Specifying a Flow Meter: 10 Common Mistakes

 

Flow Meter

In a recent Linkedin discussion, someone had asked if the future of flow meter sales lay in online marketing? There was a tremendous response from industry professionals describing why this approach could miss the advantage of interactive communication with an ‘informed user’ to guarantee an optimum flow meter solution is provided. Some of the errors that are made when people set about specifying a flow meter are listed below.

1. Specifying a Flow Meter: Flow Rate

In certain situations, users are oblivious about their process pipe’s flow rate. “That is why we want to measure it”. Plant construction is usually such that, for mechanical reasons, much larger pipes are used than just the size needed to carry the flow. This is when one has to start examining the motive force that influences the flow and plumbing considerations. Usually, users will fail to notice or forget that their process is subject to significant pulsations and peak flow may be 40% greater or even double the average.

2. Specifying a Flow Meter: Temperature

Some users do not read the maximum operating temperature specification of their meters and expect all flow meters to function up to 150 °C. When the meter fails, somehow the product is blamed! Similar to mobile phone manufacturers who put water evident markers in their phones, Titan Enterprises includes non-reversible high-temperature markers in the meters to record if the maximum operating temperature has been surpassed for any duration of time.

3. Specifying a Flow Meter: Viscosity

Many users are unaware that most flow meters are viscosity sensitive and will significantly change their calibration with variations in the fluid properties. While these Reynolds number sensitive devices are generally linear in the “turbulent” flow range, their performance reduces as the fluid runs “smoother”. For example, using a 40 cSt oil would immediately take a turbine meter, rated and calibrated for 1 cst, out of the turbulent zone and into the non-linear laminar flow region.

Temperature variations would then change the calibration factor and the viscosity. Many types of flow meters depend on higher Reynolds numbers for their linear performance including Pelton wheel, vortex shedding, variable area, and most pressure differential devices. Although other technologies such as ultrasonic meters, Coriolis and positive displacement are less sensitive to these changes, they are not entirely immune.

4. Specifying a Flow Meter: Output Type

“Will the flowmeter interface with my instrument?” the input requirements of the instrument are not known – a higher level language, NPN or PNP pulse, logic level, analogue current or voltage. This requirement has to be checked prior to specifying the meter.

5. Specifying a Flow Meter: Pipe Connections

The process flow rate should dictate the selected pipe connection. A meter to work at 100 ml per minute is unlikely to have a 1” NPT pipe thread. In general, a small flow meter will have small pipe connections. In addition to specifying the line size, the correct thread of flange detail must also be specified.

6. Specifying a Flow Meter: Pressure Drop

This is a crucial consideration. Will the available pressure head be enough to operate the flow meter that users have chosen efficiently and still leave enough available for processes afterward? This is usually overlooked with ensuing commissioning delays as either an alternative product or more pumping capacity is needed.

7. Specifying a Flow Meter: Pulsating Flow

As mentioned previously, pulsating flow can lead to issues for most flow meter types. Undoubtedly, positive displacement devices are the most immune as all other flow meter types share the honors on intolerance. It is always best practice to eliminate pulsation with a pressure regulator and a damper. Even a simple air pocket and a length of the flexible tube or a commercial bladder damper with appropriate gas charge and a high-quality pressure regulator will work. Either technique will minimize the pulsations, but only within a restricted set of operational parameters. If the stroke, frequency, or flow rate of the pump is changed, it will reduce the efficiency of the damper.

8. Specifying a Flow Meter: Environment

Is the flow meter outside in a Saharan summer or an Alaskan winter? Is it in a hazardous area on an offshore oil rig? Users sometimes forget to state all of the preceding scenarios. Environment specification is particularly vital for hazardous areas where the flow meter will require an appropriate detector and meter.

9. Specifying a Flow Meter: Power Supply Requirement

All flow meter manufacturers attempt to immunize their products from poor power supplies. A majority of experienced users will specify a quality power supply for the sensors around their plant. Some will then add noisy electrical apparatus such as solenoids and dc motors and wonder why their flow meter electronics are having problems. If users have an analogue output, a suitable voltage supply will be required because it will not function at 9 V.

10. Specifying a Flow Meter: Installation Constraints

“I did not realize I needed straight pipes before and after the flowmeter!” Prior to specifying, users should check their flow meters installation instructions as well as the meter specifications. If the operating space is tight, they should consider a positive displacement meter which will be largely immune from the surrounding plumbing. Will users need a filter? While it is good practice to install a filter close to the flow meter, only a few users do as they depend on other parts of the system. Will the fittings affect the flow profile onto the flow meter?

Before selecting a meter, users should always read the entire specification sheet and installation instructions for the flow meter being considered. If unsure, users can talk to the supplier who can help specify the optimal flow meter for their process.

This information has been sourced, reviewed and adapted from materials provided by Titan Enterprises Ltd.

For more information on this source, please visit Titan Enterprises Ltd.

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