Level-controlled tank

Where a flooded coil is located in a liquid tank, the refrigerant level will be within the tank, making it difficult to position the level control. In such cases, a gas trap or siphon can be formed in the lower balance pipe to give an indirect level in the float chamber. Siphons or traps can also be arranged to contain a non-voIatile fluid such as oil, so that the balance pipes remain free from frost. 

Where the MU water supply to cast-iron boilers does not precisely keep up with steam generation demands, the water level can quickly decrease and the problems become even more acute. Conversely, where MU does precisely keep up with steaming rates and is supplied to a common condensate receiver-FW tank via automatic level control, the tank can easily overfill when condensate finally drains back under on-off operating conditions. This gives rise to a loss of valuable hot, treated water from the system and the start of another chain of cause and effect problems. 

Figure 5.3-17. Flow-sheet of a batch process unit symbols E = heat exchanger, P = pump, R = reactor, T = storage tank, V = vessel controllers FC = flow controller, LC = level controller 0 stream number.

Liquid flows through a system of two tanks arranged in series, as shown below. The level control of tank 2 is based on the regulation of the inlet flow to the tank 1. This tank represents a considerable lag in the system. The aim of the controller is to maintain a constant level in tank 2, despite disturbances which occur in the flow F3. 

Amplitude of disturbance Initial level tank 1 Initial level tank 2 Set-point level for tank 2 Controller constant TFIN=80 RESET GOTOl... 

The feed flow-rate is often set by the level controller on a preceding column. It can be independently controlled if the column is fed from a storage or surge tank. 

We can see quickly that the system has unity gain and there should be no offset. The point is that integral action can be introduced by the process and we do not need PI control under such circumstances. We come across processes with integral action in the control of rotating bodies and liquid levels in tanks connected to pumps (Example 3.1, p. 3-4). 

Feedback is information in a closed-loop control system about the condition of a process variable. This variable is compared with a desired condition to produce the proper control action on the process. Information is continually "fed back" to the control circuit in response to control action. In the previous example, the actual storage tank water level, sensed by the level transmitter, is feedback to the level controller. This feedback is compared with a desired level to produce the required control action that will position the level control as needed to maintain the desired level. Figure 3 shows this relationship. 

In the water tank level control system in the example above, the level transmitter measures the level within the tank. The level transmitter sends a signal representing the tank level to the level control device, where it is compared to a desired tank level. The level control device then computes how far to open the supply valve to correct any difference between actual and desired tank levels. 

In this example of a proportional level control system, the flow of supply water into the tank is controlled to maintain the tank water level within prescribed limits. The demand that disturbances placed on the process system are such... 

Figure 10-20 shows two tanks in series, both with independent level controllers. This configuration will result in the lower tank inevitably overflowing. Can you explain why ... 

What have we missed in our modeling A good plant operator could take one look at the system and see what the problem is. We have not specified how the flows out of the tanks are to be set. Physically there would probably be control valves in the outlet lines to regulate the flows. How are these control valves to be set A common configuration is to have the level in the tank controlled by the outflow, i.e., a level controller opens the control valve on the exit... 

It might be worth noting that we could have considered the flow from the third tank fj as the forcing fijnction. Then the level in tank 3 would probably be maintained by the flow into the tank, f 2 Th Isvel in tank 2 would be controlled by fi, and tank 1 level by Fg. We would still have three equations. 

Gas is drawn off the top of the drum through a control valve whose stem position is set by a pressure controller (Fig. 3.7). Liquid comes off the bottom of the tank on level control. 

Steadystate error is not always undesirable. In many level control loops the absolute level is unimportant as long as the tank does not run dry or overflow. Thus a proportional controller is often the best type for level control We will discuss this in more detail in Sec. 7.3. 

Suppose the flow rate Fq increases to the first tank in Fig. 7.14. The level in the first tank will start to increase. The level controller will start to increase Fi-When F has increased to the point that it is equal to Fo.-tflc level will stop changing since the tank is just an integrator. Now, if we use a P level controller, nothing else will happen. The level will remain at the higher level and the entering and exiting flows will be equal. 

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