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How To Select Suitable Sight Glass Flow Indicator

Sight Glass Flow Indicator

Sight glass components allow operators to observe processes safely inside tanks, pipes, reactors, and vessels. However, selecting the right sight glass flow indicator for your operation can be daunting. You need to consider many factors when choosing the right sight glass flow indicator to ensure optimal performance: the process, the media, temperature and pressure, impact, physical characteristics of the process media, flow, and sanitary requirements.

Start by defining what you want to achieve with the sight glass. For example, you may want it to be used for observation inside tanks or pipes to ensure safe operation. Or you may be looking for an instrument that can be used for observation in reactors or vessels. Once you have defined this, the following guidelines give tips and step-by-step advice on selecting the ideal sight glass flow indicator.


You need to determine the structure you need.

There are two types of sight glass flow indicators.

360° view sight flow indicator.

Full view flow indicators are a traditional way to monitor process flows. They feature a glass cylinder that can be viewed from all angles, allowing ambient light to illuminate the fluid passing through it. This enables you to observe any irregularities in the process and investigate other conditions such as color, foam, and humidity. A 360° view flow indicator is ideal for monitoring high-pressure or high-temperature fluids but is not recommended for low temperatures or pressures.

View-through sight flow indicator.

This type of flow indicator has two opposing windows so that an operator can see the intervening flow of fluid simply by looking through the front window. The rear window is not necessary, and a luminaire or other light source is typically not present. This design is suited for ANSI pressure classes, high temperatures, and harsh fluid applications.


You need to decide what kind of indication function you need.

Clear glass indicators may not always be enough. The speed and direction of flow of the process material can be quickly gauged using several indicators.

Flapper sight glass flow indicator.

Flapper flow indicators check the flow direction in a pipe or conduit. The flapper deflects toward the direction of flow, and the position of the flapper changes as it is subjected to flow forces. Because there is variation in the pressure exerted on the flapper, it can be used to approximate the actual flow rate. Flapper flow indicators are an effective and affordable way to monitor flow. They are easy to install and give operators a quick and accurate indication of the current flow level.

Ball type sight glass flow indicator.

Ball flow indicators are a popular choice for high-rate and turbulent flows. The indicator is designed to indicate the presence of flow, and it does this in a way that is easy to understand: the ball suspended by the fluid suggests the presence of flow. Because gravity returns the ball to its resting position, this indicator style must be applied in vertical pipes with upward flow. Generally, this is used with slow-moving fluids or gases and not high-rate or turbulent flows.

Rotary sight glass flow indicator. 

Rotary flow indicators are a popular choice for transparent fluids and gases. They can be fitted with rotors or impellers, which turn when the fluid moves through them. The operator can observe the direction and approximate speed of flow from the window view. This is particularly useful for transparent gases and fluids, but the rotor is also visible with dark fluids.

Drip sight glass flow indicator.

Drip indicators are a type of flow indicator commonly used in industrial applications. They are designed to detect any drips or low-volume intermittent flows, typically by measuring the volume of liquid entering and leaving a pipe and indicating when this change occurs. Because gravity is utilized, drip indicators are commonly applied in vertical pipes with a downward flow.


Select the type of installation you need.

Sight flow indicators are available with three basic mounting options.

Flanged Sight flow indicator

The sight flow indicator is mounted using a flange, which secures the sight flow indicator to the inside of the pipe or vessel. This allows for easy installation and removal.

Threaded Sight flow indicator

The sight flow indicator is threaded inside the pipe or vessel. This is a secure mounting method, but it requires more effort to install and remove than flanging.

Sanitary Clamp Sight flow indicator

The sight flow indicator comes with an integral mounting system that uses a clamp to hold it securely in place. This type of mounting system is fast and easy to install, but it does not allow easy removal unless you have access to a wrench.


When choosing the materials for your sight glass device, you must consider the compatibility of the components.

Body materials should be selected based on their ability to withstand wear and tear without being damaged. Many of these materials are made in the BCST factory.

Carbon Steel Sight flow indicator

This material is widely used for general industrial applications. If the medium is without corrosive, carbon steel is a good choice; it is low-cost and durable.  

304SSStainless Steel Sight flow indicator

It is an excellent choice for any medium with some corrosion, as it can withstand it. The 304SS variety is a perfect choice for this purpose. It’s also an excellent choice for any medium with carbon steel, as it won’t corrode or rust.

