|ACTUATOR: A fluid-powered or
electrically powered device that supplies force and motion to a
VALVE CLOSURE MEMBER.
AIR SET: Also SUPPLY PRESSURE
REGULATOR. A device used to reduce plant air supply to valve
POSITIONERS and other control equipment. Common reduced air supply
pressures are 20 and 35 psig.
AIR-TO-CLOSE: An increase in
air pressure to the ACTUATOR is required to cause the valve to
close. This is another way of saying the valve is Fail Open or
AIR-TO-OPEN: An increase in air
pressure to the ACTUATOR is required to cause the valve to open.
This is another way of saying the valve is FAIL CLOSED or NORMALLY
ANSI: An abbreviation for the
American National Standards Institute.
ANTI-CAVITATION TRIM: A special
trim used in CONTROL VALVES to stage the pressure drop through the
valve, which will either prevent the CAVITATION from occurring or
direct the bubbles that are formed to the center of the flow stream
away from the valve BODY and TRIM. This is usually accomplished by
causing the fluid to travel along a torturous path or through
successively smaller orifices or a combination of both.
API: An abbreviation for the
American Petroleum Institute.
ASME: An abbreviation for the
American Society of Mechanical Engineers.
ASTM: An abbreviation for the
American Society for Testing and Materials.
BALANCED TRIM: A trim
arrangement that tends to equalize the pressure above and below the
valve plug to minimize the net static and dynamic fluid flow forces
acting along the axis of the stem of a GLOBE VALVE. Some regulators
also use this design, particularly in high pressure service.
BELLOWS SEAL BONNET: A BONNET
which uses a BELLOWS for sealing against leakage around the valve
BENCH SET: The proper
definition for bench set is the INHERENT DIAPHRAGM PRESSURE RANGE,
which is the high and low values of pressure applied to the
diaphragm to produce rated valve plug travel with atmospheric
pressure in the valve body. This test is often performed on a work
bench in the instrument shop prior to placing the valve into service
and is thus known as Bench Set.
BODY: The body of the valve is
the main pressure boundary. It provides the pipe connecting ends and
the fluid flow passageway. It can also support the seating surface
and the valve CLOSURE MEMBER.
BONNET: The bonnet or bonnet
assembiy is that portion of the valve pressure retaining boundary
which may guide the stem and contains the PACKING BOX and STEM SEAL.
The bonnet may be integral to the valve body or bolted or screwed.
The bonnet, if it is detachable, will generally provide the opening
to the valve body cavity for removal and replacement of the internal
TRIM. The bonnet is generally the means by which the actuator is
connected to the valve body.
BOOSTER: A pneumatic relay that
is used to reduce the time lag in pneumatic circuits by reproducing
pneumatic signals with high-volume and or high-pressure output.
These units may act as volume boosters or as amplifiers. A 1:2
booster will take a 3 to 15 psig input signal and output a 6 to 30
psig signal. It has also been shown that a booster may improve the
performance of a control valve by replacing a positioner. It can
provide the same stroking speed and can isolate the controller from
the large capacitive load of the actuator.
BUBBLE TIGHT: A commonly used
term to describe the ability of a control valve or regulator to shut
off completely against any pressure on any fluid. Unfortunately, it
is completely unrealistic. Control valves are tested to ANSI B16.104
and FCI 70-2-1976 which is the American National Standard for
Control Valve Seat Leakage. This standard uses 6 different
classifications to describe the valves seat leakage capabilities.
The most stringent of these is Class VI which allows a number of
bubbles per minute leakage, depending on the port size of the valve.
The correct response to the question "Will that valve go "Bubble
Tight"? is to say this valve is tested to meet Class VI shutoff
BUTTERFLY VALVE: A valve with a
circular body and a rotary motion disk closure member which is
pivotally supported by its stem. Butterfly valves come in various
styles including eccentric and high-performance valves. Butterfly
valves are HIGH RECOVERY valves and thus tend to induce CAVITATION
in liquid services at much lower pressure drops and fluid
temperatures than the globe style valve. Due to instability problems
with the older design butterfly valves, many people will limit the
travel of the valve at 60 degrees of rotation on throttling
services. This can also help keep the valve out of CAVITATION
CAGE: A hollow cylindrical trim
element that is sometimes used as a guide to align the movement of a
VALVE PLUG with a SEAT RING. It may also act to retain the seat ring
in the valve body. On some types of valves, the cage may contain
different shaped openings which act to characterize the flow through
the valve. The cage may also act as a NOISE ATTENUATION or
CAGE GUIDED VALVE: A type of
GLOBE STYLE valve trim where the valve plugs with the seat.
