Control valves tuning

Control valves tuning

Control Valves and Tuning

Table of Contents

Control Valves AND TUNING 1

Control Valves AND TUNING 2

RELATIONSHIP OF MAJOR COMPONENTS 2

Control Valve Bodies 2

Control - Valve Actuators 2

Discussion of Flow Characteristics and Valve Selection 2

QUICK - OPENING 2

LINEAR FLOW 2

EQUAL - PERCENTAGE 3

CRITICAL PRESSURE DROP 3

SIZING BY CALCULATION 3

AERODYNAMIC NOISE PREDICTION 4

LIQUID SERVICE 4

CAVITATION 4

FLASHING 5

TUNING CONTROL LOOPS 6

TUNING CONSTANTS 6

PROPORTIONAL BAND (K) 6

GAIN (K) CALCULATION 6

INTEGRAL or RESET (T1) 6

DERIVATIVE (T2) 6

TUNING 7

ADJUST PROPORTIONAL BAND 7

ADJUST RESET (INTEGRAL) ACTION 7

ADJUST DERIVATIVE ACTION (RATE) 7

FLOW CHARACTERISTICS 8

TUNING CONTROLLERS 9

GENERAL RULES FOR COMMON LOOPS 9

FLOW 9

LEVEL 9

LIQUID PRESSURE 10

GAS PRESSURE 10

TEMPERATURE, VAPOR PRESSURE, AND COMPOSITION 10

CLASSICAL CONTROLLER TUNING METHOD 11

CASCADE AND OTHER INTERACTING CONTROL LOOPS 11

DEFAULT CONTROLLER TUNING PARAMETERS 11

CONTROL LOOP SCAN RATES 12

PID ALGORITHM DEFAULT TUNING CONSTANTS 13

CONTROL VALVES AND TUNING

Selecting the proper control valve for each application involves many factors The valve body design, actuator, style, and plug characteristic are critical items for selection Proper valve sizing

is necessary for accurate, efficient, economical process control

In areas where personnel will be affected, noise prediction and control becomes a significant factor

RELATIONSHIP OF MAJOR COMPONENTS

CONTROL VALVE BODIES

The rate of fluid flow varies as the position of the valve plug is changed by force from the

actuator Therefore, the valve body must permit actuator thrust transmission, resist chemical and physical effects of the process, and provide the appropriate end connections to mate with the adjacent piping; it must do all of this without external leakage Most valve body designs are of the globe style, but other configurations such as ball and butterfly styles are available Final selection depends upon detailed review of the engineering application

CONTROL-VALVE ACTUATORS

Pneumatically operated control-valve actuators are the most popular type in use, but electric, hydraulic, and manual actuators are also widely used The spring and diaphragm pneumatic actuator is commonly specified, due to its dependability and its simplicity of design Pneumatically operated piston actuators provide integral positioner capability and high stem-force output for demanding service conditions

DISCUSSION OF FLOW CHARACTERISTICS AND VALVE SELECTION

The flow characteristic of a control valve is the relationship between the flow rate through the valve and the valve travel as the travel is varied from 0 to 100 percent "Inherent flow

characteristic" refers to the characteristic observed during flow with a constant pressure drop across the valve "Installed flow characteristic" refers to the characteristic obtained in service when the pressure drop varies with flow and other changes in the system

QUICK-OPENING

The quick-opening flow characteristic provides for maximum change in flow rate at low valve travel with a fairly linear relationship Additional increases in valve travel give sharply reduced changes in flow rate When the valve plug nears the wide-open position, the change in flow rate approaches zero In a control valve, the quick-opening valve plug is used primarily for on-off service; however, it is also suitable for many applications where a linear valve plug would

normally be specified

LINEAR FLOW

The linear flow-characteristic curve shows that the flow rate is directly proportional to the valve travel This proportional relationship produces a characteristic with a constant slope so that with constant pressure drop (∆P), the valve gain will be the same at all flows (Valve gain is the ratio of

an incremental change in flow rate to an incremental change in valve plug position Gain is a func-tion of valve size and configurafunc-tion, system operating condifunc-tions and valve plug characteristic.) The linear-valve plug is commonly specified for liquid level control and for certain flow control applications requiring constant gain

