How Does a Sight Flow Indicator Work?

How does a sight flow indicator work?

A sight flow indicator gives the operator a window to observe how a process fluid moves through the pipeline.

At a basic level, how a sight flow indicator works is simple: just look through the window to observe the flow in a pipeline. Actually, there are complexities in the many types of indication and construction of the equipment. Each style of indicator works a little differently.

To make flow easier to see, it may have components that are set in motion by the flow. The sight flow indicator is used for visual inspection by a human operator; it has no electronics for sensing, measurement or control.

Operators use sight flow indicators to inspect fluids for characteristics such as clarity, color, and foam, or to confirm that there is flow in the pipeline. Often sight flow indicators are used to visually confirm conditions reported by instrumentation and to provide a backup in case of instrument failure or power outage.

The simplest design of sight flow indicator is a glass column through which liquid or gas flow may be observed. The flow is illuminated by ambient light or LEDs and may be viewed at any angle.

A view-through flow indicator has two opposing windows so that an operator can see the intervening flow of liquid or gaslighted from behind, either by ambient light or by an attached LED or incandescent lamp.


What are the types of indication?

There are multiple types of flow indication. A sight flow indicator can be fitted with one of a variety of indication components that make flow easier to observe and give operators a general idea of flow rate. Each work in a different way and are suited to certain applications. In my experience, even seasoned process engineers are not aware of all the options available, so they are worth reviewing. These include

  • Rotary flow indicator
  • Flapper flow indicator
  • Visual flow meters
  • Drip indicator
  • Ball Flow indicator
  • Flutter indicator

Rotary flow indicators have rotors or impellers that are turned by the flow of liquid or gas. Operators can observe the direction of flow and the approximate speed of flow. This indicator operates in any position (vertical, horizontal, etc.) and with any direction of flow. I see these in many plants that I visit.

A flapper flow indicator has a hinged flapper. The flapper is deflected in the direction of flow. The angle of the flapper provides operators with an approximate gauge of flow. It operates only in one direction. This style is best applied on horizontal pipelines, but it may also be employed in vertical pipelines with upward flow.

A visual flow meter is similar to a flapper flow indicator except that the flapper has a reset spring. The force of the spring is overcome by the flow of the process fluid, and a graduated scale marked on the glass indicates the measurement of flow. Visual flow meters are an alternative to an expensive flow meter in some applications.

Drip indicators may be sight flow indicators designed specifically for drip observation or they may be conventional flow indicators installed with a drip tube. Drips may be observed in applications such as distillation. Normally drip indicators are used in vertical pipes having a downward flow of process fluid or (in the case of condensation) of process gas.

In a ball flow indicator, flow moves a ball from the bottom of the indicator housing to a position at the top of the sight window. The suspension of the ball by the fluid indicates the presence of flow. This style of indicator must be applied in vertical pipes with upward flow. Another style contains the ball inside a glass dome. When the flow diminishes, the ball drops out of view. This style must be installed in a horizontal position. I seldom see the ball style being used.

A flutter Indicator is a thin ribbon of material moved by the process flow. The intensity of flutter is a rough indication of flow speed. Unlike other indicators that cannot be fully constructed of non-reactive materials, flutter indicators can be Teflon® coated and used in Teflon coated pipelines.


How are sight flow indicators constructed?

Sight flow indicators are constructed in a variety of different configurations. To fully understand how sight flow indicators work, specifiers should have at least a general idea of how they are constructed.  How they are mounted and their range of operation affect their usefulness for a particular situation.



Sight flow indicators are mounted in-line between segments of a process pipeline. They can be mounted in a variety of ways. For view-through models, mounting styles include flange, butt-weld, socket-weld, threaded and sanitary clamp.

Pressure ratings range from vacuum up to about 3000 psi. Glass column sight-flow indicators are suitable only for low-pressure applications.

Sizes are based on the diameter of the pipeline and range from ¼ inches to 12 inches, although larger sizes may be available as a special order.



