Orifice meters are perhaps the most commonly used d/p flow meters in refineries, production facilities, and chemical plants. The accuracy of an orifice meter depends on many factors, such as fluids, upstream piping configuration, Beta ratio (d/D, or
β, the ratio of orifice bore to pipe size), Reynold’s number limits, and correction factors used. The overall accuracy ranges from ±0.5 to ±5%.
The overall accuracy of orifice metering can be affected by:
• improper meter run design (i.e. insufficient flow conditioning or straight run of pipe)
• incorrect installation of the plate (e.g., backward) and sensing line (e.g., trapped vapor in liquid service)
• rough meter tube surface
• problems of the plate (as described later) • high or low beta ratio
• pulsating flow
• liquid entrained in gas flow • transmitter problems (e.g., drift)
• inadequate secondary element (e.g., should it be a chart recorder or an elec- tronic flow computer)
• gas sampling and analysis for specific gravity measurement used in the calculation
• the inherent accuracy limitation (“uncertainty”) of the orifice discharge coeffi- cient in the calculation. The orifice coefficients used in the industry standards were developed based on limited lab and field test data.
Improperly designed and installed orifice meter systems tend to under-measure flow because of the inherent characteristics and imperfections of the current industry standards. Orifice meters tend to over-measure flow in a pulsating-flow situation. Orifice Bore. The standard thin plate orifice comes in three basic throat configura- tions: concentric, eccentric, and segmental (Figure 500-3).
The concentric throat design is the most common. Used for clean fluids in one fluid state, it is ideally suited for gas, steam, water, air, and clean liquid hydrocarbon and chemicals.
The eccentric throat design is used primarily for liquids that contain gas. The eccen- trically located orifice bore allows free passage of gas.
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Segmental throat design is used when liquids containing solids are measured. The opening allows for free passage of the solids because the bottom of the segment is tangential to the lower circumference of the pipe.
The accuracy of concentric orifice meters is generally better than that of the eccen- tric and segmental meters.
Edge. Two orifice edge configurations are in general use in the United States. The square-edge type is commonly used with gas, steam, and low viscosity liquids. The edge is sharp and can be straight or bevelled. The quadrant-edge type is a better choice for lower Reynold’s numbers (i.e., below 10,000). It is recommended if the viscosity of the liquid is above 10 centipoise (Cp) but does not exceed 50 Cp. The conical-edge type is similar to the quadrant-edge type but is not commonly used in the United States.
As a “rule-of-thumb,” Reynold’s (Re) number constraints are as follows:
The document entitled “Orifice Design Calculations by Mainframe Computer,” included as Appendix A, in Volume 1, Part I, of this manual, provides more infor- mation on applications and limitations of both designs.
Orifice Plate Specifications. Orifice plate specifications are covered in API MPMS, Chapter 14.3/AGA Report No. 3 (1992) and include for example, the following:
Fig. 500-3 Orifice Bores
Re Constraint Concentric (under 2 in.) 1000+
(2 in. and over) 5000 d+
Conical 250β < RD <200,000β
Eccentric 10,000 — 1,000,000
Integral 1000 d/D
Quadrant 250 — 3200 < RD <60,000 — 280,000
Chevron Corporation 500-11 July 1999 • Plate thickness: The minimum and maximum thickness depend on plate size,
but 1/8 inch is the minimum.
• Plate flatness tolerance: The maximum departure from flatness varies with orifice diameter (d) and size (D). It may be calculated as 0.005(D-d).
• Roughness: The orifice plate surface roughness should not exceed 50 micro- inches.
• Upstream edge: The square-edge (sharp-edge) type should not show a beam of light when checked with an orifice edge gage. Alternately, the square-edge type should not reflect a beam of light when reviewed without magnification. The orifice should not have a burred or feathered edge. Nicks on the edge can be expected to increase the measurement uncertainty.
• Centering: The concentric orifice should have the bore centered within 3% of the inside diameter of both the upstream and downstream sections of the orifice meter run.
• Beta ratio: A recent study by Chevron Petroleum Technology Company (CPTC) suggests that the optimal beta ratio is 0.4 to 0.55 for natural gas. Beta ratios departing from this range increase measurement uncertainty.
Bent orifice plate: According to a CPTC study, the measurement error of a bent orifice plate in gas service was up to 4.5% lower than the true flow rate. A bent orifice plate may be caused by plant upset, sudden valve opening or closure, or other reasons.
Materials. Orifice plates are made of “zero-corrosion” materials, usually stainless steels (304SS, 316SS), monel, or other alloys, depending on the service.
Pressure Tap Locations. The choice of pressure tap locations is dependent on many factors, such as flow conditions, pipe size, cost, and accuracy required. Orifice taps may be located as flange taps, corner taps, radius taps, vena contracta taps, and pipe taps. Their applications are discussed in Appendix A.
Piping Specifications. API Standard 2530/AGA Report No. 3, ASME Standard MFC-3M-1984, and ISO 5167 specify meter run requirements for orifice meters used in custody transfers. Plant-type orifice meters that do not require ±0.5 to ±1.5% accuracy may require less stringent flow conditioning.
The minimum length of upstream and downstream meter runs varies depending on piping configurations and beta ratio. Use of flow straightening vanes reduces the meter run length requirement. Consult the industry standards (API/AGA and ISO/ASME) for details.
Secondary Elements. Secondary elements are required to perform one or more of the following functions:
• Convert the differential pressure measurement to electrical or digital output signal
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• Record the signal
For higher accuracy, flow computers may be justified, especially when dealing with large volumes.
Specifying and Sizing. Copies of the ISA S20 Specification Forms with instruc- tions for specifying orifice meters and secondary elements are provided in Volume 2 of this manual.
