Flowmeters Are Not All Alike
- Monday, 01 December 2008
Ideally, an industrial flowmeter, whether it employs mechanical or electromagnetic principles, should respond instantly and consistently to changes in water velocity. Meters, however, are not perfect instruments and may not accurately register velocity in all measurement conditions encountered. Turbulent or pulsating flows can cause registration errors in meters.
In recent years, instrument suppliers have developed new PD meter configurations for pulsating flow streams encountered in batch applications. These meters, employing a bearingless design and non-intrusive sensors, can measure liquid flow where process pressure is above 1,000 psig and operating temperatures reach 400° F (204° C).
In situations where flowmeters are used to give an indication of the rate at which a liquid or gas is moving through a pipeline, high accuracy is not crucial. But for batching, sampling and dispensing applications, flowmeter accuracy can be the deciding factor between optimum quality and wasted product.
Typically, flowmeter accuracy is specified in percentage of actual reading (AR), in percentage of calibrated span (CS), or in percentage of full-scale (FS) units. Accuracy requirements are normally stated at minimum, normal, and maximum flow rates. Unless you know these requirements, your flowmeter’s performance may not be acceptable over its full range.
In applications where products are sold or purchased on the basis of a meter reading, absolute accuracy is critical. Otherwise, repeatability may be more important than absolute accuracy. Users should establish separately the accuracy and repeatability requirements of each application, and state both in their specifications.
Response time is another key flowmeter performance criteria. In the case of pharmaceutical dispensing, for example, the meter’s speed of response is critical to operational quality and consistency. Other applications are less concerned with response time, and are focused on accurate mass or volumetric measurements only.
Also, be sure to consider pressure drop when specifying a flow measurement instrument, especially for chemical and food plants with high viscosity fluids. Pressure drop is the decrease in pressure from one point in a pipe to another point downstream, and usually results from the friction of the fluid against the pipe. High flow rates in small pipes give large pressure drop. Low flow rates in large pipes give low pressure drop. In order to achieve a lower pressure drop, some users end up choosing a larger meter than originally planned — significantly increasing their costs.
Continuous or Total Flow Rate?
One step in flowmeter selection is determining whether flow rate data should be continuous or totalized, and whether this information is needed locally or remotely. If remotely, the user must decide whether the transmission should be analog, digital, or shared. The choice of a digital communications protocol such as HART and fieldbus adds another element to this decision. If shared, make sure the required (minimum) data-update frequency is taken into accounts.
In large chemical processing plants, flow readings are normally supplied to a Programmable Logic Controller (PLC) or Distributed Control System (DCS) for use in overall process control and optimization strategies.
In terms of flow, at room temperature and low pressures, volumetric and mass flow rates will be nearly identical. However, these rates can vary drastically with changes in temperature and/or pressure, because the temperature and pressure of the gas directly affects the volume.
When measuring the flow of compressible materials, volumetric flow is not very meaningful unless density (and sometimes also viscosity) is constant. When the velocity (volumetric flow) of incompressible liquids is measured, the presence of suspended bubbles will cause error; therefore, air and gas must be removed before the fluid reaches the meter.
From acids and juices, to argon, chlorine, hydrogen, ammonia and even phosgene, corrosive process media can mandate using a flowmeter with specialized materials of construction. In addition to 316 stainless steel and Hastelloy, standard flowmeter wetted parts are manufactured from Tantalum, Monel, Nickel, Titanium, Carbon Steel, and Zirconium.
Flowmeter manufacturers have invested considerable time and resources to develop meter designs utilizing thermoplastic materials able to handle corrosive liquids and gases. For example, all-plastic PD meters are ideal for aggressive liquid flow applications, including acids, caustics, specialty chemicals, and DI water.
Ultrasonic flowmeters have no moving or wetted parts, suffer no pressure loss, and provide maintenance-free operation— important advantages over conventional mechanical meters such as vortex meters, and also, in many cases, coriolis mass meters. Clamp-on ultrasonic meters are mounted completely external to the pipe wall, so they are not affected by corrosive or erosive liquids, and are not damaged by solids, gases or particles in the process liquid.