Jim Wolfe
Jim Wolfe

Jim Wolfe is President of John C. Ernst Co. Inc. Mr. Wolfe has 25 years’ experience in the process observation industry and has bachelor’s degrees in Business and Engineering. He can be reached at jimw@johnernst.com.

Q:Can you describe the difference between a sight-glass window and sight-flow indicator, and when one or the other would be preferable to use?

A: Sight-glass windows, sight-flow indicators, and liquid level gauges are commonly and loosely referred to as “Sight Glasses”—yet each one differs in function. A sight-glass window or sight window is a window into a tank or vessel for visual inspection of the process application. A sight-flow indicator is a window into the piping system to observe the flow stream, typically of liquid, through the glass window.

Q:In what types of applications are sight-flow indicators commonly used?

A: Liquid applications, such as water, wastewater, petroleum, lubricants, solvents, acids, and fuels, are common. Sight flows are also used in sanitary applications, such as food & beverage where crevice-free designs may be critical. They can be equipped with a mechanism for enhancing visual observation of flow, such as rotators for clear fluids like water, flappers for flow direction and intensity, and drip tubes for very low flows; or they can be plain style with no indicator. Gaseous flappers can be ideal for low velocity gas streams. Teflon-lined sight flows are used for corrosive environments and safeguard against chemical reactions.
When it comes to monitoring the flow of process media, what are some advantages of sight-flow indicators as compared to other technology options, such as electronic sensors?

Technology such as electronic sensors can be unreliable and fail or provide erroneous information. Sight flows are extremely reliable and are less likely to fail due to the design. No power is required, thus eliminating any risk of explosion. They need little maintenance and offer long product life. Direct visibility of the media to monitor any fluctuations that may affect the process, such as clarity, color, or viscosity, is an advantage. These characteristics in most cases cannot be determined without high cost. Sight flows often function as an effective, economical redundant device to high-tech monitoring systems.

Model 201
Model 201 Flat Glass Double Window Sight Flow
Model 255
Model 255 Cylinder Glass Sight Flow

Q: What are some different types of sight-flow indicators available on the market today and how do they compare?  

A: There are mainly two types of sight-flow indicators: full view cylinder glass and flat glass. Full-view sight flows offer 360-degree visibility of the process flow and can fit custom lengths. Pressure ratings on full view units are lower than flat glass models due to cylinder glass limitations. Full-view SFIs remain an economical selection with a variety of available materials and up to 12-inch pipe size. Flat glass (often called Double Window) sight flows are typically castings engineered with two circular flat glass discs which, depending on size, can offer pressure ratings in excess of 3,000 PSI. As discussed earlier, they can be fitted with different indicating mechanisms. Sizes range from ¼-inch to 16-inch pipe size. Both types can be manufactured with NPT, Flanged, BSP, Socketweld, Buttweld, or Tri-Clamp Connections.

Q:What are some general points to consider when specifying and selecting a sight-flow indicator for an application?
A: There are six main points to consider in the selection process:

  •  Wetted materials, such as body and seal components are critical for proper media compatibility;
  •  Operating pressure;
  •  Operating temperature;
  •  Indication type, such as plain, rotator, flapper, or drip tube;
  •  Type and size of connection; and  
  •  Flowrate of media.

Q: What are some common factors that would prevent a sight-flow indicator from properly functioning? What are some best practices for installation or operation?  

A: Sight-flow indicators are very tough, but avoid applications where debris or solids can become lodged inside or cause glass failure. Avoid media that can coat the glass, compromising visibility. Excessive pressure and temperatures can cause failure. Some best practices include proper gasket selection to prevent leaks and proper sizing based on the flowrate. Always replace chipped, etched, or cracked glass. When replacing glass and gaskets, always follow proper torqueing techniques to avoid “point loading” of the glass, which can cause glass failure.  Borosilicate is the preferred glass material for sight flows. During installation, proper mechanical supports should be in place. While installation is fairly simple, the manufacturer’s installation and operations manual should be carefully read.

Q: As technology advances, what role do you see sight-flow indicators playing going forward?

A: Due to simplicity, reliability, and direct visual advantage without requiring power, sight flows will always be a key component in industrial processes. Sight flows will also continue to function as a redundant failsafe backup to modern process fluid monitoring. New materials and accessories such as alloys, linings, lighting, cameras, fused glass, and glass shields will continue to develop to improve compatibility, enhance visibility, and reduce the potential for any failure.