Diaphragm Actuator:

The actuator whose element for transforming pressure into force is a diaphragm which is a flexible pressure-sensitive element that transmits force to the diaphragm plate and is at its once to the actuator stem.

The plate is a metallic assembly concentric with the diaphragm in which the stem is held. This type of actuator is normally single-acting and due to the large dimensions of the diaphragm, it is capable of producing forces large enough to operate the valves with pressures relatively small, common in automation or instrumentation systems.

Retaining ring:

The split ring is used to hold the separable flange on the body of the valve.

Spring Support:

(Spring Seat) In diaphragm actuators the spring preload is regulated with two parts, one is the spring support and the other an adjusting screw which in turn is hollow and acts as a guide bushing of the actuator in some arrangements.


The aperture through which the fluid generally passes a strong metal seat of determining diameter. It causes the passage of the fluid. Many times interchangeable for valve maintenance.

The valve plug or plug makes contact with the seat to achieve full closure, the degree of closure is classified. (Single Ported) Simple seat.

Elastomer seat

(Soft Seat): The male or Plug of the valve makes contact with the seat where it has a part made of elastomer or plastic in order to achieve a good total closure.

Bonnet or Neck:

(Bonnet, Bonnet Assembly). The main part of the valve body cover assembly is called usually with this name. That is, this piece and the set are called the same usually in the same way. For services where the valve must handle very cold or hot fluids, an extended bonnet is used. It separates the gasket receptacle from the rest of the valve body.

Detachable flange:

(Separable Flange) The flange that can be separated from the valve body on which it fits and is clamped by means of a retaining ring, the fixed or detachable body flanges allow the connection of the valve to piping. Eduardo N. Alvarez FIUBA Mechanical Control Systems Control Valves

Lower flange:

(Bottom Flange) The flange that closes the opening that some valves have in the part of the body opposite the bonnet, usually the lower one.

Guide Bushings:

The valve stem that supports the plug may be guided by a bushing in the bonnet, two bushings one in the bonnet and one in the lower flange, or by a bushing in the bonnet and by the cage, etc.

Diaphragm Housing:

(Diaphragm Case). Two-piece container (upper and lower) that contains the diaphragm allowing altogether the establishment of a pressure difference and its transformation into a force for moving the valve stem.


Jacketed to maintain the temperature in pipes surrounding space through which a fluid generally to thermostat. (Vacuum Jacketed) Jacketed with the vacuum chamber


Pneumatic actuator composed of a cylinder and sliding piston with air action in one direction and spring return (single-acting) or in two directions (double acting). The actuation can be direct or inverse depending on whether the air pressure exits the stem or vice versa. Also called Piston Actuator, Pneumatic Cylinder, etc.

The advantage of double-acting cylinders is that they can perform almost equal forces in both directions.

Stem Connector:

Clamp in two parts that integrates the stem of the actuator and that of the male of the valve.

Valve Body:

The housing of the interiors and other parts of the valve that also allows the connection of the same to the pipe are classified according to the routes and seats. The seats determine the number of internal passenger control connections.

However, the gain in a valve will depend on the operating parameters and their ratio. With the rest of the installation (see inherent characteristic).

Travel Indicator:

A pointer attached to the stem connector, often disk-shaped that allows to visualize the relative position of the male in its journey. This pointer marks the position on a scale normally attached to the valve yoke.


(Trim). The internal parts are in direct contact with the controlled fluid.


(Cage). The hollow cylindrical part allows the adjustment of the flow through the valve operating in combination with the shutter it guides. The walls of the Cage have openings whose shapes determine the flow characteristic of the control valve.

Shutter (male):

(Plug, Closure Member) The movable part allows the size of the valve passage opening to be varied.

Stuffing box or packing housing:

(Packing Box). In the bonnet, there is a housing for the gaskets that seals the space between the stem and bonnet to avoid fluid loss.

Actuator Spring:

It is the counter spring of the valve diaphragm. In other words, the control valve has a generally single-acting spring return actuator. This spring is supported by the yoke.

Stem Seals:

(Stem Seals). O-rings, Wipers, or gaskets type seals of combined materials. Normally from Synthetic rubbers in order to resist the lubricants of the actuators that prevent the escape of air to air cylinder chamber pressure.

