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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