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. Methods for controlling hydraulic surges include utilizing a gas-loaded surge relief valve, 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.