PROCESS CONTROL The process may be controlled by measuring a variable representing the desired state of the product and automatically adjusting one of the other variables of the process. In process control, the basic objective is to regulate the value of some quantity. Automatic control is the maintenance of a desired value of a quantity or condition by measuring the existing value, comparing it to a desired value, and employing the difference to initiate action for reducing this difference. Thus automatic control requires a closed loop of action and reaction operating without human aid Actuator Convert the industrial standard signal to action such as valve opening, power level, displacement, and etc. Standard instrumentation signal levels and signal conversion transmitters are used. Types of actuator •Pneumatic: simple, low cost, fast, low torque, hysteresis •Electric: motor and gear box, high torque, slow •Hydraulic: high torque, fast, expensive Valves a device for controlling the passage of fluid through a pipe or duct, especially an automatic device allowing movement in one direction only CONTROL VALVES A Control Valve is a power-operated device used to modify the fluid flow rate in a process system. 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]
Transcript
PROCESS CONTROL
The process may be controlled by measuring a variable representing the desired state of the product and automatically adjusting one of the other variables of the process. In process control, the basic objective is to regulate the value of some quantity.
Automatic control is the maintenance of a desired value of a quantity or condition by measuring the existing value, comparing it to a desired value, and employing the difference to initiate action for reducing this difference. Thus automatic control requires a closed loop of action and reaction operating without human aid
Actuator
Convert the industrial standard signal to action such as valve opening, power level, displacement, and etc.
Standard instrumentation signal levels and signal conversion transmitters are used.
a device for controlling the passage of fluid through a pipe or duct, especially an automatic device allowing movement in one direction only
CONTROL VALVES
A Control Valve is a power-operated device used to modify the fluid flow rate in a process system.
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
There are two basic types of control valvesa) Rotary motion valves having ball, butterfly or plug type closures.
b) Linear motion valves having globe, diaphragm or pinch type closures.
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
Control Valve
Linear Motion
Globe
Globe
Single Seated
Double Seated
Angle 3 way
Diaphragm Pinch
Rotary Motion
Ball Butterfly Plug
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
BUTTERFLY VALVE
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
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
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.
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).
