Control Valves Control Valves
Topics:- • Introduction • Control Valve Characteristics and Types • Control Valve Parts • Control Valve Accessories • Control Valve Operation • Valve Hand Jack and Minimum Stop
Introduction:- The control valve is composed of a valve with an externally powered actuator. The control valve is designed specifically for reliable continuous throttling with minimum backlash and packing friction. The control valve is involved with the disposition of energy in a process. It dispenses energy from the source, dissipated energy that exists within the system, or distributes energy in the system in one way or another. The chemical and petroleum industries have many applications requiring control of gases, liquids, or vapors processes. Many process operation require regulation of such quantities as density and composition, but by far the most important control parameter is flow rate. A regulation of flow rate emerges as the regulatory parameters for reaction rate, temperature, composition, or a host of other fluid properties. For this purposes the control valve is using as the process control element.
Control Valve Characteristics and Types:- The different types of control valves are classified by a relationship between the valve stem position and the flow rate through the valve. This control valve characteristic is assigned with the assumptions that the stem position indicates the extent of the valve opening and that the pressure difference is determined by the valve alone. There are three basic types of control valves whose relationship between stem position (as percentage of full range) and the flow rate (as a percentage of maximum) is shown in the Picture. 1. Quick Opening:- This types of valve is used predominantly for full ON/full OFF control applications. The valve characteristic shows that a relatively small motion of valve stem results in maximum possible flow rate
Through the valve. Such a valve, for Example , may allow 90% of the flow rate with only a 30% travel of the stem. 2. Linear:- This type of valve, as shown in picture, has a flow rate that varies linearly with the stem position. It depends the ideal situation where the valve alone determines the pressure drop. 3. Equal Percentage:- Equal percentage is the characteristic most commonly used in process control. The change in flow per unit of valve stroke is directly proportional to the flow occurring just before the change is made. While the flow characteristic of the valve itself may be equal percentage, most control loops will produce an installed characteristic approaching linear when the overall system pressure drop is large relative to that across the valve.
Depends on the construction of the valve the valves are classified in different names. Valves are classified in to two general types based on how the valve closure member is moved: by linear motion or rotary motion. The types of the valves as follows, • Globe valve • Butterfly valves • Gate valves • Diaphragm valves • Ball valves • Knife edge valves • In our plant we are using the Valtek valves and most of them are Globe or butterfly type. So we will discuss about these valves.
Control Valve Parts:- • The Valves has Two Main Parts • Body Assembly • Actuator Assembly
Body Assembly Parts • Body:The pressure retaining housing through which the service fluid flows. It has inlet and outlet connections, and houses the trim components • Seat Ring:Trim component the plug makes contact with to close the valve. • Seat Retainer:Trim component which clamps the seat ring in place. The seat retainer does not guide the plug, and should not be confused with a cage • All Gaskets:are used in control valves to prevent leakage. around the seat ring, bonnet or pressure-balanced sleeve.
Body Assembly Parts • Plug:Part that moves in and out of the seat ring to open and close the valve. It can also be used to characterize the flow. • Bonnet:The valve component which houses the guides and packing. It also seals one opening to the body. • Bonnet Flange:Flange that attaches the bonnet to the body. • Guides — Bushings contained in the packing box which align the plug with the seat ring. • Guides — Bushings contained in the packing box which align the plug with the seat ring. • Packing — Material used to seal the valve from leaking around the plug stem. • Packing Box — Internal bore of bonnet which contains guiding and packing.
Actuator Assembly Parts • Actuator — Device which develops sufficient thrust to open or close the valve. Common designs include piston,diaphragm, hydraulic, manual hand wheel and electro-hydraulic actuators. • Lifting Ring: Used for Lifting The Valve • Adjusting Screw: Part used to compress the actuator spring. • Cylinder:Actuator part used for containing air pressure and enclosing the piston. • Spring Button:The part that prevents movement of the actuator spring and permits the adjusting screw to compress the spring. • Spring:In piston actuators, the part which provides force for fail-safe operation; in diaphragm actuators, the part that provides force to counteract air pressure from the opposing chamber. • Piston:Part used to separate two air chambers of piston actuator. • Actuator Stem:Part used to connect the valve plug with the piston actuator. • Yoke:A component which secures the actuator to the valve body.
