Liquid-level control

As seen in the above equation, increasing the controller gain Kc will minimize the error. If the process gain is low, then the larger controller gain will result in a small error while still maintaining a stable loop. 

As the set point is increased, SP3 SPi, the process gain, which is the slope of the curve at SPi, SP2, and SP3, would decrease. Again, the process gain shows areciprocal relationship to the load. In this case, the load is the set point. 

Why is the dependence of process gain on load a consideration The previous discussion shows that, when a process contains a capacitance and the controller gain is adjusted to give a particular response at a given load, the response will change as the load changes. If the load on the process is reduced, then the process gain rises and. 

Yet another approach to this situation would be to adjust the controller parameters, the controller gain for example, with the variation in load of set point to compensate for the variation in process gain. This approach is termed gain scheduling [5,6] or programmed adaptation [7,8] and can be considered to be form of adaptive control [5], 

Another level control situation where a nonlinear element might be introduced into the loop is the case of level control of a cylindrical tank lying on its side, shown in 

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Fig. 13. Cascade control schemes, where TC = temperature controller FC = fuel gas flow controller and LC = liquid level controller, (a) Simple circuit having no cascade control (b) the same circuit employing cascade control and (c) and (d) Hquid level control circuits with and without cascade control,...

Anderson, G. D., Guidelines for Selection of Liquid Level Control Equipment, Fisher Controls Company. 

Specify special features and materials of construction, such as alloy or nonferrous impingement parts, or entire vessel if affected by process vapor and liquid. Specify special liquid reservoir at base of unit if necessary for system operations. Line units normally have dump traps or liquid outlet of separator, while vessel type often use some type of liquid level control. 

The unit should have backup instrumentation to ensure liquid level control to dispose of the tvaste recovered liquid. 

The best designs provide for the percentage vaporization per pass to have been completed by the time the fluid mixture reaches the upper end of the tube and the mixture is leaving to enter the bottom chamber of the distillation column. In order to assist in accomplishing this, the initial reboiler elevation should be set to have the top tubesheet at the same level as the liquid in the column bottom section. A liquid-level control adjustment capability to raise or lower this bottoms level must exist to optimize the recirculation. Sometimes, the level in the bottom of the column may need to be 25-30% of the reboiler tube length above the elevation of the tubesheet. Therefore, the vapor nozzle return from the reboiler must enter at sufficient elevation to allow for this possibility. 

We use a simple liquid level controller to illustrate the concept of a classic feedback control system.1 In this example (Fig. 5.1), we monitor the liquid level in a vessel and use the information to adjust the opening of an effluent valve to keep the liquid level at some user-specified value (the set point or reference). In this case, the liquid level is both the measured variable and the controlled variable—they are the same in a single-input single-output (SISO) system. In this respect, the controlled variable is also the output variable of the SISO system. A system refers to the process which we need to control plus the controller and accompanying accessories such as sensors and actuators.2... 

Figure 5.1. Schematic diagram of a liquid level control system.

For all commercial devices, the proportional gain is a positive quantity. Because we use negative feedback (see Fig. 5.2), the controller output moves in the reverse direction of the controlled variable.1 In the liquid level control example, if the inlet flow is disturbed such that h rises above hs, then e < 0, and that leads to p < ps, i.e., the controller output is decreased. In this case, we of course will have to select or purchase a valve such that a lowered signal means opening the valve (decreasing flow resistance). Mathematically, this valve has a negative steady state gain (-Kv)2... 

A special type of cross-flow reactor was developed in the laboratories of Vogt [16] to handle continuous gas / liquid reactions. The challenge in the reactor design was to combine efficient gas-liquid mixing, liquid level control in the reactor, turbulent flow across the membrane, and efficient gas-liquid separation to avoid gas contacting the membrane, which would lead to a shunt of gas. The total internal volume should not... 

Design liquid level control systems for the base of a distillation column and for the vaporizer shown bdow. Steam flow to the vaporizer is held constant and cannot be used to control level. Liquid feed to the vaporizer can come from the column and/or from the surge tank. Liquid from the column can go to the vaporizer and/or to the surge tank. 

The glass floats, which control the liquid levels in the distillation chambers, leaked and sank and thus caused problems with liquid level control in the distillation chambers. New floats were made, but minor leakage persisted. The float has since been eliminated, and a more sensitive level detecting device is now used. 

Emulsions can be particularly troublesome. Over a period of time emulsified materials and/or other impurities usually will accumulate at the water/oil interface. In addition to adversely affecting the liquid level control, this will also decrease effective oil or water retention time in the separator, with a resultant decrease in water/oil separation efficiency. Addition of chemicals and/or heat often minimizes this difficulty. 

Derivative control is sensitive to noise that is made up of random higher frequency perturbations, such as splashing and turbulence generated by inflow in the case of liquid level control in a vessel, so that it is not satisfactory in such situations. The variety of composition controllers arises because of the variety of composition analyzers or detectors. 

Figure 3.9. Steam heaters, (a) Flow of steam is controlled off the PF outlet temperature, and condensate is removed with a steam trap or under liquid level control. Subject to difficulties when condensation pressure is below atmospheric, (b) Temperature control on the condensate removal has the effect of varying the amount of flooding of the heat transfer surface and hence the rate of condensation. Because the flow of condensate through the valve is relatively slow, this mode of control is sluggish compared with (a). However, the liquid valve is cheaper than the vapor one. (c) Bypass of process fluid around the exchanger. The condensing pressure is maintained above atmospheric so that the trap can discharge freely, (d) Cascade control. The steam pressure responds quickly to upsets in steam supply conditions. The more sluggish PF temperature is used to adjust the pressure so as to maintain the proper rate of heat transfer.

Chettle, T. Processing (Jan/Feb 1993), 14. Choosing good solutions for liquid level control. 

A step change in set point, however, may be a poor indicator of a loop s load response. For example, a liquid-level controller does not have to integrate to follow a set-point change, as its steady-state output is independent of the set point. Stepping a flow controller s set point is... 

WARNING There is no valve immediately downstream o( the liquid outlet stream. A liquid level control will not be added automatically as Its output can not be connected You should check whether a level controller is needed for a stable dynamic simulation. 

Step 1. We are assuming in this section that the product stream from the bottom of the stripper is set on the demand of a downstream user. The bottoms stream from the stripper is flow-controlled and so we set the position of the control valve, XMV(8), on this stream (B). The rest of the liquid level controls must be chosen to accommodate this first-priority choice. Note that we could put a flow controller on this stream if necessary, but this was not done in the simulations described later. The quality specification is that component G in the product should not vary more than 5 mol %. 

For subcooling, a liquid inventory may be maintained in the bottom end of the shell by means of a weir or a liquid-level-controller. The subcooling heat-transfer coefficient is given by the correlations for natural convection on a vertical surface [Eqs. (5-33a), (5-33b)j, with the pool assumed to be well mixed (isothermal) at the subcooled condensate exit temperature. Pressure drop may be estimated by the shell-side procedure. 

To prevent flooding or emptying of the reactor, requires a liquid-level controller (LC). In this case, the pressure exerted by the liquid in the reactor measures the liquid level. The operation and installation of the liquid-level control valve is the same as the flow and temperature control valves. 

The problem is to find how the heat transfer rate can vary when the other quantities change. U is an experimental value that is known only to a certain accuracy. AT may be uncertain because of possible fluctuations in regulated steam and tower pressures. A, the effective area, may be uncertain because the submergence is affected by the liquid level controller at the bottom of the column. Accordingly,... 

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