What's Your Flow Control Valve Telling You

7/29/2019 What's Your Flow Control Valve Telling You

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What's Your Flow Control Valve Telling You?

Whether you can see them or not, the oscillations from control valves are everywhere in your

process. It's just a matter of how large and how important. When the amplitude is less than the

exception reporting setting of controllers, data highways and historians, oscillations do a

disappearing act on operator displays and trends. This does not mean they are insignificantbecause the exception reporting (percent change in a process variable that triggers an update)

is often set by systems support to minimize traffic and data storage requirements, rather than

to show change. Besides, management is much happier when they see straight lines.

However, such bliss is short-lived if the hidden cycling translates to product quality or process

efficiency issues. So what is the real deal with the source, diagnosis, impact and possible

solutions for these oscillations?

Flow Control Troublemakers

The true measure of an automation system is its ability to control change. If there were no load

upsets in feed conditions (flow or composition), equipment performance (fouling and

efficiency), ambient conditions (temperature and humidity), utilities (temperature and pressure),

or changes in set points (due to changes in product demand), we could all retire.

The measurement is the window into the process and the final element is the method of

affecting the process. The measurement should provide a fast, undistorted and reproducible

view of small changes, and the control valve must be able to make small, rapid changes and

not itself be the source of unwanted upsets. In order for a control valve to accomplish its goal,

dead band, stick-slip (Figure 1), and, in some cases, stroking time must be minimized.

Figure 1: Dead Band GraphingThe dead band and stick-slip of a control valve are greatest near the closed position.


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Dead Band Discussion

Dead band occurs only when the valve needs to change the direction of its stroke. It is

measured by how much the signal must change direction to reverse the direction of the stroke.

The official test is done for a full-scale stroke in both directions but dead band occurs for any

stroke whenever the direction is reversed. It is caused by lost motion and is due mostly to

backlash from linkages and actuator shaft and stem connections.

It is worse for rotary valves because of the gaps in rack and pinion gear teeth, the slots in

scotch yoke actuators, the key locks in shaft-stem connections, and the linkage that transfers

vertical actuator shaft motion to disc, plug or rotary ball movement. A dead band of 8% can be

common for such valves even though they are outfitted with digital positioners. More actuator

torque does not solve the problem either. The time it takes for the controller output to work

through the dead band is dead time that increases the errors from load disturbances.

The problem is not seen for setpoint changes or step changes in the controller output that are

much larger than the dead band. Thus, loop analysis or tuning based on large setpoint

changes, open-loop step tests or relay methods, will not reveal the additional dead time. For

pressure control, it can mean the possible rupture of discs or vessels.

If you consider that the peak error and integrated error for load upsets are proportional to the

dead time and dead time squared, respectively, dead band is a hidden menace. There is some

consolation, though. For pure dead band, once the valve does move, it then can respond to

small changes in signal in the same direction and dead band can cause a limit cycle only in a

loop with an integrating response (e.g., level) or a runaway response (an exothermic reactor).

A limit cycle is a sustained oscillation of nearly equal amplitude caused by a nonlinear

response such as dead band.

Stick and slip occurs whenever the valve needs to move, even in the same direction. After it

moves, it cannot move again unless the change in signal is greater than the stick. When it

does move, the valve jumps or slips by an amount that usually is larger than the change in

signal. Stick and slip generally occur together and have the common cause of friction in the

actuator design, stem packing and seating surfaces. Piston actuators, high temperature

packing, and tight shutoff in rotary valves (the so-called high-performance valve) can lead to

the worst cases of stick-slip. It also can initiate shaft windup, where the actuator shaft moves

but the ball, disc or plug does not. It is much worse at positions less than 20% where the ball,

disc or plug starts to seat.

Here's the rub: If there is stick-slip, the controller will never get to setpoint, and there will

always be a limit cycle. The big squeeze from graphite, environmental packing particularly


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when they are tightened and low-leakage classes are the biggest culprits. A bigger actuator

may help, but will not eliminate the problem. An undersized actuator can cause a huge amount

of additional slip. Stick-slip of 20% often occurs at breakaway from the seat of high-

performance valves and for any valve with Graphoil packing and no positioner. Even with a

positioner, a stick-slip of 4% is common with high-friction packing.

