Flowmeter selection - some top tips to help you make the right choice
Flowmeter selection - some top tips to help you make the right choice
The huge array of flow technology options on offer can make selecting the correct flowmeter for an application a bewildering task. A broad range of factors can influence flowmeter selection, of which cost is just one. The following is a list of top tips for selecting the best all round flow system for an application.
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1. Don’t choose on cost alone
When it comes to selecting a flowmeter, in many cases cheapest is by no means best. Although it might seem the best way to save money in the short term, opting for the lowest cost solution may potentially result in problems later down the line.
Be particularly careful where reductions in the purchase price have been achieved by cuts in supplier back-up and expertise. Ultimately, the most cost-effective installation will be the one where the supplier can offer good technical back-up, independently traceable test facilities, a long and established track record and a reputation for high-reliability products based on sound research and development.
2. Know your flow
For many straightforward flow applications such as water and waste water, measurement is a fairly simple process as long as you have installed the correct pipe size for the process and flow range, and you know the most important application aims, for example low flow leakage monitoring of regulated effluent flow measurement.
A key thing to remember when selecting a flowmeter is that every liquid or gas behaves differently when flowing through the pipeline. The main cause of this is viscosity – how much the fluid resists flow, which in turn can affect the velocity and profile of flow through the pipeline.
By profiling the flow of a liquid or gas through the pipeline, it is possible to find out how it behaves and from there to narrow down the choice of flowmeters to those best able to cope with the conditions of the application.
The flow profile of a fluid will vary according to whether it is Newtonian or non-Newtonian. Newtonian fluids include homogenised or skimmed milk, water, sugar solutions and some mineral oils and have a tendency to ‘stick’ to the pipe walls, resulting in the liquid moving more slowly at the sides of the pipe than in the middle. Newtonian liquids have a directly proportional relationship between the pressure of the liquid flowing through and the resistance, or shear force, caused by the fluid sticking to the pipe walls.
The behaviour of Non-Newtonian fluids, such as paints, shampoos and yoghurt is harder to predict, as there is no relationship between pressure and resistance. Instead, the flow of these fluids tends to vary as viscosity changes either with time or due to a change in resistance caused by the collision of two different velocities as the fluid sticks to the pipe walls. Consider the well known problem of getting tomato ketchup out of a bottle. At first, no flow even with pressure behind it, then suddenly, lots of flow even with lower motive pressure.
To select the best flowmeter, it may be necessary to calculate the Reynolds number of the application. This figure is basically the ratio of momentum against viscosity and can be calculated by using the minimum and maximum fluid flow and viscosity figures of the application. Once the Reynolds values are known, they can then be matched against a flowmeter’s Reynolds range to help pick the one that is best able to meet the demands of the application.
3. Opt for the widest turndown
Put simply, turndown is the ratio of the maximum and minimum flow rates a flowmeter can measure within its specified accuracy range. The turndown of a flowmeter is particularly important because it is virtually impossible to know in advance the exact range of flows to be measured. Selecting a flowmeter that offers the widest possible turndown will ensure that it can cover all anticipated flow variations.
4. Pay attention to installation
When selecting a flowmeter, it is important to consider exactly where and how the device will be installed, as this can significantly affect both accuracy and efficiency.
Obstructions in the pipeline such as joints, bends or valves in close proximity to the meter can all cause distortions in flow profile, affecting flowmeter accuracy and repeatability. To ensure best results, flowmeters should be installed in locations where there are several straight-lengths of unobstructed pipeline both upstream and downstream of the meter.
It is therefore important to find out the manufacturer’s installation recommendations before buying a flowmeter, particularly where installation space is limited. Remember though that there are some flowmetering techniques that require minimal or even no conditioning pipework. Use of such techniques, for example VA and Coriolis meters, can assist with accurate metering in awkward or cramped physical installations.
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5. Pick the flowmeter that will offer the best accuracy for the application
When selecting a flowmeter, it is important to find out which types are most suited to the application. Electromagnetic meters provide for the widest flow range for conductive fluids; turbine meters may provide an inexpensive capital cost option for small sizes, but suffer with the problems of mechanical wear over time and possible process contamination with worn parts (e.g. bearing swarf); while Differential Pressure meters including orifice plates, venturis and wedge meters are the most commonly used metering devices, and now come in compact integrated solutions. Coriolis mass flowmeters are ideal for measuring particularly viscous substances and their use has been steadily increasing in many applications, no longer just where the measurement of mass rather than volume is required.
Where accuracy is concerned, it is important to remember that most flowmeters are affected to some extent by the medium they are metering and by the way they are installed. Consequently, flowmeter performance in real life conditions will often be different from the reference conditions under which the flowmeter was calibrated.
It is also important to beware of manufacturers’ calibration accuracy claims. Even under stable reference conditions, the best accuracy that manufacturers can hope to achieve is 0.1%.
6. Complying with the law
We are living in an increasingly legislated environment and with an array of changes in regulations. With the onus on companies to implement a good metering regime, no-one can afford to ignore industry targets and evolving legislation. Customer and regulatory standards have seen a move towards sustainable and ‘greener’ technologies with certified and approved technologies becoming legislative criteria.
A good example are MCerts and the Environmental Permitting Regulations, which oblige all industrial and water treatment companies to self-monitor their effluent flows if they have a requirement in their EPR permit /consent for discharging effluent to a watercourse or the sea.
