ERT 321Process Control & Dynamics

Anis Atikah binti Ahmad

CHAPTER 9Control System Instrumentation

OUTLINE

• Transducers & Transmitters• Final Control Elements• Calculation of size of control valve• Transmission Line

CONTROL SYSTEM INSTRUMENTATION

Measurement Actuator

Process

C

A controller/process interface

Figure 9.3 A typical process transducer.

• Figure 9.3 illustrates the general configuration of a measurement transducer; it typically consists of a sensing element combined with a driving element (transmitter).

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Transducers & Transmitters

• Transducers for process measurements convert the magnitude ofa process variable (e.g., flow rate, pressure, temperature, level, or concentration) into a signal that can be sent directly to the controller.

The sensing element is required to convert the measured quantity, that is, the process variable, into some quantity more appropriate for mechanical or electrical processing within the transducer.

•

2

Standard Instrumentation Signal Levels

Before 1960, instrumentation in the process industries utilizedpneumatic (air pressure) signals to transmit measurement and control information almost exclusively.

These devices make use of mechanical force-balance elements to generate signals in the range of 3 to 15 psig, an industry standard.

•

•

• Since about 1960, electronic instrumentation has come intowidespread use.

Most industrial analog instrumentation now has a standandr 4 to 20 mA range.

•

Sensors

The main categories of measurements used in process control are temperature, pressure, flow rate, liquid level and composition.

Selection criteria:

1. Measurement range (Span): • The required measurement range for the process variable must lie entirely within the range of instrument performance

2. Performance: •Accuracy, repeatability, etc •The speed of response must be included in a closed loop control system.

3. Reliability: Data available from the manufacturers provide baseline conditions.

4. Materials of construction: •The instrument may need to withstand high temperatures, high pressures, and corrosive and abrasive environments. For some applications, seals & purges may be necessary.

5. Prior use: •For the first installation of a specific measurement device at a site, training of maintenance personnel and purchases of spare parts might be necessary

6. Potential for releasing process materials to the environment: •Preventing exposure to fugitive emissions for maintenance personnel is important when the process fluid is corrosive or toxic. •Sterility in bioprocesses must be maintained.

7. Electrical classification •If the measurement device is not inherently compatible with possible exposure to hazards, suitable enclosures must be purchased & included in the installation costs.

Transmitters• A transmitter usually converts the sensor output to a signal level

appropriate for input to a controller, such as 4 to 20 mA.

Transmitters are generally designed to be direct acting.

In addition, most commercial transmitters have an adjustable input range (or span).

•

•

• For example, athe input range50 to 150 °C.

temperature transmitter might be adjusted so that

of a platinum resistance element (the sensor) is

• In this case, the following correspondence is obtained:

Input Output

50 °C

150 °C

4 mA

20 mA

• This instrument (transducer) has a lower limit or zero of 50 °Cand a range or span of 100 °C.

For the temperature transmitter discussed above, between transducer output and input is

• the relation

T20 mA 4 mA

50°C T mA 4 mA m 150°C 50°C

mA°C 0.16 T °C

4 mA

The gain of the measurement element Km is 0.16

linear instrument:

range of instrument output

mA/°C. For any

K (9-1)m range of instrument input

Figure 9.4 A linear instrument calibration showing its zeroand span.

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Final Control Elements• Every process control loop contains a final control element

(actuator), the device that enables a process variable to bemanipulated.

For most chemical and petroleum processes, the final control elements (usually control valves) adjust the flow rates of materials, and indirectly, the rates of energy transfer to and from the process.

•

Control Valves

• There are many different ways to manipulate the flows ofmaterial and energy into and out of a process; for example, the speed of a pump drive, screw conveyer, or blower can be adjusted.

• However,this result automatic

a simple and widely used method of accomplishingwith fluids is to use a control valve, also called an control valve.

• The control valve components include the valve body,seat, and actuator.

trim,

Air-to-Open vs. Air-to-Close Control Valves

• Normally, the choice of A-O or A-C valve is based on considerations.

safety

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• We choose the way the valve should operate (full flow or noflow) in case of a transmitter failure.

