Brady McKay, in the Daniel Canada organization sent me a great write up on the technical details of a surge relief system on which he’s been working with a leading pipeline company. For those in the pipeline business, the workings of surge relief systems are probably obvious. For the rest of us, it may be interesting.
Surge pressure is the rapid change in pipeline pressure from the change in liquid flow rate. These pressure surges travel at sonic velocities through the pipeline and can range from 1100 ft/sec (333 m/sec) to more than 4000 ft/sec (1212 m/sec). The rate is a function of the liquid’s compressibility and the content of entrained, dissolved gas. Greater compressibility translates into lower velocities. Stabilized crude oil has a velocity around 3300 ft/sec (1000 m/sec) during surge conditions.
So what causes these sonic pressure waves? Here are three examples: starting/stopping a pump, rapidly closing an emergency shutdown (ESD) valve or motor operated valve (MOV), and slamming shut action on a non-return (check) valve. What happens in these cases is that the pipeline flow drops rapidly. For those who recall their physics, the column of liquid continues under its own momentum (p=mv) that leaves behind a low-pressure region.
Eventually, the momentum is overcome by the opposing force of a static head, which accelerates the liquid column back toward the pumping station. This reversal causes the non-return/check valve to close. This process creates another pressure surge. How large this pressure will be is a function of the initial flow rate, the static head, pipe length, material, and friction inside the pipe.
Without sufficient surge protection in place, problems that can develop include:
- Axial separation of flanges
- Pipe fatigue at weld joints
- Longitudinal pipe splits
- Pumps knocked out of alignment
- Severe damage to piping and piping supports
- Damage to specialized components such as loading arms, hoses, filters, bellows etc.
A surge relief system must be extremely fast acting, on the order of less than 50msec. A liquid surge valve needs to open quickly to respond to the initial pressure rise. It then needs to close in direct response to the pressure decay on the inlet side of the surge valve. In a typical surge relief system, the relieved flow is dumped into a large storage vessel and then later returned to the pipeline when everything returns to steady state.
Brady’s write up made great sense to me and I hope this summary has helped you. If you’re already an expert in pipeline pressure surge, please share your wisdom in the comments.