Flowmeters: Making the Right Choice

Flowmeters are essential measurement devices across industry and considering all the parameters at the outset is crucial in selecting the right one for the job, says Trevor Forster

FLOWMETER sales in all sectors increase year on year as more control is required throughout the industrial landscape. In response, innovation in flow meter technology continues to progress to meet the needs of growing demands and applications.

The basic physical techniques that flowmeters work to are well established; it’s the latest technology and materials that are applied which give the wide variety of choices in the market.

Although what we discuss here is directed at liquid flowmeters, the considerations are applicable to gases and two-phase flow measurement as well.

Flowmeters fall into six broad groups:

At the outset, one should ask, “What will using a flowmeter enable me to achieve?” Common applications include batch measuring a volume of liquid; counting the total liquid volume; determining the flow rate; and controlling the flow rate or temperature.

Defining what the flowmeter must achieve is often a moving target and considering all the parameters required for the process and environment is key to ensuring the flowmeter chosen is suitable for the process application.

Parameters that combine to determine the choice of flowmeter

1. Fluid type

The properties and process characteristics of the fluid will determine the technologies available and acceptable solutions, eg:

• What is the fluid density, viscosity, flow rate?
• What is the line pressure, temperature, pressure drop?
• Is the flow constant, intermitent, or pulsating?
• Is the fluid aggressive, conductive, or hazardous?
• Is it homogenous, have impurities and/or gas entrainment?

2. Measurement task

The flowmeter output and interface requirements will be determined by what you want the flow meter to do, eg:

• Show a simple total of fluid passed
• Show the flow rate of the passing fluid
• Deliver a pre-set quantity after receiving an input
• Control the flow rate as part of a control loop

3. Flow measurement performance

Both the mechanical properties of the flowmeter and the physical properties of the liquid combine to influence the performance of the flow measurement device. Determining the required level of performance for the flowmeter will help to pin down the suitable flowmeter types available. Ask yourself:

• Is a highly accurate flowmeter necessary?
• Is the technology’s repeatability good enough?
• Will the flowmeter maintain accuracy?
• Is the calibration uncertainty acceptable?

4. Installation and operational limitations

Poor installation can easily compromise flowmeter performance. Pipework configurations, electrical installation, pumps and other equipment introduced into the pipework that impact flow, will all have a bearing on flowmeter performance. Manufacturers specify the ideal installation requirements for each type of flowmeter and any variation from this will negate the stated performance characteristics.

Always read the flowmeter specification and installation instructions to avoid potential installation and operational issues. Ask yourself:

• Is the existing line size suitable for actual flow rate?
• Are deposits likely on the pipe wall?
• Are fluid-borne vibrations expected?
• Could a large hydraulic shock be expected?

5. Cost of ownership

The cost of ownership is not simply the purchase price but includes the cost of any repair outside of the warranty period, performance loss resulting from measurement uncertainty, the power or equipment cost of running the flowmeter, and any additional wiring requirements. Investigation into the following will give a broader picture above the simple purchase cost:

• Is specialist installation or secondary equipment required?
• Is external control or interface equipment required?
• What are the costs of a system shutdown for service or repair?
• What is the expected shelflife of the meter?

Influencing the decision

As only certain flowmeter technologies will suit certain application conditions, the liquid properties will often be the prime determinant in choice of flow measurement device. The five sets of points raised above are intertwined, and determining one parameter may lead to a compromise in another.

Although a hygienic meter could be used within a simple cooling water circuit, it would be overkill when a standard electromagnetic flow measurement device might be the perfect solution. Similarly, a turbine meter is unlikely to be suitable for a viscous oil, particularly if the temperature, viscosity and therefore the Reynolds number, constantly change. Adding the other possible variations, the window of options quickly closes.

Electronic-based meters may not perform well with pulsating flows as there could be aliasing between the meter’s cycle time and the pulsations, causing drastic reading errors. Positive displacement flowmeters would provide a better outcome where pulsating flows are integral to the process (see Figure 2).

Aggressive liquids offer their own set of problems and flowmeters such as ultrasonic and Coriolis, can withstand the effects of these type of fluids. Vortex shedding meters do not perform well at low Reynolds numbers and positive displacement meters perform poorly at high ones. Ultrasonic and Coriolis meters do not require conductive fluids, yet for electromagnetic meters it is fundamental to their operation.

The meter function may not always be clear-cut. A sight-flow, ball-in-tube, meter may be perfect for a moulding machine water flow where no extra instrumentation is required, and the process has wide operational parameters. However, for a precision medical moulding, full control over the mould temperature and therefore cooling water flow rate, may be critical for high tolerance plastic mouldings.

Flowmeter performance is an interesting point. Depending on the application, one user may require it to be as absolute as possible, whereas another would be happy with “about right”. But performance costs: the higher the performance required, generally the higher the cost of the flowmeter. Determine the actual process requirements before choosing a flowmeter purely on performance.

Very small flowmeters generally have no special plumbing requirements but with many flow measurement technologies the installation effects can be catastrophic. To demonstrate, take a turbine meter with two pipe elbows upstream of the meter. By rotating the elbows into differing upstream configurations the turbine could be stopped and indeed made to spin backwards. When plant requirements are prioritised as opposed to the fluidic ones, low flows in oversize pipes are often seen. This occurs where pipe size rather than measured flow rate is used to determine flowmeter size.

Selecting the flowmeter to suit the application

Table 1 highlights the different types of flowmeter and their suitability for various process applications.

Each type has its own strengths and weaknesses, each performing optimally under different conditions.