Orifice Tappings

An orifice plate is a restriction placed within a larger diameter pipe, almost always straight. There should be a tap placed just before, and just after the orifice. When you connect a manometer to these taps you will read a pressure drop, and from this be able to calculate actual flow. All orifice plates will have a small tab that protrudes through the Assembly, with numbers stamped on it. You do the calculation as a comparison between that ID of the pipe, and the diameter of the orifice plate.

Usually, the orifice plate manufacturer will list tables, however, these can be hard to find so you can try here:

www.NAMFG.com

Orifice flange consists of : Line Flange and Orifice Plate. Line Flange is designed, fabricated and inspected in accordance with ASME code B16.36, where you can find all details for taps and all flange dimensions. Each flange shall be furnished with 4 taps 1/2 " NPT (2 at upper half for inst. connection in case of gas lines & 2 at lower half for inst. connection in case of liquid line).

accuracy and ease of use are the varriables. The flange tap and pipe taps are the easiest to use because all the discharge coefficients are obtainable readily. the others must must be built and sent to laboratory to develop flow coefficients.

The manufacture of the flange taps is very exact and that makes it a lot more accurate if you follow the standards. the corner taps also have a swirl componet that reduces its accuracy greatly.

Twenty years ago, orifice plates and d/p cells were the dominant method of flow measurement in the process industries. From what I read, this technology is still the standard for custody transfer in much of the natural gas industry.

Today, they are seldom installed in the process industries so expertise is waning. Back in the day, a good instrument engineer knew where to find the coefficients for all sorts of tap locations and it is surprising how much error may result from seemingly similay tap locations. One outstanding resource would be an instrument engineer who is 50+ years old. The instrumemnt engineers handbook is a good place to start for any generic design and will probalby tell you more than you ever wanted to know. There used to be a few proprietary designs not addressed in the handbook (Taylor and Foxboro made them) that were extensively tested by the MFR.

Beta for a multiple holed orifice is defined by taking the equivalent beta of a single hole with the same cross section as the summ of all holes.

Tappings has to be selected on the basis of pipe diameter only.

PIPE SIZE PREFERRED TAPPING

<=1 inch corner

>2 inch flange

>6 inch radius( D,D/2 D-pipe diameter)

if these tapping locations are not possible go for pipe tappings(at 2.5 D,8 D)