White Paper | October 24, 2022
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ABSTRACT

The McCrometer V-Cone® differential pressure meter was introduced in the 1980’s. Initial customer review of the technology at that time was skeptical due to the radical change in emphasis of the flow regime from a central portion of a closed conduit (orifice plate) to fluid velocity profile around a centrally mounted cone.

This paper describes the principles and field use of the McCrometer V-Cone D.P. flow meter used in the role of wellhead metering, injection measurement allocation metering, custody transfer of both topside and sub-sea in on-off-shore oil, and gas production applications.

Introduction

The McCrometer V-Cone® meter is a differential pressure device which allows the fluid to be measured to pass around the exterior of a central dual cone the larger cone apex being upstream of a beta edge, and an acute angled smaller transition cone being on the downstream side of this edge. Flow velocities in the up-stream core of the pipe are then forced to mingle with pipe wall boundary layer velocities by radial expansion across the larger cone.

After passing this beta edge (formed between the two cones and the pipe wall) the fluid then enters into a central region in which high frequency vortices form adjacent to the pipe wall and pipe center with a low amplitude. This parameter is particularly beneficial if particulate laden fluids are to be measured which will be mentioned later in the paper.

A significant flow conditioning or flow profiling affect is obtained by the geometry of the larger cone which causes an averaging of the fluid velocities directly up-stream of the cone beta edge.

Independent testing has shown this affect to occur with close coupled out of plane elbows, and single elbows at elevated Reynolds numbers.

Consideration must be given to any installation that employs an upstream disturbance producer other than an elbow, such as a gate valve or device which can produce jetting or high velocity fluid distortion past the cone. Practical experience has shown that in these extreme cases a 3-5 diameter upstream section will suffice to allow the sufficient fluid expansion needed to allow the cone to regenerate the velocity profile. (Figure 1)

Standard design concepts

McCrometer V-Cone with Welded Construction:

This type of construction uses a support tube which acts as both a support and low pressure conduit connected to the large cone. A horizontal low pressure tube passes through the center of the cone assembly and is attached to the support tube. (Figure 2).

Machined Construction:

A machined mono-block in stainless steel is manufactured to allow a cone to be supported by a cross piece which can be removed allowing different cone diameters to be implemented. This allows flexibility of the metering system in respect to large turndowns. The meter is flangeless and sits between standard pipe flanges. (Figure 3).

Installation Effects

Pipe flow velocity profiles are rarely ideal. There are many installations where flow meters exist in which the flow is not well developed. Trying to measure disturbed flow can create some problems in certain types of DP devices. The McCrometer V-Cone overcomes this problem by re shaping and distributing the up-stream velocity profile. (Figure 4).

This feature enables the meter to be placed in a small space with minimum straight run requirement. Thus saving upstream and downstream pipe runs (real estate).

Production and Test Separators

Current design of multiphase separators allows overall uncertainty (on all three phases) of about 15% - 20% according to recent API /MMS comments.

This can be due to operator control, time delay in stabilization of the vessel, incorrect design involving fluid levels, position of vessel in respect to the pressure head requirement (on liquid side) and also on the main metering carry over of other product fractions.

In particular where orifice plates are used it may be necessary to perform, plate changes to facilitate turndown (otherwise the performance of the measurement system could be compromised), and to use upstream flow conditioners or velocity profile devices which add cost and weight to a system. Long-term vulnerability using orifice plates in production separators can be demonstrated by examining public documents in the measurement field*.

Orifice Technology Issues

Beta Edge Degradation, Stagnation Area, and Deposition

During the use of a velocity profile sensitive device (as an orifice) it is necessary to confirm that the beta edge of the device is both clean and has not been compromised due to debris and other contaminants.

Current API paper standards address a clean dry and non-polluted gas, which does not take into account generally an offshore usage condition, i.e. ”wet gas.”

This can result in high maintenance and intervention cost due to frequent plate changes.

Where paraffin or sulphate deposition occurs in the pipe, in almost all cases the shift in the Cd is caused due to a re-circulatory effect at the rear of the orifice plate. There also can be a stagnation area up-stream of the device where heavy end hydrocarbons can collect.

See enclosed photo examples from Marathon Oil site Appendix A and longevity issues later in the paper.

Wet Gas

Current AGA/API standards do not address wet gas installation conditions; hence the use of an AGA/API standard to design a possible wet gas metering system can also end in higher intervention costs not generally assumed at the start up case. Greater liquid injection over time to obtain more products on declining wells can result in an increase in wetness at the gas outlet. Examples of this can range from under-reading of the meter or over-reading depending on Reynolds Number Beta ratio, and liquid mass fraction seen at the meter.

Research in this field by Chevron and others indicates a Cd movement of over 2%-3% outside of the predicted API requirements due to wet gas with liquid loads of only 0.33bbl/MMscf as indicated in research.

Reference:

Dr V.C. Ting Chevron Corp. “Effect of Liquid Entrainment on Orifice Meters.”

Wet Gas Lab Test CEESI, Nunn, CO.

Recent Wafer-Cone® testing at CEESI indicated a low susceptibility to Cd change with liquid load. Base line values were plotted against numerous test loop instruments in a dry condition.

Flow rates from 7 –70 feet / second in a 4 inch line size where used. The liquid rate was added to a maximum of 1 and 2 Bbl per MMSCF. The liquid hydrocarbon was a Decane derivative acceptable for use in closed surroundings.

