Level Controls

Most vacuum crystallizers operate at constant liquid level and this requires liquid level controls capable of maintaining the level within about 6 inches (15 cm) of any predetermined point. This 

Many evaporator calandrias condense steam at subatmospheric pressures. Consequently, condensate must be removed by pumping, either with mechanical pumps or pumping steam traps. Large condensate loads are usually best handled when liquid levels are controlled and condensate removed with the appropriate method. 

Large condensate loads are frequently present in evaporator systems. Large quantities of condensate are best handled in an instrumented condensate pot or tank in which liquid level is controlled. Liquid-level control will reduce steam consumption by 2-17%, depending on the steam pressure and trap selected for 

Level instrumentation can be a compact pneumatic controller. Self-contained pneumatic instruments provide a positive level readout, a level sensor, a control point, and a pneumatic output control signal all in one small housing. 

For most flashing-condensate services, a standard stainless steel control valve will provide long reliable life. Special trims and seats are available for high-pressure service. The control valve should always be sized for flashing service. In many instances, this wiil require a large valve-body with reduced trim to maintain valve-position control, because the flash steam causes high back pressure in the small valve-bodies. 

Both pumping traps and liquid movers use steam as a motive force to pump condensate to the desired location. The steam must be vented between each cycle. For vacuum operation, a vacuum vent is required and usually represents an energy inefficiency unless the vent can be condensed in the process to recover the heat. The steam can be vented back to the calandria steam chest and condensed directly against the process, however, this may represent an operational Instability unless properly designed. The vent rate must be controlled at a rate which does not create problems in the calandria or evaporation system. The receiver must be adequately sized to permit stable operation. Generally, it is better to vent to a separate condensing location. 

---

The two portions of the feed stream recombine and flow into the high pressure separator where the Hquid is separated from the vapor and is fed into an intermediate section of the demethanizer with Hquid level control. The decrease in pressure across the level-control valve causes some of the Hquid to flash which results in a decrease in the stream temperature. The pressure of the vapor stream is decreased by the way of a turboexpander to recover... 

The four process control parameters are temperature, pressure, flow, and level. Modem process level detection systems are varied and ubiquitous in modem chemical plants there are thousands of processes requiring Hquid level indication and Hquid level control. From accumulators to wet wells, the need for level devices is based on the need for plant efficiency, safety, quaUty control, and data logging. Unfortunately, no single level measurement technology works rehably on all chemical plant appHcations. This fact has spawned a broad selection of level indication and control device technologies, each of which operates successfully on specific appHcations. 

There are three basic requirements that Hquid level control devices are designed to satisfy alarm functions, pump/valve control, and transmitted output signal to track level continuously. Alarm devices provide warning or shutdown functions when process levels pass a predeterrnined point in the vessel pump /valve control devices turn on/off pumps or open/close valves at predeterrnined levels in the vessel and transmitters provide a proportional output signal over a predetermined span to send to a local meter or signal back to a control room. 

Fig. 5. Buoyancy level controller. Courtesy of Magnetrol International, Inc.

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,...

Many misconceptions exist about cascade control loops and their purpose. For example, many engineers specify a level-flow cascade for every level control situation. However, if the level controller is tightly tuned, the out-flow bounces around as does the level, regardless of whether the level controller output goes direcdy to a valve or to the setpoint of a flow controller. The secondary controller does not, in itself, smooth the outflow. In fact, the flow controller may actually cause control difficulties because it adds another time constant to the primary control loop, makes the proper functioning of the primary control loop dependent on two process variables rather than one, and requites two properly tuned controllers rather than one to function properly. However, as pointed out previously, the flow controller compensates for the effect of the upstream and downstream pressure variations and, in that respect, improves the performance of the primary control loop. Therefore, such a level-flow cascade may often be justified, but not for the smoothing of out-flow. 

Many improvements have been made to streamline performance and to reduce machine operation labor. Some of these are tensionless jigs using variable speed electric motors with built-in drag for brakes, automatic reversing equipment, and automatic temperature and level controls. These machines are widely used for goods that are easily creased, such as fabrics consisting of filament acetate, heavy filament nylon, or cotton duck. They are also convenient for small dye lots and for sampling purposes. 

Process Controls If the inlet or outlet liquid to a piuTro-tnrbiue is regulated by a level controller on the hqnid-snpply vessel, a falling liquid level inside the vessel will cause this coutroUer to throttle a valve, reducing the differential pressure available to the pump-... 

Implement operating instructions to maintain liquid level above heating surface at all times Install automatic level control with low level alarm and shutdown of liquid withdrawal system to ensure liquid is above heating surface at all times... 

Provide level control system on filtrate receivers with bottoms pumps... 

Level control valve same as flow meter Porter Instr. 

The column bottom level is sometimes controlled by bottoms draw. Varying reboiler heating medium is another possibility. For some cases, bottoms draw level control works better and for others, heating medium level control. BojnowskF gives a case where heating medium level control was desired for two columns in a plant. One... 

The feed flow is often not controlled but is rather on level control from another column or vessel. The liquid product flow s (distillate and bottoms) are often on level rather than flow control. Top vapor product is, however, usually on pressure control. The reflu.x is frequently on FRC, but also may be on column TRC or accumulator level. 

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

When a small quantity of a second liquid phase is present, a drawoff pot (commonly called a bootleg) is provided to make separation of the heavy liquid (frequently water) easier. The pot diameter is ordinarily determined for heavy phase velocities of 0.5ft/min. Minimum length is 3 ft for level controller connections. Minimum pot diameter for a 4 to 8 foot diameter reflux drum is 16 inches. For... 

Make sure items such as gauge glas.ses, level controls, or pressure taps do not receive an impact head from an incoming stream. 

Our example system has a flow-controlled feed, and the reboiler heat is controlled by cascade from a stripping section tray temperature. Steam is the heating medium, with the condensate pumped to condensate recovery. Bottom product is pumped to storage on column level control overhead pressure is controlled by varying level in the overhead condenser the balancing line assures sufficient receiver pressure at all times overhead product is pumped to storage on receiver level control and reflux is on flow control. 

An alternate means of reboiler control is to remove the control valve from the steam line and provide a condensate level controller for the chest cascaded from the tray temperature. The alternate method uses A tube surface for control, with the condensate covering more or less tube surface to vary the area exposed to condensing stream. Condensing area is many times more effective for heat transfer than area covered by relatively stagnant condensate. The reboiler must have extra surface to allow part of its surface to be derated for control purposes. 

There is also another limitation of controllers that is often overlooked. A controller must be given only its one Job to do, not several. For example, a level controller might be designed to deliver feed to a process having a varying demand. A boiler level controller is such an example. This controller can be designed... 

If level controller has pneumatic transmitter, look for isolated or broken lines or leaks. 

---