Archive | Flow Tip

Expert advice on flowmeters and calibration

Charles Wemyss lists 10 reasons why you should – and should not – calibrate your flowmeter

We use the word flowmeter to describe a device that measures the flow of a fluid. Mostly we’re considering gases or liquids in a closed pipe or conduit and we need either the instantaneous flow rate or the total amount of fluid that has passed. There are many varieties of techniques dependent on the fluid being measured and dependent on the flow rate, pressure, viscosity and more. The flowmeters range from miniature positive displacement devices to large electromagnetic or ultrasonic units used for pipes over 3m diameter. The way we garner confidence in the displayed value is through calibration.

Most flowmeters are supplied by the manufacturer with a ‘laboratory’ calibration. In other words, they have been tested in close to ideal conditions. Depending on the meter type, once installed in your process, that original calibration may be valid – or it may not be.

Litre Meter’s latest rig FlowLabPro is designed for calibrating ultra-low flowmeters

There are a number of key reasons why it should be calibrated:

* To reflect the new, current conditions

* Because some component has a wear factor

* There is an accumulation of dirt or setting product, affecting the sensor

* Because the calibration frequency states it has to be

* Because the results don’t feel right compared to the rest of the process

* The process is producing poor quality product yet the flowmeter seems stable.

The best calibration is that which is performed in situ. Many of the variables are tuned out. The fluid is the same, as is the installation attitude, straight lengths, etc. That’s the precise reason why you should re-calibrate; it gives you that confidence in the device. If in situ is not possible, for example, when the fluid is hazardous or at high pressure then it has to be uninstalled and calibrated elsewhere.

Why shouldn’t it be calibrated?

Clean versus dirty is the first argument for not calibrating your flowmeter. If it comes out of the line dirty and is sent away for calibration then you’d normally expect to ship it clean. The test lab calibrates it in the clean state. However, as soon as you re-install it the process might be depositing dirt back on it. It has been calibrated for a perfect installation and is almost immediately imperfect.  In this scenario, calibration is pointless.

Next, it’s hard to compare installation to installation. All calibration laboratories pride themselves on making adequate provisions for calibration, especially good installation practice. If they’re testing a turbine meter, for example, then they should have a long length of correctly sized piping before the meter – and a length after, too. This eliminates swirl, if it’s long enough, to generate a flat flow profile and present optimum conditions to the meter. Most labs have this setup for horizontal installation – so if you have a vertical install, then watch out. Likewise, if you don’t have a long length of correctly sized pipe, or perhaps a connector that necks the diameter down a few percent, then don’t bother. The results they give you will be meaningless.

The Litre Meter low flow rig FlowLabPro delivers automatic calibration of flowmeters and instrumentation within a flow range of 0.0006 to 200 l/hr to an accuracy of ±0.2%

Next you should ask whether it is the right fluid. Unless your process is running clean water or, maybe a calibration fluid, then your average lab will not be able to calibrate with the same fluid. For some flowmeter types this may not be important. For example, if you fluid is a weak acid with a viscosity of 1.2cP and the meter is an electromag, then the calibration with water will be perfectly valid. Contrarily, if you have 10cP process fluid and it’s a turbine meter then it could be very important that the test fluid is in the 9 to 11cP range to adequately represent the effect of viscosity on meter performance at lower flows.

Traceability is next on the list. If you have been able to clear the hurdles above then it’s important you pick a lab that has the right traceability for you. If your process demands an indication of flow within +/- 4% then there’s little point on getting a UKAS-accredited laboratory with an uncertainty level of 0.22%.

We’re regularly asked ‘how often should it be calibrated?’ Recalibration periods of flowmeters are based on industry standards. In industrial applications, depending on the industry, periods of six to 12 months are recommended. We advise the user to seek out data relating to the process, other components within the process and the usage of the meter. If the measurement is critical then the recalibration should be more frequent than a non-critical, rarely used device. In the absence of any other data we advise an annual check and to vary the future calibration periods depending on results.

