Key points
Measurement instruments have a tough life. As the frontline in any industrial or municipal process, instruments are installed in conditions that can test the very limits of their endurance.
Electromagnetic flowmeters are a great example. Used in everything from potable water treatment and distribution through to measurement of industrial effluent discharges, electromagnetic flowmeters are frequently exposed to conditions that can quickly destroy or compromise other flowmeter types.
Whilst electromagnetic flowmeters can offer superior performance compared to other meter technologies, maximising their full advantages relies on them being properly installed.
To this end, this article details some of the most common pitfalls that can arise when installing and operating an electromagnetic flowmeter and explains how to overcome them.
How do electromagnetic flowmeters work?
To avoid some of the most common pitfalls beset by some flowmeter installations it helps to understand a little of how they work. Once you have a good understanding of how to get the best results, you can reduce the chances of an installation that will deliver inaccurate readings.
Electromagnetic flowmeters work on the principle of Faraday’s Laws of Electromagnetic Induction. Faraday’s research revealed that moving a conductor of some kind through a magnetic field will induce an EMF in the conductor.
It is on this this basis that an electromagnetic flow meter works.
In the equation E=BLV, E is the induced emf in the conductor (the conductor in this diagram is a wire), B is the magnetic flux density, L is the length of the conductor and V is the speed at which the wire moves through the magnetic flux.
In an electromagnetic flowmeter, the conductor is the fluid in the pipe. The flux density is maintained and controlled at a constant value as it is created by an electromagnet.
The length of the conductor is the diameter of the pipe. So, the two variables are the EMF and the speed of the fluid. They are proportional and therefore we can determine the speed of the flow from an EMF reading.
The common pitfalls with installation of the electromagnetic flowmeter will affect either the accuracy of the flowmeter or the calculation of the volume of flow. What follows are some golden rules for installation that will help to maximise the full potential of electromagnetic flowmeters.
Golden rule number 1 – Make sure your process fluid is suitable
Electromagnetic flow meters are suitable for use with conductive fluids only. Not all fluids are conductive, in fact not all water is conductive. Depending on the type of flow meter used, the typical minimum fluid conductivity required is between 5 – 50 microsiemens/cm .
There is a reason why oil is used to insulate electricity substation transformers and as such electromagnetic flow meters are definitely not the best technology for measuring the flow of oils. Care should also be taken in desalination applications as this process removes all the minerals that make water conductive.
Golden rule number 2 – Keep the flowmeter full
Below is a picture of a flowmeter in a real industrial setting. To the trained eye, it’s easy to see why it could struggle to offer accurate measurement. For those that can’t, consider a bathroom towel rail. Air will naturally find its way to the top rung as the highest point in the system. To remove it, the system needs to be periodically bled.
The same hydraulic principles apply in our picture. Ideally, the sensor tube must always remain full. A flowmeter should not be installed at the top of the system because there will probably be air in the pipe that will prevent the meter from remaining full and this in turn will cause the unit to either over-read or lead to intermittent operation.
Careful consideration also needs to be paid when mounting a flowmeter in a vertical plane. In such cases, it is highly desirable for the flow to be in the upward direction. To understand why, imagine the spinning flow of bathwater flowing down a plug hole.
In an industrial setting the same will happen, with flow spinning downwards and creating vortices that will affect reading. If you place the flowmeter in the vertical plain, an upwards flow direction will guarantee that you have a full pipe and a much smoother flow.
Golden rule number 3 – Remove static from the fluid
Electrostatic charges are a key issue that can affect electromagnetic flow meter operation. If the liquid passing through the pipe is charged in any way, it will change the induced EMF, affecting the flow reading.
The EMF induced by the coils of an electromagnetic flow meter are very small, measured in either mV or µV. For that reason, we must find a way to reduce the ‘noise’ from the fluid as even small amounts of voltage in the fluid already will affect the reading.
Traditionally, this was less of a problem in the past as fluids tended to run through metal pipes, which are conductive and allowed any static charges to be discharged to ground.
However, with the growing use of plastic piping, a significant risk of static charges caused by friction of the fluid on the pipe walls exists and this charge needs to be removed prior to the fluid entering the meter.
