Flow Meter Tutorial - Insertion - Flow Control February 2000

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How to Choose an Insertion Flowmeter Run New Search fro

by: Walt Boyes Pages: 40; February, 2000 WEFTEC September September 28 October 2, 2002 Chicago, IL ISA October 21-24 Chicago, IL Search for More Events

A consulting engineer designed a booster station for a town. However, when the bids came back, the lowest responsive bidder was $35,000 higher than the engineer's estimate. The low bidder suggested that he take out a few things. So, the flowmeter, repair bypass, and the telemetry RTU were removed. Now, the town wants to put in a new flowmeter, but the booster station serves 1,500 houses, and the flow cannot be shut off while the line is cut and a new flowmeter is installed.

•A •C •F •F •G •L •M Every municipality has these horror stories. Many industrial process plants •P have the same problem: how to put in a new flowmeter without shutting down P the plant. Fortunately, there is a class of flowmeters designed expressly for this • P • purpose. They are called insertion flowmeters because they are designed to be •R inserted into an existing line. F

There are three types of insertion flowmeters: Point Velocity Devices First, there are the point velocity devices. The theory is that the average velocity in the line is found at a point roughly 1/8 to 1/10 the pipe's diameter inside the wall of the pipe. Therefore, a device that measures the velocity at this point in the pipe can be said to be measuring the average velocity in the line. If elbows or valves, or other flow obstructions do not disturb the flow, the measured velocity times the cross-sectional area of the line equals the flow rate.

•S •S •S •S •T •T •V

There are many examples of these devices. Pitot tubes, insertion mag meters, paddlewheels and small-rotor turbines, insertion sensors, and insertion thermal dispersion sensors are all point velocity sensors. Because they measure the velocity at a single point, typically a ½ inch (13mm) cross section, they have approximately equivalent accuracy. Typically, these devices have an installed error rate of between two and five percent of full-

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scale flow (although manufacturers frequently claim better performance). These flowmeters are also susceptible to poor hydraulic conditions. In order to operate accurately, they need a lot of straight pipe, both upstream and downstream. The straight pipe run lets the natural friction of the pipe and the fluid work to correct the disturbances that elbows and valves cause. Point velocity flow products range from inexpensive (paddlewheels) to fairly expensive (insertion magmeters). Multi-Point Sensors These devices form the second class of insertion devices. They measure the velocity at several points in a line along the diameter of the pipe. This is called measuring the flow profile. The average of the readings along the flow profile is said to also be equivalent to the average velocity in the line. Multiple port pitot tubes (the first of these was tradenamed Annubar) and multiple sensor insertion magmeters belong to this class. Profiling sensors, which continuously and automatically insert to several depths, also fit the classification. These multi-point sensors can be quite accurate. Because they measure more than one velocity value, they can be linearized to bring themselves into accurate calibration over wide flow ranges. These sensors can be calibrated to be accurate and linear, even when located directly after an elbow. Multi-point insertion sensors can sometimes be as accurate as in-line sensors: 1-1/2 to two percent of indicated flow rate. Multi-point sensors can be relatively expensive in small pipe sizes (under 6" [150mm], for example), and cost effective in larger lines, up to 16" to 20" (400 to 500mm). Rotor Devices Large rotor devices, such as insertion propeller and turbine meters fall into the third class of insertion flowmeters. Because they use a larger rotor than the paddlewheels and small turbine devices, they measure more than a single point velocity. Rotor diameters ranging from 1-1/2"(38 mm) in diameter to 6" or 8" (150 or 200 mm), or even larger, make them much more easy to linearize than point velocity sensors. Moreover, they can be nearly as accurate as inline sensors, often without special calibration. Typically, rotors are used in larger line sizes, and become quite cost effective in 4" (100mm) and larger lines. Some of these devices can be inserted under pressure, but most can only be installed if the line has no fluid in it. How to Decide Deciding which of these flowmeters to install depends on three major criteria: cost, degree of accuracy required and whether the pipe can be drained for installation. If the pipe can be temporarily drained, any of these devices can be easily installed in about an hour. The next criterion is desired accuracy, followed by cost. If the pipe cannot be drained, devices that can be inserted through a hot tap valve assembly (the point velocity products and a few of the large rotor devices) are the solution. At that point, cost and desired accuracy are the deciding criteria.

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If there is a short run of straight pipe, and a high degree of accuracy is required, the multi-point insertion devices are probably the best choices (even over the large rotor devices). Characterizing the signal from these devices means that they can be used without loss of accuracy in applications like pump stations, hydronic heating systems and skid-mounted process piping. Insertion flowmeters provide flexible and useful tools for instrumenting existing lines, and their cost savings make them really suited for use in larger lines in almost all circumstances. Subscribe | Email | Site Map | Search | FAQ | Advertise

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