If it has remained unused then no recalibration may be necessary, depending on the meter type. It is wise to check that no fluid has settled in the meter that might alter the way the meter works or even cause corrosion. In the event of any doubt then the manufacturer is always your best source of advice.

{originally published in International Process Engineer in May 2016, www.engineerlive.com}

Are there any size limitations?

    When specifying flowmeters how does size affect meter selection?

  1. Can you specify the available pipe length? Some installations are very limited on installation length and meter selection can be pivotal. Assume that the number straight lengths before and after the meter (but, see below) isn’t relevant for the moment and all meters are available: If there’s only 1 diameter of straight pipe then the PD meter is probably the Number 1 choice. There’s likely very little room so larger meters like the Coriolis, which is a ‘bulky’ technology, are too long, even if they, too, need no straight lengths: that’s not strictly true but that’s another story entirely.
  2. Width: Does the unit have to fit in a narrow space? Perhaps there’s a wall one side -the meter can’t overhang that side, but is it then facing the right way?
  3. Does the installation space enable the unit to be provided with a local display – which is facing the right way? or will it need to be a remote mounted version?
  4. Is there access for maintenance? Is that all important termination panel just in front of you or is it tucked beyond a stem in a dingy corner of the installation. How good are you at holding a mirror?
  5. If it’s remote mounted – how far away can the display be? ie. what’s limit on the length of cable?
  6. If it’s remote mounted – is that panel mount, wall mount or post mount? Any special mounting considerations like weight, panel size, panel thickness?
  7. Height If it’s not a length or width then height might be an issue. Perhaps the meter can be/ needs to be installed upside down? Maybe there’s a bunch of pipe in close proximity.
  8. Are there weight limitations? On vehicle and aerospace installations the weight can be an overriding factor in meter selection. Are there weight reduction regimes? Changing the connection type or reducing a flow meter size or changing to a lightweight material can have significant effects on weight. A threaded turbine meter can be a tenth of the weight of a flanged coriolis.
  9. Does the meter type require straight lengths before (and after) the meter? Some meters are better than others. Some are much worse than others. A turbine meter needs a minimum 10 lengths before the meter and 5 lengths after. Orifice plates are meant to have more before depending on prior pipe configuration to ensure swirl is minimised.
  10. What comes before the straight lengths? If its two bends in 2 different planes then that’s a great recipe for swirl. Up to 100 lengths of pipe after that will be required to eliminate the swirl.
  11. What methods can reduce pipe lengths? One valid suggestion is to use flow straighteners or plates. These can be as little as 1 diameter long, but with a pressure loss, knock out some flow profile imperfections. They aren’t necessarily commercially available nor cheap. Perhaps, have a look at another measurement technique?

All in all consult the specialists.
Ten top tips for flowmeter selection

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Do you know your fluid?

Do you know your fluid? Is it what you think it is? Is it from a known source?

