What is safety relief devices: Safety relief devices are the valves which are used to protect equipment including the piping and its components due to overpressure scenarios or conditions. Pressure relief valves are actuated by the inlet static pressure. Safety relief valves are designed to open in an emergency or abnormal condition.
- Causes of Overpressure: The reasons for the Over Pressure scenario can be different and many. Some important causes of overpressure are described below:
Blocked discharge: It is the condition in which the outlet nozzle of any of the vessels, Pumps, compressor, or fired heater is blocked by mechanical or human error. Mechanical or Human error refers to the preliminary design of the equipment or operational mistake taken inside the plant or wherever equipment is situated.
Fire Case: A fire case is also referred to as a fire exposure event. Fire exposure in the refinery is considered the least occurring event, but the impact of the fire case event is possibly the largest. Fire cases occur in the plant which decides the relief of the entire system, but it may be limited to the specific plot plan area. Different empirical equations are available for the calculation of the relief load in a fire case scenario. The selection of the equation is based on the type of system and fluid.
Tube rupture: The rupture of the tube in a heat exchanger can take place due to many reasons which may be vibration, thermal shock, or corrosion. Rupture in the tube may overpressure the lower pressure side of the heat exchanger. Tube rupture conditions also produce a short duration of pressure shock which accelerates the Lower pressure side of the exchanger. To resolve tube rupture following points are to be taken into consideration:
- Installation of a pressure relief valve or rupture disk on the lower pressure side of the equipment.
- Provision of a sufficient path on the side of the lower pressure of equipment, so that during an overpressure scenario, the tube doesn’t rupture.
Control Failure: Control failures refer to the operation position in associated instruments and valves. In general, the control valve never fails in the required position, whereas in the case of valves that may be stuck in the wrong position and complete loop may also fail.
Thermal Expansion: Thermal expansion occurs during the isolation of the line of the hot side of the heat exchanger which transferred this heat into the Low-pressure side, causing overpressure in the Low-pressure side.
Types of Safety relief valves:
Safety relief valves are certified and approved in Section VIII of the ASME Boiler and pressure vessel code which includes unfired pressure vessels. Relief device types include conventional, balanced, pilot-operated pressure relief valves, rupture disks, and pin devices.
Safety relief valves are divided into two categories:
Non-reclosing Type relief devices: Non-reclosing relief devices remain open after being used in the operation. These types of devices are provided with the provision of manual resetting, in case if it is not provided which means the device is replaceable. Non-reclosing Type relief devices are categorized into two types:
- Rupture disc
- Pin devices
Rupture disc: A rupture disc is a non-reclosing type relief device that consists of a thin diaphragm placed between the flanges. A rupture disc is designed under the code ASME Section VIII including the tolerances stated.
The rupture disc is designed in such a manner that it will operate at predetermined pressure and temperature. A rupture disc is required in the case where an instant full opening of the Pressure relief valve is needed.
Pin devices: A pin device is also an example non-reclosing type relief device, the function of the pin device is synonymous (same) to the rupture disks. Static inlet pressure or differential pressure is responsible for actuating the pin device.
Pin devices are also designed under code ASME Section VIII, and they can be used in place of rupture disks. Pin device functions by shear acting on the pin which carries the load of pressure containing member. The main advantage of pin devices is that replacement of pins can be done without the removal of piping flanges.
Reclosing Type relief devices: Reclosing type devices that open during abnormal (excess or overpressure) conditions and close as the normal condition achieved or restored the system to prevent the fluid flow. These types of relief devices are further divided into four types based on their functions as a safety relief valve, safety valve, and relief valve.
A Safety relief valve can be used as a relief or safety valve depending on the application. Safety relief valve classification as follows:
- Conventional safety relief valve
- Balanced bellows safety relief valve
- Pilot operated safety relief valve
Conventional safety relief valve: In conventional SRV, the spring arrangement is constructed against the inlet excess pressure exerted by the fluid-stored equipment. Spring will be disturbed or uplifted in the situation when excess pressure comes to the relief (relieving) condition, during the normal pressure (below the set pressure) pressure relief valve remains in the closed position.
Most of the conventional type relief valves available in the petroleum industry consist of disks containing large surface area (disk area – AD) in comparison to the nozzle area (AN).
The effect of the backpressure in conventional type relief devices is as follows: Backpressure is the pressure that is available at the outlet of PRV. Backpressure is generally of two types.
Superimposed Backpressure: It is usually the pressure available in the discharge header before the PRV opens.
- Superimposed backpressure may be constant or variable, dependent on the system.
- Built-up Backpressure: It is the pressure developed by flow due to the open position of the Pressure relief valve.
Effect of backpressure on conventional relief valve: Backpressure directly impacts the capacity of PRV. High backpressure availability may reduce the valve capacity by up to 50%.
For Example, A Pressure relief valve is set to be relieved (Set pressure or point) at 1 Kg/cm2, but at the same time, 0.1 Kg/cm2 backpressure is available. In this case, the valve will not able to relieve the pressure until the system pressure reaches 1.1 Kg/cm2. Provision of compensation for the generated backpressure to be taken into consideration for relieving condition. If the same is not compensated, means the valve is operating at a higher pressure than the MAWP (maximum allowable working pressure).
Balanced bellows safety relief valve: This type of relief valve consist of bellow which reduces the effect of backpressure which directly affects the valve performance parameter (relief capacity, opening, and closing pressure). Balanced bellows safety relief valve is based on two types of construction bellows and pistons.
In balanced bellow type nozzle seat area (AN) is equal to the bellow effective area (AB). If any available backpressure is countered by the bellow which helps to increase the disk area expansion (extends) up to bellow and cancel the nozzle seal area, to ensure that there is no unstable force acting on the pressure relief valve downstream.
To protect the bellows from failure or leakage separate venting to provide from the discharge of the safety relief valve. The Bonnet vent must contain a larger area to assure that no backpressure is available in the relief valve for the design set of conditions.
Effect of backpressure on balanced bellows safety relief valve: In the balanced bellow type, bellow is used to counter the effect of back pressure. This type of valve can manage or withstand 30 – 35% backpressure of the set pressure Value. PRV operating at higher pressure may fail in operation.
Pilot Operated safety relief Valve: Pilot-operated safety relief valve consists of mainly two components which are the main valve and a pilot. Pilot operated safety relief valve uses a piston than the shaft. Inlet pressure from equipment is routed to the auxiliary component pilot, at the top of the piston large area is available than at the bottom, and so does the pressure.
- Normal Case: pressure available on both sides of the piston (top and bottom) is Equalizer each other and keeps the pilot-operated safety relief valve shut or in the closed position.
- Set Pressure Case: As fluid pressure reaches up to set pressure bottom portion of the piston tends to uplift from the shut position.
- Valve opening Case: fluid pressure acting on the bottom side of the pilot-operated safety relief valve is relatively greater in comparison to the pressure available at the top of the piston. This enables the piston to move upside down and release the excess pressure through the discharge port.
- Closing Position case: In this case pressure acting on the piston’s top side is getting larger with respect to the time, due to this piston moves downwards and tends to close the pilot-operated safety relief valve.
Effect of backpressure on Pilot Operated safety relief Valve: The effect of backpressure is minimum in Pilot operated type Pressure relief valve.
Image reference: https://www.spiraxsarco.com/learn-about-steam/safety-valves/types-of-safety valve#:~:text=There%20are%20two%20basic%20types,the%20diaphragm%20and%20piston%20type.
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