CAVITATION: Occurs only in
liquid service. In its simplest terms cavitation is the two-stage
process of vaporization and condensation of a liquid. Vaporization
is simply the boiling of a liquid, which is also known as FLASHING.
In a control valve this vaporization takes place because the
pressure of the liquid is lowered, instead of the more common
occurrence where the temperature is raised. As fluid passes through
a valve just downstream of the orifice area, there is an increase in
velocity or kinetic energy that is accompanied by a substantial
decrease in pressure or potential energy. This occurs in an area
called the VENA CONTRACTA. If the pressure in this area falls below
that of the vapor pressure of the flowing fluid, vaporization
(boiling) occurs. Vapor bubbles then continue downstream where the
velocity of the fluid begins to slow and the pressure in the fluid
recovers. The vapor bubbles then collapse or implode. Cavitation can
cause a Choked Flow condition to occur and can cause mechanical
damage to valves and piping.
CHOKED FLOW: Also known as
CRITICAL FLOW. This condition exists when at a fixed upstream
pressure the flow cannot be further increased by lowering the
downstream pressure. This condition can occur in gas, steam, or
liquid services. Fluids flow through a valve because of a difference
in pressure between the inlet (Pl) and outlet (P2) of the valve.
This pressure difference (Delta-P) or pressure drop isessential to
moving the fluid. Flow is proportional to the square root of the
pressure drop. Which means that the higher the pressure drop is the
more fluid can be moved through the valve. If the inlet pressure to
a valve remains constant, then the differential pressure can only be
increased by lowering the outlet pressure. For gases and steam,
which are compressible fluids, the maximum velocity of the fluid
through the valve is limited by the velocity of the propagation of a
pressure wave which travels at the speed of sound in the fluid. If
the pressure drop is sufficiently high, the velocity in the flow
stream at the VENA CONTRACTA will reach the velocity of sound.
Further decrease in the outlet pressure will not be felt upstream
because the pressure wave can only travel at sonic velocity and the
signal will never translate upstream. Choked Flow can also occur in
liquids but only if the fluid is in a FLASHING or CAVITATING
condition. The vapor bubbles block or choke the flow and prevent the
valve from passing more flow by lowering the outlet pressure to
increase the pres-sure drop. A good Rule Of Thumb on Gases and Steam
service is that if the pressure drop across the valve equals or
exceeds one half the absolute inlet pressure, then there is a good
chance for a choked flow condition.
P1 100 psig P1 (ABS) = 100 + 14.7 or 114.7 1/2 of
114.7 = 57.35
P2 25 psig
Delta P = 75
Actual pressure drop = 75
Flow is probable.
The style of valve (that is whether it is a HIGH RECOVERY or a
LOW RECOVERY style) will also have an effect on the point at which a
choked flow condition will occur.
CLOSURE MEMBER: The movable
part of the valve which is positioned in the flow path to modify the
rate of flow through the valve. Some of the different types of
closure members are the Ball, Disk, Gate, and Plug.
COEFFICIENT FLOW: A constant
(Cv) that is used to predict the flow rate through a
valve. It is related to the geometry of the valve at a given valve
opening. See Cv.
CONTROL VALVE: Also known as
the FINAL CONTROL ELEMENT. A power-operated device used to modify
the fluid flow rate in a process control system. It usually consists
of a BODY or VALVE and an ACTUATOR, which responds to a signal from
the controlling system and changes the position of a FLOW
CONTROLLING ELEMENT in the valve.
CONTROL VALVE GAIN: The
relationship between valve travel and the flow rate through the
valve. It is described by means of a curve on a graph expressed as
an INSTALLED OR INHERENT CHARACTERISTIC.
CONTROLLER: A device which
tells a CONTROL VALVE what to do. Controllers can be either
pneumatic or electronic. There are pressure, temperature, ph, level,
differential, and flow controllers. The job of the controller is to
sense one of the above variables and compare it to a set point that
has been established. The controller then outputs a signal either
pneumatic or electronic to the control valve, which then responds so
as to bring the process variable to the desired set point.
