CHAPTER IV - Petra Christian University
CHAPTER IV
FINAL CONTROL ELEMENT
➢ Final control element is actual control element. It is a part of the process itself.
4.1 Mechanical Control Elements
➢ M.C.E. is control elements that perform some mechanical operation in a process.
➢ Solid Material Hopper Valves
o Control system’s purpose : to maintain the flow of grain from the storage bin to provide a constant flow rate on the conveyer.
o This flow depends on height of grain in the bin, and hence the hopper valve must open or close to compensate for the variation.
o An actuator (motor, hydraulic cylinder, etc.) operates a vane-type valve to control the grain flow rate (by adjust shaft position).
➢ Movable Roller in Paper Thickness
o Control system’s purpose : to control constant paper thickness.
o The paper is in a wet fiber suspension and is passed between rollers. By varying the roller separation, paper thickness is regulated. The M.C.E. is the movable roller.
o The actuator (electrical, pneumatic, etc) adjusts roller separation based on thickness measurement.
4.2 Electrical Control Elements
➢ Motor Speed Control
o A process-control loop regulates this speed through direct change of operating voltage or current, as shown in Figure 7.33 for a DC motror.
o Voltage measurements of engine speed from a tachometer are used in a process-control loop to determine the power applied to the motor brushes.
o In some cases motor speed control is an intermediate operation. For example, in the operation of a kiln for solid chemical reaction, the rotation (feed) rate may be varied by motor speed control based on reaction temperature, as shown in Figure 7.34.
➢ Temperature Control
o Temperature often is controlled by using electrical heaters.
o If heat can be supplied through heaters electrically, then the process control signal can be used to ON/OFF cycle a heater or set the heater within a continuous span of operating voltages, as in Figure 7.35.
4.3 Fluid Valves
➢ Many industries depend in part on operations that involve fluids and the regulation of fluid parameters. The word “fluids” represent : gases, liquids, or vapors.
➢ The most important control fluid parameter is flow rate. Flow rate’s regulation emerges as the regulatory parameter for reaction rate, temperature, composition, etc.
➢ Process control element spesifically associated with flow is the control valve.
Control Valve Principles
➢ If a given fluid is delivered through a pipe, theb the volume flow rate is
o [pic]
Q = flow rate (m3/s) A = pipe area (m2)
v = flow velocity (m/s)
➢ The purpose of the control valve is
o to regulate the flow rate of fluids through pipes in the system.
➢ This is accomplished by placing a variable-size restriction in the flow path (Fig. 7.36).
➢ As the stem and plug move up and down, the size of the operating between the plug and the seat changes, thus changing the flow rate.
➢ There will be a drop in pressure across such a restriction.
o [pic]
K = proportionality constant (m3/s/Pa1/2)
(p = p2 – p1 = pressure difference (Pa)
Control-Valve Types
➢ They are classified by a relationship between the valve stem position and the flow rate through the valve.
o Assumption : - the stem position indicates the valve opening
- the pressure difference is determined by the valve alone
➢ Example : Figure 7.37
➢ The types are determined by the shape of the plug and the seat. They determine the amount of actual opening of the valve.
➢ Three basics types of control valves :
1. Quick Opening.
➢ This type is used for full ON/OFF control applications.
➢ A relative small motion of the valve stem results in maximum possible flow rate through the valve (show Figure 7.38).
2. Linear
➢ The flow rate that varies linearly with the stem position (ideal situation).
➢ The relationship :
o [pic]
where : Q = flow rate (m3/s) S = stem position (m)
Qmax = maximum flow rate Smax = maximum stem position (m)
3. Equal percentage
➢ Its characteristics : a given percentage change in stem position produces an equivalent change in flow, that is, an equal percentage.
➢ Valve allows a flow Qmin (minimum flow) until Qmax (maximum flow). We define rangeability R as the ratio :
o [pic]
➢ Its flow rate :
o [pic]
Control-Valve Sizing
➢ This equation is in ideal situation :
o [pic]
➢ Correction factor is involved because of nonideal characteristics (nature of the fluid).
➢ These correction factors allow selection of the proper size of valve.
➢ It is called the valve flow coefficient and is designated as Cv :
o [pic]
where : (p = pressure across the valve (psi)
SG = specific gravity of liquid
➢ Typical valves of CV for different-size valves are shown in Table 7.1.
Fluid Control Example
➢ Control of distillition column composition by regulation of a fixed point column temperature. Such regulation is achieved by controlling the feed rate as shown in Figure 7.39.
➢ A thermocouple measures …………. that is transmitted to the controller as a ………….. control signal.
➢ The controller outputs a ……………… signal proportional to proper control valve position.
➢ This is converted to a ……………… pneumatic signal by an I/P converter that operates a pneumatic actuator connected to the control valve.
➢ The valve size is determined by the characteristics of the gas or vapor that is flowing.
➢ The size of the required actuator is determined from the valve size.
Source : “Process Control Instrumentation Technology”, Curtis D. Johnson
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