Low level, switch

A safety trip can be incorporated in a control loop, as shown in Figure 5.29a. In this system the level control instrument has a built-in software alarm that alerts the operator if the level is too low and a programmed trip set for a level somewhat lower than the alarm level. However, the safe operation of such a system will be dependent on the reliability of the control equipment, and for potentially hazardous situations it is better practice to specify a separate trip system, such as that shown in Figure 5.29b, in which the trip is activated by a separate low-level switch. Provision must be made for the periodic checking of the trip system to ensure that the system operates when needed. 

Standard control methods are used for the WFI tank. A water high level switch (LSH) turns the WFI still off or on depending on whether the water level is at or below the LSH point. A water low level switch (LSL) is interlocked to the recirculation pump to shut it off should the level reach the LSL point. 

What will happen if the chemical day tanks are empty when the system calls for pumping Add low-level switches to the tanks together with appropriate software for the microprocessor to shut down the pump and signal an alarm to the operator. 

Low-level switches fail to alarm or alarm prematurely Check the operation of the level switches... 

The control arrangement is quite similar to that used with mercury cells and shown in Fig. 11.47. The pumps can be controlled by an on-off controller set by high- and low-level switches in the tank, as in part (b) of the drawing. A technically better method, as in part (a), is to control the level with a throttling valve on the pump discharge. 

The loader control panel, which may be mounted on the unit or at a remote point, contains motor starters, timers, high- and low-level switch lights and, on multiple silo systems, a selector switch to energize the proper high-level switch for automatic operation. When the operator wishes to change from loading one silo to another, the flex-hose connections to the material and vacuum line of the new tank must be changed and the tank selector switch on the control panel must be positioned to the new silo. The system is then ready to run. 

Two simple interlock systems are shown in Fig. 10.4. For the liquid storage system, the hquid level must stay above a minimum value in order to avoid pump damage such as cavitation. If the level drops below the specified hmit, the low-level switch (LSL) triggers both an alarm (LAL) and a solenoid switch (S) (or solenoid) that turns the pump off. For the gas storage system in Fig. 10.4b, the solenoid valve is normally closed. But if the pressure of the hydrocarbon gas in the storage tank exceeds a specified limit, the high pressure switch (PSH) activates an alarm (PAH) and causes the solenoid valve to open fully. 

Consider the liquid storage tank with a low-level interlock, as shown in Fig. 10.4. Suppose that an independent low-level alarm is added, with its set-point value above the value for the low-level switch. If both the low-level alarm and the low-level interlock system fail simultaneously, the pump could be seriously damaged. What is the probabihty that this occurs What is the mean time between failures ... 

All motors, contactors, and the mechanical overload torque limit on geared reduction units fitted to a conveyor drive unit, incorporate integral switch gear, in order that mechanical stoppage is monitored by visual or audible alarms. Low level switches also monitor sudden level drops, giving adequate warning to the operator to enable corrective procedures to be implemented. 

Many reported biotransformations are initially only demonstrated on a very small scale, the substrates or products may be subject to competing reactions if other enzymes are present (this can be a serious issue in whole-cell biocatalysis), or the desired enzyme is insufficiently active or produced in low levels. For many biotransformations a little care and attention is needed in the growth of the microbe to achieve the desired results. Production of a specific enzyme from a microbe can often be increased by growing the cells in the presence of a very small concentration (typically micromolar) of an inducer. The inducer could be a natural enzyme substrate, a substrate mimic or a molecule which is in some way associated with a substrate s availability or role in metabolism. This process is called induction and represents a genetic switch which cells use to respond... 

J. C. Wolff, L. Barr and P. Moss, New ultraviolet signal actuated switching valve for the measurement of low level impurities by liquid chromatography/mass spectrometry. Rapid Communications in Mass Spectrometry, 1999,13(23), 2376-2381. 

In addition, carry-over from the main peak is very often problematic for low-level late-running impurities. One way of overcoming this problem is to divert the flow to waste at the PSU for the early-running analytes, bypassing the probe and then switching the flow to the probe to analyse the minor peak(s) of interest. 

Another form of head space analysis uses a purge trapping device to trap volatile impurities. In this technique a gas, e.g. helium, is bubbled through the sample which is dissolved in suitable solvent (usually water) and the volatile impurities are thus stripped from the solution and passed in the stream of gas through a polymeric adsorbant where they become trapped and thus concentrated. The stream of gas is then switched so it passes in reverse direction through the polymeric trap, which is heated to desorb the trapped volatiles and the gas stream is then diverted into the GC. This type of procedure is used in environmental analysis to concentrate volatiles in water which are present at low levels. 

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