CRITICAL FLOW: See the
definition for CHOKED FLOW.
CV: The VALVE FLOW
COEFFICIENT is the number of U.S. gallons per minute of 60 degree F
water that will flow through a valve at a specified opening with a
pressure drop of 1 psi across the valve.
DELTA-P: Differential Pressure.
The inlet pressure (Pl) minus the outlet pressure (P2).
P1 = 100 psig DIAPHRAGM: A flexible pressure-responsive
element that transmits force to the diaphragm plate and actuator
P2 = 25 psig.
Delta-P = 75
DIAPHRAGM ACTUATOR: Is a fluid
(usually pneumatic) pressure-operated, spring-opposed diaphragm
assembly which positions the valve stem in response to an input
DIAPHRAGM PRESSURE: See Bench
DIAPHRAGM VALVE: A valve with a
flexible linear motion CLOSURE MEMBER that is forced into the
internal flow passageway of the BODY by the ACTUATOR. Pinch or Clamp
valves and Weir-type valves fall into this category.
DIRECT ACTING: This term has
several different meanings depending upon the device it is
describing. A DIRECT-ACTING ACTUATOR is one in which the actuator
stem extends with an increase in diaphragm pressure. A DIRECT-ACTING
VALVE is one with a PUSH-DOWN-TO-C LOSE plug and seat orientation. A
DIRECT-ACTING POSITIONER or a DIRECT-ACTING CONTROLLER outputs an
increase in signal in response to an increase in set point.
DIRECT ACTUATOR: Is one in
which the actuator stem extends with an increase in diaphragm
DUAL SEATING: A valve is said
to have dual seating when it uses a resilient or composition
material such as TFE, Kel-F, or Buna-N, etc. for its primary seal
and a metal-to-metal seat as a secondary seal. The idea is that the
primary seal will provide tight shut-off Class VI and if it is
damaged the secondary seal will backup the primary seal with Class
DYNAMIC UNBALANCE: The total
force produced on the valve plug in any stated open position by the
fluid pressure acting upon it. The particular style of valve, i.e.
single-ported, double-ported, flow-to-open, flow-to-close, has an
effect on the amount of dynamic unbalance.
EFFECTIVE AREA: For a DIAPHRAGM
ACTUATOR, the effective area is that part of the diaphragm area that
is effective in producing a stem force. Usually the effective area
will change as the valve is stroked - being at a maximum at the
start and at a minimum at the end of the travel range. Flat sheet
diaphragms are most affected by this; while molded diaphragms will
improve the actuator performance, and a rolling diaphragm will
provide a constant stem force throughout the entire stroke of the
ELECTRIC ACTUATOR: Also known
as an Electro-Mechanical Actuator uses an electrically operated
motor-driven gear train or screw to position the actuator stem. The
actuator may respond to either a digital or analog electrical
END CONNECTION: The
configuration provided to make a pressure-tight joint to the pipe
carrying the fluid to be controlled. The most common of these
connections are threaded, flanged, or welded.
EQUAL PERCENTAGE: A term used
to describe a type of valve flow characteristic where for equal
increments of valve plug travel the change in flow rate with respect
to travel may be expressed as a constant percent of the flow rate at
the time of the change. The change in flow rate observed with
respect to travel will be relatively small when the valve plug is
near its seat and relatively high when the valve plug is nearly wide
EXTENSION BONNET: A bonnet with
a packing box that is extended above the body to bonnet connection
so as to maintain the temperature of the packing above (cryogenic
service) or below (high-temp service) the temperature of the process
fluid. The length of the extension depends on the amount of
temperature differential that exists between the process fluid and
the packing design temperature.
FACE-TO-FACE: Is the distance
between the face of the inlet opening and the face of the outlet
opening of a valve or fitting. These dimensions are governed by
The following Uniform Face-to Face Dimensions apply.
SPECIFICATION VALVE TYPE
ANSI/ISA S75.03 INTEGRAL FLANGED GLOBE STYLE CONTROL
VALVES FAIL-CLOSED: Or
NORMALLY CLOSED. Another way of describing an AIR-TO-OPEN actuator.
