Instrumentation high liquid level

A level instrument malfunction and/or operator error had resulted in a high liquid level in the separator vessel. 

Liquid level above seal pan in bottom of column. Poor stripping of bottoms product. High pressure drop across section. Board mounted instrument improperly calibrated. Level gauge in field not properly blown down or even checked. Operation problem. 

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. 

The vertical position is preferred if the plot space is tight. Horizontal vessels are easier to support and are preferred when large liquid surge volumes are required. The liquid level displacement height for a unit volume is much less for a horizontal vessel than for a vertical vessel, which makes the control range shorter than for a vertical vessel. The displacement height per unit volume is only approximately linear on horizontal vessel when the level is near the center line, however. This can be a problem if the normal liquid level is too low or too high and the instruments are not tuned for quick response. 

The radiochemical plant, which by necessity must be operated by remote control, is highly instrumented. In addition to the usual recording and automatic control instruments for flow rate, liquid level, density, and temperature, many types of commercially available radiation monitoring instruments are used. All instruments must have a high degree of precision where measurements are required for concentration control of criticality. Reliability is improved by use of transistor components in the instrument electrical circuit. 

Figure 13.8 A faulty level sensor can cause overheating, (a) A nudfunctioning level sensor mistakenly senses a high level. It opens bottom valve. (6) Level drops and uncovers temperature sensor. If vapor temperature is lower, the controller will 1 for more heat, (c) Level is low, and bottom sump liquid is overheating, hut instruments indicate normal level and normal temperature.

Problem A safety instrumented function is identified in a SRS. If a low liquid level is detected in a separation unit, the outlet valve must be closed to protect downstream equipment from high pressure blow-by which is the identified hazard. The inlet valve must also be closed, a pump must be turned off to avoid pump damage and the inlet valve for another process unit must be turned off to minimize process disruption. The logic for this function is given in a cause and effect diagram shown in Figure 7-3. What equipment is classified as primary versus auxiliary ... 

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. 

The pressure measured in a maximum bubble pressure instrument includes the hydrostatic pressure, as shown by Eq. (48). The immersion depth of the capillary h has to be measured therefore accurately, except the capillary tip is at the liquid level (Brown 1932) or if two capillaries are used, as proposed by Sugden (1922). Due to our opinion, set-ups with two capillaries can be successfully used only in studies of pure liquids or highly concentrated solutions. However, for usual surfactant solutions, due to the dynamic character of the surface tension, significant errors can arise, caused by the fact that surface tension in the bubbles of different volume is different. 

---