13/5/2014 Types of Pressure Taps used in Orifice | PIPING GUIDE

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Types of Pressure Taps used in Orifice

Posted by Ankit Chugh on 1 :17 PM

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

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.

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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.

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.

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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.

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• Reply •

Ajmul Khan Khan • a year ago

One of the project we have implemented the AGA3 logic in controller for Flange tapping.But the

actual site is Pipe tapping.My question is that how much difference will be there between flange

and pipe tappings.If possibile any conversion factor is there to convert flange to pipe

• Reply •

ajmul • a year ago

Is there any comp factor for pipe to flange

• Reply •

Ankit Chugh • a year agoMod ajmul

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