Level control, averaging

Its most obvious application is to feed surge drums. These are included in the process design specifically to reduce the effect of upstream flow disturbances on the downstream process. Installing tight level control in this situation makes the drum ineffective. 

The main issue with averaging level control is its acceptance by the process operator. To achieve its objective the vessel level will often approach alarm limits and may take several hours to return to SP. The operator may, not unreasonably, be quite concerned by this and not entirely persuaded that the benefit to the downstream unit is worth the apparent risk. A more cautious approach can allay such concerns. Initially tuning the controller to use only part of the available capacity and demonstrating over time that it does not violate this limit will help persuade the operator to accept use of all the available capacity -particularly if the benefit is demonstrable. 

There are likely to be other similar issues. Some sites permit the operators to configure process alarms these will often then be set conservatively and the operator will need to be persuaded to relax them as far as possible. The operator may introduce asymmetry. He may be concerned about potential pump cavitation and therefore more worried by a reduction in level rather than a rise. He will increase the level SP above 50 % and may also increase the position of the low level alarm. This will mean full use is not made of the surge capacity when there is a flow increase. The converse may also apply, for example if the operator is more concerned about overfilling the vessel. 

The method used to tune the controller is very similar to that applied to tight level control. We start as before with a proportional-only controller. However, rather than eliminate the flow imbalance as quickly as possible we do so as slowly as possible. In this case the controller will take considerably more than one scan to make the correction, i.e. 

To make full use of the capacity we will allow the level to approach the alarm before steady state is reached. In other words we design for an offset of d, i.e. 

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The main problem with all four of the tuning methods presented so far is that they all have the sole objective of reaching the SP as soon as possible. With the exception of some special cases, such as averaging level control described in the Chapter 4, this usually is a requirement. But it is not normally the sole requirement Figure 3.26 shows the performance of a controller tuned to meet this aim. However, depending on the process, this might result in excessive adjustments to the MV. In our fired heater example it is unlikely that the fuel... 

It is not usual to square each error term in the controller individually. The most common approach is to multiply the controller gain by the absolute value of the error. Omitting the derivative term (since we usually do not require this for averaging level control) the control equation becomes ... 

Of course, this is a contrived example. Had the process design department consulted the plant s control engineer, the drum would have been installed upstream of the measurement of the flow of stream A Further the averaging level controller could then properly be... 

Figure 12.56 shows one of the two possible schemes (note 3). It is often used when both products are routed to downstream processes and it is a requirement that the feed to the processes is kept constant. In reality this is probably not achievable. While we can manipulate reboiler duty to provide control of composition, this will only vary separation. To vary the cut at least one of the product flows will need adjustment. One could argue that it would be better to cascade the drum level to the distillate flow. This would permit averaging level control to be applied to make smooth changes to the downstream process flows and release the reflux flow as a MV for composition control. 

A refinement is to add to / + D the rate of change of reflux drum inventory. This helps overcome the problem caused by the process not yet reaching steady state. This is particularly useful if averaging level control has been installed on the drum in order to minimise flow disturbances to the downstream process. Similarly the rate of change of the liquid inventory in the base of the column can be added to F + B. 

As an alternative other algorithms, such as bias feedforward and deadtime compensation, can be implemented in the MVC - depending on which approach is better for operator understanding and what back-up scheme is necessary if the MVC is out of service. It is also possible to move averaging level control from the DCS to the MVC. This should only be considered if it is desirable to let the MVC select which flow to manipulate (i.e. vessel inlet or outlet) depending on where the process is constrained. The DCS controller will still be required as back-up. 

This application is often referred to as averaging level control, because it is desired that the manipulated flow follow the average level in... 

Provide averaging level control of column feed. Column feed rate should also be held between maximum and minimum limits. 

If a smooth flow to the next step in the process is needed, a reflux drum, with averaging level control of distillate, should be employed. If the column top product is a vapor, takeoff should be by averaging pressure control. As an alternative, vapor may be taken off on flow control cascaded from top composition control while column pressure is controlled by heat input. 

This illustrates an advantage of PI controllers in averaging level-control service. For the same Th, only one eighth the volume is required. 

The dynamic response of the proportional-only, averaging level control system may be defined by a first-order time constant ... 

Let us look at a material-balance control scheme that is in the direction of flow. Let feed rate be set by averaging level control of the feed tank, let condensate receiver level set top-product flow, and let column base level set bottom-product flow. We will assume that each level controller is cascaded to the appropriate flow controller. 

Note that we have ignored the dynamics of level measurement and of the flow control loop. For averaging level control, this introduces little error. 

Wc assume here that averaging level control is desired. We may then prepare the signal flow diagram of Figure 15.10. Note that ... 

Since we have called for averaging level control, the natural frequency of the level control loop will be mu( lower than that of the flow control loop. Note that ... 

It has already been indicated that for averaging level control cascaded to steam flow control, one may substitute l/ T, for the flow control loop. Use of the mathematical models discussed here on commercial reboilers indicates that typical time constants range from a fraction of a second to 5-10 seconds. Practically speaking y(r), 0(r), and A(r) reduce to constants. As we will see in Chapters 16 and 17, it usually will be possible to use much simpler rcboiler models than that discussed in this section. 

As stated in Chapter 1, the functions of averaging level control are ... 

Before getting into specific applications of level control on distillation columns, let us review briefly the theory of averaging level control on simple vessels. 

The level and inflow responses to a step change in outflow of an averaging level control system with dead time and a PI controller tuned as recommended above are given in Figure 16.6. 

Responses of PI averaging level control stem with dead time to step change in outflow... 

Averaging Level Control, Proceedings, 1983 Joint Symposium, ISA, Houston, Tex., Apr. 18-21,1983. 

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