Sometimes a small second hole is drilled into an orifice plate, right at the edge of the bore, and it is there for a very specific reason: to let an unwanted phase escape past the plate. A drain hole at the bottom lets liquid or condensate drain out of a horizontal gas line so it does not pool against the plate, while a vent hole at the top lets gas pass through a liquid line so it does not build up under the plate. This guide explains when each is used, why the second hole must be kept small relative to the main bore, the measurement error it introduces, and what the metering standards say about whether a second hole is acceptable in custody service.
Orifice Drain/Vent Hole in one line: An orifice plate drain hole is a small hole placed at the bottom of the plate, at the edge of the bore, to drain accumulated liquid or condensate from a horizontal gas line; a vent hole is the same idea placed at the top to pass entrained gas in a liquid line. Both prevent a second phase from pooling against the plate and distorting the reading. The extra hole must be small relative to the main bore because it passes uncounted flow, introducing a measurement error, and the standards restrict its use in custody metering.
The two names describe the same feature applied to opposite problems, distinguished by where the hole sits and which phase it passes. A drain hole is drilled at the bottom of the bore, so it addresses liquid in a gas line. In a horizontal gas meter run, small amounts of condensate or carried-over liquid tend to collect on the upstream side of the plate and pool at the bottom because the plate blocks their path; a drain hole at the very bottom of the bore gives that liquid a low point to weep through, keeping the pool from building up and disturbing the flow the plate sees.
A vent hole is the mirror image: drilled at the top of the bore, it addresses gas in a liquid line. In a horizontal liquid meter run, entrained gas or vapor tends to break out and collect against the top of the plate; a vent hole at the top lets that gas pass through rather than accumulating in a growing bubble under the plate's upper edge. In both cases the logic is the same, give the lighter-or-heavier-than-main phase a small escape path at the point where it would otherwise trap, so the plate keeps measuring the main flow cleanly.
The choice between them follows directly from the service. A gas line that occasionally carries liquid gets a drain hole at the bottom; a liquid line that occasionally carries gas gets a vent hole at the top. The feature is only needed when a small amount of a second phase is expected and would otherwise pool against the plate, if the stream is genuinely clean and single-phase, a plain plate with no second hole is preferred, because every extra hole comes at a cost to the measurement.
The second hole is a compromise, because any hole in the plate passes flow that the main bore does not account for. The primary measurement rests on all the flow going through the characterized main bore; a drain or vent hole lets a small stream bypass that measurement through an uncharacterized path, so some flow goes uncounted or, more precisely, is counted differently than the calculation assumes. This is why the hole must be kept small relative to the main bore: the smaller it is compared with the bore, the smaller the fraction of flow it diverts and the smaller the resulting error.
Even a small hole introduces a bias, and rigorous practice accounts for it. Because the hole adds effective flow area, its presence tends to make the plate pass slightly more flow for a given differential than a plain plate would, so the true flow is slightly different from what the plain-bore calculation returns. Where accuracy matters, this is handled with a correction that adjusts for the extra area the second hole contributes, rather than simply ignoring it. Keeping the hole small keeps that correction small and keeps the residual uncertainty manageable.
The practical consequence is that a drain or vent hole is never sized generously. It is made just large enough to actually pass the small amount of liquid or gas it needs to, and no larger, because every increment of hole size trades measurement accuracy for drainage capacity. Getting that balance right, enough to keep the plate clear but small enough to preserve the reading, is the core engineering decision when a second hole is specified.
Whether a second hole is even permitted depends on how demanding the measurement is. For custody transfer, where the number carries fiscal weight and the uncertainty budget is tight, the metering standards are cautious about anything that perturbs the ideal plate, and a drain or vent hole is a deliberate departure from the clean single-bore geometry the coefficient assumes. The right answer for custody service is often to remove the source of the second phase, through separation, better drainage of the meter run, or a differently oriented installation, rather than to accept the error a second hole introduces. Where a hole is used, its effect must be accounted for to keep the measurement defensible.
In less demanding roles, process monitoring, allocation, or a check measurement, the small error from a properly sized second hole is more readily tolerated in exchange for a plate that stays clear in a stream that carries a nuisance phase. The decision is a familiar trade: a slightly biased but reliably clear measurement can be more useful than a nominally cleaner one that steadily fouls as liquid or gas pools against the plate. Which way that trade falls depends on what the number is used for.
For an operator running many meter runs through a cloud SCADA platform such as Merobix, the important things are that the presence of a drain or vent hole is recorded in the meter configuration and that any correction for it is applied consistently. A second hole that exists physically but is not reflected in the calculation is a hidden source of bias, exactly the kind of undocumented departure that surfaces later as an unexplained imbalance. Keeping the plate's full description, including whether it has a drain or vent hole and how that is corrected, in the centralized configuration keeps the reading honest and makes the plate's real geometry visible to anyone reviewing the site.
A drain hole sits at the bottom of the bore and lets liquid or condensate drain out of a horizontal gas line so it does not pool against the plate. A vent hole sits at the top and lets entrained gas pass through a horizontal liquid line so it does not build up under the plate. They are the same feature applied to opposite problems, distinguished by position and which phase they release.
The extra hole passes flow through an uncharacterized path that the main-bore calculation does not fully account for, so it introduces a measurement error. Keeping the hole small relative to the main bore minimizes the fraction of flow it diverts and the resulting bias. It is sized just large enough to pass the small amount of liquid or gas needed, and no larger, because every increment trades accuracy for drainage capacity.
The metering standards are cautious about second holes in custody service because they depart from the clean single-bore geometry the coefficient assumes and add uncertainty. The preferred answer for custody metering is often to remove the source of the second phase through separation or better drainage rather than accept the error. Where a hole is used, its effect must be corrected for to keep the measurement defensible, and its presence should be documented.
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