Approximately 80% of all spring return diaphragm operators in the
field are of this construction.
ANSI/ISA S75.04 FLANGELESS CONTROL VALVES ANSUISA
S75.20 SEPARABLE FLANGE GLOBE STYLE CONTROL
FAIL-IN-PLACE: A term used to
describe the ability of an actuator to stay at the same percent of
travel it was in when it lost its air supply. On SPRING RETURN
ACTUATORS this is accomplished by means of a LOCK-UP VALVE. On
PISTON ACTUATORS a series of compressed air cylinders must be
FAIL-OPEN: Or NORMALLY OPEN.
Another way of describing an AIR-TO-CLOSE actuator.
FAIL-SAFE: A term used to
describe the desired failure position of a control valve. It could
FAIL-CLOSED, FAIL-OPEN, or FAIL-IN-PLACE. For a spring-return
operator to fail-in-place usually requires the use of a lock-up
FEEDBACK SIGNAL: The return
signal that results from a measurement of the directly controlled
variable. An example would be where a control valve is equipped with
a positioner. The return signal is usually a mechanical indication
of valve plug stem position which is fed back into the
F1: Or PRESSURE
RECOVERY FACTOR. A number used to describe the ratio between the
pressure recovery after the VENA CONTRACTA and the pressure drop at
the vena contracta. It is a measure of the amount of pressure
recovered between the vena contracta and the valve outlet. Some
manufacturers use the therm Km to describe the pressure
recovery factor. This number will be high (0.9) for a GLOBE STYLE
VALVE with a torturous follow path and lower (0.8 to 0.6) for a
ROTARY STYLE VALVE with a streamlined flow path. On most rotary
products the F1 factor will
vary with the degree of opening of the VALVE CLOSURE MEMBER.
Note! F1 does
not equal Km.
FLANGELESS: A valve that does
not have integral line flanges. This type of valve is sometimes
referred to as a Wafer Style valve. The valve is installed by
bolting it between the companion flanges with a set of bolts or
studs called line bolting. Care should be taken that strain-hardened
bolts and nuts are used in lieu of all-thread, which can stretch
when subjected to tempera-ture cycling.
FLANGELESS BODY: See FLANGELESS
for a definition. This type of valve is very economical from a
manufacturing and stocking standpoint because a valve that is rated
as a 600# ANSI valve can also be used between 150# and 300# ANSI
flanges thus eliminating the need to manufacture three different
valve bodies or stock three different valve bodies. The down side is
that valves with flangeless bodies are not acceptable in certain
applications - particularly in refinery processes.
FLASHING: Is the boiling or
vaporizing of a liquid. See the definition of CAVITATION. When the
vapor pressure downstream of a control valve is less than the
upsteam vapor pressure, part of the liquid changes to a vapor and
remains as a vapor unless the downstream pressure recovers
significantly, in which case CAVITATION occurs. Flashing will
normally cause a CHOKED FLOW condition to occur. In addition the
vapor bubbles can also cause mechanical damage to the valve and
FLOW CHARACTERISTIC: The
relationship between valve capacity and valve travel. It is usually
expressed graphically in the form of a curve. CONTROL VALVES have
two types of characteristics INHERENT and INSTALLED. The INHERENT
characteristic is derived from testing the valve with water as the
fluid and a constant pressure drop across the valve. When valves are
installed into a system with pumps, pipes, and fittings, the
pressure dropped across the valve will vary with the travel. When
the actual flow in a system is plotted against valve opening, the
curve is known as the INSTALLED flow characteristic. Valves can be
characterized by shaping the plugs, orifices, or cages to produce a
particular curve. Valves are characterized in order to try to alter
the valve gain.
Valve gain is the flow change divided by the control signal
change. This is done in an effort to compensate for nonlinearities
in the control loop.
FLOW COEFFICIENT: See the
definition for Cv.
GAIN: The relationship of input
to output. If the full range of the input is equal to the full range
of the output, then the gain is 1. Gain is another way to describe
the sensitivity of a device.
GLOBE VALVE: A valve with a
linear motion, push-pull stem, whose one or more ports and body are
distinguished by a globular shaped cavity around the port region.
