Michael Brown Control Engineering CC 

Practical Process Control Training & Loop Optimisation

CONTROL LOOP CASE HISTORY 190

 

MEASUREMENT PROBLEM RUINS LEVEL CONTROL

 

 

As mentioned many times in my articles, the widely held belief in many plants that tuning will solve all base layer control problems is completely fallacious.  In general the main reasons for most loops that are not performing well in automatic, or even not performing properly at all, are generally not bad tuning, but are usually due to some sort of problem or problems that can occur in the various elements of the loop, or in the design or the control strategy.  The elements of the control loop consist of:

1. The process itself.

2. The transmitter.

3. The controller.

4. The final control element; which in most cases consists of a valve, an actuator, a positioner and an I/P (current to pneumatic) converter.

 

In general about 80% of all problems are due to faults in the final control element.  However all the other elements in the loop can have problems, so it is important when performing optimisation that all these various elements as well as the control strategy should be investigated, before even thinking of tuning.

The example given in this article is of a very important level control in a distillation column in a large petro-chemical refinery.  The control requirement was that the level should remain as constant as possible at setpoint in the face of frequent load disturbances.  A large number of downstream processes were adversely affected if the level varied.  The operators reported that the control was not performing properly and large variances in level were being experienced when the control was in automatic.  The control seemed to work at differing speeds, sometimes responding very quickly with cycling, and sometimes extremely slowly. They had resorted to trying to control it in manual with little success.
 
The plant’s C&I personnel had been called on and had spent a long and fruitless period trying to retune the control, with no improvement at all.

Level controls normally fall into the class of processes known as integrating or ramping, as opposed to the other class of processes known as self-regulating or self balancing, which is typified by flow control, where when you make a step change on the PD (process demand i.e. controller output) in manual the PV (process variable) changes and then settles out again.  The PV basically follows the movement of the PD.

 

Figure 1.

Figure 1.

 

An integrating process is a process where one has to balance the input and output of the process to keep the PV constant.  This is best illustrated in Figure 1 where we can see a level control on a tank.  If the controller is in manual, the level will only be constant if the input flow equals the output flow.  This is known as a “balanced process” and the PD value at this balance is called the “balance point”.  If the operator was the to make a step increase on the PD, the output flow is now bigger than the input flow and the level will start moving down in a perfect ramp.  If no further action is taken by the operator, the tank will eventually run empty.  However if the operator restores the PD to the balance point value, the in and out flows will again be equal and the level will remain constant, albeit at a lower level in the tank.

 

 

Figure 2.

Figure 2.

 

If a step change was then made on the PD in the opposite direction to close the valve a bit, the inflow would now be greater than the outflow and the level would start ramping upwards, and this would continue until the PD was restored to the balance point.  These movements are shown in Figure 2.

 

 

Figure 3.

Figure 3.

 

In the example of the level control in the distillation column, a similar open loop test was performed and this is shown in Figure 3.  It can be seen that when a 5% step change was made on the PD to open the valve a little, the level started moving down in a constant ramp.  It continued moving down until it reached 47%, and then the ramp rate suddenly and dramatically increased until the level reached 45.5%.  At this point the ramp rate slowed down to the original rate. The PD was put back to the balance point shortly after this and the level then remained constant.
 
A little later the PD was stepped down by 5%, and the level started ramping up at a rate very close to the slow ramp rate that occurred on the previous step.  After the level had moved up a little the ramp rate suddenly slowed and for about 5 minutes the level appeared to remain constant, but then suddenly started moving up very quickly at quite a high ramp rate.

What could cause this to happen?  The first thing that one would suspect would be a sticky valve that suddenly jumps.  Secondly is the vessel uniform in shape?  Thirdly is there a problem with the transmitter?

 

As mentioned earlier most faults are caused by problems in the final control element.  It is often very difficult to determine such faults on an integrating process where changes on the PD from the balance point result in ramps.  It is much easier to find such faults on self-regulating processes where the PV basically follows movements of the PD.
 
In this particular case the level control was the cascade master to a secondary flow cascade loop, and this was tested separately and found to be working well.  The vessel was uniform in shape, so that left the level transmitter as the probable cause of the problem.

 

Now I have found that plant personnel seldom seem to think that the transmitters they are using can cause problems.  This belief has become even more widespread in these days when computer technology is now also embedded into what are called “smart” transmitters.  I often ask the plant C&I people I am working with as to the details of the transmitters used on certain processes, and have seldom found that they even know what type of transmitter it is, and what it’s capabilities are.

I have found that transmitters are the cause of faulty control in quite a few cases over the years.  In this case when we managed to get details on the transmitter we found that it was a float type device, with a float that rests on top of the liquid,  and slides up and down over a tube as the level varies.  The transmitter has a sensor in the tube measuring the position of the float.  It was pretty obvious that the float was not sliding freely up and down the tube and was sticking a bit at times.

Now the methodology of tuning an integrating process incorporates what is known as the process gain of the level process.  Very roughly this is actually the slope of the fastest ramp that can be obtained on that particular process, so what was happening is the tuning which had been calculated from the slow “sticky” ramp rate was wrong when the float moved freely, and this was the main cause of the instability when the control was in automatic.

When one discovers the cause of a problem like this it seems so simple, but the fact is that many people working on base layer controls, don’t analyse the basics properly, and waste many hours, if not days, trying to fix problems with a “magic” tune. Incidentally the majority of people do not use scientific tuning and rather set the parameters by trial and error.  This seldom gives good tuning.