EQUAL-PERCENTAGE

In the equal-percentage flow characteristic, equal increments of valve travel produce equal

percentage changes in the existing flow The change in flow rate is always proportional to the flow rate just before the change in position is made for a valve plug, disc, or ball position When the valve plug, disc, or ball is near its seat and the flow is small, the change in flow rate will be small; with a large flow, the change in flow rate will be large Valves with an equal-percentage flow characteristic are generally used for pressure control applications They are also used for other applications where a large percentage of the total system pressure drop is normally absorbed

by the system itself, with only a relatively small percentage by the control valve Valves with an equal-percentage characteristic should also be considered where highly varying pressure drop conditions could be expected

The modified parabolic-flow characteristic curve falls between the linear and the equal-percentage curve

Note: Where detailed process knowledge is lacking, as a rule of thumb, use equal-percentage characteristics at 70 percent opening

CRITICAL PRESSURE DROP

Critical flow limitation is a significant problem when sizing valves for gaseous service Critical flow is a choked flow condition caused by increasing gas velocity at the vena contracta The vena

contracta is the point of minimum cross-sectional area of the flow stream which occurs just

downstream of the actual physical restriction When the velocity at the vena contracta reaches sonic velocity, additional increases in pressure drop, ∆P, (by reducing downstream pressure) produces no increase in flow

SIZING BY CALCULATION

The gas sizing equations can be used to determine the flow of gas or vapor through any style of valve Absolute units of temperature and pressure must be used in the equation When the critical pressure drop ratio, ∆P/P, causes the sine angle to be 90 degrees, the equation will predict the value of the critical flow For service conditions that would result in an angle of greater than 90 degrees, the equation must be limited to 90 degrees, as no further increase in pressure drop will cause an increase in flow; critical flow has been reached

Most commonly, the gas and vapor sizing equations are used to determine the proper valve size for a given set of service conditions The first step is to calculate the required Cg by using the sizing equation The second step is to select a valve from the manufacturer's catalog The valve selected should have a Cg, which equals or exceeds the calculated value The assumed C, value for the Cg calculation must match the C, value for the valve selected from the catalog

Accurate valve sizing for gases requires the use of dual coefficients, Cg and C1 A single

coefficient is not sufficient to describe both the capacity and the recovery characteristics of the valve

The mass flow form of the sizing equation is the most general form and can be used for both ideal and non-ideal vapor applications Applying the equation requires knowledge of one additional condition not included in previous equations, that being the inlet gas density (d)

Other valve configurations, such as ball and butterfly valves, can be sized in a similar manner using the unique C, and Cg values derived by the manufacturers

AERODYNAMIC NOISE PREDICTION

Aerodynamic noise, the most common type of control valve noise, is the result of Reynolds stresses and shear forces that are the results of turbulent flow Noise from turbulent flow is more common in valves handling compressible gases than in those controlling liquids

Noise-prediction techniques outlined below may be used to determine control-valve noise levels Predicted noise levels can then be used to select the necessary degree of noise control for each application

Graphical solution of the following equation provides a very expeditious and accurate technique for predicting ambient noise levels resulting from the flow of compressible fluids through globe valves

LIQUID SERVICE

The procedure used to size control valves for liquid service should consider the possibility of cavitation and flashing since they can limit the capacity and produce physical damage to the valve This method introduces a critical pressure ratio factor, r, which not only broadens the scope of valve-sizing techniques but also increases the sizing accuracy When used in equations, it will help

to determine more accurately the maximum allowable pressure drop for sizing purposes In order

to understand the problems more thoroughly, a brief discussion of the cavitation and flashing processes is presented in the following

CAVITATION

In a control valve, the fluid stream is accelerated as it flows through the restricted area of the orifice, reaching maximum velocity at the vena contracta Simultaneously, as the velocity

increases, an interchange of energy between the velocity and pressure heads forces a reduction in the pressure

If the velocity increases sufficiently, the pressure at the vena contracta will be reduced to the vapor pressure of the liquid At this point, voids or cavities, the first stage in cavitation, appear in the fluid stream Downstream from the vena contracta, the fluid stream undergoes a deceleration process resulting in a reversal of the energy interchange, which raises the pressure above the liquid vapor pressure

The vapor cavities cannot exist at the increased pressure and are forced to collapse or implode These implosions, the final stage in the cavitation process, produce noise, vibration and physical damage In order to avoid cavitation completely, the pressure at the vena contracta must remain above the vapor pressure of the liquid

FLASHING

If the pressure at the vena contracta remains low, the fluid will remain in the vapor state because the downstream pressure is equal to or less than the vapor pressure of the liquid