View-through flow indicators have cast bodies made of carbon steel, stainless steel, iron, or bronze. If necessary, Teflon or FRP linings can be added to protect metal bodies from reacting with process media. (PVC sight flow indicators are also available, although in industrial and chemical process applications, plastic sight flow indicators are almost never used.)



The glass used in sight flow indicators includes tempered or annealed soda-lime glass (common glass) or borosilicate glass. Soda-lime glass should not be used with corrosive chemicals.  For view-through models, each glass disc is sandwiched between gaskets in an assembly that bolts to the body or the glass are fused to a metal ring that attaches to the body with a clamp.



Gaskets are available in a wide range of materials and selected for chemical and temperature compatibility.


2019 Flow Control Innovation Awards Nominee

Vote for LumiFlo®!

Image:  LumiFlo Product Image

We’re so excited to announce that the LumiFlo® LED light has been nominated for a 2019 Flow Control Innovation Award.

TheLumiFlo LED Light is a first-to-market, exclusive patent pending LJ Star product that is designed to improve the lighting of full view sight flow indicators in process fluid lines.  It easily attaches to most inline full view sight flow indicators in process fluid lines, allowing operators to clearly see color, flow and particles moving through the sight glass in order to visually inspect product quality, clarity, viscosity and consistency.

Vote – Flow Control Innovation Awards

The LumiFlo is a tremendous product innovation, but we can’t win without YOU! Don’t miss the chance to cast your vote in the Flow Control Innovation Awards.  It’s quick and easy to do online.  No long waiting lines, no propaganda and no hanging chads.

Click here to cast your vote for innovation!

TheLumiFlo LED uses highly reliable, long-life LEDs for the perfect amount of light and optimal color temperature for a clear view of processes in the brewery, chemical, pharmaceutical, biotech and food and beverage processing industries​.  It is extremely adaptable, innovative and affordable for the customer. ​


  • Exclusive LJ Star technology – the first of its kind​
  • LED tube provides perfect color temperature light output to identify actual flow colors, with 98% Color Rendering Index (CRI)
  • Operating voltage:  24VDC with M12 connection​
  • Watertight seal for external Clean-in-Place wash-down maintenance​
  • The LumiFlo uses an FDA compliant silicone enclosure made with environmentally friendly material. It is designed to prevent yellowing, microbial and mildew growth of the silicone using nano material additives.
  • Safer to use than previous options​


  • Easily attaches to most inline full view sight flow indicators in process fluid lines​
  • Hands-free lighting​
  • Securely attaches under sight flow indicator thru-bolts without the use of additional fasteners​
  • Easy installation and replacement ​
  • No need to modify or disassemble sight glass for installation​
  • Optional battery pack delivers several hours of power for remote hard to wire locations​


  • Value Price Point for easy budget approval​
  • Improved lighting provides optimal process visibility​
  • Designed to provide high user value at low cost, no modification required to sight flow

Interested in learning more about the LumiFlo?

Alexa, Buy Me Beer

Thanks to all who stopped by to see us at the recent Interphex and Craft Brewing shows.  We hope you enjoyed talking to our process observation experts and learned a few things about our products, as well.

Raise Your Glass with us In New York City April 2-4

We’ll see all the sights and take in a show… INTERPHEX is the premier pharmaceutical, biotechnology, and device development and manufacturing event where you can “Experience Science through Commercialization.”

Join over 11,000 global industry professionals and 650+ leading suppliers, including L.J. Star, for demonstrations, and networking events to leverage quality, efficiency and cost effectiveness in today’s ever-changing pharma market.

Webcast: How to Select and Maintain Sight Glasses for Chemical and Pharmaceutical Applications

Improperly specified, installed or maintained sight glasses can represent the weakest link in a processing system.  The wrong glass or materials used will limit observation, require extra maintenance and replacements, and even fail under pressure with catastrophic results – since glass can fracture at 5 miles/second!

