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
Orifice plate flowmeters are the most widely used differential flowmeters in the process industries. These relatively simple devices have been adapted to measuring many media under many different conditions. These meters, despite their simplicity, are not quite 'set it and forget it' devices. Like any other instrument they require both calibration and maintenance.
The orifice plate is the main component of an orifice meter. The orifice plate restricts flow and develops differential pressure proportional to the square of the flow rate. Size, shape, and location of the opening in the plate depend on the type of media to be measured. Regardless of the opening design, plates come in either of two versions. These are the paddle plate, so named for its shape, and the universal plate. The paddle plate is used in orifice flanges and has a handle to aid in its installation between the flanges.
The universal orifice plate is of circular design and is used in orifice fittings or with plate holders or ring-type joint orifice flanges. Because the orifice plate is used with a holder, its outside diameter is uniform for pressure ratings in any given pipe size. Although installation technique and need for accessory items (seals, fittings, snap-rings, etc.) vary between the two styles, it is important to note that the quality, installation, and maintenance of the orifice plate are the most influential factors on measurement accuracy.
Orifice plates may be simple in design but like all precision instruments they must be properly machined to specified tolerances. Critical dimensions, which should be checked prior to installation, include:
Overall surface smoothness;
Sharpness of the upstream orifice edge;
Diameter of the orifice bore; and
Thickness of the orifice edge.
Cleanliness of the orifice plate surface should also be checked prior to installation.
An orifice plate is acceptable if it does not depart from flatness along its diameter by more than 0.010 in./in. of dam height. Dam height is defined as (D-d)/2 where D = inside pipe diameter and d = orifice bore. Placing a machinist's straightedge across the plate's face and using a feeler gage to determine the departure from flatness is an acceptable technique in this case.
The plate's surfaces should also be clean and free of any buildup or contamination. Overall surface smoothness of a clean device should not exceed 50 microinch.
Determining the condition of the upstream edge of the orifice requires is not difficult. A simple way to check is to hold the plate in front of a light source-bright sunlight is excellent for this-and view the edge. A sharp edge reflects no light when viewed with the naked eye.
Bore diameter should be measured using an internal micrometer or vernier gage. Three readings should be taken and an average calculated. Out-of-round tolerance varies with bore size from 0.0003 in. for a 0.250 in. orifice bore to 0.0005 in./in. of diameter for orifice bores greater than 1.000 in.
The diameter should determined at 68 °F (20 °C). Check that the measured bore agrees with the diameter stamped on the plate. If the orifice plate is to operate at temperatures other than this, its average bore diameter at operating temperature needs to be adjusted for thermal expansion.
Finally the edge thickness should be checked. Maximum edge thickness is defined as & or = D/50 or d/8, whichever is smaller. A downstream bevel should be machined at 30 or 45° to this edge thickness figure.
Looking for dirt and damage
It is a rare application where an orifice plate does not see dirt and damage. A harsh operating environment can subvert a dimensional check done prior to installation. Routine maintenance requires the plate be removed and dimensions verified. Before reinstallation, items to be checked, removed if necessary, or corrected include:
surface material buildup;
surface defects (deep scratches and surface etching;
liquid film of the plate surface;
presence of a liquid-level line in a gas application; and,
installation errors (backwards or off center).
If any defects require that the plate be replaced, differential pressure values should be compared before and after plate replacement to determine the extent of existing errors.
- Dick Johnson, senior editor