Switches liquid level instrumentation

In addition to the basic control loops, all processes have instrumentation that (1) sounds alarms to alert the operator to any abnormal or unsafe condition, and (2) shuts down the process if unsafe conditions are detected or equipment fails. For example, if a compressor motor overloads and the electrical control system on the motor shuts down the motor, the rest of the process will usually have to be shut down immediately. This type of instrumentation is called an interlock. It either shuts a control valve completely or drives the control valve wide open. Other examples of conditions that can interlock a process down include failure of a feed or reflux pump, detection of high pressure or temperature in a vessel, and indication of high or low liquid level in a tank or column base. Interlocks are usually achieved by pressure, mechanical, or electrical switches. They can be included in the computer software in a computer control system, but they are usually hard-wired for reliability and redundancy. 

When the nominal Uquid level is reached the level indicator (LIAH Level Indicator and Alarm High) gives an alarm. According to the operating manual the operator should then close the inlet valve and stop the pump. Should he not do this the liquid level would rise further and the instrument Level Switch and Alarm High High (LSAHH) would then close the valve and stop the pump automatically at a higher level. 

Aside from the level gauge, which is a local visual eadout instrument, many devices are installed at ves-ids to control and record liquid levels. The displacer-ype level controller or transmitter and switches are he most common. Basically, these operate by a float >r displacer in a chamber that reacts to the changing eveis of the liquid in the vessel and, in turn, operates... 

Because many research reactors are old. they still use the primary methods of liquid level measurement which consists of a float switch actuated by a mechanical arm, or a magnetic reed switch actuated by a magnet imbedded in a float. Since many of the float switches use a glass bulb with mercury, to make electronic contact between two wires when the float rotates, and because this typre of instrument is a binary instrument and does not allow the operator to monitor continuously the liquid level, it is recommend that installations using these instruments should be replaced with more modem and reliable instruments. 

A simple instrumented shutdown device would require the features shown in the above diagram i.e. a level switch set to detect extra high level in the tank causes an automatic shutoff valve to close off all liquid feed to the tank. The shutoff valve remains closed until the defect in the process control system has been rectified. 

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