Top eight considerations when selecting a flow meter

There are four major reasons flow measurements are taken:

1. Safety – if the flow rate is not controlled, temperature and pressure can reach dangerous levels

2. Product integrity – flow plays a role in ensuring the right amount of blended materials are constantly present

3. Efficiency – the efficiency of a process can be determined by measuring the amount of each input that has gone into a product compared to the amount produced

4. Process variable control – the flow rate is measured and controlled during energy transfer applications

Therefore, accurate flow measurement is important as incorrect measurement can have serious and potentially damaging consequences, and can be the difference between a company making a profit or running at a loss.

Knowing what factors to take into consideration when selecting a flow meter will ensure you’re getting the best one for your application.

1. Process media

The characteristics of the process media is one of the most important considerations as different flow meters are designed to operate at their best in different fluids and under different operating conditions. Characteristics such as conductivity, temperature, pressure and viscosity can affect certain types of flow meters, as well as how clean or dirty the water is. This makes it important to be aware of the limitations of each type of flow meter.

Flow meters with wetted moving parts, such as positive displacement or turbine meters, are unsuitable for dirty fluid applications as they will be more susceptible to mechanical wear, plugging or erosion due to the presence of solid particles. Therefore, they are best suited for clean fluid applications. For example, propeller meters are commonly used in drinking water systems, especially for measuring well water withdrawal. Propeller meters are a type of velocity meter that can measure fluids containing a certain amount of sand, dirt, iron, and other contaminants.

However, for dirty fluid applications where the fluid contains solid particles like stringy materials and wipes, such as raw wastewater, they are not recommended as the fluid would foul or damage the meter. On the other hand, non-contact meters such as electromagnetic, ultrasonic or Coriolis meters are more appropriate for dirty fluid applications. While they still have limitations, they handle the particles better. For example, electromagnetic meters are very accurate when measuring conductive materials like water and wastewater, and have no moving parts to corrode or break.

There are different styles of these meters available to suit specific fluid measurements, including drinking water, wastewater and sludges. Knowing the state the fluid being measured is in – gas or liquid – is also important. Gases can be compressed easily, so a liquid meter would be unsuitable. Furthermore, fluid compatibility with the flow meters wetted parts, such as the body, seals and gears/ rotor/paddle, need to be considered. Thermoplastics are more compatible with acids and bases which are corrosive for metals, whereas metals are more compatible with organic compounds.

2. Viscosity and flow profile

The viscosity of the fluid also needs to be considered as when flow measurement is being undertaken, it determines how well mixed the fluid is and therefore how repeatable the reading will be. For very viscous fluids, a positive displacement meter – such as an oval gear flow meter – is more suitable than a turbine meter, due to the laminar flow characteristic of these types of fluids.

On the other hand, turbine meters can be used for fluids with a turbulent flow, usually less viscous or thin fluids, such as solvent or water. This is because it is a velocity meter, and directly measures the speed of the fluid by measuring the angular velocity of the rotor which is directly proportional to the fluid velocity.

In applications where there is a high viscosity fluid moving at low flow rates, like honey or thick oils, a volumetric flow meter is most suitable. Viscosity is also linked to temperature – as the temperature increases, the viscosity decreases. Therefore, the operating temperature of the application needs to be considered to understand how the fluid viscosity will behave.

3. Flow rate information

Flow rate information – the volume or mass of a fluid flowing per unit time – is needed to determine the right size flow meter for the application and will function as designed. An undersized meter (where the flow exceeds or comes close to exceeding the meter’s capacity) will result in damage or failure of internal components, or in the worst case, will lead to complete failure.

On the other hand, an oversized meter (where the flow is below or close to the minimum range of the meter) will lead to poor accuracy or inability for it to read/ measure the flow. As a general rule, a flow meter should be sized to get between 20-80 per cent of the maximum flow rate, corresponding with the minimum and maximum flow rates.

This will allow for the flow meter to perform at its optimum, and maximise its durability and longevity. It will also allow it to cope with any peak flows that may damage the meter and any lower than average flows if there is an obstruction in the line or blockage that may not register if the meter is at its limit.

4. Accuracy

Some applications may specify and require a certain level of accuracy for the selected flow meter, as an inaccurate reading could result in financial loss or a quality issue. For example, flow meters in applications to control chemical feed may require greater accuracy than those used for the general tracking of water treated.

Similarly, flow meters used for bill tracking purposes need to be extremely accurate. Flow meter accuracy is expressed as the percentage of actual reading (the error will always be the same percentage of the actual flow), percentage of calibrated span (its absolute error will rise as the measured flow rate drops) and percentage of full-scale units (the error is consistent over the full range of the flow rate for the meter).

It is generally stated at minimum, normal and maximum flow rates. Repeatability also needs to be considered with accuracy. This measures the ability of a flow meter to produce the same reading in the same conditions, regardless of the accuracy.of the meter.

Repeatability is crucial as it is not possible to have high accuracy without high repeatability.

5. Temperature and pressure

The temperature and pressure conditions of an application also need to be considered when selecting a flow meter. Like flow rate, temperature and pressure parameters determine the meter’s material capability to withstand the effect of thermal energy and the forces exerted by the flowing fluid.