Viscosity, varies with temperature. Is flow measurement going to be affected by viscosity change due to temperature anyway? Might be if the temperature range is large and it’s a Variable Area meter… Will the fluid be changed through the life of the system, introducing different viscosities; meter choice is important here.
Viscosity change over time. due to volatility of light compounds it’s likely, especially if exposed to the atmosphere, that viscosity will increase over time. Possibly if water is leaking into the flow stream or condensation in the process that the viscosity will decrease.
Viscosity changes due to pressure. These are known but fairly small changes compared with temperature effects. Viscosity can double between atmospheric pressure and 2,500bar.
Specific Gravity, Density. These are often quoted in Material Safety Data Sheets (MSDS). For some flowmeters it’s irrelevant, especially if the measurement principle is volumetric; for others, like VA it’s fundamental. And remember density changes with temperature. In general, if you want a mass flow rate or total then use a mass flowmeter (and vice versa).
Thixotropic? A shear sensitive liquid can be tricky for some measurement principles. To preserve the fluid at normal viscosity the rate may have to radically reduced. Typical thixotropic liquids encountered are paints. When stirred they change from a ‘gel’ to a more free flowing liquid.
Corrosion issues: chemical compatibility. Perhaps the first property that is investigated in meter selection is the chemical nature of the fluid being measured. Is it going to corrode any of the components or will it react with the materials and change some dimensions or shape? If a table found on the internet indicates that polypropylene is ‘compatible’ with fluid X will it be suitable for some close fitting parts where just a 1% expansion will stop the meter going round. 1% may indicate, to some people, that it is compatible.
Build up, formation. Slow or fast deposition on the inside of the pipe and other, more sensitive parts, inside a flowmeter may affect the internal diameter used for rate calculations on velocity based devices or the weight of a rotating part or simply stop a part meshing or rotating.
Solids content and solids size. Generally expressed as a percentage, the amount of particulate and the size of that particulate will govern the metering method. And it may not be obviously so. Some of the latest paints have small amounts of additive to give the paint a special quality. These will block a tightly toleranced PD meter or it’s bearings.
Filter size. Is it filtered? Is the filter mesh in the filter bowl or has it been removed because it keeps clogging up?! What level of filtration, NAS class, mesh size, is designed in and what level has been achieved. Is it well filtered but then stored in an open container?
Lubricity. This parameter is frequently ignored and frequently not known. It can have an effect on some flowmeters.
Homogeneous? It’s usually taken for granted that fluids are homogenous i.e. the same consistency at any point. A typical non-homogeneity is air entrainment, perhaps a few bubbles or a stream of bubbles. In extremis, this might be slugs of air passing through. Most flowmeters can’t cope with this phenomena but some make a decent estimation and more than a few will recover after the air passes.
Anodic acceleration of corrosion. This problem occurs when the fluid acts in concert with two dissimilar materials in the pipeline – for example, the flowmeter body and the pipework. The measured fluid acts as an electrolyte, depositing or removing material depending whether the materials act as anodes or cathodes. In some instances another wetted part may see accelerated corrosion.

All in all, consult the specialists. www.litremeter.com

Ten top tips for flowmeter selection.

Sign up for FlowSight, the Litre Meter newsletter.

Top tips for selecting the right flowmeter for you

At Litre Meter, we want to make sure that you get the most for your money when it comes to buying a flowmeter, so we’ve put together our top tips for selecting the right device for you…

Made to quantify the rate that liquid or gas moves through it, flowmeters are required by test and measurement professionals to provide results in a wide variety of applications where accuracy is critical. This includes measurements for familiar household things like heating, ventilating and air conditioning to aerospace and agriculture.

Type of flowmeter

There are different types of flowmeter to suit different purposes and applications. By simply profiling the gas or liquid it is measuring, it’s possible to discover how it behaves when flowing through a pipe. You can then narrow down the choice of device to best cope with the conditions of the application. If you’re unsure about how to do this, get in touch with a professional and they’ll be able to help.

Purpose

There are a number of different uses for flowmeters, and as we’ve just mentioned, whatever you intend to use it for will affect your range of choice. You must consider temperatures needed, the turndown ratio, whether or not it has to be user-friendly for the workplace, and the type of liquid or gas that it is measuring the rate of. For example, if fluid containing traces of silt or sediment is flowing through the pipeline, we’d suggest that you use an ultrasonic Doppler flowmeter.

Chemical compatibility

It’s really important to take into account the materials involved in the process you intend to carry out with the flowmeter. Some materials are not compatible and this can have an effect on both the fluid or gas quality and the flowmeter’s durability. Check each material separately against a reputable chemical compatibility table, and checking your selection with the manufacturer of the fluid is also a wise idea to avoid any potential problems or issues.

Price

Whilst buying a cheaper device may tempt you by saving you money initially, it could actually end up costing you more in the long run. Don’t let short-term savings sway you and think about it practically; a higher priced flowmeter can be more cost-effective in its quality, its back-up and its durability.

LongevityVFF with FlowPod instrument.

Talking of durability, before purchasing the device, you should find out how long it typically lasts. Ask the supplier about its failure rate or the type of application you need it for. This may have an impact on the price, but by evaluating the total life cost of it, you will most likely find it to be worthwhile.

