Surge relief in liquid pipelines

Surge relief in liquid pipelines refers to mitigating or reducing the pressure surge in a pipeline system due to sudden fluid velocity or flow rate changes. A surge is a pressure wave that travels through the pipeline and can cause damage to the system, such as pipe ruptures, leaks, or equipment failure.

Consequences of high-pressure pipeline surge may include axial separation of flanges, pipe fatigue at welds, or longitudinal splits of the pipe. Pumps can be knocked out of alignment, and severe damage can occur to piping and piping supports, as well as specialised components such as hoses, filters, bellows, etc. In turn, these failures can result in unplanned downtime and product loss or spoilage, impacting profitability. There may be wider consequences, too, ranging from environmental pollution and clean-up costs to injury or loss of life.

Surge relief devices are typically installed at strategic points in the pipeline system to protect against surge pressures. These devices can include relief valves, surge tanks, or other specialised equipment designed to absorb or dissipate the surge energy and reduce the pressure spikes that can occur.

The selection and design of surge relief systems depend on various factors, including the type of fluid being transported, the length and diameter of the pipeline, the terrain and geography of the area, and the operational conditions of the pipeline system. Proper surge relief is essential to maintaining the safety and integrity of liquid pipelines and ensuring reliable and efficient transport of fluids.

Anyone responsible for the design, operation, and maintenance of fluid handling systems must therefore understand the causes of transient pressure surges and ensure adequate mitigation measures are in place.

Causes of surge

Any hydraulic system, from a simple rising water main to that of a complex petrochemical process network, will be subject to surge pressures if operational changes cause velocity fluctuations. Surge pressures may vary from being virtually undetectable to having sufficient severity to cause a catastrophic failure leading to a major disaster.

Many of us have experienced the ‘thump’ that follows when we shut off the water flow to our garden hose. This effect is often referred to as a ‘water hammer’ and a transient pressure surge causes the sound. When an appliance’s low pressure and small diameter are scaled up to the much larger piping and high-pressure arrangements commonly found in tanker terminals, oil and gas facilities, and chemical processing plants, that little thump can become a big problem. The noise alone is terrifying, and its surge is potentially hazardous.

Rapid changes in pipeline flow rate are usually caused by pump shutdown or valve closure generating pressure waves, which travel upstream and downstream from the point of origin. The pressure in the pipeline behind these propagating waves rapidly increases or decreases. Typical propagation velocities can range from 1100 ft/sec. for a water pipeline to 3300 ft/sec. for a typical crude oil line.

In addition, long pipelines are subject to a phenomenon known as line pack, where pressure slowly builds. Due to frictional losses in a pipeline, pumps must generate higher discharge pressures to move the liquid column downstream. Pump flow is maintained substantially after valve closure as the pump packs the pipeline.

In most cases, the final pressure locked in between the pump non-return check valve and the closed valve exceeds the maximum discharge of the pumps due to pressure oscillation in the line during this packing period.

Controlling surge

A complete computer model of the pipeline profile should be completed early in the pipeline design. Experienced engineers use computer-based numerical simulation techniques to determine the effects of unsteady flow of liquids transmission in pipelines and piped networks. Operational and control problems are identified during this phase, and proposed solutions are verified.

Staging pump shutdowns with the closing of main pipeline valves can help cancel the high-pressure wave travelling in the opposite direction from the closing ESD or MOV valve. Using a remote or local high-pressure or low-flow sensor can achieve pump shutdowns.

As discussed, valve closure times can affect surge pressures in the pipeline. Extending the valve closure time can achieve a more gradual flow decay. Closing cycles of different styles of valves (e.g. ball, butterfly, globe, or gate valves) can also affect flow decay. In addition, a surge pressure relief facility should be installed to protect the system. The type and location of the relief facility is governed by two factors…

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