Flow Indicators

The term “flow indicator” is used for a few different elements in the industrial instrumentation world. It is helpful to be aware of the differences because if you don’t understand them, it can make finding the right device for your needs more difficult.

The most traditional usage of the term “flow indicator” is to refer to a device that does not measure the flow rate. Nor does it monitor the flow rate in the sense that it can provide switching capabilities. It simply provides verification that flow is occurring. In essence, it answers the question “is flow happening, yes or no?”. It generally does not provide any data on the flow, although most models enable direct viewing of the media inside. KOBOLD uses the term “flow indicator” to refer to this class of flow instrumentation.

For purposes of our discussion and how we classify our products, we will be using the term “flow indicator” for the devices that simply let us know if flow is occurring. Flow indicators signal if flow is occurring within the piping system as most piping systems are not transparent to allow direct observation. Verifying flow requires a mechanism of some kind that is inserted into the piping system as a “window” and provides a visual cue as to what is going on inside.

How a flow indicator works depends on which manner of indication that it is providing. All types of flow indicators function on the same simple principle. Flow through the devices generally moves something or sometimes nothing, in the case of a simple sight glass. Typically, the flow moves a ball or spins a paddle wheel. Flaps, chains, and other less common moving mechanisms are also used. If no mechanism is present at all, then the flow can still be verified by looking closely at the glass tube to see if there is flow happening. The advantages to a flow indicator with a moving mechanism is that they can be seen from much further away.

Unless you are using the term “flow indicator” to refer to a “flow meter”, you do not “read” a flow indicator. You can view it to see if flow is occurring, but there is nothing to “read” as an indicator does not provide a flow rate measurement to “read”. You typically view a flow indicator for indication that flow is occurring by verifying movement of the indication mechanism.

Now that we have seen the different ways that the term “flow indicator” can be used, it is worth noting the other classes of flow instrumentation to ensure that you are searching for the right type of flow instrumentation.

To help further clarify which type of instrumentation is needed for your application, below is a general listing of most flow instrumentation, from simplest to most complex, with a brief description.

• Flow Restrictor: A flow restrictor simply restricts the rate of flow within a system to a predetermined amount. It cannot provide flow measurement.
• Flow Sensor: This term is more of a general catch-all terms for flow instrumentation because they all sense flow, to one degree or another. The term “flow sensor” is sometimes used interchangeably with any of the terms above, but really should be used as more of a general term for the entire class of instrumentation.
• Flow Indicator: They show that flow is happening. They answer the question, “Is flow happening at all, yes or no?”. Most models allow a clear view of the media as well.
• Flow Switch: They monitor the flow and trigger an electrical impulse at a certain rate of flow within the system. They do not provide continuous measurement unless the flow switch is part of a full flow meter. They answer the question, “is the flow too high, too low, or happening at a specific point?” or they trigger a specific action in the system. Some flow switches, like variable area flow switches, can also allow for direct viewing of the media as well through a plastic or glass tube.
• Flow Transmitters: These measure the rate of flow in the system and provide the measurement as an electrical output. Some flow transmitters may be fitted also with integral displays, controllers, or totalizers. Typical flow transmitters are sometimes referred to as “blind transmitters” because looking at the device tells you nothing, unless it connected to something that can interpret the signal produced by the transmitter. They answer the question, “how much flow is happening right now”.
• Flow Meters: These also provide a continuous measurement and are somewhat erroneously used interchangeably with “flow transmitter”. These devices also provide continuous measurement, but the rate can typically be seen directly at the device itself by a mechanical or digital means. They can also have integral switches. Some flow meter technology types allow for direct observation of the media as well. They can also have integral electronics that enable batching functions, where a precise amount of a media is dispensed. They can also have totalizers that tell you how much media has flowed in a certain period of time or for the total life of the instrument. They also answer the question, “how much flow is happening right now”, and provide many other features.

Sometimes referred to as “sight glass flow indicators”, sight flow indicators do exactly what their name states. They enable sight of the flow, usually through a glass tube or a glass window. Sometimes “sight flow indicators” refer to indicators that have an integral mechanism to clearly display flow is occurring, and sometimes they refer to clear glass tubes or windows that simply allow the media to be seen. This is really a general term for the entire class of “flow indicators” as all indicators indicate flow is occurring by something being seen by the user. You can view our sight flow indicators here.

Most standard pressure capability flow indicators are a combination of a metal and/or glass housing and typically employ a plastic means of indication within the line. The metal used is typically brass or stainless steel.

High pressure applications require a more rugged build to handle higher pressures. This requires higher-pressure capability metal, glass, and seals. It also requires a more durable flow indication mechanism than the plastic indication elements of lower pressure flow indicators.

