Correctly specifying the flowmeter that will be used to meter the flow of a chemical process is very important. Getting it wrong can have a serious impact on the quality of your product, process efficiency and ultimately your operating profit. Drawing upon decades of experience we have prepared a guide to the 10 most common areas where people make mistakes when specifying a flowmeter and how to avoid them.
1. Flow rate
In some circumstances users have no idea what the flow rate is in their process pipe. “That is why we want to measure it”. Plant construction is often such that for mechanical reasons much larger pipes are used than merely the size required to carry the flow. It is then that we have to start looking at the motive force behind the flow and the plumbing considerations. Often a user will overlook/forget that that their process is subject to significant pulsations and peak flow is perhaps forty percent higher or even twice the average.
2. Operating Temperature:
Some users do not read their meters maximum operating temperature specification and assume that all flowmeters will operate to 150°C. When the meter fails it is somehow the products fault. In a similar way to mobile phone manufacturers who put water evident markers in their phones we now include non-reversible high temperature markers in our meters to record if the maximum operating temperature has been exceeded for any length of time.
Many users are unaware that a lot of flowmeters are viscosity sensitive and will significantly change their calibration with changes in the fluid properties. These Reynolds number sensitive devices are usually only linear in the “turbulent” flow range and their performance drops off as the fluid runs “smoother”. For instance, using a 40 cSt oil would immediately take a turbine meter, rated and calibrated for 1cst, out of the turbulent zone and into the non-linear laminar flow region. Changes in temperature would then change the viscosity and the calibration factor. Many flowmeter types rely on higher Reynolds numbers for their linear performance including vortex shedding, Pelton wheel, variable area and most pressure differential devices. Other technologies such as positive displacement, Coriolis and ultrasonic meters are less sensitive to these changes but not completely immune.
4. Output type:
“Will the flowmeter interface with my instrument?” We don’t know, what are the input requirements of your instrument? Logic level, NPN or PNP pulse, analogue current or voltage or a higher-level language? Please check this requirement before specifying the meter.
5. Pipe Connections:
Your chosen pipe connection should be dictated by your process flow rate. A meter to work at 100ml per minute is not likely to have a 1” NPT pipe thread. Typically, a small flow meter will have small pipe connections. Specifying the correct thread of flange detail is essential, do not just specify the line size alone.
6. Pressure drop:
This is a very important consideration. Will the available pressure head be enough to operate the flowmeter you have chosen efficiently and still leave enough available for later processes? This is often overlooked with resulting commissioning delays as either an alternative product or more pumping capacity is required.
7. Pulsating flow:
Pulsating fluid flow can cause problems for most flowmeter types. Positive displacement devices are undoubtedly the most immune with all other flowmeter types sharing the honours on intolerance. It is always best practice to eliminate pulsation with a damper and a pressure regulator. It can be a simple air pocket and a length of flexible tube or a commercial bladder damper with appropriate gas charge and a high-quality pressure regulator. Either method will reduce the pulsations but only within a limited set of operational parameters. Change the stroke, frequency or flow rate of the pump and the efficiency of the damper will be reduced.
8. Operating environment:
Is your flowmeter outside in an Alaskan winter or subject to the Saharan summer? Is it in a hazardous area on an offshore oil rig? We have had users forget to mention all of the preceding scenarios. Environment specification is especially important for hazardous areas where the flowmeter will require an appropriate meter and detector.
9. Power supply requirements:
All flowmeter manufacturers try to immunise their products from poor power supplies. Most experienced users will specify a quality power supply for the sensors around their plant. Some will then add solenoids, dc motors and other noisy electrical apparatus and wonder why their flowmeter electronics are having problems. If you have an analogue output you will require a suitable voltage supply, it will not work off 9V.
10. Installation constraints:
“I did not realise I needed straight pipes before and after the flowmeter!” Before specifying check out your flowmeter installation instructions as well as the meter specifications. If you have a tight operating space, consider a positive displacement meter which will be largely immune from the surrounding plumbing. Will you require a filter? It is good practice to install a filter close to the flowmeter but very few users do as they rely on other parts of the system. Will your fittings affect the flow profile onto the flowmeter?
Avoiding the 10 common mistakes above will improve your chances of correctly specifying your next flowmeter. We always advise before choosing a flowmeter always read the whole specification sheet and installation instructions for the meter you are considering. If in doubt talk to your supplier who will be able you to help you specify the optimal flowmeter for your process.
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