Sunday, January 29, 2012

Types of Pressure Taps used in Orifice

Any restriction in the flow pipe causes a drop of pressure that is, the downstream pressure lowers by an amount equal to the pressure drop due to the restriction. In case of orifices, the drop in pressure varies from the location of the orifice plate to the downstream side until the flow becomes fully developed. The pressure drop is at its maximum at the vena contracta. Taps are used to sense the pressure and they are classified based on their location.

Corner Taps 
In this arrangement, openings for the pressure taps are located in the upstream and downstream flanges holding the orifice plate
orifice meter with corner

These openings should be as close as possible to the orifice plate. They however, are susceptible to dirt freezing and clogging with hydrates. They are more affected by upstream disturbances and generally are less reliable than flange or vena contracta taps.

Radius Taps
Here, pressure taps are located 1 pipe diameter upstream and ½ diameter downstream from the orifice plate (see figure 6.10). This arrangement is best from a practical standpoint because the downstream pressure tap is located at about the mean position of the vena contracta, and the upstream tap is sufficiently far upstream to be unaffected by distortion of the flow in the immediate vicinity of the orifice. (In practice, the upstream tap can be as far as 2 pipe diameters form the plate without affecting the results.)

Pipe Taps
In this, holes for the pressure taps are located 2 ½ pipe diameters upstream and 8 pipe diameters downstream from the orifice plate (see figure 6.4). Both the taps are located in the region of the fully developed flow. So they give the total pressure loss due to the orifice, and are useful for determining the overall pressure losses in the pipeline.

Flange Taps
This design places one pressure tap hole 1 inch upstream and the other 1 inch down-stream form the orifice plate. Being closely located to the face of the flange, they are accessible for inspection. Due to symmetry, they can be used to measure flow in either direction.
They should not be used in pipe sizes less than 2 inches where the ratio is high, because the downstream tap is located in a highly unstable pressure region.

Vena Contracta Taps
Here, the upstream pressure tap is ½ - 2 pipe diameters from the orifice plate, and the downstream tap is located at the position of minimum pressure, the vena contracta.
Theoretically these taps should be best, as pressure is minimum at this point and the drop in pressure for fluid passing through the orifice should be maximum. But a practical problem, such as the change of position of the vena contracta with flow rate, limits the suitability of these types of taps.

Orifice Selection Guidelines
The CPI deals with innumerable types of fluid streams for instance, clean liquids, wet steam, and liquids containing solid particles. The nature of the stream determines the suitability of the various types of orifices. Let us now summarise the type and selection of orifice.

Flow of clean liquids, gases, and low-velocity vapours is best measured by concentric orifices. For Reynolds number lower than 10,000, install square-edged orifices. At Reynolds numbers lower than 10,000, however the discharge coefficient of square-edged orifices change appreciably with either flow rate or viscosity. For this reason, either quadrant or conical-edged orifices are preferred. Quadrant-edged orifices are relatively immune to the effects of corrosion, erosion, and the deposit of solids at the surface of the orifice.

orifice meter with corner taps

Granular solids in a flowing fluid, condensate in steam, or vapour or gas in a liquid, generally do not use concentric orifices because the projecting rims of the orifice forms a dam and these foreign materials build-up in the approach pipe at the plate, causing a change in the distribution of flow.

To deal with granular solids and condensate, install an eccentric or segmental orifice plate with the hole at the bottom of the horizontal pipe to permit the free passage of those materials. If the orifice can be located in a vertical run with the flow in a downward direction, however, then the concentric orifice is preferred because of its better discharge coefficient accuracy.

When liquids containing gas or vapour must be measured in a horizontal pipe, use a segmental orifice with the opening at the top of the pipe. For these fluids, however, if suitable locations are available in vertical pipes with the flow in an upward direction, concentric orifices are preferred for greater accuracy.

Segmental orifice plates should not be used to measure liquids containing solids that are sickly or that have a density similar to the liquids. This design is affected by deposits on the face or edge of the orifice in the same way as the concentric orifice.

If fluid streams contain both heavy materials and gas or vapour in the liquid, annular orifices are recommended.

In smaller line sizes, it is impractical to install the downstream vena contracta tap when using a concentric orifice, because of interference of the flange hub. Both the segmental and eccentric orifice plates have vena contracta location farther downstream than the concentric orifice plate. In many cases, this difference is just sufficient to make use of vena contract taps feasible, and so the alternative orifices should be selected.

orifice coefficients for different types of pressure taps
orifice coefficients for different taps positions

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  1. Is there any comp factor for pipe to flange

  2. One of the project we have implemented the AGA3 logic in controller as Flange tapping.But the actual site is Pipe tapping.My question is that how much difference will be there between flange and pressure.If possibile any conversion is there to convert pipe to flange

  3. Ankit , I have 24"pipe in which orifice is to be installed. Which is the good option i.e flange taps or radius taps? If radius taps ,what is the advantage of using it or disadvantage of using flange taps on this size? Thanks in advance.



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