Water Level Gauges for Process Control in the Steam Power Industry |
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C N Shukla Training & Development Consultant Pune Techtrol Pvt. Ltd |
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Water level gauges or indicators are devices that measure, display and
transmit, water level continuously. This article discusses the role water level
gauges/indicators can play in monitoring the process and ensuring security,
reliability, efficiency, and economy. Why water level gauges are essential? During the entire operation of a boiler, a low water level condition in the boiler drum can damage the boiler tubes. On the other hand, a high water level can damage the steam separator or steam turbine due to carry over of water. The American Society of Mechanical Engineers (ASME) regards the water level in the boiler/steam drum as a crucial plant parameter. For coal/oil-fired boilers, the ASME code recommends minimum one water level visual gauge to provide a definite value of the steam drum water level. In case of feedwater heaters, a low condensate level results in the steam being discharged directly to the next heater. This minimizes the efficiency of the feedwater heater. Likewise, a high condensate level results in the opening of the steam safety valve. In this state, the feedwater heater does not function at optimum efficiency. Thus, it is of significance that the low or the high water levels in the feedwater heater steam section must be monitored. Here too, the water level gauge comes in handy. In case of steam turbines, each connection to a turbine is a potential water source, which may allow entry of water into turbine. It can be by means of collection of condensed steam or induction from external equipment. At times, the water source causes serious damage to steam turbine, which leads to reliability, security, and fiscal concerns. It is here that a water level gauge is mandatory to detect the presence of water in the interior or the exterior of the turbine prior to any damage that can occur. Specifics of the water level gauges Water level gauges can be used for measurement up to 600 degrees Celsius temperature and 200 kg/cm 2 pressure. Water level gauges used with boilers must have minimum one gauge glass. This is transparent and facilitates to visually record the water level. To differentiate between the water level and the steam level, end-connected strip gauges or ported gauges, with bi-color indication facility are used. In a b -color gauge glass, water portion is indicated by green/blue light and steam portion is indicated by means of red light. If the maximum permissible working pressure of the boiler is more than 28 kg/cm2, two disparate water level gauges or gauge glasses should be in use. If two water level gauges are in use, the one gauge glass can be maintained in usable condition and the other may be shut off during operation. It may happen that the gauge glass cannot be easily seen by the boiler operator. In such a case, the optical image of the water level must be transferred to the control area by means of mirrors or a fiber optic cable. An alternative solution is to install an independent water level gauge or arrange for perpetual transmission and display of the water level in the gauge glass by means of CCT . In feedwater heaters, visual water level gauges, magnetic level indicators and transmitters, and high/low-level switches are used to determine the water level. Design of the water level gauges While designing a power boiler steam drum visual water level gauge, the following points must be noted:
Transmission of the water level from the gauge to the control room The water level gauge reading can be transmitted to a control room in the following ways:
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