The results were plotted and the effects noted. Further work is underway to see the effect of low D.P. ranges on repeatability, accuracy, and Y factor changes (currently the welded version has a Y factor equation available to correct for density changes due to pressure).

See Appendix C, Figure 8 - CEESI = Colorado Experimental Engineering Station Courtesy of Chevron, Inc. USA

BP Amoco Field Test

During March, 2001 BP tested a McCrometer V-Cone in an upstream field condition.

A 4 inch ɸ Wafer-Cone cone was tested with a 4 inch ɸ orifice plate 36 inches up-stream and a separator downstream. The results were very impressive. See Appendix C Figure 7.

Longevity, Contamination, and Beta Edge Damage Lab Test

During late 1998 and early 1999 Marathon Oil installed test meters at their on-shore hydrocarbon facility in central Wyoming. The site was producing dirty wet gas with H2s and asphaltene contaminants. The result on the existing measurement system was not very pleasing to the client or to the local BLM office that collect royalty from these gas systems. The use of the McCrometer V-Cone was to see if the contamination would affect the meter. The assumption that it would work was a driving force to implement the installation. Three-inch (3”) meters where fitted and the most severe well used as a test site. (See Appendix A and Appendix B, “Wet Gas Photographs Wellhead”.)

On inspection, the orifice plate units showed debris build up after only three months usage with Asphaltene/Paraffin deposition at the up-stream inlet to the meter and contaminants after the plate in the low pressure region.

On inspection of the McCrometer V-Cone the unit (Appendix A) did not show the same severity, probably due to accelerated flow around the cone element. This seemed to keep the cone and sensing ports clear of deposition, thus maintaining a consistent D.P. across the meter. Entrained condensate liquid moved into slug flow condition periodically, which caused liquid to enter the orifice sensing lines and also be retained after the plate. The cone meter did not show this problem due to the straight through design. The regular “blowing” off of the plate was deemed a severe problem in man hours and traveling to the site, plus the effect on accuracy this caused. With the lack of liquid retention using the McCrometer V-Cone the system now runs within the BLM “IM” guidelines.

Damage Test

Damage testing of the Wafer-Cone meter was recently performed, this involved determining a base line on a calibration rig over several flow rates, after which intentional damage to the cone beta edge was performed in a severe manner. The photographs and data are shown in Figures 5 and 6.

Test Results (damage testing)

The deviation from the test shows the Cd shifted by app. +0.3% which is within the uncertainty of the McCrometer calibration station.

This initial test is currently being superseded by further tests with multiple damage regimes to view the effect per incident.

This work is a precurser to the use of the meter in a sub sea “non-intervention” environment. (See Appendix E Figure 8.)

Weight Penalty (platform design)

Current design of orifice systems requires large up and downstream pipe lengths. This is not desirable if you are a user since cost of real estate is at a premium offshore due to the large weight related cost in installing, large pipe runs, orifice carriers, and supporting structures.

Current weight penalty costs in the Gulf of Mexico can be greater than $25 per pound.

Installing a system including all piping requirements and a cast carrier can amount to high dollar amounts in the platform support requirements. This can be compounded on deep water platforms.

Therefore using a device, which has low weight and reduced up-downstream piping needs, can reduce overall client installation costs.

Installation cost relating to weight penalty

The McCrometer V-Cone can perform its own velocity profile conditioning as part of the meter design. The installation envelope can be reduced significantly and direct coupling to elbows can be straight to the meter flange face without Cd performance degradation due to swirl or profile skewing.

Please note that the McCrometer V-Cone weight is approximately only 1444 lbs for a 16inch #900 meter, 1161 lbs for 14 inch meter and only 945lbs for a 12 inch. This is significantly lighter than both orifice carriers and respective up-downstream piping for these size systems, which can weigh more than 2.5 tons, per stream.

McCrometer V-Cone Technology Technical Attributes

Accuracy using laboratory calibration is possible to facilitate an accurate performance over 10-1 turndown better than that of conventional differential producers. Performance of +/- 0.5% at 10-1 turndown, with debris resistance, low beta edge degradation (Beta edge is after the flow on McCrometer V-Cone), and no stagnation area make the unit ideal for high cost intervention areas and long-term performance. The low weight of the device will ensure economy of installation without having to purchase long piping lengths and high dollar flow conditioners/profilers (certain 12 inch diameter flow profile generator units can cost over $10,000).

Sub Sea Implementation and Design

Currently 35 precision tube units are in service in a sub-sea wellhead marine environment, in the U.K., Norway, Angola, Brazil and the South China Sea area. The main usage has been water injection metering; however, allocation gas metering has been a recently accepted philosophy with the device. Implementation to greater than 12000 feet is acceptable with a new configuration and special sensor housing. (See Appendix E Figure 9.)

Appendix A

Wet Gas Photographs Wellhead Metering

Appendix B

Wet Gas Photographs Wellhead

Appendix C

Gas Production Separator (Typical Arrangement)
BP Norwood Separator Site Wy. USA

Appendix D

Table 1 - Data from BP/ Norwood Separator Site Wy

Appendix E

Reference Documents / Data

Hayward: A Basic Guide and Source Book for Users, 1973

Szabo / Winarski/ Hypnar: V-Cone Meter for Natural Gas Flows, 1992

Miller: Flow Measurement Handbook (Latest Edition)

GRI: Wet Gas Research V-Cone 4 inch Diameter, 1997

Bright: BP Amoco Wy test separator data, 2001
 

*Examples cited in this document from Phillips Petroleum Embla–Platform NSFMW Gas Measurement for the Real World, 1994

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