In July 2008, Enerflex completed the engineering, procurement and construction contract for a gas turbine-driven compressor station at the midpoint of the Eastern Gas Pipeline in Mila, New South Wales.
In April 2012, Enerflex returned to the project to undertake shutdown works to replace two existing welded-in station isolation valves with flanged equivalents.
The compressor station shutdown was scheduled for a weekend in the ‘shoulder season’.
Enerflex Electrical and Instrumentation Engineering Co-ordinator Jeff Snelgar explained “This is when gas demand is at its lowest, however maintaining line pressure is critical to supplying customers. Without midline compression, the downstream pipework can drop to a low pressure over a period of days.”Article continues below…
For this reason, the shutdown had to be limited to four days.
The valves that were to be replaced provide the main isolation between the pipeline and the compressor station and are critical to safe station operations. The modifications allow for much greater flexibility if the valves require maintenance attention in the future.
Enerflex’s specialist crew used a pneumatic-powered clamshell cutter to precisely cut out the existing valves one at a time, and installed new 406 mm diameter 900# RF flanges on the pipeline side to provide connection to the new flanged valves; the station side already had existing flanges that could be utilised.
“The challenges of replacing valves on a live pipe revolve around doing the work accurately and efficiently, and the criticality of the cuts and welds,” said Mr Snelgar.
“It is imperative that the new valves, flanges, weld gaps, spool pieces and gaskets are all accurately measured and that their dimensions have been worked out to determine where the cut into the pipework needs to be made. If the gap remaining is too large then welding is difficult or impossible, and if it is left too long the cut will need to be made again, taking up time.”
“Adding to this challenge is that pipework expands and contracts with the heat of the sun, so estimating this effect plays a large part in getting it right. Our pipefitting crew managed to get both cuts within a few millimetres, which is exceptionally good.”
Mr Snelgar said that positive isolations are critical to ensure safety. “Our crew worked with the client, Jemena, to create a safe work environment when only a single double-sealing ball valve is all that separated the cut from the main pipeline, which contained explosive natural gas. Purging and continuous gas monitoring provided the required safety mitigations,” he said.
Due to the time criticality, two teams worked together simultaneously to speed up the process. The engineering and electrical and instrumentation crew worked to retrofit the actuators and swing test the valves offline while two coded welders worked on the flange and piping cuts and welds.
Upon successful non-destructive testing of the welds, the new valves were installed and the bolts tensioned. As part of the works, the existing solenoids and position switches for the actuators were required to be disconnected and bracketing and instrument air tubing modified and re-installed to suit the new dimensioned valves. The new valves were then reinstated and successfully swing tested via station controls.
Despite having a limited timeframe of only four days – the first of which was lost due to isolation issues out of Enerflex’s control – Enerflex’s congruently working teams were able to complete the works in just over two days from initial isolations through to final sign-off, bringing great satisfaction to the client Jemena.