• Hence, A-C and A-O valves often are referred to as fail-openand fail-closed, respectively.

Figure 9.7 A pneumatic control valve (air-to-open).

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Example 9.1

Pneumatic control valves are to be specified for the applicationslisted below. State whether an A-O or A-C valve should befor the following manipulated variables and give reason(s).

used

a)

b)

c)

Steam pressure in a reactor heating coil.

Flow rate of reactants into a polymerization reactor.

Flow of effluent from a wastewater treatment holding tank a river.

Flow of cooling water to a distillation condenser.

into

d)

a) Steam pressure in a reactor heating coil.

A-O (fail closed) to make sure that the transmitter failure will not cause the reactor to overheat, which is usually more serious than having it operate at too low temperature

b) Flow rate of reactants into a polymerization reactor.

A-O (fail closed) to prevent the reactor from being

flooded with excessive reactant.

A-C (fail open) if the reactor flowrate normallly is close

to the maximum flow rate of the valve so that opening

the valve fully would cause relatively little change in

operating conditions.

(c) Flow of effluent from a wastewater treatment holding

tank a river.

A-O (fail closed) to prevent excessive and perhaps

untreated waste from entering the stream.

(c) Flow of cooling water to a distillation condenser.

A-C (fail open) to ensure that overhead vapor is

completely condensed before it reaches the

receiver.

Valve Positioners

Pneumatic control valves can be equipped with a valve positioner, a type of mechanical or digital feedback controller that senses the actual stem position, compares it to the desired position, and adjusts the air pressure to the valve accordingly.

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Specifying and Sizing Control Valves

A design equation used for sizing control valves relates valveAlift to the actual flow rate q by means of the valve coefficient

Cv, the proportionality factor that depends predominantly onvalve size or capacity:

PA vq Cv f (9-2)gs

Here q is the flow rate, A is the flow characteristic, Pv is thef•pressure drop across the valve, and gs is the specific gravity of

the fluid.

This relation is valid for nonflashing fluids•

Calculation of size of control valve

• Specification of the valve size is dependent on the so-calledvalve characteristic f.

Three control valve characteristics are mainly used.•

• For a fixed pressurecharacteristic

drop across the valve, the flowA 0 A 1 ,f 0 f 1 is related to the lift that

is, the extent of valve opening, by one of the following relations:

f Linear:

Quick opening:

Equal

percentage:

f

(9-3)

Rl1f

where R is a valve design parameter that is usually in the rangeof 20 to 50.

f

Figure 9.8 Control valve characteristics.

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RangeabilityThe rangeability of a control valve is defined as the ratio of maximum to minimum input signal level. For control valves, rangeability translates to the need to operate the valve within range 0.05 ≤ f ≤ 0.95 or a rangeability of 0.95/0.05 = 19.

the

b) Calculate the valve’s rated Cv, the value that yields at least100% of qd with the available pressure drop at that higherflow rate.

Compute q as a function of A using Eq. 9-2, the rated Cv,c)Pvand from (a). A plot of the valve characteristic (q vs. )A

should be reasonably linear in the operating region ofinterest (at least around the design flow rate). If it is not suitably linear, adjust the rated Cv and repeat.

To Select an Equal Percentage Valve:

a) Plot the pump characteristic curve andpressure drop curve without the valve,

, the systemPs

as shown in Fig. 9.10.is PvThe difference between these two curves . The pump

Pv / Psshould be sized to obtain the desired value of , forexample, 25 to 33%, at the design flow rate qd.

Figure 9.10 Calculation of the valve pressure drop Pv from the pump characteristic curve and the system pressure

valve Ps .drop without the14

Example 9.2

A pump furnishes a constant head of 40 psi over the entire flowrate range of interest. The heat exchanger pressure drop is 30 psigat 200 gal/min (qd) and can be assumed to be proportional to q2. Select the rated Cv of the valve and plot the installed characteristic

for the following case:

a) A linear valve that is half open at the design flow rate.b) An equal percentage valve (R=50) that is sized to be completely open at 110% of the design flow rate.(c) Same as in (b) except with a Cv that is 20% higher than calculated (d) Same as in (b) except with a Cv that is 20% lower than calculated.

