Key points
Specifying and installing a new or replacement flowmeter can sound straight forward. It’s as simple as buying a flowmeter of the required accuracy and flow rate and then fitting it in line, right? What could possibly go wrong?
It turns out there are a few more considerations that if not covered can lead to problems. This article is by no means as exhaustive list, but it does cover the main areas that require attention.
Flow Measurement Challenges: Measurement Uncertainty
Flowmeter measurement uncertainty increases exponentially as flow rate approaches zero. This rapid fall off tends to start around the bottom 10% of the measurement range and the errors increase dramatically more than the flow meters stated accuracy (or uncertainty). Low end information is either published as an accuracy graph across the flow meters range of measurement or as a turn down ratio (the ratio between the minimum and maximum flow).
Flowmeter measurement uncertainty increases exponentially as flow rate approaches zero. This rapid fall off tends to start around the bottom 10% of the measurement range and the errors increase dramatically.
Flow Measurement Challenges: Measurement Technology
There are several fundamentally different types of flow measurement technology but not all technology types are suitable to all applications, for example:
- Turbine flowmeters are not a good choice for higher viscosity fluids and sludges.
- Coriolis flowmeters need space to vibrate, if their flow tube is touching another structure measurement can be compromised.
- Mag flowmeters can struggle to measure low conductivity fluids.
It's important to understand the limitations of a flow meters technology when considering it for a flow measurement application.
Flow Measurement Challenges: Flowmeter Size
For a flow meter to function as intended it must be completely full of the media which it is measuring, either liquid or gas. Whilst some technologies are capable of measuring both liquid and gas (i.e. Coriolis) they cannot measure a mixture of the two (i.e. mixed phase).
Particular attention needs to be given to the low end of the flow range as whilst the larger errors (as discussed above) may be tolerable, a situation can arise where the flow meter isn’t completely full because of low flow rate and a suboptimal orientation.
For example, it can be tempting to install a straight bore Coriolis flow meter with an inner bore that matches the line size as this is an ideal scenario for cleaning in place. At high flow rates this setup would likely work but at low rates there is a risk that the flow tube isn’t 100% full and the meter is measuring mixed phase.
Flow Measurement Challenges: Flowmeter Orientation
The general guidelines for flowmeter installation are that there should be a minimum straight pipe run distance either side of the flowmeter. Turbine flowmeters are at the extreme end of this typically requiring distances of 20 times the pipe diameter upstream and 5 times downstream.
Mag meters tend to require distances of 5 times the pipe diameter upstream and 3 times downstream and Coriolis meters can get away with even shorter distances. These are typical examples, even within technology types required distances can vary a little by manufacturer.
Flow Measurement Challenges: Maintenance
Typically, the most frequent maintenance carried out on flowmeters is calibration unless they are used in a process where fouling is a problem (in which case they may require periodic removal for cleaning). There are a few basic maintenance considerations:
- The process of removing a flow meter for calibration can easily result in ½ a day’s downtime or more so serious consideration should be given to inline calibration at the time of application design. This can be achieved by valve set up that accommodates a temporary hose being put inline, downstream of the flow meter (observing the minimum downstream distance of straight line the flow meter requires).
- At some point it's likely every flow meter will need removing, consideration should be given to this at the time of process design allowing sufficient access around existing pipework and equipment to facilitate safe removal.
- Even an inline calibration can require a couple of hours of downtime or more but there are Condition Based Monitoring alternatives! Endress & Hauser Heartbeat and Emerson AMS technologies monitor internal electronics and advise an external calibration check is necessary only when sufficient changes in current or voltage have occurred. This approach is complaint with FDA 21 CFR part 11, ISO 9001, IEC 61511-1 and IEC 61508.
Flow Measurement Challenges: Mixed Phase
Some processes create air bubbles and even a well sized, optimally installed flowmeter will suffer additional measurement errors as a result. It is not always possible to mitigate this with well thought out flowmeter placement, sometimes an additional device like a bubble trap (a suitable distance upstream) is required to remove the entrained air.
As already mentioned, this is by no means an exhaustive list, but it is representative of the more common challenges faced in flow measurement. Its worth taking the time to review flowmeter installation manuals as well their datasheets to give a better idea of potential challenges for the intended application. Also, flowmeter manufacturers tend to offer support (when requested) in specifying the right flowmeter for the right application.
