Tuesday, January 27, 2015

Selecting Variable Area Flow Meters for Process Measurement

Partially reprinted from a white paper by
Jim Dillon, Global VA Product Manager, Brooks Instrument
To download the entire white paper visit here.
 
VA meter
Variable Area Flow Meter
(courtesy of Brooks Instrument)
Many industrial processes require reliable, accurate and repeatable gas and liquid flow measurement and control. Variable area (VA) meters are extremely versatile in this arena. They are ideal for monitoring process flow, instrument impulse flow lines, purge gas flows, flows of flushing or cooling media, make-up flows, and reactor gas and liquid feeds.

VA meters are an inexpensive flow-metering device available for a variety of industrial applications. VA meters with indicator only are still the norm; however, more VA meters are being specified with options such as flow alarms and analog output transmitters. This

is happening for a couple of reasons. Monitoring a process remotely provides better control because the entire flow line can be monitored. Using a VA meter with an analog output transmitter allows one person to monitor several process lines remotely.

Another technique for monitoring a flow line is a VA meter with a flow switch. The switch can be set to trigger at a given flow rate. When the flow rate reaches the set point, a signal could cause a valve to close or open. A pump could be turned on or off or a simple light or some other signal could be activated.

The other major trend in VA meters is the transition from glass tube rotameters to metal tube rotameters. Metal tube meters are more durable and require less maintenance. Plus, if a glass tube breaks, not only is there the expense of the glass tube, there is downtime and maintenance technician time to consider, too.

Unlike other flow meters, VA meters will respond like a canary in a coal mine when air quality changes. If process conditions change, they usually impact flow rate. For example, if the back pressure changes on gas flows, the flow rate will change. For this reason, it’s necessary for engineers to take several factors into consideration when specifying a VA meter for their applications:
  • Normal and maximum operating temperature and pressure 
  • Flow rate — minimum, maximum and normal 
  • Reference conditions and volumetric flow type (normal, standard or other) if measuring a gas 
  • Fluid density and viscosity 
  • Accuracy 
  • Global certifications