instrumentation process 2014.12.31.pdf

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Instrumentation Process

Control System InstrumentationLecture December 31, 2014

Dr Edi Munawar

Edi Munawar 1

Dr. Edi MunawarChemical Engineering Department

Syiah Kuala UniversityContact: [email protected]

Control System Instrumentation

A typical process transducer:

General configuration of a measurement transducer, it

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typically consists of a sensing element combined with adriving element (transmitter).

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Transducers for process measurements convert the magnitude of a process variable (e.g. flow rate, g p ( g ,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

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electrical processing within the transducer.


Before 1960, instrumentation in the process industries utilized pneumatic (air pressure) signals to p ( p ) gtransmit 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.

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Since about 1960, electronic instrumentation has come into widespread use.

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Sensors

A sensor is a device that detects events or changes in quantities and provides a corresponding output, q p p g p ,generally as an electrical or optical signal; for example, a thermocouple converts temperature to an output voltage.

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Transmitters

A transmitter usually converts the sensor output to a signal level appropriate for input to a controller, such g pp p p ,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).

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djus b e pu ge (o sp ). For example, a temperature transmitter might be

adjusted so that the input range of a platinum resistance element (the sensor) is 50 to 150 C

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Transmitters

In this case, the following correspondence is obtained

This instrument (transducer) has a lower limit or f 50 C d f 100 C

Input Output50 C 4 mA

150 C 20 mA

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zero of 50 C and a range or span of 100 C

Transmitters

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

mA4CCmA16.0

mA4C50C50150

mA420)mA(T

oo

oom

T

T

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The gain of the measurement element Km is 0.16 mA/C. For any linear instrument:

(9.1)input instrument of rangeoutput instrument of rangeKm

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Final Control Elements

Every process control loop contains a final control element (actuator), the device that enables a process ( ), pvariable to be manipulated.

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.

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Final Control Elements

A linear instrument calibration showing its zero and span.p

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Control Valves

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

However, a simple and widely used method of accomplishing this result with fluids is to use a control valve also called an automatic control valve

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control valve, also called an automatic control valve.

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

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

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

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A pneumatic control valve (air-to-open)

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

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A pneumatic control valve(air-to-open)

A pneumatic control valve(air-to-close)

Specifying and Sizing Control Valves

A design equation used for sizing control valves is:

P (9.2)where:q : flow rate (gal/min)Cv : proportionality factor that depends predominantly

on valve size or capacity (unit less).

s

vv g

PlfC q

lf

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: flow characteristic (unit less).P : pressure drop across the valve (psig)gs : specific gravity of the fluidThis relation is valid for nonflashing fluids.

lf

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

Three control valve characteristics are mainly used. For a fixed pressure drop across the valve, the flow

characteristic f (0 f 1) is related to the lift l (0 l 1), that is the extent of valve opening, by one of the following relations:Linear : lf

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Linear :Quick opening :Equal percentage :where R is a valve design parameter that is usually in the range of 20 to 50.

1

lRflf

f


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Control valve characteristics

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

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To Select an Equal Percentage Valve:a) Plot the pump characteristic curve and PS, the ) p p S,

system pressure drop curve without the valve, as shown in Fig. below.

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The difference between these two curves is Pv.The pump should be sized to obtain the desiredp pvalue of . For example, 25 to 33%, at thedesign flow rate qd.

b) Calculate the valves rated Cv, the value thatyields at least 100% of qd with the availablepressure drop at that higher flow rate

sv PP

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pressure drop at that higher flow rate.


c) Compute q as a function of l using Eq. 9.2, therated Cv, and Pv from (a). A plot of the valvev, v ( ) pcharacteristic (q vs. l) should be reasonably linearin the operating region of interest (at least aroundthe design flow rate). If it is not suitably linear,adjust the rated Cv and repeat

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ExampleA pump furnishes a constant head of 40 psi over the p p pentire flow rate range of interest. The heat exchanger pressure drop is 30 psig at 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 linear valve that is half open

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at the design flow rate


Solution

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

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1) We write an expression for the pressure drop across the heat exchanger:g

(9.3)

(9.4)

2

20030

qPhe

2

20030

qPP heS

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2) Because the pump head is constant at 40 psi, the pressure drop available for the valve is:

(9.5)2

200304040

qPP hev


1) Figure below illustrates these relations. Note that in all four design cases at qd.:%333010 sv PPg qdsv

24Edi MunawarPump characteristic and system pressure drop

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3) Calculate the rated Cv :(9.6)5126200C ( )

We will use Cv = 125. For a linear characteristic valve, use the relation between l and q from Eq. 9.2:

5.126105.0

vC

ql

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(9.7)

Using Eq. 9.7 and values of Pv from Eq. 9.5, the installed valve characteristic curve can be plotted:

vPCvl


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Installed valve characteristics

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Next Lecture

Thank you for your attention and Happy New

Year 2015

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Year 2015

instrumentation process 2014.12.31.pdf

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