Find the right flow meter for your process and plant.
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
Fluid Media Type
Selecting a flow meter begins with understanding the process media fluid. Do you need to measure liquid, steam or gas? For the purpose of this article, we are focusing on liquid for volumetric measurement (flow rate or totalized flow). The question then becomes what kind of a liquid?
For example, the flow meter you choose to measure drinking water may not be the appropriate choice for wastewater treatment. Not all liquid flow meter technologies are appropriate to measure dirty fluids, particulate laden slurries, high-density, viscous fluids or sanitary liquids for food/beverage or pharmaceutical applications. The conductivity of a liquid and the presence of bubbles in a liquid are both additional factors to consider.
The chemical properties of the liquid are important, too. Corrosive and caustic liquids may require specialty materials to prevent damage to the meter. Excessive maintenance or costly replacements can result when the chemical properties of the liquid are not fully considered in advance.
Operating Temperature and Pressure
Full knowledge of the liquid to be measured is only part of understanding the overall application. Some flow meter technologies are affected by fluid temperature and operating pressure. If a flow meter's sensing accuracy is affected by temperature, then you may either need a flow meter with built-in temperature compensation or you will likely need to add a temperature sensor. Some flow meters also rely on moving parts not designed to withstand high pressure operation. While some meters work exceptionally well at a regular flow rate, others will easily outperform in high turndown applications such as those that start and stop frequently.
Flow Range
Knowing the flow range and pipeline diameter are both critical factors to consider. Will the flow rate be continuous or will it be variable? In some plants, such as municipal water treatment, the plants are often designed specifically so the flow rate has predicted fluctuations because there are daily or seasonal high and low flow periods based on consumer demand. In other operations there may be a year round continuous flow or stable flow that exists when the process runs. Not all flowmeters respond well to a sudden decrease or increase in the rate of flow. Some flowmeters operate well over a wide turndown rate.
Likewise, not all flowmeters are designed for all pipe diameters. When outfitting or retrofitting a plant, it is a good idea to use a flowmeter technology that meets the needs of all flow measurements throughout a plant. It greatly simplifies purchasing, installation, training and maintenance.
Sensor Type
The complexity of fluid flow measurement has resulted in the development of numerous flow sensing and measurement technologies. Once you start analyzing the liquid to be measured, the accuracy desired, and the process and plant requirements, however, you will usually find two or three options for your application. A brief description of the major flow sensing technologies follows:
Coriolis: Liquid flowing through a U-shaped tube results in the tube twisting, and the twisting motion or vibration is used to calculate the flow rate.
Cone: A cone is placed in the pipe, and the difference between the upstream and downstream flows is calculated with differential pressure technology to indicate flow rate.
Electromagnetic: A conductive liquid moving through a magnetic field generated in a pipe creates an electric charge, which is measured to determine the flow rate.
Orifice Plate : Differential pressure technology is used to measure flow by determining the difference in pressure from the upstream to the downstream side of the obstructed pipe.
Propeller/Turbine: Liquid flowing in a pipe spins a propeller or a turbine, and the rate of spin is measured to determine the flow rate.
Venturi: A flow element forces liquid into a smaller diameter area of the pipe and the difference between the restricted and unrestricted flows is calculated with differential pressure technology.
Vortex: An obstructive device is placed in a pipe to create vortices downstream. The vortices are measured with temperature or pressure sensors to determine the flow rate.
Ultrasonic: Ultrasonic transducers are placed in a pipe to
