Flowmeters Are Not All Alike
- Monday, 01 December 2008
Vortex Shedding – Forms fluid vortices against the meter body introduced in the pipe. These vortices are produced from the downstream face of the meter in an oscillatory manner. The shedding is sensed using a thermistor, and the frequency of shedding is proportional to volumetric flow rate.
Turbine – Incorporates a freely suspended rotor turned by fluid flow through the meter body. Since the flow passage is fixed, the rotor’s rotational speed is a true representation of the volumetric flow rate. The rotation produces a train of electrical pulses, which are sensed by an external pickoff and then counted and totalized. The number of pulses counted for a given period of time is directly proportional to flow volume.
Positive Displacement – Separates liquid into specific increments, and the flow rate is an accumulation of these measured increments over time. As the fluid passes through the meter, a pulse is generated which represents a known volume of fluid.
Ultrasonic – Operates by comparing the time for an ultrasonic signal to travel with the flow (downstream) against the time for an ultrasonic signal to travel against the flow (upstream). The difference between these transit times is proportional to the flow, and the flowmeter converts this information to flow rate and total.
Meter Selection Factors
For end users, there are a number of specific issues to keep in mind when choosing a flow-metering instrument.
When it comes to measuring the flow of liquids and gases, not all flowmeters are created equal. Different flowmeters are designed for optimum performance in different fluids — and under different operating conditions. That’s why it is important to understand the limitations inherent to each style of instrument.
Measuring the flow of liquid and gases demands superior instrument performance. From basic water to fluids with extreme viscosities, high solid concentrations and low conductivity, these applications present a wide range of flow metering challenges.
Measuring the flow of dense, viscous fluids can be difficult for many flowmeters. Thick and coarse materials can clog or damage internal meter components — resulting in poor accuracy and frequent downtime and repair.
For applications with dirty, viscous liquids, chemical processors traditionally have chosen orifice plates, venturi tubes, target meters, variable area meters, mag meters, and various mass meters. Noninvasive ultrasonic meters are also an option. For the most part, intrusive technologies such as turbine, PD and vortex meters should not be considered for these environments.
On the other end of the spectrum, pharmaceutical and biotechnology plants often have filtration equipment producing a wide range of water qualities, including USP, de-ionized (DI) water, and water-for-injection (WFI). These operations require very accurate flow measurements at various stages of the process.
Turbine meters have long been popular for obtaining precise flow readings in clean, known liquids such as ultra-pure water. They normally operate in temperature ranges up to 350°F (177°C), and can be equipped with ceramic bearings which, unlike tungsten carbide and epoxy-impregnated graphite bearings, can withstand the corrosive effects of ultra-clean water.
All flowmeter users should understand fluid viscosity. Kinematic viscosity is the ratio of the absolute viscosity to the specific gravity, usually expressed in centistokes (cst), where the resistance to flow is measured in square millimeters per second (mm2/s).
Fluid viscosity sometimes fluctuates during a given measurement period. Viscosity and density also vary due to a physical change in the mixture of the fluid. In most cases, however, these changes are due to variations in the operating temperature of the fluid itself.
Using today’s advanced flowmeter electronics, real-time volumetric flow measurements can be obtained by measuring the process operating temperature and addressing a temperature verses viscosity look up table to determine the operating kinematic viscosity. The frequency is measured directly from the flowmeter and divided by the kinematic viscosity. This ratio of frequency and viscosity is used to determine the correct K-Factor for the specific operating temperature and viscosity.
In high-pressure flow applications, the “water hammer” effect can severely damage conventional flow measurement devices. Water hammer (or, more generally, fluid hammer) is a pressure surge or wave caused by the kinetic energy of a fluid in motion when it is forced to immediately stop or change direction. For example, if a valve is closed suddenly at an end of a pipeline system, a water hammer wave propagates in the pipe.