Once a valve moves it responds as a velocity-limited exponential. For small changes, the

response looks like two time constants in series. For large changes, the response looks like a

ramp. Stroking time is a consideration for large actuators and compressor surge or pressure

control. The addition of a booster will reduce stroking time, but will introduce a relatively large

dead band (e.g., 2%), particularly when designed for piston actuators. The use of a booster

without a positioner can cause positive feedback and butterfly disc instability from high outlet-

port sensitivity. A booster in series with a positioner must have a bypass around the booster

adjusted to prevent a limit cycle.

For most applications, slip is worse than stick, which is worse than dead band, which is worse

than stroking time, although none of them are desirable.

Uncover Root Causes

Unless otherwise directed, manufacturers and technicians will make large (10%) signal step

changes (much greater than dead band or stick-slip) to their valves at 50% position--far from

the seating friction problems. For these tests, the response time is at a minimum and almost

any valve looks good.

To make the whole situation even more deceptive, smart digital positioners may give the

allusion of good response because they measure actuator shaft position and not actual

balance, disc or plug position. You can develop a lot of misleading statistics, diagnostics and

trends that are blind to dead band and stick-slip.

A good test should use signal changes that are less than 1%. To test to a rotary valve

response in the shop, you need a travel gauge on the ball, disc or plug, since positioner

feedback is prone to giving erroneous information. The most positive test is when the valve isin the pipeline at operating conditions, since high temperatures and pressure drops can make

stick-slip worse. A sensitive low-noise flow measurement shows whether the ball, disc or plug

actually moved.

Table I: Know Your Nomenclature


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A limit cycle in level can be attributed to valve dead band if its amplitude is proportional to the

dead band and is inversely proportional to controller gain, and if its period is proportional to the

controller integral time and inversely proportional to the square root of the controller gain (The

variables are described in Table I):

For stick-slip, the percent amplitude in the controlled variable seen by the controller is

proportional to the product of the slip and open-loop gain, which itself is the product of the

gains for the manipulated variable, process variable and controlled variable:


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The amplitude in the process variable [in engineering units] seen on a trend recording, is

without the controlled variable gain factor that is equal to 100% divided by the calibration span.

The manipulated variable gain is the slope of the installed characteristic of the control valve.

For oversized valves and steep slopes, the gain is much larger and consequently the effect of

slip is much greater. If you also consider that oversized valves tend to ride the seat where the

friction is greatest, you get a double hit. Ideally, the amplitude Ao from slip should be less than

A/D or D/A resolution, which for a 12-bit microprocessor with one sign bit is 0.05%. This is

essentially impossible for oversized valves, small calibration spans, or high process gains.

The stick-slip oscillation period depends upon the open-loop gain and the controller settings:

An integral time set too fast for a given valve stroking time can cause an unstable (growing)

oscillation for large upsets. The integral time must be larger than the product of the controllergain, the full-scale stroking time, and the open-loop error from a load upset. The loop goes

unstable for large load upsets:


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Figure 2: Dead Bands Tell Many TalesIf an integrating process has rounded oscillations in the controlled variable and controller output, buta clipped oscillation in the manipulated flow, it is caused by valve dead band.

If an integrating process has rounded oscillations in the controlled variable and controlleroutput, but a clipped oscillation in the manipulated flow, it is caused by valve dead band.

With a little experience, a user can diagnose a limit cycle caused by a control valve. If an

integrating process has rounded oscillations in the controlled variable and controller output, but

a clipped oscillation in the manipulated flow (Figure 2), it is caused by valve dead band. For

dead time-dominant loops, a square wave in the controlled variable and a saw tooth in the

controller output (Figure 3) are a sure sign of valve stick-slip.

Calculate the Impact

The additional dead time from valve dead band for load upsets can estimated from:

The peak error in the controlled variable for load upsets is proportional to the ratio of the total

loop dead time to the open-loop process time constant:


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To estimate the savings from elimination of excess reagent, reactant or reflux flow, the

amplitude from Equations 1 or 3 or the peak error from Equation 8 is divided by the product of

the gains of the process variable and controlled variable to get back to the change in flow:

For back mixed volumes, the amplitude of the oscillation in the reagent flow should be

multiplied by process gain and an amplitude ratio from frequency response that is proportional

to loop oscillation period and inversely proportional to the mixing time constant to get a filtered

amplitude of oscillations in a key process variable of interest:

The process gains in Equations 9 and 10 are different for different process variables or forprocesses with operating point nonlinearities (e.g., pH). The elimination of the oscillation in theprocess variable can translate to a proportional shift to a more optimum set point or a reductionin scrap or downgraded product.

