There are many kinds of flowmeters that can be used for steam and gas applications. These range from basic variable area (rotameter) and turbine flowmeters, to more advanced differential pressure (DP) and vortex flowmeters. While these are all good technologies the process will generally dictate which flowmeter should be used. Considerations are installed cost, permanent pressure loss, straight run pipe requirements and turndown. This post will focus on the various primary elements for DP flowmeters and vortex flowmeters.
A differential pressure (DP) transmitter can be used to measure flow. An obstruction, or primary element, is placed in the flow path which creates a pressure differential. The flow rate can be correlated to this differential pressure. A more technical discussion of this relationship can be found here.
The most common type of primary element used is an orifice plate. An orifice plate is a simple metal plate with a hole in the middle. The installed cost is somewhat inexpensive. However, the permanent pressure loss is significant and requires multiple pipe diameters, both up- and downstream (10 and 6 for two elbows in plane). The turndown is 8:1, but can be increase to 14:1 with more accurate pressure transmitters.
A variation of the orifice is the conditioning orifice plate. Rather than having a single hole in the plate, it uses multiple holes. The performance of a conditioning orifice plate will be similar to a conventional orifice plate, except the up- and downstream pipe diameters are reduced to 2 and 2. These can be installed in applications with short runs of pipe. Keep in mind there will be significant pressure loss and the turndown is the same.
Smaller diameter pipes can take advantage of an integral orifice plate. These include both the primary element and a short run of pipe, both up- and downstream. All other performance data is the same as orifice plates. The up- and downstream requirements are 25 and 10 for two elbows in plane. While this seems high, the largest IOP is 1-1/2″ in diameter.
An averaging pitot tube has become a widely used technology for DP flow application. The installation is quite simple which leads to a low installed cost. The permanent pressure loss is the lowest of any DP flowmeter which makes it great for high flow, low pressure applications, like ventilation ducts. It has the same turndown as an orifice plate and has a lower pipe diameter requirements (8 and 4 for two elbows in plane).
The final flowmeter to consider is the vortex flowmeter. Similar to a DP flowmeter, a vortex flowmeter uses a primary element, or shedder bar. However, rather than measuring pressure differential, it measure vortices created by the shedder bar. The impulse of the vortices is counted, and the faster the rate, the higher the flow. Vortex flow meters have low permanent pressure loss and the best turndown capabilities (25:1). It requires 10 and 5 up- and downstream pipe requirements, assuming a correct K-factor. However, the installed cost is significantly higher than DP flowmeters. Reducing models are also available to improve low flow measurement.
Other considerations are pressure (DP) and temperature (DP and vortex) compensation. These can dramatically improve the flow measurement accuracy, especially in saturated steam applications. While DP flowmeters have lower turndown capabilities they will measure low flow conditions (the accuracy is considerable reduced). A vortex meter will not measure in low flow conditions, but a reducing vortex can be utilized for these applications.
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