Back pressure
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- The pressure at the outlet of the valve.
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Blowdown
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- The pressure difference between set pressure and reseating pressure.
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Chattering
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- Rapid reciprocating motions of the disc during which the disc contacts the seat.
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Curtain area
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- The area of the cylindrical or conical discharge opening created between the seating surfaces by the lift of the disc above the seat.
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Flow area
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- The minimum net area which determines flow through the valve.
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Fluttering
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- Rapid reciprocating motions of the disc during which the disc does not contact the seat.
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Full-lift pressure relief valve
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- A pressure relief valve that opens in proportion to the rise in overpressure over a small portion of the lift only and then opens suddenly to the fully open position.
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Hammering
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- The same as chattering, but defined as full stroke movements which are severe because the disc is not carried by flow able to blow out a propagating flame, rather the disc is lifted by inertia from a shock wave, in particular in small bore piping. Danger of flashback.
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Huddling chamber
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- The annular pressure chamber located beyond the seat of full-lift pressure relief valves. Some constructions have similar features below the seat where boosting elements are located.
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Non-oscillating
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- Absence of pressure oscillations over the entire working range regardless of piping configuration.
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Operating pressure difference
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- The difference between the operating pressure and the set pressure.
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Pressure relief valve
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- A generic term for a pressure-relieving device, which automatically relieves a pressure system of excess pressure and automatically re-closes.
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Pressure/vacuum valves
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- Pressure and vacuum relief valves are relieving devices which automatically relief excess pressure when abnormal operating conditions occurs. Such valves may be actuated directly by the system pressure upstream the valve or be piloted, for instance, by a magnet. Valves can modulate open and closed over the entire or substantial portion of the lift, or modulate open over only a small portion of the lift and then open suddenly to the fully open position. Accordingly, pressure relief valves may generally be divided into modulating and full-lift valves.
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Proportional pressure relief valve
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- A pressure relief valve that opens in proportion to the rise in overpressure over the entire or substantial portion of the valve lift: modulating operation.
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Reseating pressure
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- The pressure at which the valve reseats after discharge.
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Secondary orifice
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- The ring-shaped opening at the exit of the huddling chamber.
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Set pressure
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- The start-to-open-pressure at service conditions of back pressure and temperature. Where full-lift valves open and provide the capacity needed.
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Start-to-open-pressure
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- The pressure at which the valve commences to open.
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Tightness
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- A tightness test shall be conducted at the maximum expected operating pressure, but at a pressure not exceeding the reseating pressure of the valve. When testing, a valve exhibiting no visible signs of leakage shall be considered tight (ASMA boiler and pressure code section 1)
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