A protective part of the system, against a sudden increase in pressure, is considered as the most sensitive part of any system with pressure. The typical designs of this part include a series of specific types of equipment, the main of which are: safety valves (pressure relief valves), vacuum-pressure valves and ruptured discs.
A protective device is a device designed to protect the system against abnormal pressures inside the system, and this equipment prevents a dangerous situation from occurring by its function.
Cases that cause abnormal pressure can be considered as follows:
Imbalance of fluid flow inside and outside process equipment and tanks – during use – causes fluid and vapor to accumulate, which can produce a pressure that exceeds the operating pressure. If the pressure is not controllable, the pressure must be brought to an acceptable level – as defined in the ASME standard – by discharging it by safety equipment.
The simplest case is when the liquid is pumped to the pressure tank. The discharge pressure increased by pumping can exceed the maximum allowable working pressure of the tank. In this case, pressure safety equipment must be used to protect the tank against excessive pressure entering the tank during filling.
This situation is related to equipment that is completely filled with liquid and the extra pressure is caused by the hydraulic expansion of the liquid. In the event that the temperature is rising, this potential hazard can be controlled with the right design. The nominal size of the pressure safety equipment is usually sufficient to protect the pipes and equipment against the additional pressure caused by changes in ambient temperature.
Fire can be expected in pressure tanks containing liquid or flammable gas. Pressure safety equipment must be used to protect them from excess pressure generated by the heat generated during a fire.
Compression tanks that contain only steam must be equipped with equipment that can reduce the pressure to near atmospheric pressure and prevent fire from entering the tank. Otherwise, pressure safety equipment will not be able to protect tanks that are exposed to fire, explosion and shrapnel.
Protective equipment can be considered as safety-sensitive or critical items in engineering equipment. Also, the specifications of these equipments are expressed in the form of codes and standards and their adjustment requirements can be generalized for all pressure systems.
– Accumulation pressure
The amount of pressure exiting a safety valve – when the pressure inside the container exceeds the maximum allowable operating pressure – is called accumulation.
– Return pressure
The pressure at the outlet of a safety valve is called the return pressure.
There are always two types of return pressure in a safety valve, which are:
1- Structural return pressure This return pressure is the pressure that is created in the output header of the safety valve – due to the outflow of current. This pressure occurs when the safety valve opens.
2- Cumulative return pressure
This return pressure is the pressure at the safety valve outlet head that occurs before the safety valve opens.
The difference between the regulating pressure of the safety valve and the pressure related to re-sitting is called discharge and is a percentage of the regulating pressure and is expressed in bar or kpa.
The pressure used in a device for a specific purpose and at the minimum allowable thickness of the components under pressure applied at a design temperature is defined as the design pressure.
Maximum allowable operating pressure
The maximum allowable operating pressure of a pressurized container depends on the material, thickness and service conditions that it can operate and service at a pressure higher than this value or equivalent. This pressure also refers to the regulating pressure of the valves and pressure relief devices that protect a pressurized system.
Increasing the pressure to more than the pressure is called regulating pressure. Overpressure When the pressure relief valve is set at a pressure equal to the maximum allowable operating pressure of the pressure relief vessel, it is exactly equal to the same accumulation pressure, and if it is set at a pressure less than MAWP, then this overpressure may exceed the allowable accumulation. .
This pressure is the pressure on which a safety valve is set. In fact, this pressure is the relative inlet pressure to the safety valve that is adjusted to open at this pressure.
Actual discharge coefficient
The ratio between the measured discharge capacity and the engine discharge capacity of a nozzle.
Actual discharge capacity
The calculated discharge capacity of a complete nozzle at the cross-section of the flow through it is equal to the discharge cross-section of the valve multiplied by the actual discharge coefficient.
Sudden opening pressure
The pressure at which the safety valve in a gas or steam carrier system suddenly opens to discharge pressure.
Cold test pressure
The pressure set on a pressure relief valve in a test system. Cold testing includes pressure correction in real working conditions, such as pressure behind the valve, heat, or both.
A gradual decrease in the regulating pressure in spring-loaded discharge valves, which occurs as a result of the discharge of the fluid after an increase in spring temperature.
The minimum controlled area of the valve passage that determines the intensity of the current passing through the valve.
Effective discharge level
The nominal or calculated area of the crossing area is in a pressure relief valve. This parameter, along with the effective discharge coefficient, is used in the computational formulas for sizing. The effective discharge level is smaller than the actual discharge level.
Re-leakage pressure in a valve ensures
values of static inlet pressure at which the disc is placed back on the nozzle seat.
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