Double Plate Type Boiler Gauge Glass

Boiler gauge glasses can be simple single tubular arrangements or more robust double plate types.


Boilers operating at lower pressures will have the more simple single tubular type gauge glass. Boilers operating at higher pressures (typically > 33 bar) will utilize double plate type boiler gauges. Water tube boilers in power stations will always have double plate boiler gauge glasses due to their high temperature and pressure operating conditions.

Safety Aspect

It is standard practice for larger boilers to have a minimum of two local fully independent means for monitoring of the boiler water level. A typical setup is to have two local means and several remote means of monitoring the boiler water level. The exact setup used depends upon local legislative requirements and this differs from country to country.

Large water tube boilers employed in power stations will often have critical water level protection devices fitted with a two out of three arrangements (2oo3), where one sensor failure activates an alarm and a second failure activates a safe shutdown. These 2oo3 critical protection devices are fitted in addition to the local visual boiler gauge glasses. It is not unusual to have a surveillance camera pointing directly at the boiler gauge glasses; this allows plant personnel to monitor the gauge glasses remotely.


The double plate type gauge glass is held under compression using a nut and bolt arrangement. The glass pieces sit upon joints and a mica sheet separates the glass from the water/steam.

Due to the elevated pressures and temperatures the double plate type glass plates are exposed to, it is necessary to add a mica sheet between the glass and water/steam. If not sheet is added, the hot distilled water will corrode the glass very quickly (hours) and the glass will rupture. Note that it is now possible to purchase the boiler glass pieces with the mica sheet already applied.

Gauge Glass Blow-Down Procedure

When operating a boiler gauge glass it is necessary to follow a defined procedure to reduce the probability of thermal shocking and consequent rupturing of the glass plates. The defined procedure is known as a boiler gauge glass blow down procedure and is necessary learning for anyone involved with steam generators.


This 3D model shows all major components associated with a typical double plate boiler gauge glass, these include:

  • Seals/Gaskets
  • Nuts and Bolts
  • Glass Plates
  • Water Valve
  • Steam Valve
  • Drain Valve


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