Introduced as part of a move to improve the measurement and control of discharge and waste levels from both water utility and industrial companies, the self-monitoring obligation requires operators to comply with the Environment Agency’s MCerts certification scheme. Under this scheme, companies should be able to demonstrate to the satisfaction of an MCerts inspector that they are using the Best Available Technique (BAT) to protect the environment. Where the self-monitoring of effluent flow is concerned, operators are subject to a ±8% uncertainty target for the measurement of total daily volume of effluent discharged. This covers not just the equipment used, but also other factors such as correct fitting and the training of relevant personnel to ensure that an installation is correctly set up, operated and maintained.
In practice this means that, if there are instruments or systems using a particular technology that have passed all the necessary tests and received an MCerts compliance certificate, operators must use them for new and refurbished installations.
Failing to keep abreast of the latest requirements, such as MCerts, could have serious ramifications in the event of a problem occurring. Failure to comply can be construed as a failure of a basic duty of care, potentially leading to fines or even imprisonment in serious circumstances, so it pays to keep up to date.
7. Check your accuracy
To ensure your installed flowmeters are continuing to deliver optimum measurements, it is advisable to periodically check their accuracy throughout their service life where possible.
Reputable suppliers should be able to help you achieve this. ABB offers a range of in-situ verification services for its electromagnetic flowmeters aimed at ensuring their continued accuracy throughout their lifecycle, including the new ABB Ability suite of verification tools. Delivered through ABB’s network of instrumentation service engineers, these services help operators to achieve savings in repair maintenance plans by eliminating the time and cost of removing flow sensors for testing. The most modern electro-magnetic flowmeters employ self checking diagnostics aimed at predicting possible meter or process failure in advance, thus giving process operator’s time to handle a range of problems proactively, not reactively.
8. Use the same supplier for all your flowmetering equipment
A flowmeter is often only as good as the equipment that sits alongside it. For example, a flow computer or other form of display is needed to process data from the meter and show flow rates.
Although there are many suppliers offering ancillary flow equipment, the best way to ensure a completely matched system where all components are fully compatible is to specify everything from a single reputable supplier. This will guarantee that all equipment has been manufactured to the same standards, and will also ensure that back-up is available from the same supplier for your whole flowmetering system.
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Need help in flow meter selection - Eng-Tips
We have planned to install flow meter for our RO water header lines but we are facing the flow range issue while selecting the flow meter.
Our pump capacity is 30 m3/hr (or) Liters per hour and pump head is 30 meters. we need to install this for 1 1/2" (40NB) line and 2 1/2" (65NB) line.
we have selected the metal tube rota meter for our application in this but the problem is for 40NB line they provide only 500 - LPH flow range and for 65NB line it is - LPH. why this much of variation in flow range for both sizes ? we need to measure the maximum flow rate of the pump as 30 m3/hr ?
is this possible in these two pipe sizes (40NB,65NB) ? As per vendors suggestion we could not achieve this much of flow rate in these sizes.
i will upload the vendors quotation here. can anyone please tell me why can't we achieve this flow rate ? As per Q=AXV when the pipe area reduces the velocity will increase so if we reduce the pipe area absolutely velocity will increase and same we calculated the head loss and it is very minimum range in 65NB pipe but huge head loss in 40NB line ? My doubt is due to huge head loss how can i calculate the flow rate in the 40NB line ? because then only i can select the flow range for flow meters. The pipe MOC is SS-304 ASTM grade A312.
i attached my pressure drop calculation here and vendor quotation please review when you have a free time.
Thank you all. You need to start the question with a sketch of the flow diagram from the source to the two flow meters with any proper instruments, including if any flow/pressure control valves in additional to the flow meters for better understanding of your system.
First, here are questions for you:
1. Typically the flow meter is to measure the flow range for a certain turn down ratio and better accuracy. Or, the larger size of the meter may be required for higher flow rate.
2. The flow velocities of two branches seem high in the design of the typical new facilitate. What are the required flow rates for 40NB and 65NB lines, instead of total 30m/h from the pump? Asisraja,
I'm not actually sure where to start on this rather long and rambling post with no diagrams....
First, the nominal size of a pipe, especially at these sizes, is not the same as the actual ID, which is what you need in your calculations.
Rotameters do seem very popular in India for some reason, but for liquid flow they can be restricted as all the flow needs to go through them. They work on a set velocity, so min flow for your 40NB line in velocity terms (sq area ratio) is 2.6, so the 500min becomes and the becomes . About the same velocity ratio.
So to measure 30m3/hr you prob need a 3" or 4" meter.
30m3/hr in a 40NB line for 100m is bonkers. You're doing 6.5m/sec. Pressure drop varies as a square of the velocity so no wonder it makes no sense.
Your question about negative pressure is so wrong I can't start to answer it.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it. Asisraja,
1. Determine the proper line size per the “required” flow rate on each section, which may not be the max pump flow.
2. Need to study the overall line system hydraulic and the proper design flow velocity/ pressure drop for sizing the lines of the header and each branch. The flow rate of each branch may be smaller than the pump discharge header.
3. Each branch sizing is based on how many users are on the branch line.
4. Decide how you to set up or control the flows of the header and branches, i.e. flow control or pressure control, and select the proper flow meters for better control and accuracy.
Hope these help. Asisraja,
It’s okay to have the pump of 30m3/h which may cover the necessary flow for all current users and the potential users in the future. Now the job is to size the header and branches to suit the operation needs.
For me, I may design the main discharge header size with the flow 30m3/h and the velocity 1.8m/s (about 6ft/s) to have the proper line size with pressure drop. I may not want to size the pipe with 4m/s or 6m/s velocities with higher pressure drops. To minimize the line pressure drop is to ensure the process pressure is enough at the end users with the possible future expansion.
For each branch, I need to know how many users and associated flows and calculate the branch pipe size accordingly with 1.8m/s flow rate.
Then, the size of the flow meters may be determined per line sizes accordingly, or per Vendor recommendation.