When gas transmission pipelines need to be modified or maintained, the gas within has to be removed.
Traditionally this was achieved by simply venting the gas to atmosphere or burning using temporary flare. This is now becoming impossible due to pollution legislation. Also pipelines can be many miles long and have diameter up to 56�, and so it is economic sense to save this gas. A rotary screw or reciprocating compressor can be used to suck the gas out of these lines and to re-inject it beyond an isolation valve into normal system pressure at discharge. Machine has to cope with varying pressure ratio, starting at 1:1 and increasing to very high ratio the closer line pressure gets to atmospheric. It is goal to extract as much gas as possible, and thus get as close to atmospheric as is practical. Drives for this type of compressor can be electric, possibly via generator, diesel engine, or gas engine. The compressor system needs to be mobile, and would typically be built permanently onto a truck trailer or inside ISO container.