Surge control is the use of different techniques and equipment in a hydraulic system to prevent any excessive gain in pressure (also known as a pressure surge) that would cause the hydraulic process pressure to exceed the maximum working pressure of the mechanical equipment used in the system.
What is hydraulic surge
Hydraulic surges are created when the velocity of a fluid suddenly changes and becomes unsteady or transient. Fluctuations in the fluid's velocity are generated by restrictions like a pump starting/stopping, a valve opening/closing, or a reduction in line size. Hydraulic surges can be generated within a matter of seconds anywhere that the fluid velocity changes and can travel through a pipeline at very high speed, damaging equipment or causing piping failures from over-pressurizing. Surge relief systems absorb and limit high-pressure surges, preventing the pressure surge from traveling through the hydraulic system. Devices for controlling hydraulic surges include gas-loaded surge relief valves, spring-loaded pressure safety valves, pilot-operated valves, surge suppressors, and rupture disks.
Typical applications Surge control products have been used in many industries to protect the maximum working pressure of hydraulic system for decades. Typical applications for surge relief equipment is in pipelines at pump stations, receiving manifolds at storage facilities, back pressure control, marine loading/off loading, site specific applications where pressure surges are generated by the automation system, or any location deemed critical by an engineering firm performing a surge analysis.
Surge suppressors Surge suppressors perform surge relief by acting as a pulsation dampener. Most suppressors have a metal tank with an internal elastic bladder in it. Within the tank they pressurize the top of the bladder with a compressed gas while the product comes in the bottom of the pressure vessel. The gas in the bladder is supplying the system with its set point. During normal operation, as the process conditions begins to build pressure; the internal bladder contracts from the pressure gain allowing liquid to move into the surge suppressor pressure vessel adding volume to the location. This increase in physical volume prevents the pressure from rising to dangerous levels. Advantages:
Very fast speed of response. Zero loss of product from the pipeline from a surge event. Can be used as both a surge suppressor and for surge relief. Disadvantages:
Limited capacity of volume for surge relief. The surge suppressor must be as physically close as possible to the area where the surge is generated. Surge suppressors can become very large depending on line size. Has a limited maximum working pressure.
Rupture discs A rupture disc, also known as a burst disc, bursting disc, or burst diaphragm, is a onetime use, non-resealing pressure relief device that, in most uses, protects a pressure vessel, equipment or system from over pressurization or potentially damaging vacuum conditions. A rupture disc is a sacrificial part because it has a one-time-use membrane that fails at a predetermined differential pressure, either positive or vacuum. The membrane is usually made out of metal, but nearly any material can be used to suit a particular application. Rupture discs provide instant response (within milliseconds) to an increase or decrease in system pressure, but once the disc has ruptured it will not reseal. Due to the one time usage of this disc it requires someone to replace the plate once it has ruptured. One time usage devices are initially cost-effective, but can become time-consuming and labor-intensive to repeatedly change out. Advantages:
Isolates equipment from the process conditions, protecting the equipment until it is needed for a surge relief event. Cost effective installation. Very fast response time. Disadvantages:
One time use. Requires down time to replace. A rupture disk has only one set point. Uncontrollable release of large amounts of harmful substances.
Spring-loaded pressure safety valves Spring-loaded pressure safety valves use a compressed spring to hold the valve closed. The valve will remain closed until the process pressure exceeds the set point of the spring pressure. The valve will open 100% when the set point is reached and will remain open until a certain blow down factor is reached. Oftentimes the blow down is a percentage of the set point, such as 20% of the set point. That means that the valve will remain open until the process pressure decreases to 20% below the set point of the spring-loaded relief valve. Advantages:
Opens 100% when set point is reached. Easy to install and maintain. High flow capacity or Cv value in gas service. Disadvantages:
Has a blow down factor inherent to the design of the valve. The spring takes a set, making the set point drift over time. May release product to atmosphere.
Surge relief valves Surge relief valves are known for their quick speed of response, excellent flow characteristics, and durability in high pressure applications. Surge relief valves are designed to have an adjustable set point that is directly related to the max pressure of the pipeline/system. When the product on the inlet of the valve exceeds the set point it forces the valve to open and allows the excess surge to be bled out in to a breakout tank or recirculated into a different pipeline. So in the event of the surge, the majority of the pressure is absorbed in the liquid and pipe, and just that quantity of liquid which is necessary to relieve pressures of unsafe proportions is discharged to the surge relief tank. Some valve manufactures use the piston style with a nitrogen control system and external plenums, while others use elastomeric tubes, external pilots, or internal chambers.
Pilot operated valves Pilot operated surge relief valves are typically used to protect pipelines that move low viscosity products like gasoline or diesel. This style of valve is installed downstream of the pump/valve that creates the surge. The valve is controlled by an external, normally closed pilot valve. The pilot will be set to the desired set point of the system, with a sense line that runs up stream of the valve. When the upstream process conditions start to exceed the pilot set point, the valve begins to open and relieve the excess pressure until the correct pressure is met causing the valve to close. Advantages:
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