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What are the considerations of flow meter?

30 Apr.,2024

Six Top Factors to Consider When Selecting a Flow Meter

Article | June 15, 2023.
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Water utilities rely on accurate and dependable flow measurement for critical process controls. Regulatory agencies also require flow monitoring and reporting, with specific accuracy limits.

Flow metering technology is continually improving, and a variety of meter types and styles are available. Each flow meter application is unique, and flow meter selection should not be based on “low bid.” While cost is always important, other factors carry more weight when it comes to selecting a flow meter.

Here are six important considerations for choosing the best flow meter.

1. Understand the Process

Before specifying a meter, it’s important to clearly understand the entire process and where the meter fits in that process. Involve plant operators during design to discuss issues of maintenance, calibration, and access. Does flow need to be totaled? Does information need to be transmitted to a supervisory control and data acquisition (SCADA) system or be available on the Web? Knowing who will be using the meter and specifically how it will be used is the starting point.

2. Media Being Measured

The media being measured is one of the most important considerations when choosing a flow meter. Conductivity, temperature, pressure, and viscosity can affect certain types of flow meters. How clean or dirty the water is may also impact the type and style of meter.

So, the next step in choosing a flow meter is to thoroughly understand the characteristics of the flow to be measured. In an existing facility, data on the matrix should be readily available for review. Standard engineering criteria can be used for new facilities if needed.

For example, propeller meters are often used in drinking water systems, especially for measuring well water withdrawal. These velocity meters can measure fluids containing a certain amount of sand, dirt, iron, and other contaminants. However, these meters would not be recommended for raw wastewater containing stringy materials and wipes, which would foul or damage the meter.

On the other hand, electromagnetic “mag” meters are very accurate when measuring conductive materials like water and wastewater. They have no moving parts to corrode or break. Different styles of mag meters are available for specific fluid measurements, including drinking water, wastewater, and sludges.

3. Accuracy, Range, And Certification Requirements

How accurate must the flow measurement be? This depends on the meter’s purpose. Flow measured to control chemical feed may need greater accuracy than that used for general tracking of water treated. Flow measurement for billing purposes, such as a water supply authority selling water to a utility, must be extremely accurate.

Regulatory conditions may also apply. Most water and wastewater systems have permit limits for maximum flows and must measure and report daily flow totals. Be sure to check all existing permits for flow measurement accuracy requirements.

Measurement range and turndown rates must be considered as well. Engineers often size equipment and pipe for a 20-year growth period, which can lead to problems when current flows are much less than design rates.

Some utilities may require meters to have NSF-61, NSF-372 approvals or ISO 9001certification. Manufacturers may also accredit their calibration laboratories through the National Voluntary Laboratory Accreditation Program (NVLAP). NVLAP provides NIST accreditation to testing and calibration laboratories using management and technical requirements of the international standard ISO/IEC 17025:2005.

4. Meter Location and Installation

Proper location and installation of flow meters are critical for providing accurate measurement. Even the most sophisticated meter will read erroneously if installed improperly. Improper installation occurs most often when trying to “force” a flow meter into an existing plant or network or make it fit in a tight site.

Flow Disturbances

Most flow meters require a specific amount of straight run pipe to prevent flow disturbances. Pipe bends, valves, tees, and reducers can create significant flow measurement errors—up to 50 percent for certain meters. In a new system, straight runs can usually be designed appropriately. However, sufficient straight runs may be difficult to obtain in existing systems or constrained sites. Flow conditioners may help reduce inaccuracies. Also, some meters can measure more accurately than others under these conditions.

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Piping

Pipe size, material, and direction are part of the meter selection equation. For instance, downward flow should be avoided when measuring liquid. As noted previously, systems are sometimes overdesigned to accommodate future growth. In most cases, the pipe must be full for accurate measurement.

5. Reporting/Data Recording

What information needs to be monitored and recorded, and how? Some applications require continuous recording of the flow, plus total flow readings. Do operators need event notification, such as high flow or zero flow alarms? Data must often be sent to a SCADA system. In some cases, flow meters are located remotely and need battery-powered devices.

During design, determine what type of output is needed. While 4 to 20 milliamp is most common, some facilities may need MODBUS cards or other instrumentation.

6. Consult With A Technical Partner

Flow meter manufacturers’ representatives have a great depth of expertise. They know the right questions to ask and can recommend the best solutions for each unique installation. Use them as a trusted technical partner.

Your technical partner can help to make cost-effective decisions as well. Manufacturers can help by outlining lifecycle costs, including installation, maintenance, and calibration in addition to purchase price.

Most of all, your technical partner can help you choose the right meter up front rather than trying to make the wrong meter work after it’s installed.

Considerations in Flow Meter Selection

This is the second of two parts. Read the first part here

Flow measurement is a critical aspect in a wide range of plant operations. Users choosing equipment to meter the flow of liquid or gas processes must consider various factors to arrive at an optimal solution. Experience has shown there are significant differences between meter technologies, with each type of device having its own advantages and disadvantages for industrial facilities.

