types of control valves

7/27/2019 Types of Control Valves

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TYPES OF CONTROL VALVES

Sliding-Stem Control valveGlobe Valves - Single-Port Valve

Single port is the most common valve body style and is simple in construction. Generally single-port

valves are specified for applications with stringent shutoff requirements. Because high-pressure fluidis normally loading the entire area of the port, the unbalance force created must be considered in

selecting actuators for single-port control valve bodies. Although most popular in the smaller sizes,

single-port valves can often be used in 4-inch to 8-inch sizes with high-thrust actuators.

Figure. Single-Ported Globe-Style Valve

Balanced-Plug Cage-Style Valve:

This popular valve body style, single- ported in the sense that only one seat ring is used, provides the

advantages of a balanced valve plug often associated only with double-ported valve bodies. Cage-

style trim provides valve plug guiding, seat ring retention, and flow characterization. Addition a

sliding piston ring-type seal between the upper portion of the valve plug and the wall of the cage

cylinder virtually eliminates leakage of the upstream high pressure fluid into the lower pressure

downstream system.

Downstream pressure acts on both the top and bottom sides of the valve plug, thereby nullifying

most of the static unbalance force. Reduced unbalance permits operation of the valve with smaller

actuators than those necessary for conventional single-ported valve bodies.

Interchangeability of trim permits choice of several flow characteristics or of noise attenuation or

anticavitation components. For most available trim designs, the standard direction of flow is in

through the cage openings and down through the seat ring.

Figure. Valve Body with Cage- Style Trim, Balanced Valve Plug, and Soft Seat


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Rotary shaft Control valve

Butterfly ValveBodies require minimum space for installation. They provide high capacity with low pressure loss

through the valves. Conventional disks provide throttling control for up to 60-degree disk rotation.

Patented, dynamically streamlined disks suit applications requiring 90-degree disk rotation. Butterfly

valve bodies might require high-output or large actuators if the valve is big or the pressure drop ishigh, because operating torques might be quite large.

Butterfly valves exhibit an approximately equal percentage flow characteristic.

Standard butterfly valves are available in sizes through 72-inch for miscellaneous control valve

applications. Smaller sizes can use versions of traditional diaphragm or piston pneumatic actuators,

including the modern rotary actuator styles. Larger sizes might require high-output electric or long-

stroke pneumatic cylinder actuators. Butterfly valves exhibit an approximately equal percentage flow

characteristic. They can be used for throttling service or for on-off control. Soft-seat construction can

be obtained by using a liner or by including an adjustable soft ring in the body or on the face of thedisk.

These control valves have good rangeability, control, and shutoff capability. The paper industry,

chemical plants, sewage treatment plants, the power industry, and petroleum refineries use such

valve bodies.

Figure. High-Performance Butterfly Control Valve

Eccentric-Plug Control Valve

In this type of rotary control valve the constriction is in the form of eccentric plug which is rotated in

the valve body by an external lever. Path of eccentric plug minimizes contact with the seat ring when

opening, reducing seat wear and friction, prolonging seat life, and improving throttling performance.

Figure. Sectional of Eccentric- Plug Control Valve Body


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FLOW CHARACTERISTICS OF VALVES

The three most significant type of inherent flow characteristics are-

1. Linear Flow characteristic

2. Equal percentage Flow characteristic

3. Quick-Opening Flow Characteristic

LINEAR FLOW CHARACTERISTIC

In the valves of linear characteristic, the flow is directly proportional to the valve travel. A valve

with an ideal linear inherent flow characteristic produces flow rate directly proportional to the

amount of valve plug travel, throughout the travel range. For instance, at 50% of rated travel, flow

rate is 50% of maximum flow; at 80% of rated travel, flow rate is 80% of maximum; etc. Change of

flow rate is constant with respect to valve plug travel. Valves with a linear characteristic are often

specified for liquid level control and for flow control applications requiring constant gain.

For valves having linear flow characteristic

Cv= KL ;Where K = constant and

L = valve travel

Cv= valve flow coefficient

EQUAL PERCENTAGE CHARACTERISTIC

With an equal percentage plug each increment of plug movement produces a change in flow which

is, proportional to the amount flowing before the change occurred.

Ideally, for equal increments of valve plug travel, the change in flow rate regarding travel

may be expressed as a constant percent of the flow rate at the time of the change.

The change in flow rate observed regarding travel will be relatively small when the valve plug isnear its seat and relatively high when the valve plug is nearly wide open. Therefore, a valve with an

inherent equal-percentage flow characteristic provides precise throttling control through the lower


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portion of the travel range and rapidly increasing capacity as the valve plug nears the wide-open

position.

