Introduction to Pressure Safety Valve
One of the most critical automatic safety devices in a pressure system is the pressure safety valve. The primary purpose of a pressure safety valve is the protection of life, property and environment during an over-pressure event in a pressurized vessel or equipment. An over-pressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or maximum allowable working pressure. A pressure safety valve is designed to open and relieve excess pressure from vessels or equipment and to re-close and prevent the further release of fluid after normal conditions have been restored.
A pressure safety valve is a safety device and in many cases the last line of defense.
It is important to ensure that the pressure safety valve is capable to operate at all times and under all circumstances. A safety valve is not a process valve or pressure regulator and should not be misused as such.
Pressure safety valve should have to be operated for one purpose only: over-pressure protection.
Reasons for Excess Pressure in a Vessel
There are a number of reasons why the pressure in a vessel or equipment can exceed a predetermined limit. The most common are:
- Blocked outlet
- Exposure to external fire, often referred to as “Fire Case”
- Thermal expansion of fluid
- Abnormal process conditions (Chemical reaction)
- Cooling system failure
- Heat exchanger tube rupture
- Pipework component failure
- Control Valve failure
Each of the above listed events may occur individually or simultaneously. Every cause of over-pressure will create a different mass or volume flow to be discharged. For e.g. small mass flow for thermal expansion and large mass flow in case of a chemical reaction. It is the process engineers responsibility to determine the most worst case scenario for the sizing and selection of a suitable pressure safety device.
Types of Pressure Safety Valves
Spring Loaded Pressure Safety Valves
In a Spring loaded Pressure Safety Valve the closing force or spring force is applied by a helical spring which is compressed by an adjusting screw. The spring force is transferred via the spindle onto the disc. The disc seals against the nozzle as long as the spring force is larger than the force created by the pressure at the inlet of the valve. Figure 2 shows all the steps in working of Spring Loaded Pressure Safety Valves.
In an upset situation a safety valve will open at a predetermined set pressure. The spring force Fs is acting in closing direction and Fp, the force created by the pressure at the inlet of the safety valve, is acting in opening direction. At set pressure the forces Fs and Fp are balanced. There is no resulting force to keep the disc down on the seat. The safety valve will visibly or audibly start to leak (initial audible discharge).
As the pressure inside the system increases, the force Fp increase above the set pressure and the additional spring force required to further compress the spring is overcome. The valve will open rapidly with a “pop”, in most cases to its full lift.
In most applications a properly sized safety valve will decrease the pressure in the vessel when discharging. The pressure in the vessel will decrease at any subsequent point, but not later than the end of the upset situation. A decreasing pressure in the vessel will lower the force Fp. At set pressure however the flow is still acting on the enlarged disc area, which will keep the valve open. A further reduction in pressure is required until the spring force Fs is again greater than Fp and the safety valve begins to close. At the so called reseating pressure the disc will touch the nozzle again and the safety valve closes.
Pilot Operated Pressure Safety Valves
Pilot Operated Safety Valve is controlled by process medium. To achieve this, the system pressure is applied to the pilot valve (= control component for the main valve) via the pressure pickup. The pilot valve then uses the dome above the main valve piston to control the opening and closing of the main valve. Figure 4 shows all the steps in working of Pilot Operated Pressure Safety Valves.
During normal operation, the system pressure is picked up at the main valve inlet and routed to the dome. Since the dome area is larger than the area of the main valve seat, the closing force is greater than the opening force. This keeps the main valve tightly closed.
At set pressure, the pilot valve actuates. The medium is no longer routed to the dome. This prevents a further rise in dome pressure. Also, the dome is vented. As a result, the closing force ceases as a precondition for the system over-pressure to push the main valve open. Depending on the design of the pilot valve, this opening is either rapid and complete (Pop Action) or gradual and partial following system pressure (Modulate Action).
If system pressure drops to closing pressure, the pilot valve actuates and again routes the medium to the dome. The pressure in the dome builds up and the main valve closes either rapid and complete (Pop Action) or gradual and partial following system pressure (Modulate Action).
Difference between Pressure Safety Valve and Pressure Relief Valve
“Pressure Safety Valve” and “Pressure Relief Valve” are commonly used terms to identify pressure relief devices on a vessel or equipment. Although freely used interchangeably, these terms differ in the following aspect:
Pressure Safety Valve
Pressure safety valve is the term used to describe relief device on a compressible fluid or gas filled vessel. For such a valve the opening is sudden. When the set pressure of the valve is reached, the valve opens almost fully.
Pressure Relief Valve
Pressure relief valve is the term used to describe relief device on a liquid filled vessel. For such a valve the opening is proportional to increase in the vessel pressure. Hence the opening of valve is not sudden, but gradual if the pressure is increased gradually.
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