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Blog series: How to handle low liquid flows? Part 5

May 19, 2020 Bart de Jong

Part 5 - How to deal with external conditions?

In the world of flow control and measurement, we distinguish between ‘low flows’ and ‘high flows’. But what does this really mean? At Bronkhorst, we supply flow meters and controllers in the ‘low flow’ range. Do you know what ‘low flow’ means? In our blog series, we explain the difference and share our tips & tricks for low liquid flow setups. In this final part of the blog series, you can read about the influence of external conditions.

Read part 1 of the blog series
external conditions vibrations in flow-meter or coriolis

How can external conditions influence your flow meter? 

Bronkhorst flow meters mentioned in the previous parts of this blog series, that are capable of measuring (ultra) low flows, are very sensitive. This implies that even the tiniest disruptions in a customer’s process or ambient conditions can be detected. So any disturbances that may have been present in the process now emerge, because the measurement accuracy is much better than before this sensitive flow meter was applied. A customer might then respond with “there is something wrong with the meter”. However, don’t shoot the messenger! Instead, use the information in this blog series to optimise your own process. Check external aspects such as the tubing to and from the flow meter, the influence of surrounding vibrating equipment or the presence of solid particles in the liquid flow.

To put the previous consideration in a practical context: when choosing a Coriolis-based mass flow meter or flow controller, a relatively high pre-pressure will be necessary to overcome the relatively high pressure drop over the device. This is mainly the case when Coriolis instruments are operating in their nominal flow range. However, because Coriolis instruments have a large turndown ratio down to 1%, the pressure drop in the lower region is usually negligible and comparable with thermal. 

Although the measurement conducted with such a Coriolis flow meter is much more accurate than with a thermal-based flow meter, a high pre-pressure from a pressure vessel will allow more gas to be dissolved in the liquid. This is released as gas bubbles downstream in the process at a lower pressure, resulting in instability. These low-flow blog series are therefore intended to create awareness of all the things you can do to improve the setup of your process, each with their own pros and cons and possible effects.

What tubing should I choose?

Choose the smallest possible tubing. By minimising the length and diameter of the liquid supply tubing between the flow meter and the process, the fill and refresh times will be as short as possible. The pressure drop over Coriolis-based mass flow meters is much larger than over thermal-based flow meters, because the capillary in the latter is about 20 times shorter and its diameter is larger. 
Find an optimum between the pressure drop and the smallest possible internal volume of the tubing. For low flows up to 100 g/h, tubing with an outer diameter of 1/16 inch (~ 1.6 mm) is recommended. For higher flow rates, we recommend 1/8 inch (~ 3.2 mm) tubing to limit the pressure drop. Try to use as few connectors, bends or T-parts as possible, because they can cause air accumulation which results in flow instability. If necessary: use small-volume connectors.

Choice of piping in flow meters
Piping in flow meters

The choice for hard tubing like stainless steel or flexible tubing is mainly based on the operational pressure. In a production environment at high pressures, flexible hoses are rarely used. For flow rates up to 2 g/h, the use of hard tubing is strongly recommended because it prevents internal volume changes. For flow meters with Hastelloy capillaries, tubing made of Hastelloy is recommended. PEEK, PolyEther Ether Ketone, is preferably applied for aggressive liquids that attack stainless steel. 

Would you like more information about: ''Why the choice of piping important is for thermal mass flow meters?

Read the blog about choice of piping

Prevent water hammer by avoiding sudden changes in tubing diameters

A phenomenon that needs special attention is water hammer. Also familiar from your toilet or dishwasher at home, this is a hydraulic shock that occurs when a liquid in motion is suddenly forced to stop, start or change direction. This will result in pressure surges that are much higher than (static) pressure values for which a system has usually been dimensioned. 

Prevent water hammer by avoiding sudden changes from large to small tubing diameters, by applying a small pulsation damper (where an isolated gas column has a cushioning effect), by gradually increasing an applied pressure or by avoiding the pump running against a closed valve.

How to deal with vibrations?

Vibrations from a pump or other surrounding equipment can negatively impact the performance of Coriolis-based mass flow meters. The working principle of Coriolis instruments is based on vibrations, so make sure that pumps and other surroundings machines are vibrating at a different frequency than the frequency of the Coriolis instrument. To prevent these external vibrations from reaching the Coriolis flow meter, you can use (slightly flexible) PEEK tubing, or the flow meter/controller can be mechanically uncoupled by applying a curl in the hard tubing (‘pigtail tubing’). For the Coriolis-based instruments, Bronkhorst has 2kg and 4 kg mass blocks available with vibration dampeners, an extra buffer to absorb vibrations. 

Read our blog ‘How to deal with vibrations using Coriolis mass flow meters’ to find our more how vibrations can affect the working of your flow meter.

Read the blog about vibrations
Mass block for Coriolis flow meters against vibrations
Mass block for Coriolis flow meters

What about Calibration?

We recommend calibrating thermal-based flow meters such as μ-FLOW and LIQUI-FLOW devices once a year. For Coriolis devices like the mini CORI-FLOW ML120, no calibration is required because their measurement principle is less vulnerable to ageing. However, in sectors like automotive, pharmacy and food, calibration is required by legislation or by standardisation. In these industries, it is vitally important that a measuring device indicates the correct value.
For calibration purposes, it may be useful to apply transparent flexible tubing such as Teflon, so that any gas bubbles in the liquid flow can be detected visually.

Bronkhorst Calibration Centre
Bronkhorst Calibration Centre

Use particle filters to avoid clogging

To prevent the small diameter tubing and capillaries of the flow meters from clogging, or to prevent damage to piezo control valves, it is recommended that one or more particle filters are incorporated. This becomes more important when channels and control valves are used for the lowest flow rates. The filter pore size should be at least ten times smaller than the smallest capillary, orifice or control gap in the system, and upstream of a piezo control valve the recommended pore size should be 5 microns. A large filter surface area can compensate for a high pressure drop caused by a small pore size.

Do you want to read more?

This blog was the final part of a blog series of 5. Check out the previous parts:

Would you like more information about low flow?

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