on September 20th, 2010

Dear Colleagues,

It is all quite frustrating - the more you learn about a particular technology, the less you feel you know – esp. in the context of measuring flow rates. The story of engineering, I guess. Flowmeters touch us in every engineering discipline; hence it is definitely worthwhile pondering the latest and numerous developments in flowmeter technology and how you can take advantage of them. Obviously, the main improvements in all flowmeters have been in the electronics, advanced diagnostics, wireless networking and using long-life battery based power .

There is naturally some disagreement about which flow meters are gaining the greatest traction; especially with the rather uncertain economic situation clouding forecasts. But the key flow metering technologies are still differential pressure, ultrasonic, coriolis and vortex. Arguably, turbine and positive displacement technologies are declining.

Differential pressure (dp) – the old workhorse prospers The principle is that pressure drop across a restriction is proportional to the square of the flow rate. Easy to calibrate and thus lower maintenance costs.Improvements range from less straight pipe up and downstream required; advanced diagnostics to pick up items such as lines being blocked and changes to fluid composition. Multivariable dp flowmeters now can calculate mass and energy flow with a single instrument; without the need for multiple devices.

Magnetic flowmetering with no pressure loss

The principle here is based on a conductive fluid passing through the sensor’s magnetic field will generate a voltage proportional to its velocity. Highly accurate and no pressure loss make them sought after. But the fluids have to be conductive; so not good for gas or steam. But a solid performer esp. with additional diagnostics available. Coriolis and oscillating tubes The principle here is that as a fluid moves an oscillating tube vibrating at its resonant frequency, forces are induced which cause the tube to twist. The amount of twist is proportional to the mass flow rate. These are highly accurate devices over a wide turndown and are unaffected by pressure, temp., viscosity and density. There is rapid growth in the use of these flowmeters but they are somewhat more expensive. Recent improvements include ability to measure with entrained gas.

Down the Vortex

When flow passes a bluff body, it generates vortices downstream with a frequency proportional to the flow velocity. Accurate, reliable and very affordable but unable to measure at very low flow rates.

Ultrasonics in two flavours

There are two approaches here: Doppler where the frequency shift is measured in signals reflected off moving particles and transit time where an ultrasonic signal is transmitted with the flow and against the flow; and the difference in transit time measured. Recent developments are for excellent results for gas, low flows and for custody transfer of petroleum fluids.

OK; so are there any other real benefits with these new technologies ?

The use of industrial data communications technologies (HART, Fieldbus, Wireless) to the flow meter allows considerably more information to be transmitted to the user than the traditional 4-20mA technique. And without calibration, testing and various other operations can be conducted on the flowmeter with stopping the operation of collecting flow data (as in the ‘old days’, when the flowmeter had often to be removed from operation).

Typical diagnostics features (which contrary to some old hands’ opinions, can be really useful) are detecting when the sensor has failed or is not operating properly (e.g. reading too high or low; PD orifice plate is eroding); identifying a plugged line (e.g. coating buildup which needs to be removed); eliminating spikes in the measurement (e.g. due to entrained gas, cavitation); composition changes in the fluid and finally, electrical problems such as ground loops and defective power supplies.

Finally, to wrap up with a short discussion on that old chestnut where confusion abounds when talking about flowmeters….

What is the difference between rangeability and turndown ratio of flowmeters?

 Normally both terms are used interchangeably but they are actually subtly different: Turndown ratio is the ratio of the maximum flow to the minimum flow that a flowmeter will measure to a stated accuracy. Rangeability is the ratio of the maximum full scale range to the minimum full scale range of the flowmeter (thanks, David Spitzer). The turndown ratio will be less than the rangeability.

Thanks to Control Engineering, Controlglobal and Spitzer & Boyes for some great reading resources on flow which I have referenced in the above discussion.

Perhaps a trifle tedious to hear in the context of flow meter improvements, but nonetheless true, as Lloyd Dobyns and Clare Crawford-Mason remark: Continual improvement is an unending journey.

What are the next major improvements likely in flow meters ?

Yours in engineering learning


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