Sizing can be accomplished by any of the following three methods:
1. By manual calculation—Methods of manual sizing are given in Appendix B, Volume I, Part I, of this manual. Understanding the manual sizing methods will give the engineer a better sense of what the computer programs are doing. 2. By consulting vendors—Some vendors may size an orifice meter for you;
some will charge a fee while others will not. If sizing is done by the user, ask vendors to confirm the calculation.
3. By computer—A mainframe computer program called “ORIFICE” is avail- able. The user’s manual, “Orifice Design Calculations by Mainframe Computer,” is included as Appendix A in Volume 1, Part I, of this manual. Users can run the calculations in two ways:
– Through VM mainframe computer program “ORIFICE.” The logon proce- dure is described in Appendix A.
– Through Plant Equipment Information System (PEIS), which is used by some Refining locations in Chevron1.
A number of PC-based (personal computer) orifice design calculations - both inside and outside Chevron - have been developed and some are being used. For example,
– The Orifice program by Pascagoula refinery as part of the plant meter data- base. The program was written based on the ISO/Miller equations as used in the mainframe program, and it also includes other calculations
– TechData (“Kyle”)
– ISA’s FLOWEL by Kenonics Controls Ltd. – Gulf Publishing’s INSTRUCALC
These programs are in general suitable for plant orifice meter design calcula- tions. Since it is not practical to keep track the accuracy of various PC-based orifice programs on a continuous basis, the following criteria may be helpful to evaluate the accuracy of a “new” PC-based orifice calculation.
If a “new” PC-based orifice program is “based on ISO 5167/ASME-MFC 3M,” it can be tested to determine if the calculations and implementation are accu- rate and consistent with the ISO/ASME equations by comparing the results from the mainframe ORIFICE program.
Chevron Corporation 500-13 July 1999 In general, the following criteria can be used to evaluate the calculations by a new PC-based orifice program to determine if it is adequately accurate for plant operations:
a. Multiple test cases should be included covering normal and worst situa- tions (e.g., low Re, small pipe size—near the limits of the equations in the industry standard).
b. If the PC-based program is supposed to be based on ISO-ASME/Miller equations, the difference between the orifice discharge coefficients calcu- lated by the PC-based program and by the mainframe ORIFICE program should be within 0.1%, and the difference on flow rates within 0.15%. c. If the PC-based program is supposed to be based on the API MPMS,
chapter 14.3/AGA-3 (1992) equations, the orifice discharge coefficients calculated by the PC-based program and by the GAZ program (for gas flow) should be within 0.1% and the difference on flow rates within 0.15%.
Note For custody transfer gas flow calculations, the GAZ program developed by CPTC follows the equations in the current API Manual of Petroleum Measurement Standards (MPMS), Chapter 14.3 (and AGA 3). It can be used for custody transfer orifice calculations. The GAZ calculations can also be used to verify calculations by a gas supplier or buyer, or the calculations by commercially available electronic flow computers.
Appendix A in Volume I, Part I of this manual provides test cases with results calculated by the mainframe VM ORIFICE program, for users who want to test their new PC-based programs.
Integral Orifice. An orifice meter comprises an orifice installed integrally with a differential pressure transmitter (Figure 500-4). Although it provides for a compact installation, the overall accuracy is lower: ±2% to ±5%.
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Integral orifices are used for 1½ inch, 1 inch and ½ inch pipe and will work on the same fluids as standard orifice assemblies. Care must be taken when using orifices under 0.125 inch bore; 0.125 inch and smaller bored orifices are vulnerable to plug- ging or dirty fluids.
At present, no API, ASME, or ISO standards for integral orifice are available.
V-Cone Meters
Like an orifice meter, a V-Cone meter is a differential type meter based on the prin- ciple of correlating the observed pressure drop due to an obstruction in the line to the volumetric flow rate. As the name implies, the obstruction is a V-shaped cone hanging in the center of the pipe (Figure 500-5). The size of the central V-Cone determines the beta ratio of the meter which is so defined as to have the same opening area as a same-size orifice meter at the same beta ratio.
Discharge Coefficient. While the discharge coefficient of an orifice meter (which is approximately 0.6) is calculated by an elaborate equation based on various parame- ters about the meter and the fluid properties, the discharge coefficient of a V-Cone meter (which is approximately 0.85) is usually supplied by the manufacturer based on factory calibration.
Performance. One manufacturer claims a ±0.5% accuracy of reading for the primary element. Depending on secondary instrumentation, possible V-Cone meter system accuracy ranges from ±1% to ±2%. The V-Cone primary element exhibits repeatability to ±0.1% or better. The turndown of the V-Cone meter is claimed to be 15 to 1—exceeding the traditional differential head type meters.
The V-Cone meter is not significantly affected by non-ideal flow conditions. This is in contrast to orifice meters which are known to be sensitive to non-ideal flow conditions that are usually caused by upstream elbows, valves, and other fittings. Tests conducted at CPTC in 1995 have showed that the V-Cone flow rate measure Fig. 500-5 Cutaway Drawing of V-Cone Differential Pressure Flow Meter (Courtesy of
Chevron Corporation 500-15 July 1999 ments are within 0.5% of the no-swirl baseline measurement even in highly swirling flow (e.g., swirl angles up to 40 degrees).
Applications. Since their accuracy is not compromised by short pipe lengths between the disturbances (elbows, valves, etc.), V-cone meters may find process measurement applications at locations where long meter runs are not available due to space limitation, for example, on platforms and congested process plants. On an offshore platform, with shorter pipes, the space and weight requirement of a
metering system can be reduced. This is a very important consideration for offshore platform construction and maintenance. The cramped platform deck area precludes long straight pipes to condition the gas flow before being measured by the tradi- tional orifice meters.