Types of Control Valves
Types of Control Valves
Types of Control Valves
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 fluid is 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.
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. Cagestyle 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
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 is near 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
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 this reason, 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
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
LINEAR
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 springand-diaphragm pneumatic actuator is most commonly specified due to its dependability and simplicity of design. Pneumatically operated piston actuators provide high stem force out- put 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.
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 or close 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:
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.
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 threevalve 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 globestyle 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