Control valves tuning
CONTROL VALVES AND TUNING Page 1 of 13 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 Berry’s Commissioning Handbook CONTROL VALVES AND TUNING Page 2 of 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 Berry’s Commissioning Handbook CONTROL VALVES AND TUNING Page 3 of 13 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 configuration, system operating conditions 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. Berry’s Commissioning Handbook CONTROL VALVES AND TUNING Page 4 of 13 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. Berry’s Commissioning Handbook CONTROL VALVES AND TUNING Page 5 of 13 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 Berry’s Commissioning Handbook CONTROL VALVES AND TUNING Page 6 of 13 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. GAIN = 100% (the entire span of measurement) % of span being used as a proportional band Assume 50% proportional band, PB = PROPORTIONAL BAND GAIN = 100% (span) 50% (PB) GAIN = 2 Honeywell uses letter “K" to represent GAIN, therefore K = 2. INTEGRAL OR RESET (T1) • 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. Berry’s Commissioning Handbook CONTROL VALVES AND TUNING Page 7 of 13 TUNING 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. Berry’s Commissioning Handbook CONTROL VALVES AND TUNING Page 8 of 13 FLOW CHARACTERISTICS Very fast. P + I Controllers. Most lags are in the control Low gain, fast reset, high PB system. Derivative hurts. FLOW Non Linear (square) Linear valves for differential pressure measurement common. measurement. Noisy. Equal percentage valves for linear measurement. Valve is the major dynamic element. Fast. P + I controllers. PRESSURE Most lags are in the control Gain near 1, fast reset rate. (Liquid) system. Derivative of no value. Linear. Noisy Linear valve. Single capacity. Self acting or high gain proportional No dead time. controllers. PRESSURE Linear, no noise. Reset seldom necessary. (Gas) Simple process. Derivative unnecessary. Valve characteristic relatively unimportant Dynamics vary. Three response controllers. PRESSURE Dead time possible. Settings vary. (Vapor) Slow compared to other pressure processes. valves. Equal percentage Linear, no noise. Single capacity Precise control: No dead time. High gain controllers. LEVEL Linear. Averaging control: Infrequent noise. Low gain, specialized controllers. Valve characteristic unimportant. Multiple capacity system. Three response controllers. Dead time possible (especially in heat exchangers). Settings vary but gain usually above 1. TEMPERATURE Derivative of limited value if dead time is large. Non linear. No Noise. Equal percentage valves. Measurement dynamics are important. Dynamics vary. P + I Controller. Dead time usually present. Low gain, variable reset rate. Usually linear. Derivative sometimes useful. COMPOSITION Sometimes noisy due to poor On line analyzers fast. often noisy, pH mixing. nonlinear. Sampling systems complicate both measurement and control. add dead time. Linear valves. Berry’s Commissioning Handbook CONTROL VALVES AND TUNING Page 9 of 13 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. Berry’s Commissioning Handbook CONTROL VALVES AND TUNING Page 10 of 13 If close control of level is important, set the proportional band to as small a value as possible (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. Berry’s Commissioning Handbook [...].. .CONTROL VALVES AND TUNING Page 11 of 13 CLASSICAL CONTROLLER TUNING METHOD When the process is reasonably stable and no plant upsets are expected, switch the controller to manual Then set D, (derivative or rate on some controllers) to minimum (if provided on the controller) and I, (integral or reset on some controllers) to maximum Select a set point equal... start-up values only, and each controller will still require additional tuning This tuning may occur several times on individual controllers, depending on plant start-up conditions In fact, it can be some time (the plant has to stabilize) before all controllers have their Berry’s Commissioning Handbook CONTROL VALVES AND TUNING Page 12 of 13 final (normal operations) tunings There are no values shown... calibration offsets in the field device Master controller output in cascade loops shall be 0% to 100% CONTROL LOOP SCAN RATES The control loops shall be configured to achieve the functionality and philosophy of the P&IDs Following are the basic types of control loops: - Analog Indication Only Loop; - Single Control Loop; - Cascade Control Loop; - Complex Control Loop; - Discrete 1/0 Loops within APM... 0% and open as 100% When a control valve is tripped on abnormal condition (Low-low level, etc.), the PID controller shall be configured to switch to manual output mode and the controller output to the fail-safe condition value In case of sensing element failure, a "Bad PV" alarm will be generated and if it is a control point, controller shall switch to manual output mode Controller output to field... Handbook CONTROL VALVES AND TUNING Page 13 of 13 The controllers (APM) base scan rate will be 0.5 seconds Some fast loops (according to EPC contractor) will run at 0.25 seconds PID ALGORITHM DEFAULT TUNING CONSTANTS The PID algorithms will be configured with the following default values unless otherwise specified by the EPC contractor It is understand that these are initial values, final loop tuning. .. should be added in the final tunings of the applicable controllers The following is a list of typical start-up tunings: PROCESS GAIN K PB REPEATS/MIN 5 200 12 PRESSURE (Liquid) 1 100 1 1 Min PRESSURE (Gas) 2 50 5 2 Min LEVEL 1 100 2 5 Min TEMPERATURE 1.3 75 3 3.5 Min ANALYZERS 75 2 5 Min FLOW 1.3 MINS/REPEAT T1 083 or 5 Sec CONTROLLER INPUT/OUTPUT INDICATION Output to valves viewed by the operator... temperature controller To test for interaction when two -loops cycle together at the same period, place one loop in manual If the cycle stops, interaction is probably the problem Rearrange the loops or use the technique outlined above to minimize cycling DEFAULT CONTROLLER TUNING PARAMETERS For the start-up of any plant, there are default tuning parameters that can be entered into each controller These... small D These instructions are for controllers adjusted in terms of minutes per repeat Some manufacturers use an inverse relationship so large becomes small and vice versa If the measurement is noisy (Ph loops in particular), derivative cannot usually be used; never under any circumstances set the derivative greater than the integral CASCADE AND OTHER INTERACTING CONTROL LOOPS Tune the secondary loop... loop first using the local set point mode Reduce the integral as much as possible Transfer to remote set point and tune the primary loop Never use a primary controller integral value less than four times the integral value used for the secondary controller The same rules hold true for interacting loops such as pressure and pressure compensated temperature used for a distillation tower Tune the pressure... or reset on some controllers) to maximum Select a set point equal to the measurement and adjust the proportional band to 100% (or gain at 1.0) to start Change the output a small amount and transfer the controller to automatic Note the starting valve position If oscillations do not develop, repeat step 2 reducing the proportional band, perhaps to half the value tried before Continue to reduce the proportional . CONTROL VALVES AND TUNING Page 1 of 13 Control Valves and Tuning Table of Contents Control Valves AND TUNING 1 Control Valves AND TUNING 2 RELATIONSHIP. DEFAULT TUNING CONSTANTS 13 Berry’s Commissioning Handbook CONTROL VALVES AND TUNING Page 2 of 13 CONTROL VALVES AND TUNING Selecting the proper control