“It’s like cooking – but on an industrial scale you don’t weigh on a kitchen scale, you measure continuous flow to understand quantities.”
Norman Glen, service leader for densitometers and physical properties of fluids at Glasgow-based consultancy and calibrator NEL, is explaining the importance of accurate flow measurement in process engineering.
“Although most of the built environment we interact with is solid, much of it is processed in the liquid state,” he says. “If you’re sitting at a desk, it’s probably chipboard and the chair is most likely plastic; both are processed in the liquid state. So liquid is the most important state for processing and you need to know quantities.”
As well as process control – ensuring consistent results by closely monitoring input levels, as per the cooking example – product value is another reason for accurate flow measurement: “You need to know how much of it you are producing and selling.”
Often the instrumentation is the last thought about when plant is designed. Sometimes the plant builder may not have an interest in product quality and just wants the cheapest option for flow measurement
Jonathan Humphrey, product marketing manager for flow, Endress+Hauser
In a process plant where a quantity of a material is being paid or charged for, you want to be as accurate as you can be about the volumes for hard-nosed financial reasons. Small errors can mean huge losses when the process is repeated over a year.
Legal requirements exist in some sectors to measure volumes, to ensure correct taxes are paid. And safety regimes often require knowledge of flow rates to ensure plant is set up to eradicate risk.
So flow measurement can be seen to be central to the successful management of a process plant. But it’s not always treated as such.
Endress+Hauser product marketing manager for flow, Jonathan Humphrey, says flow meters are often not considered until it’s too late for their potential to be reached.
“Often the instrumentation is the last thought about when plant is designed,” he says. “Sometimes the plant builder may not have an interest in product quality and just wants the cheapest option for flow measurement.”
Silvia Cuesta, flow product manager at Emerson Automation Solutions, says flow meters are often positioned incorrectly or not looked after properly.
“Mistakes occur during instrument installation and specification,” she says. “Devices are often not positioned with recommended upstream or downstream diameters or with minimum fluid velocity configurations. Operational issues are often due to routine maintenance procedures not being followed correctly.”
Several types of technology compete to measure flow rates in process plants and it is important to select the correct method for your particular requirements.
Glen says ultrasonic flow meters are widely used for industrial applications. “The most common type is a transit time meter, where a beam is fired across the flow in a pipe and back at a slight angle,” he says. “The two transit times are different, as the beam travels faster when it goes with the flow, and from the difference you can calculate the flow rate.”
Coriolis flow meters, based on the principle of the Coriolis force, measure mass rather than volume, removing the need for complex conversion calculations in many applications.
“Over the past few years this has become quite popular in everything from food to chemical processing,” says Glen.
“You have two tubes forced to vibrate. When there is no flow, they vibrate in phase, but flow is accelerated and decelerated around bends, causing the pipes to vibrate out of phase, and the more out of phase the higher the mass flow rate.”
Flow meters will offer more diagnostic capabilities and form part of an Industrial Internet of Things strategy that provides operators with valuable insight into their process for more predictive outcomes
Silvia Cuesta, flow product manager, Emerson Automation Solutions
Humphrey says Vortex flow meters are commonly used for steam applications where the nature of the material can cause problems for other methods.
“The Vortex has a bluff body, which is a prism shape, the flow goes across it and you get an alternating vortex or region of low pressure either side,” he says. “Each time it does that it gives you a fixed volume. It is a robust technology.”
Differential pressure technology can often be used for applications with extremely high temperatures or long line sizes, such as power plants. “It can become more cost effective,” says Humphrey.
Natural gas flow is often measured using thermal mass meters. These involve a probe heated at a specific difference to the gas. The faster the gas flow, the more power is needed to maintain the temperature of the probe, so this power is proportional to flow.
Magnetic flow meters generally have no moving parts and work according to a mathematical formula known as Faraday’s law. They are often used for water-based or conductive liquid applications.
Once a plant manager has chosen the best technology and the right model flow meter to suit their needs, it is important that they install it correctly to maximise its accuracy.
“Skill is required during the installation and commissioning of flowmeters,” says Cuesta. “End users occasionally install systems themselves, but in general it is recommended that this is performed by experienced service technicians or integrators.”
People put flow meters on their machines and think they’ll work forever. But like any other instrument it should be calibrated; and the calibration should be fit for purpose
Norman Glen, service leader for densitometers & physical properties of fluids, NEL
Humphrey has seen plenty of errors made at this stage. “Mistakes often happen with installation of flow meters,” he says. “You sometimes need a lot of straight pipework yet you see them coming around bends or through junctions. If we were brought in to advise when plant was built then we could help more.”
Of course even when the right kit is chosen, installed and commissioned, the job is far from done.
“People put flow meters on their machines and think they’ll work forever,” says Glen. “But like any other instrument it should be calibrated; and the calibration should be fit for purpose.
“So if you’re going to use a meter to measure flow of a fluid at 50°C and 30 bar pressure, don’t calibrate it at room temperature and ambient pressure.”
With inaccurate flow measurement causing a raft of problems to plant managers, but calibration often requiring a process to be temporarily halted, there is high demand for flow meters that can detect their own problems.
“The use of diagnostic techniques on meters allow them to perform checks on their own health,” says Glen. “For example, an ultrasonic flow meter can look at signal strength to see whether any deposits are occurring on the receiver that would indicate problems measuring.”
Diagnostics can help plant managers look beyond the health of a flow meter itself and identify issues elsewhere in a process.
“An ultrasonic meter might use multiple beams, and as well as giving an average, the diagnostic software might look at each one and see if there was significant deviation that might show gas where you thought you only had liquid.
“With the advent of more processing power there is so much more that can be done. You can spot trends.”
In spring this year Emerson introduced the Rosemount 8712EM wall mount magnetic flow meter transmitter that, the company says, has “powerful diagnostic capabilities and usability features to help users… gain quick and easy insight into their processes”.
“Access to the meter verification diagnostics from the display simplifies the steps needed to verify meter health,” says Cuesta.
He predicts that, in the future, there will be an even greater demand for flow meters. “Flow meters will offer more diagnostic capabilities and form part of an Industrial Internet of Things strategy that provides operators with valuable insight into their process for more predictive outcomes.”