Sight glasses are used in piping systems where it is necessary to observe the product flow. Sight glass allow operators to observe the fluid flow direction, approximate rate of flow and condition of process fluids. Sight glasses should be designed to provide clear, continuous and unobstructed visibility of processes or products in piping, tanks or vessels.

Sight Glass

Types of Sight Flow Indicators

Sight flow indicators are available in various configurations. The type of indicator used depends on the process fluid and the visibility required. Sight flow indicator fall in two broad categories:

  • Full view indicators 
  • View through flow indicators

View through flow indicators are further available in following types

  • View through flow indicator with single glass 
  • View through flow indicator with double glass
  • View through flapper flow indicator
  • View through rotary flow indicator
  • View through drip indicator

Full view Indicators

They are also called 360 view flow indicators or cylindrical or tubular type flow indicators. This type of flow indicator is suitable for low pressure applications, moderate flow rates and where maximum visibility is required. The design has no corners where debris can get collected. The indicators are available with different glass lengths. This type of sight flow indicators should not be subjected to mechanical strain.

View through flow Indicator with glass

Sight Glass

This type of flow indicators are supplied with a single glass or two glasses on opposite sides. Two opposite glasses allow the operator to see through the process medium. If the facilities does not have adequate ambient light, a light may be fixed on the opposite of the viewing side which allows a better view of the flowing medium. When compared to the full view sight indicator, this flow indicator can be designed to withstand full ASME pressure rating.

View through Flapper flow indicator

This type of flow indicators are fitted with a hinged flapper which is visible through the sight glass. The flapper is deflected towards the flow direction. The extent of deflection gives an indication of flow rate to the operator. These indicators are suitable for horizontal and vertical installation. This type of indicators are ideal for use with transparent fluids and gases which are hard to detect by direct observation and for opaque or dark fluids which are difficult to observe.

View through Rotary flow indicator

This type of flow indicators are fitted with rotors or impellers which rotate due to flow of fluid. The speed of impeller or rotor gives an indication of flow rate through the pipe. These flow indicators are suitable for horizontal and vertical installation positions.

View through Drip indicator

Drip models are used for vertical downward flow applications in processes involving gravity flow, extremely low or intermittent flow. A strong advantage of a drip model is absolutely no obstruction to downward flow. Drip tubes are positioned in the center window area for ease of observation.

Properties of different type of glasses used for Sight Flow Indicators

Properties Acrylic Glass Soda-Lime Glass  Borosilicate Glass Quartz Glass 
Suitable for temperature up to 80 ºC 150 ºC 300 ºC 1100 ºC
Chemical Resistance Low cost design High resistance to chemicals Resistance to aggressive chemicals Resistance to aggressive chemicals