An orifice meter measures the pressure drop across the plate, but exactly where you tap into the pipe to sense the high and low pressures changes the number you read - because pressure is still recovering downstream of the plate and the profile is not flat. The two common conventions in gas metering, flange taps and pipe taps, place those connections in different spots and therefore behave differently. This guide compares where flange and pipe taps sense the differential pressure, explains why the tap type changes the discharge-coefficient equation, and stresses why the flow computer must be configured to match the physical taps actually installed.
Flange Taps vs Pipe Taps in one line: Flange taps and pipe taps are two conventions for where the high and low pressure connections are located relative to an orifice plate. Flange taps sit close to the plate, about one inch from each face, and are built into the orifice flanges or fitting. Pipe taps sit farther out - the upstream tap around two and a half pipe diameters before the plate and the downstream tap around eight diameters after it. Because the pressure profile differs at those locations, the tap type changes the discharge-coefficient calculation, so the flow computer must be told which taps are physically present.
The differential an orifice meter reads is the difference between an upstream pressure and a downstream pressure, and neither of those is a single fixed value along the pipe - the pressure dips sharply as the flow accelerates through the bore and then partially recovers downstream as the flow spreads back out. Where you tap therefore matters, because a tap near the plate reads a different pressure than one placed farther away on the recovery curve. The two conventions simply make different choices about where on that curve to sense the high and low pressures.
Flange taps place both connections close to the plate, at a short fixed distance - about one inch - from the upstream and downstream faces. They are drilled into the orifice flanges or built into the orifice fitting itself, which makes them mechanically convenient because the tap location is fixed by the hardware regardless of pipe size. This is the dominant convention in modern custody gas metering, and it is the arrangement most orifice fittings are built around. Because the distance is set in absolute inches rather than pipe diameters, flange taps behave consistently across a range of line sizes.
Pipe taps sit much farther from the plate and are located in terms of pipe diameters rather than fixed inches: the upstream connection is placed roughly two and a half pipe diameters ahead of the plate and the downstream connection roughly eight diameters behind it. The downstream location is chosen well out on the recovery curve, closer to where the pressure has largely recovered, so pipe taps sense a different differential than flange taps do for the same flow. Because their placement scales with pipe diameter, pipe taps require drilling into the pipe run at those computed distances rather than into a standardized fitting.
The discharge coefficient exists to relate the pressure drop the meter measures to the actual flow, and that relationship is anchored to where the pressure is measured. Since flange taps and pipe taps sense the pressure at different points on the recovery curve, they present the flow computer with different differential pressures for the very same flow, which means the coefficient that converts that differential into a flow must differ too. The discharge-coefficient correlation is therefore not tap-agnostic - it is derived and expressed specifically for a given tap configuration.
In practice this shows up in the flow equation as tap-specific terms. The Reader-Harris/Gallagher correlation used for flange taps has a form fitted to the flange-tap pressure locations, while pipe-tap metering uses a coefficient relationship developed for pipe-tap locations. They are not interchangeable: the same measured differential, interpreted with the wrong tap's coefficient, yields the wrong flow. This is not a small correction buried in the noise but a structural difference in how the coefficient is computed, because the two tap types are literally measuring different things about the same pressure field.
It follows that the tap type is a piece of fixed configuration that has to travel with the meter, alongside the bore and pipe diameters that set the beta ratio. Knowing that a meter run uses flange taps or pipe taps is as fundamental to the calculation as knowing its plate size, because the coefficient correlation branches on it. A meter shop specifying or documenting a run records the tap type deliberately, precisely because the downstream flow calculation cannot be correct without it.
Because the discharge-coefficient calculation depends on the tap type, the flow computer must be configured with the tap arrangement that is actually installed on the pipe - and a mismatch here is a classic, quiet source of measurement error. If a run is physically plumbed with flange taps but the flow computer is set to pipe taps, or vice versa, the computer applies the wrong coefficient correlation to a correct differential and produces a flow that is biased. The instruments are healthy, the differential is real, and the reading still looks entirely normal, which is what makes the mistake so easy to miss.
This is why matching the configuration to the hardware is part of the discipline of commissioning and auditing an orifice run. When a meter shop builds or reworks a run, it verifies the physical tap arrangement and confirms the flow computer's tap-type setting agrees with it, together with the bore diameter, pipe diameter, and tap locations. The tap type is not something the computer can infer from the live signals - it can only be told, so the setting has to be right and has to stay right through any change to the run.
In a cloud SCADA such as Merobix, the meter's configuration, including the tap type, sits alongside the live differential and static pressures and the computed flow, where it is visible and auditable rather than buried in a device that no one inspects. That transparency helps catch a tap-type mismatch during setup or review - an unexpected shift in a meter's factor after a rework, or a configuration that does not match the documented run, becomes something an operator or auditor can see and correct before it accumulates into a custody imbalance that only surfaces later as an unexplained difference between parties.
Flange taps sit close to the plate, about one inch from each face, and are drilled into the orifice flanges or built into the orifice fitting, so their location is fixed in absolute inches regardless of pipe size. Pipe taps sit much farther out and are located in pipe diameters - the upstream tap around two and a half diameters ahead of the plate and the downstream tap around eight diameters behind it. The downstream pipe tap is placed well out on the pressure-recovery curve.
The pressure downstream of an orifice plate recovers gradually, so flange taps and pipe taps sense the pressure at different points on that recovery curve and present different differentials for the same flow. Because the discharge coefficient converts the measured differential into a flow, it must be computed differently for each tap arrangement. Flange-tap and pipe-tap metering therefore use coefficient correlations fitted to their specific tap locations, and the two are not interchangeable.
The computer applies the coefficient correlation for the wrong tap arrangement to an otherwise correct differential pressure, producing a flow that is biased. Nothing looks wrong in the field - the instruments are healthy and the reading appears normal - so the error can persist unnoticed and accumulate into a custody imbalance. This is why commissioning and auditing an orifice run include verifying that the flow computer's tap-type setting matches the physical taps actually installed.
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