After the first vapor cavities are formed, the increase in flow rate will no longer be proportional to

an increase in the square root of the body differential pressure When sufficient vapor has been formed, the flow will become completely choked As long as the inlet pressure (P1) remains constant, an increase in pressure drop (∆P) will not cause the flow to increase

The first stages of cavitation and flashing are identical; that is, vapor forms as the vena contracta pressure is reduced to the vapor pressure of the liquid

TUNING CONTROL LOOPS

TUNING CONSTANTS

PROPORTIONAL BAND (K)

• If Proportional Band is 100%, each percent of change at the input to the controller will produce the same percent of change at the controller's output

• If a Proportional Band is less than 100%, each percent change of input signal to the controller will produce a greater percent of change at the controller's output

• If a Proportional Band is larger than 100%, each percent change in input signal to the controller will produce a smaller percent of change at the controller's output

• The Proportional Band that is selected for a particular operating situation determines how much corrective signal the controller can produce for each percent of change in the variable controlled by the controller

• The controller's output signal determines the amount of movement that will be produced

at the control valve

GAIN (K) CALCULATION

Ratio of entire span of measurement to percent span being used as Proportional Band

% of span being used as a proportional band

50% (PB) GAIN = 2 Honeywell uses letter “K" to represent GAIN, therefore K = 2

• Integral action repeats the proportional controllers initial corrective signal until there is no difference between the PV and Setpoint

• Integral ( T1 ) is expressed in "Minutes per Repeat"

DERIVATIVE (T2)

• Changes the output of a controller in proportion to the "RATE" or "SPEED" at which the controlled variable is moving towards or away from the setpoint

• Derivative action is expressed in minutes

• Represents the time that the proportional plus derivative will take to reach a certain level

of output, in advance of the time proportional action alone would produce the same output

i.e: When derivative is applied to a two mode controller ( PI ), to make it a three mode controller ( PID ), it's action consists of decreasing the number of repeats per minute required to drive the error back to setpoint

ADJUST PROPORTIONAL BAND

Always tune proportional band with very little reset action That is, for instance with a speed control loop, always set the reset (integral) adjustment at, say twenty or thirty seconds or more before adjusting the proportional band

Then, adjust the proportional band to a smaller value (higher gain) until cycling or instability begins

EXAMPLE: Start with 40% proportional band (a gain of 2.5); then halve the proportional band

to 20% (a gain of 5); then halve the proportional band to 10% (a gain of 10); etc

When cycling just begins, increase the proportional band by 50 percent That is, from 10% to 15%; from 18% to 24%; etc Cycling should stop The proportional band adjustment should now

be properly set and should be left at this value

ADJUST RESET (INTEGRAL) ACTION

This is done by reducing the time value (in seconds) Say the reset is at twenty seconds Then reduce the reset to ten seconds; then reduce the reset to five seconds; then reduce the reset to two seconds; etc When cycling or instability begins, increase the reset adjustment by 50%

Example: If cycling is observed at two seconds, increase the reset to three seconds If cycling is observed at 8 seconds, increase the reset to 12 seconds, etc The reset action should now be properly adjusted and should be left at this value

ADJUST DERIVATIVE ACTION (RATE)

If a derivative adjustment is felt necessary, adjust the derivative action by beginning at a setting of one second, then two, then three, until improvement is observed and seems to be optimal Normally, derivative action is not needed and does not help the situation

FLOW CHARACTERISTICS

Most lags are in the control Low gain, fast reset, high PB

measurement

Valve is the major dynamic element

Valve characteristic relatively unimportant

Linear, no noise

Valve characteristic unimportant

Dead time possible (especially in

large

Non linear

Measurement dynamics are important

Dead time usually present Low gain, variable reset rate

Sampling systems complicate both measurement and control add dead time Linear valves

TUNING CONTROLLERS

Since there are a very large number of combinations of the two or sometimes three, "knobs" provided for controller tuning, many methods have been developed over the years to aid in their proper adjustment A few require upsetting the process to some extent, often an unacceptable practice in real life These notes are intended to provide a few simple rules to use in tuning

controllers which will minimize upsets and still get the job done

THE CONTROLLER MUST BE ADJUSTED TO BALANCE THE PROCESS

If the process is fast to respond (i.e a flow loop), then the controller must be tuned fast too Fast or slow for a controller refers to integral (or reset)

