Water Level Gauges for Process Control in the Steam Power Industry
C N Shukla
Training & Development Consultant
Pune Techtrol Pvt. Ltd

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:
  • The pipes used for connection purpose must be minimum one inch in diameter to allow quick and free flow of water into and out of the level gauge.
  • The minimum water column drain size must be 0.75 inches in diameter.
  • To avoid stoppage of water by sediment deposits, 'Through Flow' valves that indicate the open or the closed position must be chosen. At times, double block valves are essential.
  • A water column should be included to minimize the amount of condensate flowing through the gauge. If the condensate quantity is excess, the water level reading becomes difficult to read and also the reading is unstable.
  • For high-pressure boilers, the optimum design is of 'Ported Bi-color' gauges.
  • The inclusion of gauge isolation valves is useful in case of gauge failures for isolation of the gauge from a secure lower level. These valves are frequently used with chainwheel operators.
  • To differentiate the water level, a 'Bicolor Illuminator' is recommende . For optimum visibility, the LEDtype focused design is advisable. The added benefit is that it has less maintenance.
  • The 'Illuminator Display' is to enable a fixed viewpoint.
  • To alter the line voltage to the low voltage that the Illuminator requires, an 'Illuminator Power Supply' is used.
For an electronic water level gauge, note the following:
  • A probe column is designed to hold the probe covers, junction box, and the probes at predetermined levels. Typically, the probe column is welded to the drum-connecting piping. To decide the probe column length, the total length of level visibility essential is the deciding factor.
  • The probe column is at a lesser temperature than the steam drum. To view the correct drum water level, the probe position in the column is corrected for density.
  • The junction box is used as a terminal point for wires from the electronics control unit to the probes.
While designing a feedwater heater electronic water level gauge, the following parts must be included:
  • A probe column drain
  • A probe column including or excluding a probe wire junction box, which is usually configured with 3 to 4 probes at the trip level and the critical alarm level
  • Pipes for connectivity purpose
  • Column isolation valves
  • Electronic Control unit: The position of this unit is within 30.48 meters of the probe column.
Turbine Water Induction Protection (TWIP) equipment is used to avoid damage by water to steam turbines. The water level gauges can be installed in the condensing pot directly. In this situation, maintenance of these gauges can be performed only when the steam system is not in operation. For frequent maintenance, the water level gauges must be isolated by installation in a separate column.

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:
  • Mirrors: Install a sequence of wall/floor-mounted mirrors or gaugemounted mirrors and use sheet metal ducting to ensure no -obstruction of the water level image. This arrangement is similar to a periscope.
  • Fiber optic cable: Install this cable on the water level gauge's display side and run it to the control room connecting it to a panel display:
    • This is an expensive method.
    • This system is difficult to adjust.
    • Heat and dust have an adverse effect on the image quality.
  • Closed circuit camera with television: Use a color camera and television to view red/green bi-color water gauges:
    • This is an economical method.
    • This system is easy for long cable distances.
We can conclude that in Thermal Engineering, water level gauges/indicators can play a significant role to monitor the process and ensure security, reliability, efficiency, and economy.