This type of valve is characterized by a torturous flow path and is
also referred to as a LOW RECOVERY VALVE because some of the energy
in the flow stream is dissipated; and the inlet pressure will not
recover to the extent that it would in a more streamlined HIGH
HANDWHEEL: A manual override
device used to stroke a valve or limit its travel. The handwheel is
sometimes referred to as a hand jack. It may be top mounted, side
mounted, in-yoke mounted or shaft mounted and declutchable.
HARD FACING: A material that is
harder than the surface to which it is applied. It is normally used
to resist fluid erosion or to reduce the chance of galling between
moving parts. Hard facing may be applied by fusion welding,
diffusion, or spray coating the material. Alloy #6 or Stellite is a
common material used for this purpose.
HARDNESS: A property of metals
that is discussed frequently when speaking of various component
parts used in valve construction, particularly valve trim. There are
two hardness scales which are commonly used, Rockwell &
that 316 SST is on the Rockwell B scale which means it is a much
softer material than the others shown.
|#6 Stellite (Alloy 6)
HIGH RECOVERY VALVE: A valve
design that dissipates relatively little flow stream energy due to
streamlined internal contours and minimal flow turbulence.
Therefore, pressure down stream of the valve VENA CONTRACTA recovers
to a high percentage of its inlet value. These types of valves are
identifiable by their straight-th rough flow paths. Examples are
most rotary control valves, such as the eccentric plug, butterfly,
and ball valve.
HYSTERESIS: The difference
between up-scale and down-scale results in instrument response when
subjected to the same input approached from the opposite direction.
Example: A control valve has a stroke of 1.0 inch and we give the
valve a 9 psig signal. The valve travels 0.500 of an inch. We then
give the valve a 12 psig signal, and the valve travels to 0.750 of
an inch. When the valve is then given a 9 psig signal, the stroke is
measured at 0.501. That represents hysteresis. Hysteresis can be
caused by a multitude of variables, packing friction, loose linkage,
pressure drop, etc. If someone asks you what the hysteresis of your
control valve is, it is a bum question because hysteresis is more
aptly applied to an instrument than to a control valve. There are
simply too many variables in the valve and the system to answer the
question properly. The control valve only responds to the controller
signal and will move to a position to satisfy the controller - thus
negating the effects of hysteresis.
INCIPIENT CAVITATION: Is a term
used to describe the early stages of CAVITATION. At this point the
bubbles are small, and the noise is more of a hiss, like the sound
of frying bacon. There is normally no mechanical damage associated
with incipient cavitation although it could have an effect on the
corrosive properties of some fluids.
INHERENT DIAPHRAGM PRESSURE:
The high and low values of pressure applied to the diaphragm to
produce rated valve plug travel with atmospheric pressure in the
valve body. This is more commonly referred to as BENCH SET.
INHERENT FLOW CHARACTERISTIC:
It is the relationship between valve capacity and valve travel and
is usually expressed graphically. It is derived from testing a valve
with water as the fluid and with a constant pressure drop across the
valve. The most common types of inherent flow characteristics are
LINEAR, EQUAL PERCENTAGE, MODIFIED PARABOLIC, and QUICK
INSTALLED DIAPHRAGM PRESSURE:
The high and low values of pressure applied to the diaphragm to
produce rated travel with stated conditions in the valve body. The
"stated conditions" referred to here mean the actual pressure drops
at operating conditions. Example: A control valve may have an
INHERENT DIAPHRAGM PRESSURE or BENCH SET of 8 to 15 psig. But when
subjected to a 600 psig. inlet pressure, it may start to open at 3
psig. and be full open at 15 psig. It is because of the forces
acting on the valve plug and the direction of flow through the valve
(FLOW-TO-OPEN or FLOW-TO-CLOSE) that the installed diaphragm
pressure will differ from the inherent diaphragm pressure.
INSTALLED FLOW CHARACTERISTIC:
The flow characteristic when the pressure drop across the valve
varies with flow and related conditions in the system in which the
valve is installed. The purpose of characterizing a control valve is
to help compensate for nonlinearities in the control loop.
INSTRUMENT PRESSURE: The output
pressure from an automatic controller that is used to operate a
control valve. It is the input signal to the valve.
INTEGRAL SEAT: The flow control
orifice and seat that is an integral part of the valve body or cage.