Installation

It’s also important to think of the installation before selecting your flowmeter. Consider exactly where and how it will be installed as this can hugely affect its accuracy and efficiency. You must think about the type of meter and whether it’s affected by any obstructions in the pipeline like joints, bends or valves as these could cause distortions to the flow.  This is all worth doing because if the device is installed correctly in a suitable application, it will be more accurate and will ultimately save you money.

If you need help in selecting the best flow meter for the job, our Litre Meter team will be happy to help. Simply get in touch via our Contact Form or give us a call on 01296 670200.

Positive displacement meters: pros, cons and selection

Positive displacement flowmeters, sometimes known as PD meters, have been around for more than 100 years. They are commonly used in a wide range of applications from domestic water measurement to measuring ultra flow rates of chemical at high pressures subsea.

First off – what is a “positive displacement” meter? Well, as the name suggests it involves the positive displacement of a volume of fluid – this is usually a liquid but there are some units suitable for gas. There is a chamber and inside the chamber, obstructing the flow, is a rotor.
The shape of the rotor and chamber vary greatly with each meter type but they all provide an output for each rotation. Most meter designs therefore lend themselves to being totalisers. Most can have the flow rate calculated from this primary data.
Advantages
An accurate PD meter will have minimal ‘leakage’ across the rotor seal. This is generally minimised with the use of more viscous liquids and accuracies of ±0.1per cent are sometimes quoted. On the other hand rotary piston flowmeters are used by the water industry in the UK for measurement of water over a normal flow range to accuracies of ±2 per cent.
Because they measure a volume precisely it does not matter if the flow is pulsing. They will follow the increase and decrease of flow found in reciprocating pumps of all types. With higher viscosities the turndown ratio can be high. Even with water 100:1 is not uncommon and 3000:1 is possible at 250cSt. Few applications require this but it does enable measurement of ultra low flow rates without miniature parts or normal flow measurement at minimal pressure drop.
Most meters are simple to maintain as they have only one or two moving parts and are coupled with simple readouts that are easily understood in the field. There is no requirement for straight pipe lengths like that might be needed for electromagnetic or turbine devices. They can be connected directly to elbows or valves and in most cases in a variety of orientations.
Designs are relatively easy to adapt for high pressure applications eg over 100 bar.
 
Disadvantages
All PD meters require clean fluid so a filtration level of 100 micron is usual. Some meters can actually block the flow if a larger particle is trapped in the wrong place. Many meters are not made in high specification materials and therefore corrosion can be a concern. An all plastic or all 316SS meter is the exception rather than the rule. As the application flow rate increases the size of the PD meter seems to increase by a square law! It is rare to find meters over 12-in in size although they exist at these elevated sizes for the prime reason of accuracy – frequently being utilised for custody transfer reasons.
In the author’s opinion, the most common PD meters are as follows:
  • Rotary Piston: As mentioned above these form the basis of domestic water measurement but the design of the rotary piston that oscillates in a circular chamber with a fixed web has been modified and extended to ultra low flows and high flows, as well as high pressures and for food applications. A good all-rounder.
  • Spur gear: The fluid rotates two gears and is forced around the outside of the gears and the inside of the chamber. Depending on the location of the sensor these can yield very high pulses per litre values useful in batching and fast acting processes.
  • Diaphragm (or bellows meter): These are common in many people’s home as their domestic gas meters. When the gas flows through it alternately fills and empties bellows causing levers to crank a shaft providing an output. Very useful for wide-ranging gas totalisation.
  • Oval Gear: Quite similar to the spur gear where two oval gears mesh together and sweep the chamber. The volume displaced is much larger than the round gear. Fairly low cost and some designs available in plastic.
  • Nutating Disc: This meter is the hardest to understand but is effective. The rotor is a circular disc attached to a ball. The shaft on the ball is inclined. As the disc rotates in a spherically sided chamber the disc and therefore the shaft wobble creating an output.
  • Helical Screw: Possibly the most accurate PD: meter two intersecting cylindrical bores are fitted with 2 interlocking helical screws. As the fluid passes through they rotate. On standard applications the author has observed differences of just ±0.37 per cent of reading over 50:1 turndown over annual recalibrations over 10 years – quite an achievement. Also common nowadays fitted on petrol pumps.
  • Slide Vane: Historically the most accurate of PD meters with the rotating element having a number of moving blades that rotate about a fixed cam. Linearities have been claimed of ±0.02 per cent.
  • Others: If we go back to Felix Wankel’s seminal work on rotary machines we see that there are as many designs for PD meters as there are pumps. He explored in a rational way the various shapes of rotor and chamber. While the majority don’t see the light of day in the marketplace this brief essay illustrates the variety in general use, and this is without discussing the Roots meter, wet gas meter and multi rotor designs.
Two decades ago the PD meter was considered to be old technology and likely to be overtaken by more modern electromagnetic and ultrasonic devices. Nowadays the PD meter still represents good value and can provide excellent measurement in a wide variety of duties.

