DEFINITIONS
Safety Valve
A valve which automatically discharges gases and vapours so as to prevent a predetermined safe pressure being exceeded. It is characterised by a rapid full-opening action, and used for steam, gases or vapour service.
Relief Valve
A valve which automatically discharges fluid, usually liquid, when a predetermined upstream pressure is exceeded. The term is commonly used for pressure relieving valves in which the lift is proportional to the increase in pressure above the set pressure.
Safety Relief Valve
A valve which will automatically discharge gases, vapours and liquids so as to prevent a predetermined safe pressure being exceeded. It is characterised by a rapid full-opening action. The type 3500, 2600, Ni/19.1 and Ni/32.1 are of this class.
PRESSURE TERMS
Set Pressure
The pressure measured at the valve inlet at which a safety relief valve should commence to lift under service conditions.
Differential Set Pressure
The pressure differential between the set pressure and the constant superimposed back pressure, is the pressure at which the valve is set. It is applicable only when a conventional type safety relief valve is used to discharge against a constant superimposed back pressure.
Cold Differential Test Pressure
The pressure at which a safety relief valve intended for high temperature service is set on a test rig using a test fluid at ambient temperature. The cold differential test pressure will be higher than the set pressure, in order to compensate for the effect of the elevated temperature on the valve.
Lift
The actual travel of the valve disc away from the seat when the valve is relieving.
Capacity
Actual mass flow rate of discharge, can also be expressed in volumetric terms.
Equivalent Capacity
Mass or volumetric flow rate of a fluid calculated from the capacity of the valve for a test fluid. The fluids commonly used for test purposes are steam, air and water.
Accumulation
The pressure increase over a maximum safe working pressure of the vessel or system when the safety relief valve is discharging at its rated capacity is called accumulation. The terms refer to the vessel or system to be protected and not to the valve. Accumulation is the same as overpressure when the valve is set at the design pressure of the vessel.
Overpressure
The pressure increase above the set pressure at the valve inlet at which the discharge pressure is attained.
Re-seat Pressure
The pressure measured at the valve inlet at which the safety valve re-seats.
Blow-Down
Otherwise known as pressure drop of a safety relief valve, it is the difference between the set pressure and re-seating pressure expressed as a percentage of the set pressure or as a pressure difference.
Simmer
The pressure zone between the valve set pressure and the popping pressure. In this pressure zone the valve is only slightly open, and therefore discharging a small percentage of its rated capacity.
Popping Pressure
The Pressure at which the valve disc rapidly moves from a slightly open (simmer) position to a practically full open position.
PRESSURE CONDITION
The following are the pressure conditions illustrated by Fig. 1.
a) Inlet pressure is the normal system operating pressure which is usually 10% below the set pressure, or the re-seating pressure of the valve. Diagram indicates that the valve is closed and there is no flow through the valve, and consequently zero back pressure.
b) Inlet pressure is equal to the set pressure of the valve, valve is slightly open, small flow which is defined as simmer.
c) Inlet pressure equal to ‘relieving pressure’ which is the ‘set pressure’ plus ‘over-pressure’, valve is fully open and discharging at full flow rate, back pressure is built up in the discharge piping, this is defined as ‘built up back pressure’.
d) Inlet pressure – as condition a) but with back pressure present in discharge pipe, defined as ‘superimposed back pressure’.
e) Inlet pressure – as condition b) but with superimposed back pressure marginally above condition d).
f) Inlet pressure – as condition c) but superimposed back pressure increased due to flow through valve. Defined as ‘superimposed’ plus ‘built up back pressure’.
Note: If the back pressure at condition d) and e) is variable, a balanced bellow valve
must be used.
VALVE ACTION – SERIES 3500
The valves are designed to have a short ‘simmer’, then to open rapidly to the open position, and to re-seat at a controlled pressure. Fig. 2 shows a diagram which demonstrates the valve action, and shows the flow through the valve corresponding to the pressure at the valve inlet. Both the ‘simmer’ and the re-seating pressures are adjustable by means of a blow-down ring. When the valve is in its fully lifted position, the discharge area is controlled by the bore of the nozzle, which ensures that flow calculations for various fluids can be readily made.
The nozzle, blow-down ring and reaction hood have been carefully designed and tested to obtain the best flow efficiency and the highest lifting forces. The following explains the various forces involved when the valve opens. See Fig. 3.
The valve disc is in the closed position (1), the following forces act on the disc:
a) Spring compression, tending to close the valve.
b) The back pressure, if any, which also tends to close the valve.
c) The fluid pressure, acting on the disc area limited by the seating surface which tends to open the valve.
When force X becomes greater then force Z exerted by the spring, the valve disc lifts but in doing so it further compresses the spring thereby increasing load Z. At the same time the force which tends to open the valve also increases, and it is essential that the latter be the maximum possible.
This is obtained by full advantage of the fluid pressure, by allowing it to act on the disc surface outside the annular seating surface as indicated (2) thus increasing the area against which the pressure acts, and also using kinetic energy of the discharge fluid.
The shape of the reaction hood deflects the out-flowing fluid by approximately 180° as indicated by (3), part of the fluid kinetic energy is transformed into a disc lifting force.
The valve lifting forces are increased more than the closing forces, until the valve reaches the full open position.
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