Know what you want and you’ll get it!

To the uninitiated flow meter selection can seem very difficult, however, it can also be a rewarding learning process. The ultimate flowmeter is one that is safe, can be used in all applications regardless of size and material compatibility, imparts no measurable effect on the flow stream, installs simply, is affordable and readily available in all line sizes. Unfortunately, no such device exists and therefore the flowmeter market is fragmented into as many products to choose from as there are applications. An orderly process of elimination can be used to select the best available technology for your particular application. Although the weight given to each attribute may differ in your application, proper selection should not overlook any of the seven following considerations.

1. SAFETY

A proper safety evaluation will require an analysis of the mechanical and material compatibility and the electrical specifications of the flowmeter.

· Mechanical safety considerations will address such topics as where and what type of support is required?

· What is the effect of the installation on the piping system in question? It is critical not to go against the recommendations offered by the piping system supplier.

· Does the sensor meet or exceed the design criteria for pressure, temperature, and coefficient-of-expansion?

· What amount of fluid pressure is experienced upon the installation both upstream and downstream, and could any one component failure cause catastrophic failure?

· If the sensor passage should become clogged what effect would water hammer have on the piping system?

· What recommendations are made with regard to sensor use?

· Is it possible to isolate the sensor for service, or must the pipe run be isolated to access the sensor?

Media Compatibility is a primary concern. Will aggressive acids, bases, or temperature extremes cause a weakening or failure of the sensor material? Abrasive fluids (slurries) can rapidly deteriorate sensor chambers, bearings, or internal sensor surfaces. If abrasives are known to exist, get clarification from the vendor on velocity limits and maintenance recommendations.

Electrical safety is also an important aspect since water and electricity DO NOT MIX.

· Can the sensor pose a shock hazard if immersed?

· Does the sensor have the required agency approvals for the area of operation such as F.M. or Cenelec for explosive environments?

2. RELIABILITY

Everyone wants to know they have purchased the best value for their money. Are there unknown reliability aspects that must be considered prior to purchase? This usually requires an in-depth study of the sensing technology and matching the right sensing principle to the application. A simple scoring system, shown below, is used to tabulate the best flowmeter fit for particular application. Remember that the weights given to each attribute will be application specific and therefore should be adjusted based on the selector’s needs.

3. INITIAL EXPENSE

All expenses are constrained by budget and therefore it is critical to be able to quickly determine the approximate cost of a particular sensor. Generally, the sensing technology and pipeline sizes are the predominant factors affecting price. It's important to note that some technologies are not available in all line sizes. Also, do not overlook the cost of installation such as mounting, electrical hook-up or PLC/DCS programming - these costs can be substantial.

4. COST OF OWNERSHIP

Many people know of this in principle, but fail to factor the cost of ownership into the selection process. Be sure to get an honest evaluation from your vendor, so that all costs associated with the ownership of your sensor are known and fall within your initial and on-going budget constraints. Hidden costs that can rapidly accrue are installation cost, cost to shut down system, and in the worst case scenario, the cost of complete retrofit and replacement of a sensor along with the process fluids. Getting a clear picture of the maintenance interval will help you plan for the inevitable service requirements and should be factored into the purchase price.

5. SURVIVABILITY

The ability of a sensor to maintain accuracy and reliability over time will depend on the materials of construction and the type of moving parts, if any, in the sensor assembly. Some highly sophisticated sensor electronics can suffer from high levels of electrical noise or temperature fluctuations. The liquid to electrical coupling mechanisms in some sensor types can change over time, and thus affecting the accuracy or even the reliability of the sensor.

6. INFLUENCE ON THE PROCESS BEING MEASURED

Some sensors will not just measure the flow, but they must also alter its characteristics in order to operate. Knowing how a particular sensor will affect the flow stream is critical in certain applications such as gravity fed flows.

7. SIGNAL COMPATABILITY

In this rapidly evolving industry, it is essential to choose sensors that have the ability to interface with both present and future (proposed) system architectures. Most sensors purchased today do not fully utilize their capability due to lack of common signal standards or due to the time and cost in implementing the necessary software changes. Knowing the instrumentation compatibility is essential to seamless system integration.

Additional Resources Available:

With the advent of the worldwide web and cellular communications, the ability to gather information has become much more efficient. Past reliance on local expertise is giving way to widespread dissemination of information. This in turn allows greater application of the products and services available. Today, the challenge is in wading through all the information available. Nothing will replace knowledge gained through experience, so don’t be afraid to ask questions and gauge responses accordingly. Through a systematic analysis of your particular application, you will surely find the best flowmeter for your specific needs.