What’s the Right Thickness for your Sight Glass?

Used in a range of industries and applications, sight glass is used to check the level of liquid in a reservoir or supply line. Critical tools for ensuring operations are running safely and smoothly, these transparent tubes or windows allow users unimpeded observation in order to monitor conditions in piping, vessels, chemical reactors, and other industrial equipment.

Because sight glass is made from sturdy glass substrates, it can be used in harsh conditions and can withstand high temperatures and pressures, as well as chemical attacks. Sight glass may be subjected to thermal shock, caustics, abrasives, and impacts. But only properly designed, appropriately thick sight glasses will offer reliable resilience.


Sight Glass Thickness Design Considerations

sight-glassSpecific sight glass design will depend on the specific application, so it’s important to carefully consider the exact conditions the glass will be exposed to and any potential hazards or special requirements. Various internal and external forces can impact the strength of glass components, and even minute flaws in the glass can lead to failure, so selecting the appropriate thickness is crucial.

Glossing over these preliminary considerations can result in a subpar design, increasing the risk of sight glass failure. This wastes both time and money, as projects must be stopped until the equipment can be repaired or replaced. Even more importantly, these failures can be extremely dangerous to the operator and surrounding workers, resulting in injury or even death.

There are several factors to consider when selecting sight glass thickness, but temperature and pressure, in particular, play key roles.


When designing sight glass, it’s important to consider any temperature extremes in which it will operate, determine whether the temperature will fluctuate, and pinpoint the highest and lowest possible temperatures. Certain glass types perform differently depending on the temperature range. For example, soda lime glass can be used at temperatures less than 300 °F, while borosilicate glass can be used at temperatures up to 500 °F. Quartz or sapphire glass should be used for temperatures greater than 500 °F. 


Pressure can also fluctuate greatly during operations, but it’s first important to understand that pressure is classified in multiple types: Working pressure is the greatest pressure allowable in an operating environment; design pressure is the maximum pressure the specific system has been designed to withstand; test pressure is generally specified by end users in order to ensure that components not just meet but exceed safety requirements; and burst pressure is essentially what it sounds like — the amount of pressure that can be applied before failure. (Burst pressure tests are usually only conducted in highly sensitive or critical applications, such as nuclear facilities.)

The various pressure capabilities of sight glass assemblies are determined by the material, thickness, and unsupported diameter, and both normal operating pressure and peak upset pressure should be taken into consideration when creating sight glass thickness specifications.

Find the Glass Thickness You Need

As a worldwide leader in the manufacture of quality fabricated glass parts, Swift Glass has years of experience fabricating sight glass for many applications. To make the process for our customers even easier, we’ve created a glass-thickness calculator; simply type in the unsupported diameter of the glass you’re working with, as well as the PSI it needs to withstand, and it will provide the recommended thickness to ensure optimal strength and durability. If you already have the unsupported diameter and the thickness of the piece of glass you’re working with — enter those values and calculate the PSI it will be able to withstand.


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