Deaerator Level Control

Q: My question is this: Can we replace a P controller with a linear function generator in

controlling the level on a deaerator?

General information about the control loop(s):

1. The issue is related to a deaerator (DA) level control. There are currently two loops

controlling the DA level, i.e. the make-up control loop and valve and the surplus control

loop and valve. The setpoints of the loops are set at 142 mm and 213 mm, respectively.

The total DA level range is 0 mm to 355 mm. Make-up water is supplied indirectly to the

DA, i.e. it will go first through the condenser and condenser level loop. The surplus goes

back to the condenser. 

2. The loop design specification states that there should be a linear relation between the

level control error and the corresponding valve's position. The position of the make- up

valve should be between 0% to 100% for a corresponding level error range of 0 mm to

142 mm; it should start controlling when level falls below 142 mm; and fully open when

level reaches 0 mm. The position of the surplus valve should be 0% to 100% for a

corresponding level range of 213 mm to 302 mm; that is, it should start controlling when

level increases above 213mm, and fully open when level reaches 302 mm. 

3. Between 142 mm and 213 mm both valves should be closed. 

4. Due to above requirements, P controllers have been used for both make-up and surplus

loops, with gains of 2.5 for the make-up and 4.0 for the surplus. Both loops are

implemented in digital control system (DCS) controllers.

ADVERTISEMENT

Unfettered Blog Control Systems Cybersecurity Expert, Joseph M. Weiss, is an international authority on cybersecurity, control systems and system security. Weiss weighs in on cybersecurity, science and technology, security emerging threats and more.

The operational issues: 

The issue, according to the operator, seems to be related to the used P type controllers

in both loops.

The operator has the view that the control loops very often seem to ignore the SP, and

seem sometimes to operate the valves in a direction opposite to the expected. For

example when the level is <142 mm, instead of opening the make-up valve, the loop will

close it.

The above often happens when the loop has tripped to manual, and operators change it

back to auto.

As a solution, some more experienced colleagues have proposed replacing the P

controllers with a linear function generator, and ensure the required logic to achieve

manual-auto bumpless transfer is in place. That solution is expected to always produce

the desired relationship between level control error and the valves' positions, and also

ensures both valves are closed between 142 mm and 213 mm.

I have concluded that the behavior seen by the operator is "normal" for P controllers. I

am not sure if replacing the P controllers with the linear function generators is correct, but

I can't figure out the reason or what consequences will appear (especially bad ones) in

operating the loops if we implement this change. 

Do you agree with the proposed change? What control issues could that change generate? Any

suggestions for any changes in the loop would be very welcome.

Guido Villacis 

guido.villacis@autograf.pl

A: Gap level control can be provided by using two controllers or by splitting the output of a single

controller among two valves with a gap in the center of the control signal where both valves are

closed.

Deaerator gap levelFigure 1: Deaerator gap level control using two proportional controllers. The setpoints are in the center of the controller throttling ranges, and the deaerator level follows the operating lines shown as a function of load. To obtain these operating lines, the setpoints have to be 71 mm for LIC-1 and 258 mm for LIC-2

In your deaerator level control system, the make-up valve must have an open-failure position, which means that the controller has to be reverse-acting, and therefore its gain has to be negative (Figure 1). If it's not, that explains your problems. When a proportional-only (P) controller is started up, the bias is normally adjusted to 50%, so that when the level is on the setpoint, the valve will be 50% open, and this opening will provide the "normal" flow. If the load (the required water flow) moves away from the 50% setpoint, the proportional controller is incapable of changing the flow to return the level to setpoint because it needs an error (e) in order to change its output signal (m) to the required new opening. This output is m = (controller gain)(e) + b, so that the output "m" equals the 50% bias (b), when the error is zero.

Therefore, whenever the required valve opening (the load) changes, an error must develop, and

this error is called the "offset." For example, if the load "m" has changed to 60%, while the

normal load (b) was 50%, the offset error is Offset Error = (60% - 50%)/(controller gain).

Consequently, if the controller gain is high, (the slope of the operating curve in Figure 1 is steep),

and proportional control is acceptable because the level will not change much while the valve is

throttled (the offset is small).

Therefore, as shown in Figure 1, the two-controller system will "bump" the valve by the amount of

the offset when the loop is switched between manual and automatic, but this does no harm to the

level control, and the operator should simply be educated to accept it.

Another option is to provide logic that guarantees bumpless transfer, but it will not improve the

quality of level control, only eliminate the "offset bump."

The correct system parameters are listed in Table 1.

Table 1: Gap level control using two proportional controllers.

Béla Lipták

liptakbela@aol.com

A: I'm guessing that the current implementation does not use true P controllers. Digital systems

based on incremental (velocity) algorithms have a floating output bias that changes whenever the

controller is placed in manual or hits a limit. True proportional control is what you need, and you

can get it with simple linear functions. The only caveat is that you can't transfer bumplessly from

manual to auto. But it will control level. Teach the operators to accept the variable offset.

