel installations. Should liquid condense in the PE pipe, it will eventually re-vaporise, expanding some 270 times in volume, increasing the pressure and subsequently tripping off the over pressure protection on the lp gas control equipment.

At a 75mbar distribution pressure, there is far less chance of this happening. Currently, if you distribute at over 75mbar, you are obliged to insert excess flow valves into your pipework according to IGE TD4. This is a costly exercise and limits the ease of maintenance.

At a distribution pressure of 75mbar or below, although the pipework size may increase slightly, the requirement for excess flow valves is dispensed with, increasing the safety of the service (bringing the distribution pressures inline with natural gas standards) and giving the added advantage of being able to run pipework inside the property. For those properties that are listed buildings or have requirements that all services should be hidden, or even for piping lp gas into flats, it can be easily seen that this method is the way forward. We currently have several sites in the UK that have used this three-stage breakdown and also have several more in the pipeline.

A basic breakdown of the complete system is as follows:

From the tank farm, the lpg vapour is broken down from a typical 7 bar pressure to between 0.7 and 1 bar using a twin stream, active/slam regulator module. This unit incorporates two regulating streams, one working and one as a standby. These streams should be alternated when normal maintenance is carried out.

From the first stage module, the lpg gas is piped underground to a second stage module. This could be either twin stream active monitor or single stream. The typical regulator in this module would be a J125 type with integral limited relief valve, OPSO and optional UPSO. Distribution pressure from here would be 75mbar or less. The siting of this second stage module should be considered carefully so as not to be easily accessible to the public, or it should be enclosed in a lockable kiosk.

An alternative to the two units would be a combined first and second stage module. This combines all of the lp gas control in one lockable stainless steel box and subsequently makes the system more secure.

As the second stage distribution pressure is 75mbar, an OPSO is not required at the house end. We use the J42U domestic regulator for the third stage, which has the added advantage of incorporating a self-resetting UPSO valve. This means that the customer does not even have the upset of having to reset the regulator in the event of a break in the lpg supply.

The third stage meter box design can be incorporated into all the usual types of meter box, and also as a stand-alone unit for installing in flats or places of historical importance.

Contact LPG Provida on 01246 269807 www.meterprovida.com