Figure 9.9 A control valve placed in series with a pump anda heat exchanger. Pump discharge pressure is constant.

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Solution

First we write exchanger

an expression for the pressure drop across the heat

2Phe

30 q (9-5) 200

2q

30 P P (9-6) s he 200

Because the pump head is constant at 40 psi, the pressure dropavailable for the valve is

2q

P 40 P 40 30 (9-7) v he 200

Figure 9.11 illustrates these relations. Note that in all four designcases at qd.Pv / Ps 10 / 30 33%

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Figure 9.11 Pump characteristic and system pressure dropfor Example 9.2.

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a) First calculate the rated Cv.

200 C 126.5 (9-8)v

0.5 10

We will use Cv = 125. For a linear characteristic valve, use theℓrelation between and q from Eq. 9-2:

q ℓ (9-9)Cv Pv

PvUsing Eq. 9-9 and values of from Eq.plotted.

9-7, the installedvalve characteristic curve can be

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Calculate the rated Cv (valve fully open) at 110% of qd.

Use a value of Cv = 115.

For the equal percentage valve, rearrange Eq. 9-2 as follows:

b) An equal percentage valve (R=50) that is sized to be completely open at 110% of the design flow rate.

4.1147.3

220vC

1

RPC

q

vv

RPC

q

vv

loglog1

(9.10)

(9.11)

Substituting Cv = 115, R = 50, & values of q and ΔPv yields the installed characteristic curve in Fig. 9.12.

Figure 9.12 Installed valve characteristics for Example 9.2.

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(c) Same as in (b) except with a Cv that is 20% higher than calculated

(d) Same as in (b) except with a Cv that is 20% lower than calculated.

Use a value of Cv = 1.2(115) = 138.

Cv = 0.8(115) = 92

From these results we conclude that an equal percentage valve with Cv~115 would give a reasonably linear installed characteristics over a larger range of flows and have sufficient capacity to accommodate flows as high as 110% of the design flow rate.

• Electronic controllers (digital or analog) can be located relatively far from their instruments with little concern for the impedance of the intervening transmission lines or for the time of transmission, which for all practical purposes is instantaneous.

• Most transmitter analog signals are in the form of current rather than voltage because voltage is affected by wire and connector resistances, which change with wire length, temperature & aging.

• Voltage level control & instrumentation signals are better restricted to laboratory environments where short distances are normally involved.

Transmission lines

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Accuracy in instrumentation

Accuracy requirements are inherently related to control system objectives.

•E.g. cooling water flow errors on the order of 10% might be acceptable in a control loop regulating the temperature of a liquid leaving a condenser;

• errors in the feed flow rate to a process on the order of 1 or 2% might be unacceptable if throughput/inventory calculations must be made with these data.

• Error is the difference between a perfect measurement and the measurement that actually is made.

• It is often expressed as a percentage of full scale (%FS) or less commonly referred to the input (RTI).

Accuracy in instrumentation

Figure 9.13 Analysis of types of error for a flow instrumentwhose range is 0 to 4 flow units.

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In Figure 9.13:

•For this instrument, the precision is limited to ±0.01 flow units because we cannot read the instrument any more precisely.

•Precision is related to resolution, which is defined as the smallest change in the input that will result in a significant change in transducer output. In this case, resolution = 0.01.

•Systematic error (bias) = 0.37-0.25 = 0.12 units.

• The maximum error can be as large as 0.14 units.

•The accuracy (inaccuracy) is no worse than 0.14 at these conditions.

•The repeatability of the measured value is ±0.02 flow units at the constant conditions of this experiment.

Note:

•The precision (resolution) of a transducer can be good while it accuracy is poor.

•The accuracy cannot be good if the precision is poor.