Valves with equal-percentage flow characteristics are used on pressure control applications, on

applications where a large percentage of the pressure drop is normally absorbed by the system itself

with only a relatively small percentage available at the control valve and on applications where

highly varying pressure drop conditions can be expected. In most physical systems, the inlet pressure

decreases as the rate of flow increases, and an equal percentage characteristic is appropriate. For thisreason, equal percentage is the most common valve characteristic.

In the valves of equal percentage characteristic, the flow change to the lift is small when the valve

travel is small. As the valve travel becomes larger, however, the flow change corresponding to the

same lift tend to become greater.

QUICK-OPENING FLOW CHARACTERISTIC

A valve with a quick opening flow characteristic provides a maximum change in flow rate at low

travels. The curve is basically linear through the first 40 percent of valve plug travel, and then

flattens out noticeably to indicate little increase in flow rate as travel approaches the wide-open

position. Control valves with quick-opening flow characteristics are often used for on/ off

applications where significant flow rate must be established quickly as the valve begin to open.

Consequently, they are often used in relief valve applications.

Quick-opening valves can also be selected for many of the same applications for which linear flow

characteristics are recommended, because the quick-opening characteristic is linear up to about

70percent of maximum flow rate. Linearity decreases sharply after flow area generated by valve plug

travel equals the flow area of the port. For a typical quick-opening valve shown below, this occurs

when valve plug travel equals one-fourth of port diameter

Typical Construction to Provide Quick-Opening Flow Characteristic

CHARACTERIZATION OF CAGE GUIDED VALVE

In valve bodies with cage guided trim, the shape of the flow openings or windows in the wall of the

cylindrical cage determines flow characterization. As the valve plug is moved away from the seat

ring, the cage windows are opened to permit flow through the valve. Standard cages have been de-

signed to produce linear, equal percentage, and quick-opening inherent flow characteristics. The flow

rate/travel relationship provided by valves using these cages is equivalent to the linear, quick-

opening, and equal percentage curves.


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Cage-guided trim in a control valve provides a distinct advantage over conventional valve body

assemblies in that maintenance and replacement of internal parts is much simplified. The inherent

flow characteristic of the valve can be easily changed by installing a different cage. Interchange of

cages to provide a different inherent flow characteristic does not require changing valve plug or seat

ring.

EQUAL PERCENTAGE

QUICK OPENING

LINEARFigure. Characterized Cages for Globe-Style Valve Bodies

VALVE ACTUATORS

A pneumatic, hydraulic, or electrically powered device that supplies force and motion to open or

close a valve is known as actuator. Pneumatically operated control valve actuators are the most

popular type in use, but electric, hydraulic, and manual actuators are also widely used. The spring-

and-diaphragm pneumatic actuator is most commonly specified due to its dependability and

simplicity of design.

Pneumatically operated piston actuators provide high stem force output for demanding service

conditions. Adaptations of both spring-and-diaphragm and pneumatic piston actuators are available

for direct installation on rotary-shaft control valves .Electric and electro-hydraulic actuators are more

complex and more expensive than pneumatic actuators. They offer advantages where no air supply

source is available, where low ambient temperatures could freeze condensed water in pneumatic

supply lines, or where unusually large stem forces are needed.


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Diaphragm Actuators

Pneumatically operated diaphragm actuators use air supply from controller, positioner, or other

source. Net output thrust is the difference between diaphragm force and opposing spring force.

Diaphragm actuators are simple, dependable, and economical.

Various styles include: direct- acting (increasing air pressure pushes down diaphragm and extends

actuator stem); reverse-acting (increasing air pressure pushes up diaphragm and retracts actuator

stem); reversible (actuators that air pressure pushes down on diaphragm, which may either open orclose the valve, depending on orientation of the actuator lever on the valve shaft).

The following figure shows the various parts of direct acting diaphragm actuator:

Figure. Diaphragm Actuators

Electro hydraulic Actuators:-

Electro hydraulic actuators require only electrical power to the motor and an electrical input signal

from the controller. Electro hydraulic actuators are ideal for isolated locations where pneumatic

supply pressure is not available but where precise control of valve plug position is needed.

Figure. Control Valve with Double-Acting

Electro hydraulic Actuator and Hand wheel


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Manual Actuators

Manual actuators are useful where automatic control is not required, but where ease of operation

and good manual control is still necessary. They are often used to actuate the bypass valve in a three-

valve bypass loop around control valves for manual control of the process during maintenance or

shut- down of the automatic system. Manual actuators are available in various sizes for both globe-

style valves and rotary-shaft valves.

Dial-indicating devices are available for some models to permit accurate repositioning of the valve

plug or disk. Manual actuators are much less expensive than automatic actuators.

SLIDING-STEM VALVES ROTARY-SHAFT VALVES

Figure. Typical Manual Actuators


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types of control valves

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