The seat is machined directly out of the valve body and is normally
not replaceable without replacing the body itself - although some
can be repaired by welding and remachining.
INTEGRAL FLANGE: A valve body
whose flange connection is an integral or cast part of the body.
Valves with integral flanges were traditionally known to have the
ANSI short FACE-TO-FACE dimension ANSI/ISA S75.03. However many
manufacturers now produce valve bodies with both integral and
SEPARABLE FLANGES that will meet both the ANSI short and long
I/P: An abbreviation for
current-to-pneumatic signal conversion. This term is commonly used
to describe a type of transducer that converts an electric (4-20
m.a) input signal to a pneumatic (3-15 psig.) output signal.
LANTERN RING: A rigid spacer
used in the packing with packing above and below it. The lantern
ring is used to allow lubrication to the packing or allow access to
a leak off connection. On some of the new fugitive emission packing
systems, it also acts as a stem guide.
LAPPED-IN: A term that
describes a procedure for reducing the leakage rate on
metal-to-metal seated valves and regulators. The plug and seat are
lapped together with the aid of an abrasive compound in an effort to
establish a better seating surface than would normally be achieved
by means of machining.
LEAKAGE CLASSIFICATION: A term
used to describe certain standardized testing procedures for CONTROL
VALVES with a FLOW COEFFICIENT greater then 0. 1 (Cv).
These procedures are outlined in ANSI Standard d B16.104-1976, which
gives specific tests and tolerances for six seat leakage
classifications. It should be remembered that these tests are used
to establish uniform acceptance standards for manufacturing quality
and are not meant to be used to estimate leakage under actual
working conditions. Nor should anyone expect these leakage rates to
be maintained after a valve is placed in service. There is no
standard test for SELF-CONTAINED REGULATORS at this time.
Note! You will see many instances where regulators are
specified using the above criteria.
LEAK-OFF: A term used to
describe a threaded connection located on the BONNET of a valve that
allows for the detection of leakage of the process fluid past the
LINEAR FLOW CHARACTERISTIC: A
characteristic where flow capacity or (Cv) increases
linearly with valve travel. Flow is directly proportional to valve
travel. This is the preferred valve characteristic for a control
valve that is being used with a distributive control system (DCS) or
programmable logic controller (PLC).
LINEAR VALVE: Another name for
a GLOBE VALVE. It refers to the linear or straight-line movement of
the plug and stem.
LIQUID PRESSURE RECOVERY: See (F1).
LOADING PRESSURE: The pressure
used to position a pneumatic actuator. It is the pressure that is
actually applied to the actuator diaphragm or piston. It can be the
INSTRUMENT PRESSURE if a valve positioner is not used or is
LOCK-UP VALVE: A special type
of regulator that is installed between the valve POSITIONER and the
valve ACTUATOR, where it senses the supply air pressure. If that
pressure falls below a certain level, it locks or traps the air
loaded into the actuator causing the valve to FAIL-IN-PLACE.
LOW RECOVERY VALVE: A valve
design that dissipates a considerable amount of flow stream energy
due to turbulence created by the contours of the flow path.
Consequently, pressure downstream of the valve VENA CONTRACTA
recovers to a lesser percentage of its inlet value than a valve with
a more streamlined flow path. The conventional GLOBE STYLE control
valve is in this category.
MODIFIED PARABOLIC: A FLOW
CHARACTERISTIC that lies somewhere between LINEAR and EQUAL
PERCENTAGE. It provides fine throttling at low flow capacity and an
approximately linear characteristic at higher flow capacities.
NORMALLY CLOSED: See
NORMALLY OPEN: See
P1: Is used to designate Inlet
P2: Is used to designate Outlet
PACKING: A sealing system that
normally consists of a deformable material such as TFE, graphite,
asbestos, etc. It is usually in the form of solid or split rings
contained in a PACKING BOX that are compressed so as to provide an
effective pressure seal.
PACKING BOX: The chamber
located in the BONNET which surrounds the stem and contains the
PACKING and other stem-sealing components.
PACKING FOLLOWER: A part that
transfers a mechanical load to the PACKING from the packing flange
PISTON ACTUATOR: A
fluid-powered, normally pneumatic device in which the fluid acts
upon a movable cylindrical member, the piston, to provide linear
motion to the actuator stem. These units are spring or air opposed
and operate at higher supply pressures than a SPRING RETURN
PLUG: See CLOSURE MEMBER.