Probably the most common type of flow indicator, a paddle wheel flow indicator, employs a paddle wheel to indicate flow. The same simple technology is also used in paddle wheel flow meters, except in paddle wheel flow meters, flow rate is provided by measuring the speed of the paddle rotation. Depending on the model of paddle wheel flow indicator, visual flow indication can also be provided by a paddle wheel flow meter.

Although no flow measurement is provided by a paddle wheel flow indicator, the speed of the turning wheel can very, very roughly indicate whether the flow is low or high by observing the speed at which the paddle is revolving around its axis. Visit our product page to learn more about the paddle-wheel flow indicators that we offer.

Rotameter technology is also referred to as “variable area” technology. Simple flow indicators are not usually created in variable area technology. Most variable area flow devices are flow meters and not indicators, although variable area flow meters provide visual flow indication if they are built with a translucent tube that houses the float. Because the only extra step needed to create a flow meter out of what would be a “rotameter flow indicator” is a scale printed on the glass tube by which the top of the float can be read against for a flow rate, most variable area devices are flow meters.

Most liquid flow indicators are compatible with water and can be used as water flow indicators. Besides the general application parameters such as pressure and temperature, the other consideration to be aware of when choosing a water flow indicator is accounting for the characteristics of the water in the system.

Is it clean, ultra-pure, dirty, or mixed with other media that causes it to be aggressive or viscous? These factors can determine the suitability of certain models of flow indicators for a specific water application. To see which of our water flow indicators are compatible with different kinds of water, view our flow product comparison page or speak with one of our knowledgeable engineers.

Most gas flow indicators are suitable for use as air flow indicators. To determine whether an air flow indicator is right for your application, make sure that it meets the pressure and temperature requirements of your system and pay attention to whether or not the air has any characteristics that could damage the visual indication mechanisms, such as large particulate matter that may be carried in the air flow stream.

Our suggested air flow indicator is our DKB Air Flow Indicator that offers indication via a ball mechanism.

Water cooling systems protect expensive equipment by removing heat produced by the process by transferring it to water and moving the water into a cooling process where the temperature is reduced and then the cooled water is sent back to reduce the heat once again. Without proper water flow, the system can become compromised and cause a lot of costly damage. Flow indicators can be used in various parts of the process to verify flow.

Flow meters or flow transmitters, go one step further than a flow indicator and provide a continuous rate of flow. They are an even better safeguard for water cooling systems as they can offer more information and they can control functions that a simple flow indicator cannot.

For example, a flow meter would be able to detect leaks by sensing a significant unexpected drop in the flow rate. Flow meters also offer more automation to the process because they can transmit the data to control centers for continuous monitoring. A simple flow indicator usually requires on-site visual confirmation of flow.

Flow meters can also provide switching options, that in tandem with the pumps in the system, can keep the flow rate steady to enable consistent cooling to the process. For a simple water-cooling flow indicator, we recommend the DA Industrial Flow Indicator.

Because flow indicators typically only use a mechanical means to indicate flow and no electrical components are part of the device, most any liquid flow indicator can be used as a liquid fertilizer flow indicator as long as the liquid characteristics are taken into account. Certain indicators will not tolerate particulate matter or higher viscosity liquids very well. Because of the corrosive nature of liquid fertilizer, flow indicators made of stainless steel are usually recommended, although the chemical composition of liquid fertilizer can vary, and a flow indicator should be specified for the exact composition of the fertilizer.

Because liquid fertilizer can also accumulate build up on the sight glass, our unique model DAA Sight Glass Flow Indicator with an integral cleaning mechanism, which does not require system shut down or device removal for cleaning, delivers an extra value for liquid fertilizer.

Installing a flow indicator is relatively easy as they are mechanical in nature do not require any menu programming. With little to no exception, all simple flow indicators, that are providing visual indication or flow only with no flow rate measurement, are inline devices that are inserted into the piping of the system.

Each end of the device will have an inlet for the flow and an outlet for the flow that are on opposite sides of the device. The inlets and outlets will have a connection type, here in the US it is usually an NPT thread or an ANSI flange that will connect with the pipe on each side of the device, effectively inserting itself directly into the piping system and the flow path of the media.

Here are some other considerations for installation and commissioning a flow indicator:

• Ideally it is best if the pipe remains full so that it properly engages the indication mechanism.
• Vertical flow up is an ideal installation as this orientation ensures a full pipe during flow.
• Suddenly starting the inflow may cause pressure peaks exceeding the working pressure of the instrument. This could cause damage to the indicator.
• Remove any coarse foreign matter before installing the instrument in the pipe.

Flow indicators are generally low in price compared to most other industrial flow instrumentation. The most prominent pricing variables will be the materials used in the construction. For example, high pressure models require higher grade materials to ensure longevity and will cost more as a result.

It is important to verify that the flow indicator that you are considering is an industrial grade flow indicator and not a flow indicator meant for more residential use. Cheap, all-plastic flow indicators are not going to stand up to the rigors of industrial applications.