Industrial water flow meters; advantages and disadvantages
Our flow meter expert, CEO Charles Wemyss looks at nine essential questions to ask when considering water flow meters for your application.
Ease of installation?
Flow meters come in many different packages with many different requirements. Some need to be fitted between flanges but at the other end of scale some are strapped on. Some may need considerable downtime for fitting or are a no-go because of clearance issues, proximity to walls and other instrumentation. It’s not uncommon for flow meters to be fitted in pits for underground piping. A high IP rating is required as some pits can flood.
Straight length requirements?
This is related to meter type, but also accuracy expectation (see below for that one, too). An orifice plate meter, to take the most needy of meter types, requires 100 diameters of straight pipe after two bends in different planes to ensure that the flow profile has resolved itself. Even with a simple bend 25 diameters might be necessary.
Modern electromagnetic meters are the opposite of this requiring maybe a diameter or so. They bend the rules by squishing the flow profile in the heart of the meter, but still, it enables great freedom of installation.
Are flow rate, pressure and temperature ranges known?
The simplest of questions at first but loaded. Frequently, the operator will think they have a good idea of flow rate based on pump speed but often the pump curve can be misleading and is generally very dependent on pressure and therefore, the rest of the pipeline. Temperature should be one of those simple measurements. If the process is run 24/7 then will the temperature (and pressure) be the same night-to-day and summer-to-winter? Will that affect flow just on its own?
Is the line designed, can we create a good installation?
Most meter types will have recommended straight lengths before and after, installation attitude i.e., horizontal line, vertical line etc and commentary about the item that precedes the straight length. With a fresh installation, space permitting, the pipe can be optimised for cost, pressure drop and accuracy.
Does it need to be portable?
If so, the options on meter types reduces dramatically.
What is left, is clamp-on meters and some insertion meters. With clamp-on then the transducer can be easily moved to a new installation. This may seem ideal, and is excellent for troubleshooting, but getting the installation correct, with the right amount of acoustic compound and in a precise line will require some training and/or diligence. Insertion meters can have hot-tap installation which makes changing them that much easier.
In larger lines/higher pressures they will need a mechanism to move them in and out – there is a lot of force involved. Clearly, they’re not as portable as clamp-on but
they’re great for medium-term measurement with infrequent location changes. Some insertion devices require a precise insertion depth, to be at a specific point in the flow profile.
Is power available?
One might think that this is the preserve of mechanical meters and a readout only at point of installation. There are battery powered devices, especially of the electromagnetic type that have been on the market a while. These can be hooked up to battery powered telemetry and the readings will then be available on the ‘net. Look, no wires, no maintenance for 10 years.
Is a display or output OK at the meter or will it feed back into the process?
Most meters nowadays are electronic in some respect or other. If power is around then the world of digital communications is a short step away. The old staples of 4-20mA and pulse are joined by HART, FieldBus, ProfiBus and more. If one of these is an absolute requirement then this may be the place to start the search.
What accuracy is required to fulfil the user’s needs?
It’s a good rule of thumb to reckon that an inline meter installed fully in the pipeline will win in terms of accuracy over something clamped on. There will be no accidental knocks or questionable flow profile to contend with or an approximation of the pipe ID – remember, just 1% error in pipe diameter is straight away a 2% error in the volume flow calculations.
Is maintenance an issue?
This is likely a part of the installation conversation, as well. A clamp-on or insertion device is probably fairly easy to remove for a workshop examination or calibration. The opposite, hardest to maintain and install, is a fully immersed meter fitted within flanges, or even threaded versions.
There is a balance between accuracy, portability and installation. This balance is complicated, in another dimension, by cost considerations. A fully immersed mechanical meter will have a higher pressure drop, and therefore energy cost, than an ultrasonic or electromagnetic meter.
The orifice plate is simple to install but has the highest pressure drop of any option so will be expensive in terms of running costs but, like the non-intrusive meters, has pretty much nothing to wear out. When the line is drained then a flanged – or wafer-fit – meter can be accessed by loosening the flange bolts and winching or lifting the device out. A threaded meter is a trickier proposition, maybe requiring disassembly of longer lengths of pipe up to a bend, pump or valve.
- LMag-F is 3″ to 12″ 0.75% accuracy, flanged
- LMag-I1 is 6″ to 12″ 1% accuracy, insertion
- LMag-I2 is 4″ to 12″ 2% accuracy, saddle mount
- LSonic is 1/2″ to 2 1/2″, usually threaded
Our team of Specialist Applications Engineers have an average of over 25 years’ experience in flowmeter selection for industrial processes, grab your advantage and contact us today to discuss your options further.