1. introduction · 2018. 11. 28. · pid tuning case study switching control handles on two...

PID Tuning Case Study

Switching control handles on two interacting control loops using Aptitune

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1. Introduction The LGO pump-around flow of a crude distillation tower is under consideration in this case study. The

liquid is pumped from tray 17 of the distillation column and cooled by two heat exchangers. The first

heat exchanger (E15) serves as re-boiler to the downstream debutanizer column. The second

exchanger (E07) is located in the crude oil pre-heat train. There is a by-pass around both these

exchangers, but only the by-pass around the E07 exchanger is relevant to this case. The cooled LGO

pump-around is returned to tray 22 of the distillation column.

The total pump-around flow, FC002, is measured before both the exchanger by-pass lines split off,

and controlled by the direct valve through exchanger E07 (FV002 that has a valve opening of 29% in

the screen print below). The pump-around return temperature, TC009, is controlled by the by-pass

valve around exchanger E07 (TV009 that has a valve opening of 43% in the screen print below). These

are two highly interactive control loops.

There is often a problem with controllability of the total pump-around flow, as the flow control valve

operates at a low valve opening, on average below 20 %. The valve tends to saturate fully closed at

times, especially at lower plant throughput. This leaves the flow uncontrolled, hanging above the

desired flow setpoint. The pump-around return temperature control also deteriorates in this



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situation, and as a result the by-pass valve can sometimes fully open if the return temperature

remains below SP for a longer period of time.

The only way to restore controllability, would be for the operator to reduce the temperature setpoint

by a few degrees, so that the by-pass valve (TV009) would close and eventually the total pump-

around flow would drop below the current flow setpoint. This would allow the PA flow control valve

(FV002) to open again. The operators usually do not do this because it is a time-consuming exercise.

The data plot below shows saturation of the total PA flow valve (pink) and the flow PV (blue) hanging

above its SP. The SP shown comes from a plant feed to PA ratio control master loop. The

deterioration in PA return temperature control (red) is also evident, and the by-pass valve (orange)

fully opens for some period, although this does not necessarily happen every time.

2. Description of what was done It seemed from historical data that the by-pass valve was larger than the direct valve and could be a

better handle to keep control of the total pump-around flow. The solution proposed was to swap the

two control valves for flow and return temperature. Although it was clear that the controllability

issue could still arise, especially at lower plant throughput, it made sense to give preference to

keeping the flow in control. After all, without good flow control, the return temperature was also not

really controlled well anymore, as the flow through the exchanger did not vary that much compared

to the flow through the by-pass. Note that even with the direct valve fully closed, there was always

still a minimal liquid flow through the exchanger E07, and it was never cut off completely.



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Should the temperature controller reach saturation in future, when the control handles have been

swapped, and the direct control valve would fully close, it made sense to then rely on the operator to

either: 1) increase the total flow setpoint so that the duty load on E07 would increase, or 2) decrease

the return temperature setpoint so that more cooling could take place in exchanger E07. Saturation

on this case would simply mean that the return temperature setpoint is unrealistic.

To identify the degree of interaction between the two control loops, and to identify which control

handles to use for each controlled variable, open loop steps were made on the two valves.

The following 2x2 model matrix was identified based on these steps:



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The models do not inconclusively point out that the by-pass valve (TV009) is a stronger handle to

control the total PA flow, as the gain is only slightly larger (2.08 compared to 1.65). However, since

the aim is to keep the flow controller from the kind of saturation that it often underwent when

controlled by the direct valve (FV002), it still seemed a good idea to swap the manipulated variables

for the flow and the return temperature. As the absolute value of the model gains for the return

temperature are roughly equal in magnitude (0.796 compared to 0.815) this change would not be

detrimental to the temperature control.

The capability of Inca Aptitune to calculate optimal tuning for interactive loops, was used with

success in this case. The response curves for SP changes on both loops are shown below. Because the

two valves are indeed roughly equal in size, the optimal tuning did not change all that much. Both the

flow and temperature were tuned much faster than before though. The final tuning is shown in red,

whereas the initial tuning is shown in blue.



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The valve responses due to the above setpoint changes are shown below. The naming of the two

valves were changed in this exercise. FV002N is actually the new flow control valve, namely the by-

pass valve. TV009N is the valve directly after the exchanger E07, which will function as the new

temperature control valve.



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3. Results Some step test data is shown below for these two interacting loops.

The data above does not show any occurrences of saturation on the direct valve (FV002). This did still

occur though at lower plant throughputs, where a lower pump-around flow was specified to keep the

PA/feed ratio at its high limit. The difference is that the PA flow remained tightly in control, while the

return temperature control got saturated. This meant that the PA return temperature has dipped

below its current SP and the direct valve closed to try and reduce the cooling duty.

The operator could then take action. Since both these control loops were included in an APC

application as manipulated variables, the desired control strategy could also be taken care of by APC,

depending on other system constraints. The flow was now always tightly in control as the by-pass

valve was never prone to saturation. To get the return temperature back in control, the operator (or

the APC) could either reduce the temperature SP; or increase the PA flow SP up to a high limit of the

specified PA/feed ratio.

4. Conclusion The swapping of the control valves of these two interacting loops, the LGO pump-around flow and the

return temperature, helped to always guarantee a tightly controlled PA flow. Periodic controllability

problems due to valve saturation was moved from the flow control to the temperature control. This

provided a situation which was clearer in understanding to the operator and easier to recover from by

either the operator or the APC application. The Inca Aptitune software was indispensable in the

controllability analysis and re-tuning of these two interacting loops, after having swapped the control

valves on DCS.

1. introduction · 2018. 11. 28. · pid tuning case study switching control handles on two...

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