What is Surge Relief Valves?
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, by the way using this pressure is discharged is called surge relief valves.
Surge Relief Valve Skids
Typical Surge Relief System
Surge relief valves
- Hydraulic surges are created when fluid velocity changes suddenly and becomes unsteady or transient.
- Pressure surges occur from sudden events, such as a valve closure or a pump trip, often triggered by an emergency shutdown (ESD)
- Surge control is the use of different techniques and equipment to prevent pressure surges
- Methods include utilizing
- Gas-loaded Surge Relief Valve
- Spring-loaded Pressure Safety Valves
- Pilot-operated Valves
- Surge Suppressors
- Rupture Disks.
- Surge relief valves have quick speed of response, excellent flow characteristics, and durability in high pressure applications.
- Designed to have an adjustable set point that is directly related to the max pressure of the pipeline/system.
- 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.
- Types;
- Pilot operated typically used in low viscosity product applications. Pilot valve will be set to the desired set point of the system, with a sense line that runs up stream of the valve.
- used for pump protection duty
- for applications where the relief valve is required to maintain pressure at a given set point.
- Piston style (balanced piston design) gas-loaded with a nitrogen control system and external plenums
- Nitrogen, is loaded on the back side of the piston forcing the valve closed
- Rubber boot-style gas-loaded relief valves – nitrogen pressure loaded on the outside diameter of a rubber boot
Controlling Surge Relief Valves
- Spring-loaded pressure safety valves and pilot-operated valves are controlled mechanically using the pressure from a compressed spring.
- Design is limited by the pressure that can be generated by the spring in the valve.
- Gas-loaded relief valves are controlled by the nitrogen pressure loaded into the relief valve.
- Electronic control system to add and vent nitrogen pressure from the gas-loaded surge relief valve
- Assures the required set point accuracy and repeatability needed in this critical application.
Pilot operated
- Used for pump protection duty and for similar applications where pressure relief is required to maintain pressure at a given set point
- Pressure is controlled to within ±2 psi regardless of upstream conditions
- The pilot and main valve are single seated valves with high capacities.
- These valves protect the line against excessive pressure and surge or as a pump bypass to maintain a constant pump discharge pressure.
Balanced Piston Nitrogen Loaded Surge Relief Valves
- Normally closed and open on increasing inlet pressure. Nitrogen gas is used to pressurize the valve piston to keep it in the closed position.
- Incorporates an integral oil reservoir mounted on the external surface of the cylinder head, which upon installation is partially filled with a light oil.
- Gas under pressure is applied to the reservoir. The pressure of the nitrogen gas, minus the 4 psi (force of the valve spring) is the effective set point of the valve.
- Line pressure on the nose of the piston is equally transmitted to the spring side of the piston.
- When the nitrogen pressure is applied to the top of the oil, plus the spring pressure is greater than line pressure, the valve will be in the closed position.
- Oil reservoir is supplied with two sight gauges.
- When the valve is closed the oil level in the reservoir will show oil covering the lower sight glass only
- Oil visible in both the lower and upper sight gauges indicates that the valve is in the open position
- Absence of oil in the lower sight gauge indicates that proper oil capacity has not been reached or that reservoir oil is leaking into the product system
- When the pressure against the bottom of the piston exceeds the pressure plus the force of the spring exerted against the top of the piston, spring tension is overcome, and the valve opens.
- Opening and closing speed is controlled by a unique check valve mounted to the internal surface of the cylinder head.
- Opening of the valve is relatively unrestricted, and response time is typically under 100 msec
- The opening and closing speed is controlled by an orifice in the check valve
Surge relief valve design
- Surge valve sizing depends on
- Location
- Set point pressure
- Factors that need to be considered include
- modulating valve’s response time – refers to the time it takes for a relief valve to open when the pressure set point is exceeded
- valve flow coefficient (CV) – quantity of fluid that will flow through a wide open valve with a 1 psi pressure drop
- excess pressure above set point to reach required flow rate – defines pressure needed to open the valve
- valve characteristic control curve.
- Gas loaded systems, required pressure will depend upon the system relief pressure and the amount of fluid the valve must pass when it relieves.
Designing a surge relief system
- Dependent upon a complex range of factors
- Potential for pressure increases
- Volumes which must be passed by the surge relief equipment in operation
- Capacity of the system to contain pressures
- should also work in concert with other safety and control systems
- Control or ESD valve closure times can also affect surge pressures in a pipeline.
- By extending valve closure time, a more gradual flow decay can be achieved.
- Pump and pipe sizing are other factors to be considered in hydraulic design.
- Predicting transient behaviour-commonly termed “surge analysis”-involves detailed computer modelling.
- This is not yet subject to codes of practice, so surge relief designs must be based on industry best practices combined with knowledge and experience.
- Design will include components to dampen or slow the relief valve on closing (a check valve provides unrestricted flow when opening and a reduced orifice to limit closing speed)
- In nitrogen-loaded valves, nitrogen system must supply a constant pressure (set point) to the modulating valve under varying ambient temperature and have its own control system to regulate the nitrogen supply pressure.
- Supply pressure should be set at the relief pressure required.
- The gas pressure set point, minus the force exerted by the spring, is the surge relief pressure for the system.
- The nitrogen gas tank could be buried underground or insulated to keep the gas at a constant temperature. Thermal expansion, caused by the increases in temperature of the nitrogen gas, can change the relief set point.
- Surge relief systems often require correctly sized inlet and outlet manifolds with isolation valves upstream and downstream of the surge relief valves. These are normally full-port ball valves, as they provide least resistance path in the line of relief.
- A full bore liquid ultrasonic flow meter can also be employed at the outlet of each surge relief valve to determine the relief quantity during each valve cycling
Surge control on a skid
- Preferable is a factory-tested skid-mounted surge relief systems that have properly sized surge relief valves, manifolds, and piping
- A typical surge relief system skid has redundant, parallel surge relief valves; inlet and outlet manifolds sized to minimize the pressure loss; and a nitrogen system.
- All of these components are integrated on the skid, along with other required equipment, control system, and instrumentation.
- Piping runs include the necessary instrumentation, including a nitrogen control system