Actuator Stem: This threaded cylindrical piece links the actuator diaphragm to the disc stem from the valve. An extension can be added to this part to transmit the movement of the stem of the valve to the positioners.

Angle Valve:

It is a valve in which the inlet and outlet axes form an angle. Usually perpendicular.

Separable Globe Valve:

(Split Style). Valve whose construction is distinguished by having the body divided into two parts, one upper one in which the bonnet is fixed, and a lower one, between them, is the seat easily interchangeable thanks to this arrangement.

Plug Stem:

(Plug Stem). Bar that extends through the valve neck assembly and that allows the shutter to be moved from outside said set


The structure allows the actuator to be fixed to the neck of the valve.


Adjustment Rings:

(Shim Seals) 0.125mm metal packing washers were used in varied numbers for adjusting ball seal position on V-notch rotary valves.

Ring Seal:

(O Ring). Generic name of ring seals. In the valves, there are special rings for the seal of the butterfly discs and ball valves (Seal Ring). There are also them as the last closing in the shutter valves in order to obtain lower losses in total closure. In some cases, the seal Rotary valves must be specially designed to allow flow in both directions.

Severe or Heavy Duty Seal Rings:

Connecting rod:

(Connecting Rod). In general, the part is used to transform linear movement into rotary and vice versa. In Rotary Disc Valves with pneumatic linear actuators, a connecting rod is necessary to achieve the rotation of the stem integral with the disc or ball.


(Ball) Control member of a rotary shaft valve when its shape is spherical. It can be from Ball complete (Ball, Full) or in partial forms (Ball Segmented). The most commonly mentioned advantage of ball valves is that they are fully open its interior is congruent with that of the spout in which it is placed.

 V-notch ball:

In this case, the rotary valve plug consists only of a part of the ball hollowed out and handles the law of flux variation through a V-shaped notch, which is one of the most popular types of modulating valves. Allows a wide range of control and a characteristic equal percentage. Its behavior is similar to that of the linear displacement plug valve.

Connecting rod end bearing:

(Rod end Bearing). The connection between actuator stem and connecting rod, in specially designed cases to reduce loss of motion when switching from linear to rotary displacement in valves rotary with linear actuators.

Flangeless Body:

Also known by the name of Style Body for Water. These valves are fastened between pipe flanges with long bolts.

Conventional or normal disc:

It is the most normal in butterfly-type rotary control valves, it is symmetrical Regarding the flow, the dynamic torques that appear on the shaft in its operation normally limit it to 60 degrees inflow modulation operation (throttling service).

Disc designed to reduce dynamic stress:

(Disk, dynamically designed). It is the butterfly valve disc with a hydro-dynamically designed profile to generate lower shaft torques, in order to perform throttling service up to 90 degrees of rotation in operation.

Eccentric disc:

It is the butterfly valve disc in which the axis of rotation does not pass through the center of symmetry of the same so that the trajectory of the edges of the same is such that they come out of the contact seal. As soon as opening begins, reducing friction and wear.

Full Sphere:

(Full Ball). One of the kinds of rotary valve plugs, we will see the advantages and disadvantages of each type of construction.

Reverse Flow:

Flow in the opposite direction to the normal process. The conventional butterfly valve, due to its symmetry, it admits flow in both directions.

Whereas, not in some asymmetric ones, in which case it is necessary to consult as some seal rings allow the reverse flow by design.

Normal or Standard Flow:

(Standard Flow) Direct flow or normal process direction, (the most common), inlet side where the seal ring is located. Also called direct flow.

Mounting on journals (trunnions)

(Trunnion Mounting) How to link the ball with trunnions or journals by bushings to the body of the valve, or the disc with its axis, so that the bushings remain on the same axis diametrically opposites in said body.

Eccentric Shutter

(Plug, eccentric) A type of rotary valve that has an eccentric plug that is applied to the seat in a form that reduces friction and wear, its use stands out in particular to erosive fluids.

Actuator Lever:

Arm rigidly attached to the rotary valve shaft by means of which the linear movement of the rotary actuator.

Sliding Seal:

Front seal of a pneumatic cylinder specially designed for rotary valve actuator. It allows the stem of the same to move longitudinally and transversely without loss of tightness of the cylinder front chamber.

