Orifice plates: Out of sight not out of mind
In the absence of tube bundles (straightening vanes) the length of upstream and downstream straight-line pipe required to achieve optimal orifice plate performance is determined by the beta ratio. Orifice beta is the ratio between orifice diameter and inside pipe diameter.
B ecause orifice plates are so unassuming in appearance, and because they are out of sight, their impact on flow accuracy is often ignored or even worse, not understood. The fact is orifice plates are precision industrial grade devices.
Seldom does one encounter the words "precision" and "industrial grade" used to describe the same device, but in the case of orifice plates that's exactly what they are, and like any precision device, they require proper design, installation, inspection, and maintenance.
The American Gas Association's Report No. 3, "Orifice metering of natural gas and other related hydrocarbon fluids" (AGA 3, Washington, D.C., www.aga.org), describes the assumptions and conditions necessary to achieve accurate measurements using orifice plates. Companies using orifice plates to measure flow should own, understand, and use the AGA 3 report to help produce accurate and repeatable results.
Orifice plate performance related topics discussed in AGA 3 fall into two broad performance effect categories-pipe and plate.
Pipe effects include pipe roughness, misalignment of mating parts, and poorly designed installations that cause flowing media to swirl.
Orifice plate accuracy is seriously affected when the flow of media does not pass through the orifice in a straight line; thus anything that adversely affects straight-line flow introduces measurement errors. For example, AGA 3 includes tables and calculations for determining the length of straight-line, up-, and down-stream pipe and how and when to use tube bundles (straightening vanes) to ensure straight-line flow through an orifice. (See "Straight Lengths" diagram.)
Internal pipe roughness specifications for various beta (orifice diameter to pipe inside diameter) ratios are also included in AGA 3. Failure to comply with the AGA 3 roughness specifications can introduce as much as 1% swirl-induced errors.
Orifice plate performance effects can be defined in subcategories including:
Blockage or obstructions at up- and/or down-stream meter-taps and can cause positive and negative errors depending on the tap blocked and the amount of blockage;
Dull or nicked orifice plate edges can introduce 1% to 10% error and is dependant on the beta ratio and extent of the plate edge's dullness or nick;
Bent or deformed plates usually are the result of actual flow conditions dramatically exceeding design conditions, even for short periods of time. When the plate deforms the orifice enlarges (beta ratio changes) and can produce as much as a 6% unregistered flow error;
Orifices not centered in the pipe can produce as much as a 2% error;
Flexing plates is caused by operating just above plate design conditions causing the plate to deflect or flex in the stream resulting in an every-changing beta ratio. Depending on the amount of flex, the error can be as much as 20%;
Pulsing flow, often resulting from positive displacement pumps, or centrifugal pumps with one or two faulty impeller sections, will always result in registering more flow than actually passed through the orifice;
Reversed plates are simply those installed with the sharp orifice edge on the exit or downstream side of the plate. Depending on the beta ratio, errors introduced by reversed plates can be as much as 20%; and
Debris and lubricants on the plate upstream face have been found to produce errors as great as 15% with downstream surface buildup contributing up to 3% error.
Orifice plates are much more than a chunk of steel with a hole in the middle, they are precision industrial grade devices capable of producing accurate, repeatable measurements. They deserve to be treated as precision devices.
- Dave Harrold, senior editor
Care and feeding of orifice plates
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