Valtek Tiger-Tooth: The Valtek Tiger-ToothTM ntrol valve effectively attenuates gaseous and hydrodynamic noise, and eliminates the damaging effects of cavitations. Tiger-Tooth trim features a series of circular stacked discs; each of which has concentric grooves (or teeth) machined on the face. Flow passes from the center of the disc stack across the face of the discs, causing a series of expansions and contractions of the flow. This mechanism reduces pressure gradually across the face of the discs without the sharp pressure drop and subsequent pressure recovery typical of conventional, singlethrottling- point trims. Tiger-Tooth trim is constructed to be forgiving of small entrained particles that typically plug competitive trims or cause them to stick. The wide acceptance of the Tiger-Tooth valve in the process control industry confirms its ability as a highly successful noise and cavitation reduction device.
Valve Accessories • Positioner • I/P Transducer • Volume Booster • Quick Exhaust • Lockup Realy • Solenoid • Limit Switch
Positioner:- • A valve Positioner is like a proportional controller. The set point is the control signal from the primary controller and the controlled variable is the valve position. The Positioner compensates for disturbances and nonlinearities. • The use of positioner are as follows, • When the signal pressure is not enough to operate the control valve. • To make split range between the valves. • It can be used to reverse the action of the actuator from air to open to air to close and vice versa. • To minimize the effect of hysterisis effect. • To minimize the response time for the valve. • If the actuator is spring less positioner will be used. • If the valve has high friction.
Operation:- The operation of the most common positioner as follows. In construction, pneumatic valve positioners have diaphragms or bellows to sense the incoming signal from the controller and feed back devices from the valve stem. The unit may be position balanced or force balanced. Any error in the two signals causes a proportional change in the output of a pilot valve. In our plant we are using Valtek beta positioner and the main parts are shown in the picture.
Instrument Signal Capsule: It will receive The Signal from I/P Transducer & move The Pilot Stem. • Spool Valve: • Feedback Spring • Cam • Range Arm • Range Adjustment Locking Screw • Range Adjustment Gear • Zero Adjustment Locking Knob • Zero Adjustment
The Beta Positioner is a force-balanced instrument. Figure 2 shows a Beta Positioner, with either a pneumatic or electro-pneumatic (I/P) module, installed on adouble-acting actuator for air-to-open action. Positioning is based on a balance of two forces; one proportional to the instrument signal and the other proportional to the stem position. With the I/P model, the current signal is first converted to a 3-15 psi air signal. For the pneumatic model, the 3-15 psi signal is passed directly into the positioner. The pressure signal acts upon the diaphragms in the instrument signal capsule creating a downward force. The motion of the actuator stem is transmitted to the top end of the feedback spring through the follower arm and cams. As a result, tension in the feedback spring will vary as the stem position changes. When these opposing forces balance exactly, the system will be in equilibrium and the stem will be in the position called for by the instrument signal. If these opposing forces are not in balance, the summing beam will move up (or down) and, by means of the spool valve, will change the output pressures and flow rate. This will cause the piston to move until tension on the feedback spring equalizes with the instrument signal pressure.
An increase in the instrument signal forces the instrument signal capsule and summing beam downward. This motion of the summing beam also pulls the pilot valve spool downward from its equilibrium position. This opens the pilot valve ports, supplying air to port 1 and exhausting air from port 2. This causes the actuator piston to move upward. This upward motion of the piston is transmitted back to the positioner through the feedback linkage and cam resulting in the spring being stretched proportionally to the valve position. The piston continues to stroke upward until the force in the feedback spring increases sufficiently to counter the force generated by the instrument signal capsule. At this point, the summing beam and spool begin to return to their equilibrium position. As the valve spool ports start to close, the air flow rate to the actuator is decreased. After the piston has reached the required position, the feedback spring tension force will equal the force generated in the instrument signal capsule. The summing beam and instrument signal capsule will remain in their equilibrium positions with no air flowing to the actuator until a change in the instrument signal is made. A decrease in the instrument signal reverses the described actions causing a proportional downward movement of the actuator piston and stem.