Conventional Disc Valve:

Commonly called Butterfly Valve, in them the part that controls the flow. It is a disk in most cases, with the shaft passing through a diameter, also like those of balls are rotary valves. The dynamic pairs developed in them make them have limitations in some control ranges of the passage area. Rotary shaft control valve.

(Rotary-Shaft Control Valve). A valve that modulates the flow through a part that rotates around an axis.

Dynamically Designed Disc Valve:

It is a Butterfly Valve, but in this case, the part that controls the flow is a disc that takes into account the dynamics of fluids in its design, so there are no limitations in no control range of the passage area, being able to handle up to 90 degrees.

Eccentric Disc Valve:

It is a Butterfly Valve, but in this case, the part that controls the flow is a disc that has its axis that does not pass through the plane of symmetry of the disk (slightly eccentric). This arrangement allows that when slightly rotating from its closed position, the disc is separated from its seat allowing a longer duration due to reduced friction.

Control Valve Characteristics and Functions Terminology

Double Acting Actuator

The actuator that allows forceful movements in both directions, in the control valves is customary to be accompanied by the positioner. These actuators are usually pneumatic pistons.

Effective Area:

In diaphragm actuators, it is the part of the total area of the diaphragm that produces the force actual on the stem. This effective area varies in the course and is greater at the beginning of the course than at the final. Molded diaphragms have less effective area variability than those cut out from Sheets of cloth and flat rubber are therefore recommended.

Dead Band:

It is the range in which the input can be varied without producing an observable response to some of the systems. In diaphragm valves, it is the pressure variation at the inlet without the stem starting to move. It is usually expressed as a percentage of the diaphragm pressure range.

For example, 1.5% which will represent approximately 0.2 psi


It is the flow that passes through the valve under defined, normalized conditions.

Flow Characteristic:

It is the algorithm that relates the path of the shutter with the flow through the valve. It is usually expressed as a percentage of both the maximum travel and the maximum flow rate. Flow rates can also be expressed in terms of Flow Coefficients. (Cv).

There are two possibilities of flow characteristic: installed or inherent.

Inherent Flow Characteristic:

It is the characteristic obtained in the valve when the difference in pressures between flanges is constant.

Installed Flow Characteristic:   

It is the characteristic that is obtained in the conditions in which it works the valve in the installation, of course with pressure variation between flanges and variations inflow.

Linear Flow Characteristic:

It is the inherent characteristic in which it is a direct proportionality between the control valve opening and flow-through from the valve.

Equal Percentage Flow Characteristic:

 It is one of the typical inherent characteristics of flow control valves. In this case, with a control valve operating under ideal conditions, the characteristic of equal percentage responds to equal percentage variations in the path of the plug stem with equal percentage variations in flow through the valve.

Fast Opening Flow Characteristic:

It is the inherent characteristic in which it is obtained maximum flow with minimum stroke.

Seat Load:

Force acting on the seat due to excess power of the actuator in front of the static pressure difference, spring force, and dynamic effects that could influence the shutter.

In practice, a diaphragm actuator is chosen so that its force can overcome inertia, the friction, static and dynamic unbalance, and have a certain rest of force to exert the closure on the seat.

Spring Constant:

Deformation load ratio of the spring of a valve to the diaphragm. Usually in countries, Anglo-Saxon has given in Pounds per inch (PSI) and in European standards in Newton/meter or in systems Practical in Kgrs / cm.

Calibration curve:

Results from graphically representing the steady-state output of a device regarding the entry into permanent regime as well. Normally the range of input values in percentage form and in the other also in percentage form the output is represented, in such a way that in the case of valves it would be the percentage of the Valve capacity with respect to the shutter aperture percentage.


(Cycling or Hunting) The oscillation occurs in a sustained way even if the stimuli are removed externally. An undesired closeness of instability emerges as a conclusion of the presence of the cycled.

Static Unbalance:

It is the net force acting on the plug stem due to the difference in pressures exerted by the fluid when the valve is closed.

Total unbalance:

(Stem Unbalance). It is the net force acting on the plug stem in all operating conditions.

Dynamic Unbalance

It is the force exerted by the fluid on the obturator, a value that will depend on the position of the same since this force depends fundamentally on the difference of water pressures upstream and downstream of the shutter.