When considering temperature, viscosity and density need to be taken into account as these may fluctuate when there is a physical or temperature change in the fluid. Operating temperature also needs to be considered as the wetted components, such as the seals, have temperature limits and some materials will not be able to withstand extreme temperatures or extended periods of time at higher temperatures.

Furthermore, the temperature will determine if an electronic instrument can be mounted on the meter or if it will need to be remote mounted as these too have temperature limitations. Pressure needs to be considered as the operating pressure of the application must not exceed the maximum allowable of the flow meter as it has the potential to create a hazard, as well as cause the meter to deform and likely increase inaccuracy over time.

Pressure drop should also be considered, especially in applications with high viscosity fluids. When a flow meter’s pressure rating is calculated, safety is taken into consideration, allowing for small pressure spikes to occur without causing the meter to fail. Both temperature and pressure need to be considered together as high temperatures will affect the pressure capacity of the meter, causing metals to become more ductile and likely to stretch, while maximum pressure ratings allow for the maximum temperature rating of a meter.

6. Location and installation

In order to achieve optimal performance and the desired accuracy, proper location and installation of the flow meter are crucial as even the best meter will record inaccurate readings if installed improperly. Incorrect installation usually occurs when a flow meter is forced into an existing plant or network or to make it fit in a tight site.

The piping size, material, configuration and direction all play a part in the selection of a flow meter. Piping configuration is crucial as it must be constructed in a way that the flow meter is always full of fluid in order to provide an accurate reading. Pipe direction is also important to determine if the flow meter will be installed horizontally or vertically. Where a meter is mounted vertically, the flow should be from bottom to top so that the meter is always full of liquid and to prevent air entrapment.

These issues are most commonly seen where a system has been over-designed to accommodate future growth. Most flow meters require a specific amount of straight run pipe to prevent flow disturbances. For example, velocity meters require a straight run pipe in the upstream and downstream to get a stable flow profile. This is important to consider as an irregular velocity profile caused by pipe bends, valves, tees, and reducers will impact the accuracy and repeatability of the meter – up to 50 per cent in some meters.

In a new system, straight runs can usually be designed into the system as appropriate. However, in existing installations or constrained sites, sufficient straight runs may be difficult to find. In these situations, flow conditioners may help reduce inaccuracies by reducing swirl and disturbances, and some meters can measure more accurately than others in these conditions.

Another consideration is the orientation of the flow meter, which should be strictly followed according to the manufacturer’s instructions.

7. Output/indication

Whether flow measurement data is needed locally or remotely and how the meter will translate the flow rate into a usable data form need to be taken into consideration in the selection process. These are determined by what the data will be used for i.e. billing, regulatory reporting or monitoring, or process control.

Remote indication can be transmitted via analog, digital or shared through protocols such as HART, FOUNDATION Fieldbus or Modbus. In some applications like large industrial facilities, readings are usually supplied to an industrial automation and control system for use in process control and optimisation strategies.

8. Necessary approvals

Before selecting a flow meter, end users need to ensure that it meets any necessary approvals for the application, such as approvals for use in hazardous areas.

Meeting flow objectives

Understanding the key considerations when selecting a flow meter and why they are important are crucial to achieving success with flow objectives for an application.

In particular, in applications where the wrong decision could have greater consequences for operational and business performance, care should be taken during the selection process and a knowledgeable instrumentation supplier should be engaged in the early stages of a project.

Knowing what factors to take into consideration when selecting a flow meter will ensure you’re getting the best one for your application.

Related articles

Selecting a liquid flow meter to measure volumetric flow rate or totalized flow can be a complex process. There are many factors to consider, including the fluid type, application environment, operating parameters like temperature, pressure and flow rate, flow meter technology, accuracy and repeatability requirements, reliability, installation constraints, maintenance requirements and instrument life cycle.

For example, many types of flow meters measure liquid, and some are better suited to clean water than wastewater treatment environments. Some are more accurate and repeatable than others. Some require less frequent or more complex maintenance. Some last longer than others.

In choosing a liquid flow meter, it is important to consider all the selection criteria rather than focusing on one aspect alone, such as price. Low purchase price alone can often be a misleading indicator considering required performance, maintenance costs and life cycles. A better consideration would be total cost of ownership, which takes into account not only purchase price but also the cost of installation, maintenance, calibration and meter replacement.

On the other hand, sometimes an inexpensive flow meter with simple features does the job adequately. When the application is simple, performance may be less critical, and there might be no compelling reason to consider a more sophisticated solution.

Selection Considerations

Developing an application-specific comparative flow meter evaluation tool is a good place to start. Table 1 is an example of a flow meter selection matrix, in worksheet format, that will help in comparing various types of flow meters to specific flow meter criteria. Time invested upfront in thoroughly understanding the fluid to be measured and the process or plant environment where the flow meter must operate will ultimately pay dividends.

Table 1. Flow meter selection worksheet

Evaluation Criteria

Requirement/Goals

Manufacturer 1

Manufacturer 2

Application

Process Influent

Fluid Type

Raw Water

Fluid Temperature

40 - 50 deg F

Fluid Pressure

20 – 40 PSI

Flow Range

100 – 1000 GPM

Pipe Diameter

12 Inch

Flowmeter Technology

Cone, Mag or Propeller

Accuracy