NOT PROPORTIONAL BAND (or gain)

Do not confuse these actions or grief will be your constant companion during your controller tuning efforts If the process is slow (i.e temperature control of a tray part way up a distillation column), then the controller must be tuned slow TO MATCH THE PROCESS If you do not have

a feel for the process characteristics or cannot find someone to enlighten you, leave controller tuning to someone else who can get the needed information

GENERAL RULES FOR COMMON LOOPS

FLOW

Usually, at least half of the control loops in a plant are flow loops Set integral (I) at 0.1 minutes Adjust the proportional band so that the measurement is not too noisy, usually about 300%

although an occasional poor meter run installation may require as much as 1000% A loop where

a valve positioner has been used will require a proportional band setting two to three times larger than for a loop without a positioner Slow moving or sticky control valves may require 0.2 or 0.3 minutes but are rare exceptions If these settings do not work, inspect the valve and orifice

installation to find the, problem Fix the problem Do not adjust the controller to some ridiculous setting such as a 10 minute reset time Use the controller in manual or a hand valve if you think a

10 minute reset time is necessary

IMPORTANT NOTE: No controller will work when the valve is almost closed or almost wide open Don't attempt tuning under these conditions Have the operator open or close a bypass (if one exists) or wait until process conditions change enough to get the valve back within its

operating range (from 5 to 95% of travel as extreme limits with 10 to 90% as a safer range) Never use derivative action in a flow loop

LEVEL

The next most common loop after flow is level DO NOT EVER USE A SHORT INTEGRAL VALUE IN A LEVEL LOOP If you do, you will find the loop will always cycle, often with a period (time from the peak of one cycle to the peak of the next) of 10 to 15 minutes The shorter the integral time, the longer the period Set the integral at 10 minutes This will satisfy 80 to 90%

of the level applications in a plant, if the vessel time constant (volume/flow) is 1 to 2 minutes, then

a shorter integral time can be used but remember that a large value is safer If the vessel is large and the controlling flow is a trickle, then a greater value of integral must be used

(20-50%) without causing cycling Use a larger proportional band (perhaps 100%) if smooth flow control to a downstream unit is more important than tight level control Never use derivative action in a level loop

Level loops will usually show a limit cycle when the level controller sets a valve, which is not equipped with a positioner A limit cycle looks like a saw blade, sometimes with flat bottoms and/or tops

Limit cycle will show about 5% change There is absolutely nothing you can do to tune out such a limit cycle Changes in tuning will shorten or lengthen the period but only a positioner or level cascaded to a flow controller will eliminate the problem When the flow is used to control the level going to tankage, cycling is usually unimportant If it is the reflux or feed to a distillation tower, then such a limit cycle may be unacceptable Please note that a valve cycling almost closed

or fully open will also produce a limit cycle, usually of the flat bottom type (when almost closed)

or of the flat top type when almost fully open

LIQUID PRESSURE

Tune the same, as flow loops Noise should not be as severe as for flow and proportional bands will usually be smaller

GAS PRESSURE

Tune the same as level loops using a large integral value Proportional bands can be quite small (under 100% and often as small as 20-30%.)

Well now that you've tuned over 90% of the loops in the typical plant, on to the more difficult control tuning applications These are temperature, vapor pressure, and composition Included are the temperatures used to infer composition for so many distillation columns

TEMPERATURE, VAPOR PRESSURE, AND COMPOSITION

There are several ways to tune these more difficult loops The first is to use starting settings of 100% proportional band, a 5 or 10 minute integral time, and no derivative Switch the controller

to automatic when the measurement is close to the desired set point If a cycle develops, measure the time from peak to peak (high to high or low to low) This is the period of the control loop Divide by two If the starting integral value is less than one half of the period, the integral time is too short and is causing the cycle Increase the integral time If each peak is higher than the one before, increase the proportional band (double, triple etc.) until the cycles damp out The period will get shorter as the integral time is increased When the period is about twice the integral time and the cycles are dampening out, you're pretty well finished If the measurement is not noisy, set the derivative at one quarter of the integral time Readjust the proportional band if required to get

a damped oscillation after an upset (wait for a bump or ask the operator to make a small set point change in a safe direction)

If the shortcut method described above is unsuccessful or you want to be a bit more methodical, follow the procedure given below It will always work and will leave no doubt as to the

characteristics of the control loop