(Leakage). It refers to the amount of fluid that passes through the valve when it is closed, with specified differential pressures, absolute pressures, and clamping force.


Occurs when the response of the system differs depending on whether it is a response when the output stroke responds to an upward variation of the input with respect to the exit stroke and a downward variation of the entry.

Failure Position:

The position adopted by the valve when there is a lack of compressed air that handles it by construction. (Failsafe). Safe fail position.

Additional instruments may be required connected to the actuator to obtain it, for example, pneumatic three-way valves in order to vent the air from the actuator in an emergency.

(Fail Closed) Fail Closed When lack of air, it closes.

(Fail Opened) Open Fail When short of air it opens.

Failure positions are chosen to obtain an emergency under safe conditions.

Normal Position of a Valve:

Normally Closed Control Valves

(Normally Closed Control Valve). They are those in which the diaphragm is brought to ambient pressure

They close by the action of one or more springs placed for this purpose.

Normally Open Control Valves

(Normally Opened Control Valve). They are those in which, when the diaphragm is brought to ambient pressure, they open by the action of one or more springs placed for this purpose.

Inherent Diaphragm Pressure Range:

(Inherent Diaphragm Pressure Range). It consists of the specification of the maximum pressure extremes and minimum that must be applied to the diaphragm to achieve the complete travel in the shutter with the valve at atmospheric pressure, that is to say, bench test without fluid circulation inside, by Example range is 3 to 15 psi.

Diaphragm Pressure Differential:

(Diaphragm pressure Span). Difference between the maximum and minimum pressure in the range in which the valve controls with the inherent or installed characteristic that it has, for example, if the range is from 3 to 15 psi the Span or differential is 12 psi.

Maximum useful travel:

(Rated travel). It is the travel of the shutter from the closed position to the maximum opening recommended by the manufacturer.

Pressure Recovery in Control Valves.

High Recovery Valve:

(High Recovery Valve) Refers to valves that, due to their internal design, dissipate little energy from the fluid, so that at the outlet there is a high recovery of the gauge pressure.

Low recovery valve:

(Low Recovery Valve) Refers to valves that, due to their internal design, dissipate a lot of energy from the fluid, so that at the outlet there is low recovery of the gauge pressure, in general, despite the logical differences due to the different designs, globe valves have more energy losses


Relationship between the maximum value that can be processed by a control instrument and the minimum value processable by the same within which the deviation with respect to the real value remains below of the specified error.

Installed Diaphragm Pressure Range:

Limit pressures that make the shutter travel under operating conditions (for example 3 to 15 psi). Taking into account the borderline cases of maximum and minimum flow and therefore the various pressure drops in the valve. As the forces are exerted on the shutter, the pressures will be different from those inherent.


It refers to the low deviation of values obtainable in multiple repetitions of a measurement made under the same conditions, does not include or consider hysteresis.


It is the minimum appreciable division by means of an instrument.

Output resolution: it is the smallest change in the output that can be obtained from the instrument in question.

Input resolution

It is a necessary change in the input of the instrument to obtain the minimum output. In general, it is desirably expressed as a percentage of the operating span or differential.


(Sensitivity) Steady-state refers to the relationship between the change in the output and the change in the input is usually identified with static gain.

Direct Acting Valve:

(Direct Acting, Push down To Close). Construction of a globe valve such that the plug is between the actuator and seat so that the actuator must drive the plug towards the seat to close.

This terminology is also used in rotary valves when the actuator pushes to close.

Reverse Acting Valve:

(Reverse Acting, Push down to open). Construction of a globe valve such that the seat is between the actuator and plug so that when the actuator drives the plug it moves away from the seat.

Vena-contracta (expression in Latin):

The minimum cross-section of the flow stream, where the fluid velocity is maximum

Response Speed:

(Speed Response, Stroke Speed) regarding the operation of control valves, this is the expression that indicates the speed with which the valve travels. For example, there may be a case time in which we will be referring to the travel time from closing to total opening.

Control Valve Actuators

The most common are diaphragm actuators with an antagonistic spring. Less used but accessible and applicable to cases where the effort must be able to be applied in both senses are pneumatic piston actuators.


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