The solution to this is to use fluid contact rings, also known as earthing rings.
As flowmeters work in both directions by design, placing the earthing rings on either side, as shown below, will help to accommodate any future changes.
Golden rule number 4 – allow a straight run either side for best results
The design of a pipeline can have a significant impact on the behaviour of a fluid flowing through it.
An ideal flow profile would look a bit like the one in the below diagram, with a fast flow in the middle and a slightly slower flow at the edges caused by the effects of friction against the pipe wall.
In reality, the combined impact of the pipe system and the type of fluid being measured are likely to cause distortions that can affect the flow profile and the resulting measurement accuracy.
The best way to avoid swirling flows and profile distortions is to place the flowmeter in a straight run of pipe. There are variations to the rule, but as a guide most sensors require a minimum number of straight and unobstructed pipe diameters both upstream and downstream of the meter.
Typically, the minimum recommended requirement is five diameters upstream and two diameters downstream.
However, some disturbance sources may require significantly more than this. Obstructions in the flow such as temperature or pressure sensors or junctions between pipes can cause substantial turbulence that can disrupt the performance of the flowmeter.
To overcome this, the fluid needs to be able to return to a normal profile before it enters the meter either by providing extra lengths or artificially conditioning the flow through the use of suitable devices such as flow straighteners.
Golden rule number 5 – Keep things clean
Just like a human artery, pipelines can become clogged with deposits over time, reducing the diameter of the pipe and restricting the space available for liquid to flow through.
Where electromagnetic flowmeters are concerned, the impact of this will be a change in the ‘L’ component of Faraday’s equation – that is, a change in the length of the conductor, which will impact on the flow measurement accuracy.
The way that the flowmeter is positioned is also important. The picture below shows what happens when a flowmeter is placed at the lowest point of the system. If air rises to the top of the system, it is very likely that silt will build up at the bottom, which can then accumulate over time and cause potential problems.
Golden Rule Number 6 – Avoid electromagnetic field interference
In the above diagram, it should be clear to anyone with an understanding of how electromagnetic flow meters work that the potential exists for there to be interference between the magnetic fields of EMF 1 and EMF 2 if they are lined up next to one another on adjacent pipes.
While lining things up neatly in a row will look nicer, staggering the placement of the two flowmeters, either by increasing separation between the pipes or offsetting the sensors, like in the diagram below, will help avoid performance issues caused by interference.
Golden Rule Number 7 – Beware of pumped flow
Pumped flow is a familiar challenge for those in the business of water treatment.
Discontinuous fluid flow created by pumps can be hard for flowmeters to handle as many simply cannot measure fast enough to make a good approximation of the average flow considering all the peaks and troughs.
There are a couple of ways to take these lumps and bumps out. The first would be a pulsation dampener which works in a similar way to a car’s shock absorber, creating a much smoother flow.
The other way to tackle the issue is to use an electromagnetic flowmeter that can take many more readings per second. Compared to DC flowmeters, which change the magnetic field direction between six to 12 times a second, AC sensor excited electromagnetic flowmeters change the direction of the magnetic field up to 70 times a second. It takes readings at the same frequency, ensuring a much more accurate overall average flow.
This form of sensor excitation is ideal for use with rapid flow pulses, which are essentially lumps of flow within the pipe followed by periods of zero flow.
In the diagram above, you can see the comparative output readings from DC and AC sensor excitation principles of flowmeter. The middle section shows the actual flow. The two flow spikes, separated by a period of zero flow, represent pulsating flow.
The waves at the top indicate the frequency of readings from the DC flowmeter. The flow readings are taken just over six times a second, but despite this the average flow rate recorded is very different from the actual flow with significant periods of flow being “missed” and causing the flow meter to under-read the real volume passed through the pipe.
By contrast, the increased frequency of the AC flowmeter, highlighted in the bottom of the diagram, produces a reading which is much more closely related to the actual flow conditions and hence the flow volume.
Summary – Make sure it doesn’t happen to your flowmeter
If you are planning on fitting a flowmeter it is worth considering these common pitfalls of installation to ensure you get the best possible accuracy from your instrument. These are only the most common and it is worth speaking to your sales representative about other examples they may have seen.
