Michael Brown Control Engineering CC 

Practical Process Control Training & Loop Optimisation

 CONTROL LOOP CASE HISTORY 159

 

BAD VALVES AGAIN HAMPERING CONTROL
 

It still fascinates me that so many control problems are caused by poor operation of control valves.  I have encountered and recorded hundreds of cases of such problems.  What is amazing is that many plant people still don’t realise that these control problems are valve related and in most cases waste a lot of time trying to resolve the problems by playing with the tuning knobs.

This article shows a couple more examples of controls being completely hampered by the valve performance

The first example is of a very important flow control in a refinery.  The operators were complaining that the loop was not controlling and they even had trouble trying to control it in manual.  They wished to control the flow between 25% and 35% of the measuring range.  They reported that they had no real problem existed with the control once the SP was above 32%, but they found they could not get any control below that point.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure1.

 

The first test one always does is to test the loop in automatic without changing any of the tuning parameters or other controller settings.  This test is shown in Figure 1.  It can be seen that the control initially looked fine and followed a SP step change upwards and back again reasonably well.  Then after a while the flow started spiking downwards.  This was intermittent until the SP was stepped down a few percent and then the spiking became continuous.

The reasons for this can be seen by looking at the values of the PV and the PD (controller output) at the point where the spiking starts.  The PV is at approximately 31% and the PD is at 2%.  If the valve/positioner combination had been calibrated correctly then it means that the valve is just about shut.  Now control valves should never be operated in a control region with their position close to seat.  For a variety of reasons valve manufacturers are not able to get valves to operate successfully at very low openings.  An old lore of control suggests that for reliable control you should always operate valves above 20% of opening.

Two of the main problems with valves operating close to seat are firstly that from the linearity aspect it is very difficult to get the valve to follow their design characteristics when the valve is really near the seat.  Secondly and probably in this case even more important, many valves when installed in the line have increasing differential pressure as the valve moves towards the closed position.  If the actuator is not powerful enough to deal with the differential pressure near seat, cycling commonly occurs, which is probably what we are seeing here, and would support the proposal that the valve is in fact calibrated correctly.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2.

 

The next thing to consider is why the flow is at 31% when the valve is at 2%.  This may be better answered by looking at Figure 2 which is an open loop test, which is performed by placing the controller into manual, and removing any filters (damping), if there are any, in both the transmitter and control system, and then making a series of step changes.

The first thing that one should consider is if the transmitter calibration was correct.  (Unfortunately in the plant where these tests were being carried out, for several reasons it is not possible to have an instrument mechanician standing by to answer these questions quickly, and our reports are passed to their department for them to sort out the problems as soon as they can manage to do so.)  It would however appear that there is something definitely wrong with the valve, as the flow does drop to zero when the PD drops below 2%, so it doesn’t appear that the valve is passing.  However the instant the PD rose over 2% the flow jumps straight to around 31%.  This is very strange and our recommendation was that the valve should be checked thoroughly, as there may probably be a problem in the positioner or actuator.

The next thing seen from this open loop test is that there appears to be non-linear installed characteristics as the PV steps increased in size as the PD moved up in steps of equal magnitude.  Unfortunately we were not allowed to move the flow up past 50% to examine this in more detail, which would also have given us an idea if the valve was sized correctly, and also allow confirmation of the non-linearity.  It would appear likely however that the valve is very oversized.

In any event the tests have confirmed the problem and no proper control can be achieved until the problems with the valve and possibly measuring system.
 
The second example of a poor valve is of a flow loop that was a cascade secondary to a level control.  The level had to be tuned “tightly” i.e. to try and keep it on SP as closely as possible under all conditions.  The operators had noticed that the flow control was not working well, and that they would welcome improvement in its performance.  They said the level control itself was still reasonable

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure3.

 

Figure 3 is the “as found” closed loop test which is similar to the first test described in the previous example.  The test showed the PV did follow SP albeit quite slowly.
 However several significant facts can be concluded from the test.

Firstly the PD (controller output) moved significantly more than the resultant moves in the PV.  There could be two reasons for this.  The one could be that the process gain of the loop was very small which is usually caused by a transmitter with too wide a span.  The other reason could be that the valve is extremely sticky.  In view of the fact that the PV readings were above 60%, it is more likely that the valve is sticky.

Secondly the response of the PV to step changes in SP were quite slow which is probably due to poor tuning.

Thirdly one can see that when the PV over or undershot the SP it took quite a while for the PD to be able to correct the error.  The very last step in the test illustrates this extremely well.  The PV moved slightly above the SP.  The PD then started ramping down to bring the PV back again, and actually moved several percent before any effect was seen, which was the PV literally slipping down  very quickly under SP.  This is a definite sign of a valve with hysteresis and/or stickiness.

These observations were definitely confirmed in the next test which was an open loop test, as shown in Figure 4.  It can be seen here very nicely how the valve was very sticky at times, but seemed to move better when closing.  However when stepped in the opening direction, it responded terribly slowly.  The valve is definitely extremely sticky.  (No real evidence of hysteresis was seen in this test).

Once again the valve needs a lot of work before really good control can be obtained.  However it should be noted that if the level did not have the secondary flow loop and was going directly to the valve, it is very unlikely that any proper level control would have been achieved.  Even with such a bad valve, cascade secondary flow control helps to overcome bad valve problems unbelievably well.
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure4.

 


These observations were definitely confirmed in the next test which was an open loop test, as shown in Figure 4.  It can be seen here very nicely how the valve was very sticky at times, but seemed to move better when closing.  However when stepped in the opening direction, it responded terribly slowly.  The valve is definitely extremely sticky.  (No real evidence of hysteresis was seen in this test).

Once again the valve needs a lot of work before really good control can be obtained.  However it should be noted that if the level did not have the secondary flow loop and was going directly to the valve, it is very unlikely that any proper level control would have been achieved.  Even with such a bad valve, cascade secondary flow control helps to overcome bad valve problems unbelievably well.