PORT-GUIDED: A valve plug that
fits inside the seat ring, which acts as a guide bushing. Examples:
Splined Plug, Hollow Skirt, and the Feather-Guide Plug.
POSITION SWITCH: A switch that
is linked to the valve stem to detect a single, preset valve stem
position. Example: Full open or full closed. The switch may be
pneumatic, hydraulic, or electric.
POSITION TRANSMITTER: A device
that is mechanically connected to the valve stem and will generate
and transmit either a pneumatic or electric signal that represents
the valve stem position.
POSITIONER: A device used to
position a valve with regard to a signal. The positioner compares
the input signal with a mechanical feed back link from the actuator.
It then produces the force necessary to move the actuator output
until the mechanical output position feedback corresponds with the
pneumatic signal value. Positioners can also be used to modify the
action of the valve (reverse acting positioner), alter the stroke or
controller input signal (split range positioner), increase the
pressure to the valve actuator (amplifying positioner), or alter the
control valve FLOW CHARACTERISTIC (characterized positioner).
POST GUIDE: A guiding system
where the valve stem is larger in the area that comes into contact
with the guide busings than in the adjacent stem area.
PUSH-DOWN-TO-C LOSE: A term
used to describe a LINEAR or GLOBE STYLE valve that uses a DIRECT
ACTING plug and stem arrangement. The plug is located above the seat
ring. When the plug is pushed down, the plug contacts the seat, and
the valve closes. Note! Most control valves are of
PUSH-DOWN-TO-OPEN: A term used
to describe a LINEAR or GLOBE STYLE valve that uses a REVERSE ACTION
plug and stem arrangement. The plug is located below the seat ring.
When the plug is pushed down, the plug moves away from the seat, and
the valve opens.
PRESSURE RECOVERY FACTOR: See
QUICK OPENING: A FLOW
CHARACTERISTIC that provides maximum change in flow rate at low
travels. The curve is basically linear through the first 40% of
travel. It then flattens out indicating little increase in flow rate
as travel approaches the wide open position. This decrease occurs
when the valve plug travel equals the flow area of the port. This
normally happens when the valve characteristics is used for on/off
RANGEABILITY: The range over
which a control valve can control. It is the ratio of the maximum to
minimum controllable FLOW COEFFICIENTS. This is also called TURNDOWN
although technically it is not the same thing. There are two types
of rangeability - inherent and installed. Inherent rangeability is a
property of the valve alone and may be defined as the range of flow
coefficients between which the gain of the valve does not deviate
from a specified gain by some stated tolerance limit. Installed
rangeability is the range within which the deviation from a desired
INSTALLED FLOW CHARACTERISTIC does not exceed some stated tolerance
REDUCED TRIM: Is an undersized
orifice. Reduced or restricted capacity trim is used for several
reasons. (1) It adapts a valve large enough to handle increased
future flow requirement with trim capacity properly sized for
present needs. (2) A valve with adequate structural strength can be
selected and still retain reasonable travel vs. capacity
relationships. (3) A valve with a large body using restricted trim
can be used to reduce inlet and outlet fluid velocities. (4) It can
eliminate the need for pipe reducers. (5) Errors in over sizing can
be corrected by use of restricted capacity trim.
REVERSE ACTING: This term has
several deferent meanings depending upon the device it is
describing. A REVERSE-ACTING ACTUATOR is one in which the actuator
stem retracts with an increase in diaphragm pressure. A
REVERSE-ACTING VALVE is one with a PUSH-DOWN-TO-OPEN plug and seat
orientation. A REVERSE-ACTING POSITIONER or a REVERSE-ACTING
CONTROLLER outputs a decrease in signal in response to an increase
in set point.
REVERSE FLOW: Flow of fluid in
the opposite direction from that normally considered the standard
direction. Some ROTARY VALVES are considered to be bi-directional
although working pressure drop capabilities may be lower and leakage
rates may be higher in reverse flow.
ROTARY VALVE: A valve style in
which the FLOW CLOSURE MEMBER is rotated in the flow stream to
modify the amount of fluid passing through the valve.