316 Stainless Steel Sight flow indicator

316 stainless steel is a good choice for food and sanitary applications. It has excellent corrosion resistance and can be used in drinking water, food processing, and pharmaceutical manufacturing.

LCB Sight flow indicator

LCB, or “low-carbon boron carbide,” is a material with excellent heat resistance, making it ideal for industrial use. The operating temperature range of LCB is from -46°C to 345 °C. Because of its high resistance to heat and low thermal expansion coefficient, this material can withstand temperatures as high as 950°C without cracking.

Duplex Steel Sight flow indicator

Duplex steel is a type of stainless steel containing carbon and chromium. The difference between a duplex and other types of stainless steel is that it has more chromium than different types, making it more resistant to corrosion. Therefore, duplex steels are suitable for high corrosion mediums like salt water, whereas 304 or 316SS are not workable for these applications.


You need to confirm what level of pressure must the sight glass flow indicator withstand.

The allowable pressure/temperature limits for a visual flow indicator depend on the life of the materials used in the body, retainer, window, and gasket. When talking with the vendor, specify the temperature and pressure the sight flow indicator must be capable of enduring.

You should confirm the working pressure of the pipeline or tank at the site. The sight glass flow indicator body structure and flange selection are based on this operating pressure. If the working pressure of the pipeline or tank is higher than the sight glass flow indicator body pressure rating, it will shorten the sight glass working life and create waste in the factory.

BCST sight glass flow indicator is a measuring instrument that can determine the viscosity of a liquid. It is usually used in petroleum, chemical, and other industries. Each sight glass flow indicator is made of high-quality stainless steel. We can produce different sight glass flow indicators according to customers’ needs.

The test pressure for the sight glass flow indicator is from 150# to 1500#. In addition, we carry out a hydraulic pressure test at our factory before delivery to ensure that each sight glass flow indicator meets the requirements for use.


The spare parts selection should be paid attention to when ordering.

Selecting suitable gasket:

The integrity of the seal is essential to the excellent performance of the indicator, so make sure that the seal material is compatible with the process media with which it will come in contact.  Standard gaskets for most industrial applications are made of an enhanced PF material or Teflon. Again, gasket integrity is crucial to successful sight glass performance. Unfortunately, selecting suitable gaskets can be challenging because they are not always available in standard sizes or shapes that are compatible with your process equipment. Ensure that the gaskets are appropriate for each application, as specific processes require different thicknesses or diameters than others.

Selecting suitable glass:

  • Fused windows are made of glass pressed into a cylindrical shape and then heated to fuse the glass. The process makes the glass resistant to bending stresses that would otherwise cause cracks or failure. Fused glass can withstand high-pressure ratings, impact resistance, and safety margins. These fused glass windows are popular in the chemical, pharmaceutical, and food industries.
  • When working with soda-lime glass, it is vital to understand the limitations of that type of glass. Soda-lime glass is a soft transparent material that can be used in conditions less than 300°F and when there are no corrosive materials, and there are no drastic changes in temperature  (which could lead to thermal shock). Traditional soda lime discs and borosilicate glass discs should be replaced. They should not be reused after servicing(disassembly) because tiny fractures may occur during mounting that is not visible to the naked eye.  Under certain conditions, soda lime glass can infiltrate the process medium.
  • Borosilicate glass is the best choice for temperatures up to 500°F because it has the highest resistance to thermal shock. It’s also less likely to experience clouding or erosion than soda-lime glass, so it’s a better choice for applications involving contact with caustic or acidic chemicals.  If the glass is fused to its metal frame,  reuse is not a problem because the metal ring takes over the assembly constraints.
  • Quartz and sapphire glass are the best options for high-temperature applications that require a clear, smooth surface.
  • Quartz glass is recommended for temperatures greater than 500°F, such as in high-temperature steam applications. Quartz glass is made of quartz sand and natural gas—so it’s non-toxic and powerful. It can withstand temperatures up to 3,000°F, so it’s excellent for furnaces, boilers, and other appliances used at high temperatures.
  • Sapphire glass has been around since ancient times: it was used as a jeweler’s stone because it was challenging and scratch resistant. It’s now used to make everything from windows to television screens because it allows light through more clearly than other types of glass and doesn’t reflect much heat into its surroundings.

The selection and application of sight glass specifications are integral to any fluid transport and control process.

You should quickly and successfully navigate the sight glass specification process by carefully considering the abovementioned guides.

For more information on the selection and application of sight flow indicators in process applications, visit our website at or email us at [email protected].

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