How well do you need to know it?

What factors should you look for in flowmeter selection relating to the output or display?

  1. Precision. Often misunderstood, but in the most part, it’s what matters in measurement. It’s the degree to which repeated measurements under unchanged conditions show the same results.
  2. Accuracy. The degree of closeness of measurements of a quantity to that quantity’s actual (true) value.
  3. Linearity. For flowmeters it’s the curve of accuracy compared against flow rate.
  4. Resolution. If the digital display only has 3 digits then selection of the units has more effect than the accuracy (etc.) of the meter itself. For example, set up with a maximum of 110 US gallons/min the resolution of ±1 US gallon per minute is 1% at best and 5 or 10% or worse at lower flows. Changing over to litres improves the resolution by a factor of 4. More importantly this shows the value of having enough display digits to match the users requirements and, probably, the accuracy of the meter.
  5. Traceability. So the supplier gives you a set of data, a claim of performance. All meaningless without reference to something solid, something comparable like a National Standard.
  6. ISO 5725. According to ISO 5725-1, the terms trueness and precision are used to describe the accuracy of a measurement. Trueness refers to the closeness of the mean of the measurement results to the actual (true) value and precision refers to the closeness of agreement within individual results. Therefore, according to the ISO standard, the term “accuracy” refers to both trueness and precision.
  7. ISO17025. Simply a laboratory standard: General requirements for the competence of testing and calibration laboratories.
  8. Repeatability. Another word meaning precision but often taken as the closeness of one set of results to some more with conditions unchanged. Probably should include some reference to time and:
  9. Hysteresis. In some systems the precision varies according to whether the flow is increasing to the measurement point or decreasing. In particular near the start-up flow rate it may be found that, with the flow increasing from zero, the meter provides an output at ‘x’ whilst, when the flow decreases the meter may continue to provide an output at lower than ‘x’.
  10. Long term accuracy. This could be restated as: will it measure the same tomorrow as it does today and what about next year?
  11. Recalibration. The periodicity at which the meter should be recalibrated is not set in stone. Some meter types are less stable than others. Where the meter is used to calculate tax or fiscal amounts then a daily recalibration is sometimes necessary. In a benign fluid, with flow rates kept within bounds then others might need checking every 10 years. Litre Meter recommend a yearly check at first, analysis of the results, then an increased period depending on the customers needs.
  12. On site calibration. Whilst every flowmeter that Litre Meter manufactures is calibrated in laboratory conditions on a similar fluid and at a steady flow rate there are differences such as meter orientation and pressure pulsation.  Whilst pressure pulsation won’t affect positive displacement meters it can have a severe effect on turbines, for example. So Litre Meter recommend that each meter is calibrated in-situ. Various techniques are described in the flowmeter manual.

 

What flow units do you want to use?

  1. Gas measurement: what’s the difference between Standard and Normal? It’s simply the reference conditions. Standard and Normal are different though depending which country you’re in and what industry it’s applied to. However, if you know the reference conditions eg 0°C and 1 bar then everything can be calculated. Mass flow of gas is often expressed as standard Continue Reading →