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Baker Hughes
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V. Terminology for Safety Valves (Contd.)
• Blowdown
Blowdown is the difference between set pressure
and resetting pressure. It is generally expressed as a
percentage of set pressure.
• Chatter
Chatter is abnormal, rapid reciprocating motion of the
movable parts of a safety valve in which the disc contacts
the seat.
• Closing pressure
Closing pressure is the point at which the valve recloses.
• Cold Differential Set Pressure (CDS)
This is the test pressure at which the valve is adjusted
to open on the test stand. This pressure includes the
corrections for temperature and/or back pressure service
conditions. Nameplate shows “cold set press” (see Figure
31).
• Disc
A disc is the pressure containing movable element of a
valve which affects closure.
• Flutter
Flutter is abnormal, reciprocating motion of the movable
parts of a valve in which the disc does not contact the seat.
• Leakage
See “Seat Leakage”.
• Lift
Lift is the disc rise when a valve is open.
• Maximum Allowable Working Pressure (MAWP)
The maximum pressure permissible in a vessel at the
design temperature.
• Nozzle
A nozzle is the pressure-containing element which consists
of the inlet flow passage (orifice) and includes the fixed
portion of the seat.
• Operating Pressure
The normal pressure required to operate the vessel or
system. A suitable margin must be provided between the
operating and the valve set pressure.
• Operating and Set Pressure-Differential
Valves in process service will generally give best results
if the operating pressure does not exceed 90 percent of
the set pressure. On pump and compressor discharge
lines, however, the differential required between the
operating and set pressures may be greater because of
pressure pulsations coming from a reciprocating piston. It
is recommended that the valve be set as high above the
operating pressure as possible.
• Orifice
The orifice is the minimum cross-sectional area of the
controlling nozzle bore.
• Overpressure
Overpressure is any pressure in excess of the set pressure,
usually expressed as a percentage of set pressure. For
certification, accumulation is the percent of overpressure
at which the valve is to be fully open as specified by the
ASME Code.
• Safety Valve
A safety valve is an automatic pressure relieving device
actuated by upstream pressure. A safety valve is designed
to open to prevent a rise of system pressure above a
specified value and to reclose after normal conditions have
been restored.
A safety valve is a two-chamber component having primary
and secondary zones. Each chamber has different design
criteria (i.e., pressure and temperature). The nozzle and
disc are the primary members. The base and bonnet are
the secondary members.
• Rated Capacity
This is the rated flow at an authorized percent
accumulation permitted by the applicable Code. Rated
capacity is generally expressed in pounds per hour (lbs/hr)
or kg/hr for vapors; standard cubic feet per minute (SCFM)
or m
3
/min for gases; and in gallons per minute (GPM) or L/
min for liquids.
• Relief Valve
A safety valve which opens in proportion to overpressure. A
relief valve is used primarily for liquid service.
• Reseating Pressure
See closing pressure.
• Safety Valve
A safety valve characterized by rapid full opening or pop
action. It is used for gas or vapor service.
• Safety Relief Valve
A safety valve that may be used as either a safety or relief
valve, depending upon the application.