A: It sounds to me like the P controllers rebalance their bias term when the controller is in

manual mode. Some control systems allow the option of disabling the bumpless rebalance. (I do

not know what is meant by "tripping" to manual, or what would cause that. A boiler trip to no fire?

The deaerator level must always be controlled, no? A leaking valve could fill or drain it.)

If the disable option does not exist, by all means use linear function generators (characterizers)

to provide the required control action.

I'd build in some dead band, so that the valve does not creep off its seat near zero. Let it get a

few percent away from zero output, and then pop it open to the required percent. Do this by

putting a kink in the function. This is not usually possible in a P controller.

The function generator will not provide any way to rebalance during manual mode. Even if it did,

the rebalance is not desired because it would bias the function, as it does now with the P

controllers. That is, you do not want bumpless transfer to a calculated value. The operator should

adjust the valve to the calculated value before resuming automatic operation. Actually, this will

work with the P controllers, if the bias value is displayed, so that the operator can manually

adjust the output to make the bias value go to zero.

Foundation fieldbus control blocks use a timed ramp to bring the manually set output back to the

calculated value on transition to auto. This provides a bumpless transfer that uses the ramp to

bring the valve to the calculated value without adding a permanent (until the next transition to

manual) bias. Perhaps blocks are available to do this in the control system that is being used.

If the number of function blocks is not constrained, another way to solve this is to build two

function generators, and connect a PI valve position controller between the function output and

the valve. If the operator needs manual control of the valve, the PI will rebalance to zero error at

the integral rate when automatic control is restored.

Bill Hawkins

bill@iaxs.net

A: I believe the problem is related to bumpless transfer.

Some controllers have a feature called "setpoint tracking" (some manufacturers call this "PV

tracking"). This means that the setpoint does not remain constant always, but when the controller

is in manual, the SP is forced to whatever the current PV is. So when the operator changes back

to auto, the SP and PV are already equal, hence there is no "error," therefore, no change in valve

position. This is one way of achieving bumpless transfer, but it is not the only way.

I assume—and hope—that you do not have setpoint tracking on your system. If the operator sets

a setpoint of 40% (displayed as 142 mm), then that setpoint stays there. If this is the case,

bumpless transfer must be achieved in some other way.

A proportional control algorithm first determines the error (SP - PV, for a reverse acting

controller-makeup valve) or (PV - SP, for a direct-acting valve-surplus loop), where both PV and

SP are in % of measurement value. (That is, 142 mm = 40% of 355 mm, 213 mm = 60% of 355

mm. Then the controller output is computed as (controller gain x error + an added value which I

will call "bias"). This "bias" is an operator-adjustable setting on some controllers; on others it is

not. In essence, the bias value is the signal sent to the valve when the controller is in auto and

the SP and PV are equal.

The manual-auto switch comes after the normal control algorithm, so in what follows, I will talk

about two different outputs:

1. The controller output, which is the signal sent to the valve, whether the controller is in

Auto or Manual

2. The control algorithm output, which is the computed output that is used if the controller is

in Auto.

Consider the make-up controller. You want the controller output to be 0% (valve closed) when

the PV is at 40% (142 mm). At that point, there is 0% error, so the bias value should be 0%, and

it should never be changed; that is, you do not want bumpless transfer.

Consider this. Suppose the level is at 71 mm (20%). The error is 20%. (SP is at 142 mm, or 40%.

SP - PV = 40% - 20% = 20%.) If the controller is in Auto, the controller output would be Error x

Gain + Bias = 20% x 2.5 + 0% = 50%. But suppose the controller is in manual and the operator

has set some other value for valve position, say 55%. In order to have bumpless transfer, the

controller has to change the bias to 5%, so that the control algorithm output matches his manual

setting. (Error x Gain + Bias = 20% x 2.5 + 5% = 55%). Now, when PV = 40% (the SP) the valve

will be 5% open, not 0 % open, as you desire.

If you did not have bumpless transfer, in the situation above (the PV is at 71 mm, the controller is

in manual, and the operator-set valve position is 55%), when the operator goes to auto, the valve

position will go to 50%. There will be a bump, but the operator will know what the controller is

doing at all times.

Going to a linear function generator will not help. A proportional controller without bumpless

transfer is already a linear function generator. Going to a linear function generator, then

implementing the logic for bumpless transfer will get you right back to the place you are now.

I would seriously consider why the system ever trips to manual. What has gone wrong to cause

that. Could there be additional controls that would mitigate these circumstances? I'm thinking of

something like a feed-forward action here. I think there should be an in-depth analysis of the

entire system, plus some operator training. I have seen cases where operator understanding of

the system went a long ways toward solving problems.

Harold Wade

Hlwade@aol.com

A: The control scheme that you describe is a bit different than what we would typically implement

for deaerator level control. In our implementation we would use a single level controller whose

output would drive the two valves in a split-range fashion. In some applications, we would also

add a feed-forward to the controller based on deaerator outlet flow. If the specification for your

control scheme is as you describe, however, there is no reason why you could not use linear

function generators in place of the proportional-only controller if you provide the manual-auto

transfer related functions with additional logic.