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CONTROLLING VESSELS and TANKS © Walter ********, P. Eng., 2001 Sept 07. walter(at)********(dot)ca

First published in Hydrocarbon Processing , July 1995.


INTRODUCTION. It would seem that controlling a vessel should be a very simple matter -- They don't really do anything! But then, if they didn't do anything why are there so many of them? And why do they have so many different names? Going through a typical set of Piping and Instrumentation Diagrams (P&IDs) I see the following vessels:

· Degassing Drum · Gas Separator · Storage Tank
· Feed Flash Drum · Reflux Accumulator · Day Tank
· Surge Drum · Suction Scrubber · Slug Catcher
· Lube Oil Separator · Head Tank · Deaerator

Although each of these is essentially a simple vessel or tank without any special internal structure, each serves a different purpose. Once it is clear what the purpose of a piece of equipment is, and how it functions, it will also be clear how to control and protect it. Different purposes require different controls.
SURGE TANKS. The most common function of a vessel or tank is to match two flows that are not identical in time but are expected to average out over the long run. Take a feed surge drum, for example. Flow into the unit is more or less steady but is subject to interruption. The flow to the processing unit should be as constant as possible, avoiding sudden change. Nevertheless, it, too, may be subject to interruption due to downstream conditions.
The purpose of the surge drum is to maintain sufficient inventory to feed the process and to maintain sufficient void capacity to continue receiving feed as it arrives. Clearly the tank must be large enough to accommodate any normal discrepancies between input and output over a reasonable period of time. Between the upper and lower bound, the exact value of the level does not matter.
Two separate control parameters are implied: Level and flow. Level control is no problem. Greg Shinskey ¹ refers to "The easy element -- capacity". A high gain, level controller connected to a valve at either the inlet or the outlet will maintain the level very accurately at its setpoint. The only problem with this approach is that it absolutely defeats the purpose of the vessel. The same effect would be achieved by blocking in the vessel and bypassing the inlet directly to the outlet.
To control flow alone is also quite simple. A flow controller at the outlet, properly tuned, will maintain a steady flow to the process. Unfortunately, there is nothing to make this flow equal to inflow. It will not even equal the average inflow unless there is something to make it do so.
What we need is an instrument that measures the accumulated error between inflow and outflow. The tank itself is that instrument!

Level = Starting Level + [IMG]http://www.********.ca/ce6_v&t/int.jpg[/IMG] (Inflow - Outflow) dt / Tank Area

(To a process controls engineer, every piece of equipment is just a big, non-tuneable instrument!) The level transmitter only transmits the process value to the control system. If we now cascade the output of the level controller to the flow controller, we have a system that has one process variable: Accumulated flow imbalance. It has only one point of control: Outflow to the process.
To start this simple process:
[IMG]http://www.********.ca/ce6_v&t/6-1.jpg[/IMG]· Fill the tank about half full.
· Give the level controller the current level as its set point. (PV tracking does this automatically.)
· Switch the flow controller to automatic with an estimated average flow as its setpoint.
· Switch the flow controller to cascade.
· Switch the level controller to automatic.

The control system will keep the flow "constant" but that constant varies in response to the imbalance between outflow and inflow. It is not important that the initial estimate of average flow be exact. A low guess will result in the tank level rising a little. A new, higher, estimate will result and the outflow will be adjusted accordingly. In the long term the average flow out is not an independent variable at all. It will be exactly equal to the average flow in. This can be accomplished at any arbitrary tank level. The level setpoint is based on the operator's estimate of the nature of the flow interruptions and whether the most probable upset will require additional flow or void capacity.
Should a pump be necessary to transfer the liquid from the vessel to its destination it should be placed between the vessel and the flow measurement. Further information on the control of pumps is found in Controlling Centrifugal Pumps² . This article also includes a section titled "On/off Control" for less critical level applications.
There is a long discussion on the special requirements for level control of steam heat exchangers and condensate receivers in Controlling Steam Heaters³.
Surge drums are sometimes used for gas. The abrupt flow variations of a Pressure Swing Absorption (PSA) unit, for example, often need to be smoothed out before the tail gas can be introduced into a down-stream process. In these cases, pressure takes the role that level has in a liquid process. That is, a pressure/flow cascade is the appropriate solution.
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