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 |
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|
Fluid Type |
Raw Water |
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|
Fluid Temperature |
40 - 50 deg F |
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|
Fluid Pressure |
20 – 40 PSI |
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|
Flow Range |
100 – 1000 GPM |
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|
Pipe Diameter |
12 Inch |
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|
Flowmeter Technology |
Cone, Mag or Propeller |
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|
Accuracy |
+/- 2% of Rate |
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|
Repeatability |
+/- 0.5% |
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|
Installation Considerations |
Retrofit with pump and valve in close proximity |
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|
Maintenance Schedule |
Inspect/Repair Verify Calibration |
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Expected Installation Life |
> 25 Years |
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Budget |
$7500 |
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|
Meter Purchase Price |
$3500 |
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|
Installation Cost |
$1000 |
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|
Annual Maintenance & Calibration Costs |
$500 |
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|
Meter Life Expectancy |
10 Years |
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Replacement Cost |
$5000 |
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|
1 Year Cost Of Ownership |
$5000 |
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|
5 Year Cost Of Ownership |
$7000 |
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|
10 Year Cost Of Ownership |
$14,500 |
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 measure the velocity of a passing liquid. Flow rate is determined based on the velocity measurement.
Accuracy
How accurate does your measurement really need to be? While highly precise flow meter technologies can measure within ±0.01 percent of full scale, there is generally a price to be paid for this type of performance. If you are measuring chemical additive injections into pharmaceutical, biotech or food/beverage products, then this type of accuracy is essential. On the other hand, many other processes are less critical, and “good” rather than “precise” accuracy is all that is needed to get the job done.
| Evaluation Criteria | Requirements/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 | ||
| Flow meter Technology | Cone, Mag or Propeller | ||
| Accuracy | +/- 2% of Rate | ||
| Repeatability | +/- 0.5% | ||
| Installation Considerations | Retrofit with pump and valve in close proximity | ||
| Maintenance Schedule | Inspect/Repair Verify Calibration | ||
| Expected Installation Life | > 25 Years | ||
| Budget | $7,500 | ||
| Meter Purchase Price | $3,500 | ||
| Installation Cost | $1,000 | ||
| Annual Maintenance & Calibration Costs | $500 | ||
| Meter Life Expectancy | 10 Years | ||
| Replacement Cost | $5,000 | ||
| 1 Year Cost of Ownership | $5,000 | ||
| 5 Year Cost of Ownership | $7,000 | ||
| 10 Year Cost of Ownership | $14,500 |
Repeatability
When you consider accuracy, do not forget to ask your flow meter manufacturer about repeatability. The term repeatability in flow instrumentation is equivalent to consistency of accurate measurement. Because flow meters are typically calibrated to pipes flowing at a specified rate, then the accuracy of measurement can drop too. The manufacturer's repeatability specification will help in comparing accuracy specifications among different devices.
Installation
The requirements for flow meter installation vary by the type of flow meter technology. The three basic types of installation in order of complexity from most difficult to simplest are: inline, insertion and clamp-on. An inline meter requires cutting the pipe; in contrast, insertion and clamp-on types can be installed under flowing conditions.
Nearly all major flow meter technologies require a manufacturer's specified pipe diameter straight run upstream and downstream from the meter to ensure a stable flow profile. Failure to comply with the manufacturer's straight pipe run installation requirements often leads to either poor accuracy or inconsistent performance (repeatability problems).
When flow meters are placed too close to pumps, valves and other equipment, unstable or irregular flows can impact performance and eventually result in maintenance problems. If you find yourself in a tight spot in terms of a plant retrofit or limited space in a complex pipe gallery, a limited number of flow meter technologies, such as electromagnetic devices or self-conditioning differential pressure meters, will offer the appropriate solution. Either meter type requires virtually no straight run due to the sensing technology, or they feature built-in flow conditioning technology that remove swirl and other flow distortions without the need for straight pipe conditioning.
Maintenance
Ask the manufacturer about the required maintenance of any flow meter under consideration. These requirements can range from periodic inspection and cleaning with devices such as orifice plates to replacing moving parts that wear to calibration checks to maintain accuracy. Increasingly, environmental and safety regulations at the federal, state and local levels specify maintenance procedures for all types of plant instrumentation including flow meters.
Life Cycle
What is the expected life of your flow meter? In some applications such as subsea oil/gas production, your flow meter must have a life expectancy of 25 years or more with no possibility for maintenance. In other applications, a simple disposable device with a one to two year lifespan is perfectly acceptable. Your application probably falls somewhere in between. As you compare different flow meter technologies, be sure to calculate the cost of installation and maintenance and also amortize the cost of the flow meter over its lifespan. These cost comparisons can be revealing.
Conclusion
While choosing a flow meter can be a complex task, simplify the task by using a comparison table like Table 1. Do not hesitate to ask your flow meter manufacturer for product information, demonstrations and training. Flow meter manufacturers are happy to help you find the best flow meter solution for your process and plant.
Pumps & Systems, September 2010