I/PTransducer: • I/P Transducer:Transducers convert a current signal to a pneumatic signal. The most common transducer converts a 4-20 mA electric signal to a 3-15 psig pneumatic signal. An increase in the dc signal to the coil increases the magnetic field around the coils. This field increases the magnetic strength in the armature and the magnetic attraction across the air gap between the armature and the pole pieces. The magnetic attraction will therefore downward at the nozzle end and upward at the feed back bellows end, resulting in a torque that rotates the armature about the torsion rod to cover the armature nozzle. The resulting restriction produces an increased pressure in the nozzle, in the upper chamber of the relay, and in the feed back bellows. The relay responds to the increase in nozzle pressure to increase the output pressure to the actuator and control valve. Volume Booster: Volume Boosters are used on throttling control valves to provide fast stroking action with large input signal changes. At the same time, the flow boosters allow normal positioner air flow (and normal actuation) with small changes in the positioner input signal. Depending on actuator size, packing set and the number used, boosters can decrease valve stroking times up to 90 percent.
Quick Exhaust: • Quick exhaust valves allow the cylinder actuator to quickly vent one side to atmosphere, resulting in an almost immediate full-open or full-closed position. This sudden movement generally limits quick exhaust applications to on/off services where positioners are not used. Lockup Relay: • is designed to hold the actuator in the last operating position upon air failure. Solenoid: • Used To Open The Valve Or Close it Limit switch: • Limit switches are available to indicate a valve open or closed position. Switching Valve: • It is used in fail lock up system for to set the air pressure in the required level for lock the valve.
Valve Operation:- • Air to Open • Air to Close • Air fail to Lock in the same position
Fail Safe System for Valves:- Where service conditions exceed the capabilities of the standard fail-safe spring to drive the valve to its failure position, and where specially designed, extra-strong failure springs may be both mechanically and economically unfeasible, air spring fail-safe systems on Valtek control valves provide the thrust necessary to drive the plug to its failure position. An air spring provides a pressurized volume of air to drive the actuator piston in the failure direction. The volume of air is sometimes provided within the actuator itself, or where the cylinder volume is insufficient, a separate external volume tank is provided. Air spring systems are used primarily to close valves upon air failure. And sometimes they must open valves upon air failure. A fail-closed Valtek valve is customarily operated with the flow direction over the plug. Thus, with the plug on the seat, the upstream pressure acts to hold the valve closed.
Fail-open Valtek valves customarily operate with the flow direction under the plug. Thus, when a general system failure occurs, the upstream pressure will keep the plug off the seat and the valve open. Air springs on Valtek valves are practical because the locked-up air is used only at the instant of air failure to drive the valve to the fail position. Line pressure will insure that the valve stays either closed or open. Occasionally, service conditions require that the valve remain in the last operating position upon loss of air supply. For such applications, Valtek valves can be equipped with a fail-in-place lock-up system. If air failure occurs, the system activates two pilot-operated lock-up valves that trip and lock the existing cylinder pressures on both sides of the piston, thus maintaining the last throttling position.
Signal-to-open, Fail-closed Signal-to-close, Fail-open
For air-to-open (air-to-retract) air action, tube “output 1” to the bottom and “output 2” to the top of the cylinder. For air-to-close (air-to-extend) action, tube “output 2” to the bottom and “output 1” to the top of the cylinder.
Introduction:- Control valves are often equipped with auxiliary hand wheels of the continuously connected or push only type, allowing for manual operation of the valve in case of air failure. Continuously connected hand wheels can also be used as a limit stop to limit valve travel in either the open or closed position. Push only hand wheels are designed to close the valve or limit the opening. The different types of hand wheels are described below.
