Variable Area Technology
1. What is Variable Area?
Variable area (VA) flow meters, commonly called rotameters, are a cost-effective solution for flow measurement of liquids and gases. The VA meter has excellent measurement repeatability, which provides process stability and consistency.
External power is not required for operation; therefore, they deliver a fail-safe flow rate under any circumstance.
VA flow meters can be made from a variety of wetted materials for process fluid compatibility, high-pressure and high temperature conditions, and hazardous locations.
The variable area technology was invented in Germany more than 100 years ago. A variable area flow meter is often called a rotameter because the float spins (rotates) as the flow passes it. The spinning float is a clear indication that the fluid is flowing.
The Brooks Rotameter Company began in 1946 by introducing an innovative rotameter design that not only used bolts to fasten the side plates but also dowel pins. This rugged design earned Brooks the reputation it has today of making meters that are rugged, durable and suitable for difficult applications.
In 1958, Brooks introduced the Sho-Rate, which is an industry standard for low-flow purge meters. The need for meters that were capable of higher process pressures and temperatures led to metal tube meters in the 1960s and 1970s.
The industry need for remote monitoring of the process created more opportunities for rotameters. Brooks introduced meters with flow alarms that could be set for a specific flow point and meters that provided pneumatic and analog outputs that corresponded to flow. HART and Fieldbus communications were later additions to the product.
Today, the variable area meter is capable of measuring flow rates with process pressure approaching 20,000 psi/1350 bar and process fluid temperatures of 750 °F/420 °C. Even though the technology is more than 100 years old, it is still a respected technology.
Brooks Instrument has a broad line of variable area meters that serve many industries and handle the toughest applications.
3. Principle of Operation – Variable Area
There are five primary elements to a variable area flow meter (rotameter):
- Metering tube – a tapered chamber that contains the fluid and the float
- Float – a spherical or symmetrically shaped object that moves up or down based on flow rate
- Scale – a device marked with increments of the flow units used to read the flow rate
- Process connections (end fittings) – the flanged or threaded assembly that the customer uses to connect the process fluid lines – along with meter size, this is very important to the customer
- Body or housing – the frame or structure that contains the metering tube, float, scale and customer connections
A variable area meter (rotameter) is a flow meter that measures volumetric flow of liquids and gases. Its operation is based on the variable area principle, where flow raises a float in a tapered tube, increasing the area for passage of the fluid flow. The float will reach a stable position when the force exerted by the flowing fluid + the buoyancy equals the gravitational force.
A change in flow rate upsets this force balance and the float will move up or down until it again reaches a position where the forces are in balance. The pressure drop across the float is low and remains essentially constant as the flow rate changes.
Float response to flow rate changes is linear and usually a 10 to 1 flow range is standard. Variable area meters can be installed directly after pipe fittings or valves without adverse effects on the meter accuracy. The meters are also inherently self-cleaning since the flow of gas or liquid between the tube wall and the float provides a scouring action that discourages the build-up of foreign matter.
The forces or influences involved in the rotameter can be seen in the diagram below.
It is common to use rotameters for their repeatability, which is defined as the closeness of the measurement points taken under the same process conditions. The diagram at right illustrates this principle.
Accuracy of a variable area meter is calculated using the full scale accuracy method as opposed to the percent of rate method. The difference is shown in the diagram below.
4. How are variable area (rotameters) used?
VA meters are widely used in a diverse range of applications, from simple to sophisticated.
Here are details of a few typical applications:
Process analyzers are designed to accurately and continuously measure a target analyte in a process stream. Sampling is the single most critical issue for process analysis. When the sampling system becomes plugged, it can cause big problems and costly, unscheduled maintenance.
The variable area flow meter – with 4-20 mA output – continuously monitors sample flow to the analyzer, unlike a flow alarm or flow switch. When flow starts to drop – indicating the onset of plugging – users can schedule maintenance to clear the problem before the analyzer is starved of the sample. Rotameters without 4-20 mA output are often used for sample flow in analyzer applications.
Large rotating equipment requires effective and reliable flow monitoring on a number of fluid supplies – like lubrication fluids, coolants, and dry gas seal gasses – to ensure efficient and safe operation. Metal tube variable area meters are commonly used to monitor lube oil and coolant flows. The optimum solution is using a 4-20 mA transmitting variable area meter so that flow can be continuously monitored. But glass tube or plastic body variable area meters are frequently used to ensure proper flow to dry gas seals.
Offshore Oil Platforms
Operators of offshore platforms require reliable products that operate under extreme conditions such as high pressure and difficult environmental conditions. Common offshore applications involve injecting a fluid (many times a proprietary fluid) into the high pressure extraction fluid to either prevent corrosion, prevent freezing of the extraction fluid, or add lubrication. In all cases, the goal is to provide local monitoring of the extraction process, which improves the overall process yield. Since the extraction process normally operates under very high pressures, the flow meter choices are limited. The model 3809 variable area meter is an excellent choice because it is simple, proven, reliable (only one moving part) and is available for operating pressures to 20,000 psi/1350 bar. The unique 3809 meter will have heavy wall construction with stainless steel indicator housing. If remote monitoring is required, the model 3809 can be supplied with 4- 20 mA analog output and worldwide approvals.
Additional variable area meter applications:
- 1. Chemical injection/dosing – controls the flow rate of the fluid to be mixed (added) to the primary fluid
2. Boiler control – measures steam flow to a boiler or measure the gases that heat the boiler
3. Purge applications –controls the purge gas or liquid to keep process lines clear, to create a positive pressure and/or be the shielding gas in a contained welding application, or to create a positive pressure where electrical components are installed in an area where hazardous gases could explode if they entered the electrical compartment
4. Tank blanketing – inert gas is the “blanket” over the liquid in a tank, which prevents the liquid from giving off vapors which could ignite and then explode
5. Simple flow measurement – options such as flow switch (alarm) or continuous electronic output (4-20 mA output with HART) allow flow conditions to be monitored and controlled remotely; a valve is also available to maintain flow at a given set-point.