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A sightglass, also called a control or inspection device, is a window in a pipe, boiler, tank or silo for the visual observation of the presence, flow or level of a medium. This allows a device or the entire process to be checked / inspected.

A welded sight glass generally consists of a stainless steel or steel base flange (block flange, welding flange) and a blanket flange, 2 seals and the glass. The flanges are bolted together.

The sight glasses in the piping system (inline mounting) can be supplied with various process connections such as flanges, wire connection or welding connections.

Depending on the pressure, temperature and the medium, one of the two most commonly used borosilicate or soda lime glasses is often chosen for the glass. Ebora can use this information to calculate the minimum thickness of the glass for you.

Please note that, contrary to popular belief, every sight glass is subject to wear and tear.
We therefore advise you to check the glass regularly and to always order spare glass and seals.

If safety is compromised, you can always use double glazing. In the case of double glazing, the glass is always laid out in such a way that it can withstand the entire pressure. In case of aggressive media or steam, we recommend adding a mica disk to protect the glass!

Transparent media are more difficult to see. The following has been thought of for this purpose. In a rectangular sight glass one often uses reflex glass. This is a glass with ribs on the inside. The refractive index of the medium makes even the clear media visible. For the other sight glasses, a flap, ball or rotor is often used to show the flow of the clear transparent media.

A sight glass can also be fitted with lighting to allow a better view into the tank or pipe.

Ebora is a major player in the field of sight glasses and has divided them into the following 4 main groups.

  • Round sight glasses
  • Rectangular sight glasses
  • Oval sight glasses
  • Flow sight glasses

The range includes sight glass luminaires from vacuum to high pressure and/or temperature applications.

If the medium can cause deposits in the sight glass, a wiper or sprinkler can be used. We would like to receive these options before ordering, because the glass needs to be adjusted for this purpose. The same applies to the use of lighting, because the flange is provided with extra boreholes.

Installation guideline:

After welding the base flange onto / into the pressure vessel, the sealing surface must be checked for deformations! Finishing may be necessary!

The pressure class does not take into account the basic flange. This will have to be checked together with the pressure vessel according to AD 2000 instructions B9 or an equivalent standard!

For sight glass lighting and specials, please contact us at +31(0)26-3706830 or info@ebora.nl

Please note: Almost all problems that occur with sight glasses concern the incorrect choice of materials in combination with the medium!

A few points of attention with sight glasses:


  • Rarely is the pressure of the medium the cause of breakage or leakage of the sight glass.
  • Most cracks are caused by the mechanical forces on the seals and the glass.
  • The medium pressure on the glass is nothing compared to the pressure of the seals on the glass.
    Often the crack will start outside the seal. (Not directly visible)
  • The safety factor in the pressure is based on the medium pressure.
  • The pressure of the seals often exceeds the permitted long-term surface pressure.
    These cracks will occur slowly.
  • Fractures caused by impact almost always come from outside.

Chemical resistance

  • If you estimate the max pressure realistically, you have a chance of better resistance.
  • Corrosion is the most common cause of breakage and leakage.
  • General fluids also corrode the glass (even water!!!)
    Higher temperatures, alkalis and hydroxides separate the silicon compounds. Think of drinking glasses that no longer shine after a while.
  • Temperature, pressure and tension accelerate the corrosion.
    This is why it is the strongest among the seals.

Pressure - temperature

  • The safety factor on sight glasses is the pressure, not the temperature.
  • Increasing the pressure in liquids hardly leads to an increase in temperature.
    Pressure and temperature are not dependent on each other for liquids.
  • Most high temperature applications contain gases.
  • Pressure changes in gases also cause temperature changes.
  • Pressure shocks therefore cause temperature shocks in gases.
  • At higher temperatures, the glass can very quickly withstand less pressure.
  • Pressure and temperature peaks can therefore not be viewed independently in the case of gases.

Get in touch!

+31(0)26 370 6830

Request an account!


Get in touch!


Request an account!

+31(0)26 370 6830

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