CONTROL VALVE Control valves are valves used to control conditions such as flow, pressure, temperature, and liquid level by fully or partially opening or closing in response to signals received from controllers that compare a "setpoint" to a "process variable" whose value is provided by sensors that monitor changes in such conditions. The opening or closing of control valves is usually done automatically by electrical, hydraulic or pneumatic actuators. Positioners are used to control the opening or closing of the actuator based on electric, or pneumatic signals. These control signals, traditionally based on 3-15psi (0.2- 1.0bar), more common now are 4-20mA signals for industry
TYPE OF VALVE Ball Valve Butterfly Valve Gate Valve Globe Valve
Parts of a Valve1. Closure member: part of the valve that closes flow (disk, ball, gate, etc.).2. Actuator: means of operating the valve – hand, gear, chain wheel, motor, solenoid, pressure and flow of the media, air pressure.3. End fitting: must be specified when buying the valve - butt weld end, compression flange, pipe thread, quick disconnect4. Material: closure member, housing, seat – stainless steel5. Packing/seals: seals stem, replaced6. Seat: where the closure members seals against the valve housing
BALL VALVE Sphere with a port in a housing, rotate to expose channel. Applications: Flow control, pressure control, shutoff, corrosive fluids, liquids, gases, high temp. Advantages – low pressure drop, low leakage, small, rapid opening Disadvantages – seat can wear if used for throttling, quick open may cause hammer
BALL VALVE WORKINGball valve consisting of a spherical ball located between twosealing rings in a simple body form. The ball has a hole allowing fluid to passthrough. When aligned with the pipe ends, this gives either full bore or nearlyfull bore flow with very little pressure drop. Rotating the ball through 90°opens and closes the flow passage. Ball valves designed specifically for controlpurposes will have characterized balls or seats, to give a predictable flowpattern.Ball valves are an economic means of providing control with tight shut-off formany fluids including steam at temperatures up to 250°C (38 bar g, saturatedsteam). Above this temperature, special seat materials or metal-to-metalseatings are necessary, which can be expensive. Ball valves are easily actuatedand often used for remote isolation and control. For critical controlapplications, segmented balls and balls with specially shaped holes areavailable to provide different flow characteristics
TYPES OF BALL VALVEHALF BALL VALVE FULL BALL VALVE
BUTTERFLY VALVE rotating disk on a shaft, in a housing Low pressure, large diameter lines where leakage is unimportant Advantages – low pressure drop, small and light weight Disadvantages – high leakage, high actuation forces so limited to low pressures
BUTTERFLY VLAVE WORKINGbutterfly valve, which consists of a discrotating in trunnion bearings. In the open position the disc is parallel to the pipe wall,allowing full flow through the valve. In the closed position it is rotated against a seat,and perpendicular to the pipe wall. Traditionally, butterfly valves were limited to lowpressures and temperatures, due to the inherent limitations of the soft seats used.Currently, valves with higher temperature seats or high quality and speciallymachined metal-to-metal seats are available to overcome these drawbacks. Standardbutterfly valves are now used in simple control applications, particularly in largersizes and where limited turndown is required.A fluid flowing through a butterfly valve creates a low pressure drop, in that the valvepresents little resistance to flow when open. In general however, their differentialpressure limits are lower than those for globe valves. Ball valves are similar exceptthat, due to their different sealing arrangements, they can operate against higherdifferential pressures than equivalent butterfly valves.
GATE VALVE Sliding disk, perpendicular to flow Applications: Stop valves, (not throttling), high pressure and temp, not for slurries, viscous fluids Advantages – low pressure drop when fully open, tight seal when closed, free of contamination buildup Disadvantages – vibration when partially open, slow response and large actuating force
GATE VALVE A Gate Valve, or Sluice Valve, as it is sometimes known, is a valve that opens by lifting a round or rectangular gate/wedge out of the path of the fluid. The distinct feature of a gate valve is the sealing surfaces between the gate and seats are planar. The gate faces can form a wedge shape or they can be parallel. Gate valves are sometimes used for regulating flow, but many are not suited for that purpose, having been designed to be fully opened or closed. When fully open, the typical gate valve has no obstruction in the flow path, resulting in very low friction loss.
GLOBE VALVE three types (globe, angle and Y), disc or plug moved perpendicular to flow and closes on a ring seat Throttling, general purpose flow control valve Advantages – faster than gate, seat less wear and tear, high pressure drop for pressure control Disadvantage high pressure drop, require considerable power to operate (gears and levers), heavy
What are Globe Valves? Globe valves are named for their spherical body shape. The two halves of the valve body are separated by an internal baffle which has an opening forming a seat onto which a movable disc can be screwed in to close (or shut) the valve. In globe valves, the disc is connected to a stem which is operated by screw action. When a globe valve is manually operated, the stem is turned by a handwheel. Although globe valves in the past had the spherical bodies which gave them their name, many modern globe valves do not have much of a spherical shape, but the term globe valve is still often used for valves that have such an internal mechanism. In plumbing, valves with such a mechanism are also often called stop valves since they dont have the global appearance, but the term stop valve may refer to valves which are used to stop flow even when they have other mechanisms or designs.
Globe valves are named for their spherical body shape.The two halves of the valve body are separated by aninternal baffle which has an opening forming a seat ontowhich a movable disc can be screwed in to close (orshut) the valve. In globe valves, the disc is connected toa stem which is operated by screw action. When a globevalve is manually operated, the stem is turned by ahandwheel. Although globe valves in the past had thespherical bodies which gave them their name, manymodern globe valves do not have much of a sphericalshape, but the term globe valve is still often used forvalves that have such an internal mechanism. Inplumbing, valves with such a mechanism are also oftencalled stop valves since they dont have the globalappearance, but the term stop valve may refer to valveswhich are used to stop flow even when they have othermechanisms or designs.
Controlled VariablesControlled variables are variablesthat we want to maintain at constantor specified values (T, P, flowrate, level, etc.).
Manipulated VariablesManipulated variables are variablesthat we intentionally change tomaintain our controlled variable at aconstant value. We often manipulatethe values by opening or closing avalve.
Measured VariablesMeasured variables are variables thatwe measure with a meter (often oftenthe controlled variable or a variablethat we use to calculate the controllervariable).
Controller A unit that reads an INPUT signal and a SET POINT, compares the two, performs a calculation on the difference, and sends out an OUTPUT signal. Common type of controller: PID = proportional, integral, derivative
ControllerINPUT OUTPUT meter controller valve SET POINT (commonly from the operator)(Electrical or pneumatic signalindicated by dashed line.)
Types of Control: Feed Back When the controlled variable changes or is different than the set point, the controller adjusts the manipulated variable to bring the controlled variable back to the set point value. The controlled variable must change (be different than the set point) for control action to be taken.