Which types of sight glasses are available?

Ebora divides the assortment into four main groups:

• Round sight glasses — compact, suitable for standard pipe diameters and high pressure
• Rectangular sight glasses — larger viewing area, commonly used on tanks and boilers; suitable for reflex glass
• Oval sight glasses — a combination of viewing area and compactness, widely used in medium-sized installations
• Sight flow indicators — inline mounting in the pipeline, with process connections such as flanges, threaded or welded connections

Select the type based on the desired installation situation (weld-in or inline), the required viewing area and the process requirements in terms of pressure and temperature.

Which glass do you choose: borosilicate or soda lime?

The glass selection is determined by pressure, temperature and the medium. The two most commonly used glass types are:

Ebora can calculate the minimum glass thickness based on your process data. Please provide the pressure, temperature and medium when submitting your enquiry.

How do you install a sight glass?

A weld-in sight glass consists of a base flange and cover flange (stainless steel or carbon steel), two gaskets and the glass. The flanges are bolted together. After welding the base flange onto the pressure vessel, check the sealing surface for deformation — rework may be necessary.

Inline sight glasses are mounted directly in the pipeline via flange, threaded or welded connections. In piping systems where backflow is a risk, a sight glass is often combined with check valves to monitor the process direction. Note: the pressure rating of the sight glass does not account for the base flange, which must be assessed together with the pressure vessel in accordance with AD 2000 guideline B9 or an equivalent standard.

When do you use double glazing or a mica disc?

Choose double glazing when safety may be at risk. With double glazing, each individual pane is dimensioned to withstand the full process pressure independently. For aggressive media or steam, we recommend adding a mica disc to the inner side of the glass. If you are working with overpressure and also want to protect your installation downstream, take a look at our range of safety valves.

Making transparent media visible

Clear or transparent media are difficult to observe with the naked eye through a standard sight glass. Two solutions are available:

• Reflex glass — used in rectangular sight glasses: the inner surface features prismatic grooves that use the difference in refractive index to make even clear media visible
• Flap, ball or rotor — used in round or oval sight glasses: a mechanical element that moves with the flow to indicate whether the medium is flowing or stationary

Choose reflex glass for continuous visual monitoring of transparent liquids. Choose a rotor or ball when the main requirement is detecting flow or no-flow conditions. For isolation of your piping system, also view our range of butterfly valves.

Accessories for sight glasses

Depending on your application, the following accessories are available:

• Lighting — for improved visibility in dark tanks or pipelines; the flange is fitted with additional drillings
• Wiper or flushing device — for media that may cause deposits on the glass; the glass is adapted accordingly
• Mica disc — for protection of the glass with aggressive media or steam

Please specify accessory requirements before ordering, as the flange and glass are adapted to accommodate them.

Wear and maintenance: always order spare parts

Every sight glass is subject to wear — even when process conditions remain within specifications. The most common cause of damage is not the medium pressure, but corrosion caused by the medium itself. Even water attacks glass over time; higher temperatures, alkalis and hydroxides accelerate this process.

Inspect the glass regularly for discolouration, haziness or hairline cracks, particularly beneath the gaskets. Cracks most often start there, as the gasket clamping force far exceeds the medium pressure. When ordering a sight glass, always include spare glass and gaskets.

Pressure and temperature: what you need to know

For gaseous media, pressure and temperature are not independent of each other. Pressure surges also cause temperature shocks, which can temporarily reduce the pressure resistance of the glass significantly. Take this into account when selecting the glass type and pressure rating.

For liquids, this does not apply: an increase in pressure causes virtually no change in temperature. The safety factor for sight glasses is based on medium pressure, not temperature. If you are in doubt about the correct specification for your application, please contact our technical department.

Sight glasses from ACI Industriearmaturen

Ebora is an official representative of ACI Industriearmaturen, a specialist manufacturer of sight glass fittings. The ACI programme covers sight glasses for vacuum to high pressure applications, in standard sizes and as custom solutions. From round weld-in sight glasses to sight flow indicators with lighting: the complete ACI range is available through Ebora. Select the right sight glass based on pressure, medium, temperature and installation situation or contact us for technical advice or a tailored quote.

Frequently asked questions

What are the most common causes of glass breakage in a sight glass?

Most glass breakages are not caused by the medium pressure, but by the mechanical force of the gaskets on the glass and by chemical corrosion. Cracks therefore often appear outside the gasket, in a location that is not immediately visible. Inspect the glass periodically and replace gaskets and glass in good time.

When do I choose a sight flow indicator instead of a weld-in sight glass?

A sight flow indicator is mounted inline in the pipeline and is suitable when you want to monitor flow or flow direction in an existing piping system. A weld-in sight glass is used when you want to install a window in a tank, boiler or silo. The choice depends on the installation situation and the required field of view.

Can a sight glass be used with aggressive media such as acids or alkalis?

Yes, provided you select the correct combination of glass type, gasket material and, if necessary, a mica disc. Alkalis and hydroxides attack soda lime glass relatively quickly; borosilicate glass offers greater resistance. Please provide the medium, concentration, temperature and pressure with your enquiry so we can advise on the correct material combination.