The previous article in this series (Pumps & Systems, October 2017) described common flow applications and types of flow technology. This article will look at important selection criteria.

In a typical industrial facility, fluid characteristics (single or double phase, viscosity, turbidity), flow profile (laminar, transitional, turbulent), flow range and accuracy requirements are all important considerations in determining the right flow meter for a particular measurement task. Additional considerations such as mechanical restrictions and output-connectivity options impact the user’s choice.

Some of the key criteria in flow meter selection include:

Process media. Different flow meters are designed to operate best in different fluids and under different operating conditions. That’s why it is important to understand the limitations inherent to each style of instrument. Fluids are conventionally classified as either liquids or gases. The most important difference between these two types of fluid lies in their relative compressibility (i.e., gases can be compressed much more easily than liquids). Any change that involves significant pressure variations is generally accompanied by much larger changes in mass density in the case of a gas than in the case of a liquid.
Type of measurement. Industrial flow measurements fall under one of two categories: volumetric or mass. Volumetric flow rate is the volume of fluid passing through a given volume per unit time. Mass flow rate is the movement of mass per time. It can be calculated from the density of the liquid (or gas), its velocity and the cross-sectional area of flow. Volumetric measuring devices, like variable area meters or turbine flow meters, are unable to distinguish temperature or pressure changes. Mass flow measurement would require additional sensors for these parameters and a flow computer to compensate for variations in these process conditions. Thermal mass flow meters are virtually insensitive to variations in temperature or pressure.
Flow rate information. A crucial aspect of flow meter selection is determining whether flow rate data should be continuous or totalized. A flow rate has to do with the quantity of a gas or liquid moving through a pipe or channel within a given or standard period of time. A typical continuous flow measurement system consists of a primary flow device, flow sensor, transmitter, flow recorder and totalizer.
Desired accuracy. Accurate flow measurement can be the difference between on-spec quality and wasted product. Flow meter accuracy is specified in percentage of actual reading (AR), percentage of calibrated span (CS) or percentage of full-scale (FS) units. It is normally stated at minimum, normal and maximum flow rates. A clear understanding of these requirements is needed for a meter’s performance to be acceptable over its full range.
Application environment. Flow meters can be employed under a host of varying conditions in an industrial plant. For example, users must decide whether the low or high flow range is most important for their metering application. This information will help in sizing the correct instrument for the job. Pressure and temperature conditions are equally important process parameters. Users should also consider pressure drop (the decrease in pressure from one point in a pipe to another point downstream) in flow measurement devices, especially with high-viscosity fluids. In addition, viscosity and density may fluctuate due to a physical or temperature change in the process fluid.
Fluid characteristics. Users should make certain that the selected flow meter is compatible with the fluid and conditions they are working with. Many plant operations involve abrasive or corrosive fluids, which move under aerated, pulsating, swirling or reverse-flow conditions. Thick and coarse materials can clog or damage internal meter components—hindering accuracy and resulting in frequent downtime and repair.
Installation requirements. Planning a flow meter installation starts with knowing line size, pipe direction, material of construction and flange-pressure rating. Complications due to equipment accessibility, valves, regulators and available straight-pipe run lengths should also be identified. Nearly all flow meters must be installed with a run of straight pipe before and after their mounting location. Where this is not possible, a flow conditioner can be used to isolate liquid flow disturbances from the flow meter while minimizing the pressure drop across the conditioner.
Power availability. Pneumatic instrumentation was once used in most hazardous area applications, since there was no power source to cause an explosion. Today’s installations normally call for intrinsically safe instruments, which are “current limited” by safety barriers to eliminate a potential spark. Another option is to employ fiber optics. Turbine flow meters offer an advantage in environments where a power source is not available. They do not require external power to provide a local rate/total indicator display for a field application, and instead rely on a battery-powered indicator. Solar-powered systems can also be used in remote areas without power.
Necessary approvals. Approvals for the use of flow measurement equipment in hazardous plant locations include FM Class 1 Division 1, Groups A, B, C and D; and FM Class 1, Zone 1 AEx d (ia) ia/IIC/T3-T6. Standards such as the Measuring Instruments Directive (MID) in the European Union (EU) apply to fiscal and custody transfer metering for liquids and gases. In terms of environmental emissions, industrial flow meters must meet the Electromagnetic Compatibility (EMC) Standards EN55011:1992 and EN61326-1:1997.
Output/indication. Flow meter users must decide whether measurement data is needed locally or remotely. For remote indication, the transmission can be analog, digital or shared. The choice of a digital communications protocol such as HART, FOUNDATION Fieldbus or Modbus also figures into this decision. In a large industrial facility, flow readings are typically supplied to an industrial automation and control system (IACS) for use in process control and optimization strategies.

Summary

Choosing the right flow measurement solution can have a major impact on operational and business performance. For this reason, companies anticipating a flow meter purchase should consult with a knowledgeable instrumentation supplier in the early stages of a project. The effort spent learning about basic flow measurement techniques, and available meter options, will ensure a successful application once the equipment is installed.

Read the first part of this series here

For more information, please visit Gas Flow Meters.

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