Control
Loop Case History 111
SURGE
LEVEL CONTROL
Recently a delegate on one of my
courses who works in control on mining mineral processing plants
related to me that he was doing some work on a mill sump level
control and had found that the input error signal to the
controller was the cube of the actual error signal, (SP-PV)3.
This resulted in instability and he couldn’t understand why
the system designer had done this.
I am pretty sure that the reason this had been done was to
attempt to provide a form of “surge” level control, but the
way it had been done was incorrect. This article will
investigate surge level control.
Level controls are special cases where one has to be extremely
clear as to the purpose of the control. The reason for this is
that in the majority of cases of level control, the vessels have
relatively long retention times . In other words the volume of
the vessel is very large compared with the volume of liquid
passing through the control valve, which may be on the input, or
on the output of the vessel, as the case may be. This results in
relatively tiny process gain. (Refer Loop Signature P1-3,
available on CD for persons outside Southern Africa.)
If one is truly wishing to keep the level as constant as
possible (known as “tight level control”), then it becomes
mandatory to use high controller proportional gains, so that the
valve moves very quickly in response to changes in load or
setpoint. It is not uncommon to see gains as high as 10 – 20
in such cases. This means that if a change in error occurs, one
can get very big changes on the output of the controller which
in turn results in big changes in the flow through the valve. An
example of this is given in Figure 1 where the controller has a
gain of 8. A 12% step change in setpoint results in a 96% change
in the controller output.
Fig. 1
In the previous Case History article 110 published 2 months ago,
it was shown that a controller actually transfers the variance
from the one side to the other side of the loop. The higher the
proportional gain in the controller the more “reactive” the
output of the controller to changes in error on the input of the
controller, and the more the valve will move around to correct
theses changes.
This may be highly undesirable in certain cases, and many of
these can occur in many level control situations, which
generally are when the actual level is of secondary importance
to keeping the control element from changing too quickly in
order to keep the flow out of the vessel as constant as
possible.
This may sound strange initially. Why put in a level control at
all? Why not rather use a flow control? The reason is generally
because of two other important and overriding considerations:
a.The tank should never overflow; for obvious reasons.
b.The tank should never run empty as that may damage the pump.
A lot of vessels including many sumps used in mining mineral
processing are in fact used as “buffer” or storage vessels
to ensure that processes downstream can be fed with the liquid
from the vessels at a constant rate. In other words, the vessels
are there to absorb surges occurring in the upstream processes.
The actual level in the tank is not of primary importance as
long as the two criteria above are satisfied. The main aim under
normal operating conditions is to keep the output flow out of
the tank as steady as possible, and if it does change, then the
changes should be as slow as possible, so that controls
operating downstream can keep up with the changes in flow and do
their job in a satisfactory manner.
A typical example of this is a mill sump – refer Figure 2.
Although one tries to keep the feed into and out of the mill
constant, the sump is there to average out any dips or surges on
the output feed, so that a really nice constant feed can be
supplied to the downstream processes.
Fig. 2
To try and achieve all three of the requirements mentioned
above, many of the manufacturers have come up with a variety of
relatively fancy controllers. Some of them like notch gain
controllers, and error squared on gain controllers are standard
blocks in many of the DCS's. However as normal, the
manufacturers usually give little information on their use, and
I have found very few people know what they are for, or how to
use them. I am typically asked how one tunes such a controller.
(Some of the more ignorant manufacturers offer error squared on
both gain and integral controllers, which should never be used
on level applications, as they will very likely result in
instability.)
In my opinion, one should generally not use a controller at all
for this application. (The one exception may be in cases where
integral is needed to bring the level slowly back to a
particular level). In fact, I believe controllers used in
this application, in reality create a great deal of confusion
amongst plant operating personnel. The reason for this is that
you do not actually want to control the level at a particular
setpoint. So if you do set the controller up properly, the
chances are that the process will never be at setpoint, and this
worries most operators. They generally believe that a controller
is only working correctly if the process is on setpoint.
So what happens in most plants I have been in is that they
complain to the Instrumentation and Control department, and an
instrument technician or mechanician goes out and retunes the
controller to operate quickly, thus destroying the desired
control strategy. (As mentioned in previous articles I have
found that the average instrument technician and mechanician do
not themselves really understand the process and the control
requirements).
What I would suggest is that the controller be dispensed with
entirely, and that the operator be provided with a level
indicator only. If it is absolutely essential that he
should be able to control the level valve in manual if the need
arises, then an auto/manual "hand station" can also be
provided.
Say we specify that the tank level must never exceed 90% and
never drop below 10%. Then to achieve the control requirements
one connects the valve to the transmitter via a curve-shaping
block (like a polynomial or a "look-up" table). Refer
to Figure 3. This sets the valve to a desired position according
to the level in the sump.
Fig. 3
The simplest method
of programming the block is by joining the 2 end points by a
straight line. This is shown in Figure 4.
Fig. 4
Basically it
works just like a ball valve that operates between 10% and 90%
of the tank level. Thus if the level rises the valve starts
opening until it is fully open at 90%, and visa versa for
falling level.
This works well, but an even better way of doing it is to design
the shape so the valve moves very little at all over most of the
level range, and only really moves when the level is approaching
the high or low limit. The most effective means of achieving
this is to use an “error cubed” curve which is illustrated
in Figure 5. Using this method allows very slow and very little
movement of the valve over most of the level range, but if the
level does reach towards the bottom or top limits, the valve
starts moving smoothly, but relatively slowly to prevent the
limits being exceeded. (To calculate the graph, one takes the
cube of the deviation from 50% level.)
Referring to the question posed by the delegate on the course in
the first paragraph above, I strongly suspect this was what the
control strategy was supposed to achieve. However for reasons
that are outside the scope of this article, it cannot be
achieved by cubing the error signal and applying it to a P+I
controller, as instability will result. Also as mentioned above,
the use of a controller is not a good idea.
Fig. 5
Some interesting feedback came from another delegate who was on
one of my courses a year ago and was subsequently appointed the
senior instrument engineer on a mine in Zambia. The following is
taken from an email he sent me recently:
I could not get any stability on the floatation circuits because
of all the surges, and the recycles.
The biggest upsets were caused by the VSD transfer pumps pumping
into the cells. There was great variability in pump speed and
the flow/speed non-linearity did not help. I put an error
squared algorithm for sump surge control. This worked like a
dream. The sump took in the surges and the speed stayed fairly
constant. It was very easy to setup, just decrease the gain,
until required pump speed variability was reduced enough.
This made a quantum jump managing the level in the float cells.
From something that would cycle between max and min to a level
hovering around setpoint.
This feedback really made my day!
Index to articles
Michael Brown is a specialist in control loop optimisation, with many years of experience in process control instrumentation. His
main activities are consulting, and teaching practical control loop analysis and optimisation. He gives training courses which can be
held in clients' plants, where students can have the added benefit of practising on live loops. His work takes him to plants all over South Africa, and also to other countries. He can be contacted at:
Tel (011) 486-0567
Fax (011) 646-2385
E-Mail: michael.brown@mweb.co.za
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