Know your flow rate/ flow range

Perhaps this is obvious but you need an idea about what your flow rate might be to size the meter correctly. Not only the maximum and minimum but also the normal flow rate expected. Maybe there will be a unique flow rate at start up?

  1. How high a flow rate? This value is normally what decides the meter size. The supplier can optimise the accuracy and rangeability by selecting a meter that has a maximum flow rate just above the customers maximum. Remember that this value could change over the life of the plant so maybe you need to have some percentage in hand.
  2. How low a flow rate? Just like the selection above this can be crucial in meter size selection. If it is essential that 0.02 l/min is to be measured there’s no point offering a meter that will meet the max flow but not the low flow. Maybe there is space for two meters or, more likely, another technology is required.
  3. What’s the normal flow rate? More often than not the user will know his process is optimised for a specific flow rate or range so this is what most manufacturers will use as a flowmeter size guide. But, usually by dint of the fact a flowmeter is required, the user may not now the actual flow rate that well.
  4. What’s the start up flow rate regime? At the start of the day, or the start of the process, as things get warmed up there can be some long acceleration times perhaps even below the minimum flow rate considered above. At the other end of the flow spectrum the instantaneous acceleration may be detrimental to the meter itself. This might dictate the type of meter selected.
  5. If it’s batching how does the flow rate change at the end? Sometimes the process will be designed to slow down at the end to lessen the effects of splashing, for example, and linearisation may be a boon.
  6. Average or pulsing flow? Sometimes the flow is quoted as, say, 0.5 litres per hour. What isn’t necessarily stated is the flow profile i.e. whether it’s a constant flow or one that varies through a time period. It might be 10 short bursts of 0.05 litres every hour. So the meter needs to be sized accordingly. Perhaps 0.05 litres per minute/3 litres per hour would be better for meter size selection?
  7. Is linearisation required? If the flow rate is always the same then linearisation won’t bring any benefit. If the flow is changing or can be at different values through the day then linearisation can bring out the best in the flowmeter.
  8. Is the flow limited at some times of day by other processes? In a ring-main situation there maybe other processes that draw off flow and pressure at odd times that unpredictably change the fundamental maximum and normal flows.
  9. Does the leakage flow need to be measured? In some applications the normal flow is, say, 3 litres per minute. A hydraulic actuator is filling so it’s a short cycle. But wouldn’t it be great, wouldn’t it be the customers need to monitor any leakage flow down to 20 ml/min? What meter type would be required?
  10. Accidental flow surge? During the normal course of operation the flow is between minimum and maximum as described above. However, in an emergency flow may suddenly increase or even change phase to steam, for example. Is the meter required to measure this or just survive?
  11. What’s the flow direction? Is it always one way, does it sometimes go backward? Does the flow backward need to be deducted from the forward – or added? Does the reverse flow need to be measured or, like above, be survivable.
  12. What are the lifetime requirements? There may be flow degradation, process reductions, planned or unplanned. Litre Meter have seen data from oil wells where the scale inhibitor injection flow rate varies from 17.7 to 9 to 7 to 33 ml/hr over 8 years. So if the meter was specified for a minimum of 17 or 15 then it would stop registering later that year and only start ‘working’ again 6 years afterwards.

All in all it’s best to ask the experts….

Ten Top Tips for Flowmeter Selection

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