SEAT LOAD: The contact force
between the seat and the valve plug. When an actuator is selected
for a given control valve, it must be able to generate enough force
to overcome static, stem, and dynamic unbalance with an allowance
made for seat load.
SEAT RING: A part of the flow
passageway that is used in conjuction with the CLOSURE MEMBER to
modify the rate of flow through the valve.
SELF-CONTAINED REGULATOR: A
valve with a positioning actuator using a self-generated power
signal for moving the closure member relative to the valve port or
ports in response and in proportion to the changes in energy of the
controlled variable. The force necessary to position the CLOSURE
MEMBER is derived from the fluid flowing through the valve.
SEPARABLE FLANGE: Also known as
a SLIP-ON FLANGE. A flange that fits over a valve body flow
connection. It is generally held in place by means of a retaining
ring. This style of flange connection conforms to ANSI/ISA 275.20
and allows for the use of different body and flange materials.
Example: A valve with a stainless steel construction could use
carbon steel flanges. This type of valve is very popular in the
chemical and petro-chemical plants because it allows the use of
exotic body materials and low cost flanges.
SOFT SEATED: A term used to
describe valve trim with an elastomeric or plastic material used
either in the VALVE PLUG or SEAT RING to provide tight shutoff with
a minimal amount of actuator force. A soft seated valve will usually
provide CLASS VI seat leakage capability.
SPLIT BODY: A valve whose body
is split. This design allows for easy plug and seat removal.
Split-bodied valves are made in both the straight-through and angle
versions. The Masoneilan 2600 or ANNIN is an example of a split body
SPRING RATE: A term usually
applied to SELF-CONTAINED REGULATORS describing the range of set
point adjustment available for a particular range spring.
STATIC UNBALANCE: The net force
produced on the valve stem by the fluid pressure acting on the
CLOSURE MEMBER and STEM within the pressure retaining boundary. The
closure member is at a stated opening with a stated flow condition.
This is one of the forces an actuator must overcome.
STELLITE: Also called #6
Stellite or Alloy 6. A material used in valve trim known for its
hardness, wear and corrosion resistance. Stellite is available as a
casting, barstock material and may be applied to a softer material
such as 316 stainless steel by means of spray coating or
STEM: The VALVE PLUG STEM is a
rod extending through the bonnet assembly to permit positioning of
the plug or CLOSURE MEMBER. The ACTUATOR STEM is a rod or shaft
which connects to the valve stem and transmits motion or force from
the actuator to the valve.
STEM GUIDE: A guide bushing
closely fitted to the valve stem and aligned with the seat. Good
stem guiding is essential to minimizing packing leakage.
SUPPLY PRESSURE: The pressure
at the supply port of a device such as a controller, positioner, or
transducer. Common values of control valve supply pressures are 20
psig. for a 3-15 psig. output and 35 psig. for a 6-30 psig.
STROKE: See TRAVEL.
THROTTLING: Modulating control
as opposed to ON/OFF control.
TRANSDUCER: An element or
device which receives information in the form of one quantity and
coverts it to information in the form of the same or another
quantity. (See I/P)
TRAVEL: The distance the plug
or stem moves in order to go from a full-closed to a full-open
position. Also called STROKE.
TRIM: Includes all the parts
that are in flowing contact with the process fluid except the body,
BONNET, and body flanges and gaskets. The plug, seats, stem, guides,
bushings, and cage are some of the parts included in the term
TRUNNION MOUNTING: A style of
mounting the disc or ball on the valve shaft or stub shaft with two
bushings diametrically opposed.
TURNDOWN: A term used to
describe the ratio between the minimum and maximum flow conditions
seen in a particular system. Example: If the minimum flow were 10
G.P.M. and the maximum flow were 100 G.P.M. the turndown would be
10:1. This term is sometimes incorrectly applied to valves. See
VALVE: A device which
dispenses, dissipates, or distributes energy in a system.
VALVE BODY: See BODY.
VALVE FLOW COEFFICIENT: See
VALVE PLUG: See CLOSURE
VENA CONTRACTA: The location
where cross-sectional area of the flow stream is at its minimum
size, where fluid velocity is at its highest level, and where fluid
pressure is at its lowest level. The vena contracta normally occurs
just downstream of the actual physical restriction in a control