Control Valve
Control Valve
Control Valve
INTRODUCTION
The Control Valve plays a very important role in the automatic control of modern plants. Controls like exchange of energy, reduction of pressure or simply to fill a tank depends on some form of Final Control Element to do the job. Final control element may be considered as the muscle of automatic control. The furnish the necessary power amplification between the low energy levels in the controllers and the higher energy levels needed to perform their function in controlling flowing fluids. The control valve is most widely used type of final control element. The control valve functions as a variable resistance in the pipeline.It provides a pressure drop by changing the turbulence in the process fluid.
Regulation of the plug travel can be done by various methods :- Manually, Pneumatically, Hydraulically and Electrically operated actuator or any combination of these operating devices.
TRANSMITTER
REC
The majority of control applications are valves with linear, equal-percentage, or modified-flow characteristics. Linear - Flow capacity increases linearly with valve travel. They are used : # For slow processes # When more than 40% of the system pressure drop occurs across the valve Equal Percentage -Flow capacity increases exponentially with valve trim travel, equal increments of valve travel produce equal percentage changes in the existing Cv. The are used : # For fast processes # When high rangeability is required Quick Opening # For On / Off control # When max. valve capacity must be obtained quickly. Modified Parabolic Characteristic is approximately midway between linear and equal-percentage characteristics. It provides fine throttling at low flow capacity and approximately linear characteristics at higher flow capacity.
TORTUOUS FLOW PATH LOW RECOVERY CAN THROTTLE SMALL FLOW RATES OFFERS VARIETY OF SPECIAL TRIM DESIGNS SUITED TO HIGHPRESSURE APPLICATIONS USUALLY FLANGED OR THREADED SEPARABLE BONNET
DISADVANTAGES :
Relatively high leakage rate. Low pressure recovery characteristics. Erosion may occur due to leakage characteristics
DISADVANTAGES :
Can not control total flow May requires different size ports
DISADVANTAGES :
Normally made in sizes 2 & above.
DISADVANTAGES :
Poor control characteristics Low rangeability Short diaphragm life Slow response speed
BUTTERFLY VLAVE
ADVANTAGES :
High Capacity Economical specially in larger size Low pressure drop. Does not permit sediment buildupGood for slurries services
DISADVANTAGES :
Operating torque is high- Requires large actuator Tight shutoff depends on use of resilient seat which is temperature dependant.
BALL VALVE
ADVANTAGES :
High Capacity Good control characteristics Good Rangeability Good for Slurry services
DISADVANTAGES :
Limited operating pressure Not good for high pressure drop
PINCH VALVE
ADVANTAGES :
High capacity Economical Good for slurries Flowing medium does not contact working parts
DISADVANTAGES :
Poor control characteristics Not good for high pressure drop application
Cv = q
G P
EROSION : A wearing action on valve trim and body resulting from high velocity of fluids and fluids containing solid particles. Erosion occurred in steam service, in flashing service, and where high pressure drops occur across valve body .
CORROSION : It is chemical reaction between valve body material and fluids carrying in it and causes deterioration of valve material.
Cavitation may be noisy but is usually of low intensity and low frequency.
This situation is extremely destructive and may wear out the trim and body parts of the valve in short time.
Class I. Identical to Class II, III, and IV in construction and design intent, but no actual shop test is made. Class II. Intended for double-port or balanced singe-port valves with a metal piston ring seal and metal-to-metal seats. Air or water at 45 to 60 psig is the test fluid. Allowable leakage is 0.5% of the rated full open capacity. Class III. Intended for the same types of valves as in Class II. Allowable leakage is limited to 0.1% of rated valve capacity. Class IV. Intended for single-port and balanced single-port valves with extra-tight piston seals and metal-to-metal seats. Leakage rate is limited to 0.01% of rated valve capacity. Class V. Intended for the same types of valves as Class IV. The test fluid is water at 100 psig or operating pressure. Leakage allowed is limited to 5 X 10 ml per minute per inch of orifice diameter per psi differential. Class VI. Intended for resilient-seating valves. The test fluid is air or nitrogen. Pressure is the lesser of 50 psig or operating pressure. The leakage limit depends on valve size and ranges from 0.15 to 6.75 ml per minute for valve sizes 1 through 8 inches.
VALVE POSITIONER
A valve positioner is basically a relay that senses both an instrument signal and a valve stem position. Its primary function is to ensure that the valve plug position is always directly proportional to its controller output signal. For example if the positioner receives a 35% signal, it will supply sufficient pressure to the actuator to cause it to stroke 35% of its travel. It is used to split range valves, to reverse the signal to a valve, to overcome friction forces within a valve on high-pressure drop application and on application requiring fast, accurate control. It is normally mounted on the control valve.
I / P CONVERTOR
The electro-pneumatic (I/P) signal converter is used as a linking component between electric or electronic and pneumatic systems. It converts standard electric signal 4-20mA into the standard pneumatic signal of 0.2 kg/cm to 1.0 kg/cm or 3-15 psi.
BOOSTER RELAY
The Booster relay is used to increase the volume of the signal to overcome the time lay due to large volume of the actuator of the control valve or due to large distance between the transmitter and receiver instrument. With this the speed of transmission is improved. The amplifying pilot provides a gain of one so that out put pressure is the same as the input pressure. The booster relay is simply inserted in the output line. Its advantage derives from the large part of the pilot valve so that the large flow of air is allowed when change of pressure takes place.
REVERSING RELAY
Reversing relays are used when the action from controller need to be reversed. For example, split range valve may be operated from a controller where one valve is air to open while the other is air to close. A reversing relay can be used on one of the valves to achieve the desired action.
LOCKUP RELAY
Certain application may required a diaphragm control valve to remain in its last controlled position in the event of air supply pressure failure. This purpose is achieved by incorporating lock valve in the air output line from valve positioner to actuator of that valve.
HANDWHEEL
Hand wheel may be supplied for manual operation of control valves for emergency use, during start up or in the event of the air failure. They are used infrequently & primarily in critical services or when block & bypass valves are not provided.
LIMIT SWITCH
A switch that activates when the position of a device crosses a given physical limit.
SOLENOID VALVE
A Solenoid valve is an electromagnetic device, which moves a plunger up or down when a current is passed through a coil that surrounds the plunger.