Before you choose your next sight glass, there are a variety of key specifications and glass material questions to answer, based on application temperature, how any chemicals used react to the glass, and the level of pressure involved. ​And, you’ll want to understand the differences between true borosilicate glass – its performance and certifications – compared to others that may be suggested as equivalent alternatives.

Watch this tutorial webcast from the process engineers at LJ Star to see how others are specifying and maintaining sight glass installations in their facilities.

Webcast: Best Practices in the Selection and Use of Sanitary Clamps

Keeping a Firm Grip on Sanitary Clamp Safety

For pharmaceutical and biotech companies, maintaining sterile process control is an ever-increasing challenge – one that can lead to cross-contamination, short term loss of product, long term drug shortages, lawsuits and even loss of life should sterile process be breached.

Sanitary clamps are crucial to keeping processing systems clean while allowing maintenance personnel to disconnect and reconnect piping quickly and securely. However, as simple as that sounds, making secure and sanitary process system connections consistently requires maintenance personnel to have the right clamps, the right tools, and the right training. A connection that isn’t properly secured can call the hygienic integrity of the whole processing system into question.

In addition to the risk of contamination due to manufacturing equipment issues, improper handling practices can be equally problematic. Pressure, temperature, and caustic process materials are all potential workplace hazards that can injure. When it comes to process equipment, maintenance and training are critical to ensuring reliability and safety.

This webcast, How to follow Sanitary Clamp Best Practices, shows how to help maintain sterile process control and offset the risk of improper connections from clamps, gaskets and ferrules by educating pharmaceutical and biotech workers on proper clamp assembly, inspection, installation and maintenance.


Webcast: The Top Three Steps to Ensuring Sight Glass Safety

Keeping an Eye on Sight Glass Safety

If improperly specified, installed or maintained, sight glasses can be the weakest link in a processing system because they can limit observation capabilities, require frequent maintenance or replacement, or make it difficult to illuminate the vessel in which they’re installed.


Additionally, sight glasses can fail, endangering workers and causing extensive destruction and system downtime. When glass fails under pressure, it is sudden and catastrophic. Glass can fracture at 5 miles per second which can cause irreputable damage to both your workers and your facility.


In systems made primarily of metal, such as pressure vessels or process piping runs, the weak spots are generally sealing joints and glass.  Typically, failure of an industrial sight glass on a piece of equipment, or within a piping system, will halt the entire manufacturing process and force downtime until the equipment can be repaired or replaced.


 In this webcast, How To Ensure Sight Glass Safety, you’ll learn how to choose the right sight glass for the application and applicable standards, how to install the sight glass correctly and how to routinely and carefully maintain sight glasses.


In our efforts to continually improve our products we are introducing a change to the Domed Hexagon Nut used to tighten our range of clamps. These nuts are used to replace the Wing Nut in certain applications. The existing Domed Hexagon Nut, we currently supply, will become obsolete to be replaced by the new Hexagon Flat Nut.

The Hexagon Flat Nut has the following advantages over the Domed Hexagon Nut:

The contact point of the hexagon flat nut is only on the face of the clamp segment. This means the force is applied solely to a plane parallel to the nut face. This has benefits over the domed hexagon nut which has a contact point inside the legs of the clamp segment. In addition to the above, galling tends to be reduced as the thread at the tip of the nut is not deformed. This often results in a tighter thread which has been seen on multiple clamps. The hexagon flat nut also maintains the perpendicularity of the thread to the clamp body. This reduces the off collinear misalignment of the thread further reducing the tendency to gall.

You can read more on the entire LJ Star line of Sanitary Clamps, or contact us  for a quote and more information on product specs and performance.


Surface Finish and 3-D Printing Are Big Topics at Recent BPE Meetings

Aarash Navabi, CPIP, Director of Research + Development, Research Dynamics, Massachusetts Division of L.J. Star, participates on several critical ASME Bioprocessing Equipment (BPE) Committees, the Surface Finish Subcommittee, the Polymers Subcommittee and is leading another Task Group for BPE. At meetings held in San Diego, California in May 2018, Aarash reported on a variety of committee discussions and decisions that drive the processing industry.