1) SIDE-MOUNTED, CONTINUOUSLY CONNECTED HANDWHEELS, SIZE 25, 50, 100 AND 200 (Linear Actuators) Operation If air failure should occur, or if manual control of the valve is desired, the side-mounted, continuously connected hand wheel can be operated as follows: 1. Set the three-way bypass valve (located on the supply line to the positioner) to 'manual' to vent the air pressure from the actuator. NOTE: Three-way valves are installed in the supply line only when there is no lock-up system or volume tank. On volume tank or lock-in-place systems, the cylinder ports.
2. To open the valve, turn the hand wheel counter-clockwise to retract the plug. 3. To close the valve, turn the hand wheel clockwise to extend the plug. 4. To return the valve to automatic control, return the hand wheel nut to 'neutral' as shown by the hand-wheel position indicator and set the three-way bypass valve to 'auto.' 5. Adjusting the hand wheel nut to a position other than neutral provides a limit stop function, either limiting opening or closing
TOP-MOUNTED, CONTINUOUSLY CONNECTED HANDWHEELS Size 100 and 200 (Linear Actuators) Operation If air failure occurs, or if manual control of the valve is desired, the top-mounted, continuously connected hand-wheel can be operated as follows: 1. Set the three-way bypass valve (located on the supply line to the positioner) to 'manual' to vent the air pressure from the actuator. NOTE: three-way valves are installed in the supply line only when there is no lock-up system or volume tank. On volume tank or lock-in-place systems, the bypass valve is located between the top and bottom cylinder ports. 2. To open the valve, turn the hand wheel counter-clockwise to retract the plug. 3. To close the valve, turn the hand wheel clockwise to extend the plug. 4. To return the valve to automatic control, return the hand wheel nut to 'neutral' as shown by the hand-wheel position indicator and set the three-way bypass valve to 'auto.' The neutral position is indicated when the top of the screw aligns with the red line on the cap liner. 5. Adjusting the hand wheel nut to a position other than neutral provides a limit stop function, either limiting opening or closing.
TOP-MOUNTED, PUSH-ONLY HANDWHEELS (Linear Actuators Only) Operation The push-only hand wheel can be used to close the valve or as a limit stop to limit valve opening: 1. If provided, set the three-way bypass valve on 'manual' to vent air pressure to the actuator. 2. Turn hand wheel in a clockwise direction to extend the actuator stem and to lower the valve plug. 3. To return the valve to automatic control, turn the hand wheel counter-clockwise until the hand wheel runs out of threads and stops (neutral position), and set the three-way valve on 'auto.' 4. To limit the opening of the valve, move the hand-wheel from neutral to the desired limiting position.
DECLUTCHABLE HANDWHEELS (Rotary Actuators Only) Operation The declutchable hand wheel used on rotary actuators can be operated as follows: 1. Set the three-way bypass valve to 'manual' to vent the air pressure from the actuator. 2. Engage the hand wheel mechanism by rotating the clutch handle 90 and allowing it to be fully seated in the clutch indicator’s deep slot. 3. Rotate the hand wheel until the spring loaded clutch key engages the hand wheel gear. At this point the hand wheel mechanism is fully engaged. 4. To open, turn the hand wheel counter-clockwise.
5. To close, turn the hand wheel clockwise. 6. To declutch the hand wheel, rotate the hand wheel until there is little or no load on it. Pull the clutch handle out and index it 90 until it seats in the clutch indicator’s shallow slot. NOTE: It will be difficult to pull out the clutch handle if the hand wheel mechanism is transmit-ting torque.
Operation:- The push-only limit stop is used to limit valve opening. To set the limit stop: 1. Loosen the limit stop jam nut (see Figure 6). 2. To lengthen the stroke, screw the limit stop bolt counter-clockwise from the locking stop hub until the desired position is reached. To shorten the stroke, screw the limit stop bolt clockwise into the locking stop hub until the desired position is reached. 3. Tighten the limit stop jam nut firmly against the locking stop hub.