Figure 3: Evidence in OscillationFor dead time-dominant loops, a square wave in the controlled variable and a saw tooth in the controller outputare a sure sign of valve stick-slip.

Solution in the SelectionControl valves designed for on-off operation generally are a poor choice for a control valve.Conversely, throttling valves should not be used for isolation or interlocks.

A calculation can be made to compensate for backlash by adding a bias to the controller outputwhenever a signal reversal exceeds a noise band. However, the exact dead band is a moving


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target and a bias greater than the actual dead band effectively creates slip. Also, stick-slipgoes hand-in-hand with dead band in high-performance valves.

The control valve that responds best to small changes is a judiciously sized sliding stem(globe) valve with a digital positioner and a correctly sized diaphragm actuator and properlytightened Teflon packing. It has negligible backlash and a stick-slip of 0.1%. If you must use a

rotary valve, avoid tight shutoff and high-friction packing and use a diaphragm actuator with ashort shaft and splined connections between the actuator shaft and the ball, disc or plug stem.Make sure the ball, disc or plug are cast with its stem otherwise the junction between the ball,disc or plug and its stem can become another source of backlash. If high temperature orenvironmental packing must be used, increase the actuator size and positioner gain to help itbetter deal with the packing's increased friction.

Big Problems for Small Valves

Small valves are more prone to improper sizing, irregular flow characteristics, greater

stick-slip and plugging. Here, size does matter because most of these problems originate

from extremely small Reynolds Numbers, clearances, and stem diameters.

For a CV less than 0.01 or extremely viscous fluids, the valve may be operating in the

laminar flow regime or in the region of transition from turbulent to laminar flow. Liquidflow moves from being approximately proportional to the square root of the pressure

drop towards being proportional to the pressure drop as the flow goes from being fully

turbulent to completely laminar. For anything other than a 1-psi pressure drop this can

translate to an enormous sizing error. The result is often an oversized valve that ridesthe seat, where the high seating friction causes excessive stick-slip. Even worse,

operation in the transition region where the flow for a particular valve position has poor

reproducibility, because normally insignificant disturbances such as microscopicchanges in roughness or small vibrations can trigger a switch between turbulent and

laminar flow and an erratic installed flow characteristic.

If you also consider the possibility of a significant distortion of the inherent flow

characteristic caused by machining tolerances that are an appreciable portion of theclearances for such tiny trim sizes, the scene is set up for an unknown and extreme

nonlinearity. Tiny clearances can pose all sorts of problems because small particles and

coatings cause plugging and sticking. The low flow velocities at the surface that isnormally associated with laminar flow makes the likely hood of coating much greater.

Finally, tiny stems are likely to be bent from normal handling both before and after

installation. Slight deflections of the stem can cause huge amounts of stick-slip. What

good is a control valve if you can't drop it or step on it?

Is the situation hopeless? Not if you go with a manufacturer who specializes in tighter

machining tolerances and minimizing stick-slip. I would stay away from stems smaller

than 3/8 inch and I would insist on getting response and flow test results. I would use

computer programs now available that properly deal with laminar flow and offer aninstalled characteristic curve for your piping and operating conditions. Even though a


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smart digital positioner may be larger and cost more than the valve, it is more importantthan ever that it be used and properly tuned for the small actuator volume. The

challenge here is for packaged platforms that are going for low bid not to get cheap

where it hurts.

If there is a tendency for plugging, pulse-width modulation is a solution if there issufficient back-missed volumes to attenuate the pulses. This also can provide a linear

flow characteristic and a flow large enough to be turbulent. The ratio of maximum to

minimum pulse width establishes the rangeability. The maximum pulse width, andhence, cycle time determine the degree of variability that needs to be filtered and the

additional dead time from the pulse off time. Now stroking time can be an issue because

it is desirable to have the minimum pulse be as short as possible.

A variable-speed drive is another possible solution, but the user needs to be aware of a deadband that is artificially introduced into the electronics and the minimum discharge headrequirements to prevent reverse flow for varying static heads.

Interestingly enough, valve specifications do not require that a control valve move. A responserequirement should be added to the control valve specification that details the stick-slip, deadband, and the response time (63) for a small step in the throttle range. Ideally, a ramp at theexpected rate of change of the loop should be used rather than a step, to reveal the hiddendead time from dead band and the saw tooth from stick-slip.

What's Your Flow Control Valve Telling You

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