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.[1]

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 Valve

1. 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 disconnect

4. Material: closure member, housing, seat – stainless steel

5. 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 WORKING

ball valve consisting of a spherical ball located between two

sealing rings in a simple body form. The ball has a hole allowing fluid to pass

through. When aligned with the pipe ends, this gives either full bore or nearly

full bore flow with very little pressure drop. Rotating the ball through 90°

opens and closes the flow passage. Ball valves designed specifically for control

purposes will have characterized balls or seats, to give a predictable flow

pattern.

Ball valves are an economic means of providing control with tight shut-off for

many fluids including steam at temperatures up to 250°C (38 bar g, saturated

steam). Above this temperature, special seat materials or metal-to-metal

seatings are necessary, which can be expensive. Ball valves are easily actuated

and often used for remote isolation and control. For critical control

applications, segmented balls and balls with specially shaped holes are

available to provide different flow characteristics

TYPES OF BALL VALVE

HALF 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 WORKING

butterfly valve, which consists of a disc

rotating 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 low

pressures and temperatures, due to the inherent limitations of the soft seats used.

Currently, valves with higher temperature seats or high quality and specially

machined metal-to-metal seats are available to overcome these drawbacks. Standard

butterfly valves are now used in simple control applications, particularly in larger

sizes and where limited turndown is required.

A fluid flowing through a butterfly valve creates a low pressure drop, in that the valve

presents little resistance to flow when open. In general however, their differential

pressure limits are lower than those for globe valves. Ball valves are similar except

that, due to their different sealing arrangements, they can operate against higher

differential 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 don't 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 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 don't 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.

Valve Symbols

Butterfly Valve

Ball Valve

Gate Valve

Globe Valve

Controlled Variables

Controlled variables are variables that we want to maintain at constant or specified values (T, P, flow rate, level, etc.).

Manipulated Variables

Manipulated variables are variables that we intentionally change to maintain our controlled variable at a constant value. We often manipulate the values by opening or closing a valve.

Measured Variables

Measured variables are variables that we measure with a meter (often often the controlled variable or a variable that we use to calculate the controller variable).

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

Controller

controllermeter valve

(Electrical or pneumatic signal indicated by dashed line.)

INPUT OUTPUT

SET POINT (commonly from the operator)

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.