30 June 2020
FLOW METER FOCUS
Common Q&A’s from flow meter users
In this issue: How Do We Achieve Best Accuracy?
• Data Accuracy In this second article in a series of four, GM Flow
will address further questions which commonly get
• Swirl
raised by customers while discussing Adjusta-
• Installation Requirements
Cone. Normally we are comparing a dual chamber
• Cost of Ownership orifice fitting, the most widely used device for
• Flow Calculations measuring gas during well testing operations. This
• Calibration issue centers around accuracy of measurements.
Data Accuracy
Orifice meters are designed to be manufactured and used according to ISO-5167 and AGA-3 standards. Both
of these standards specify that a minimum length of straight pipe must be installed upstream and downstream
of the meter to maintain accuracy and limit uncertainty. AGA recommends that when using a suitable flow
conditioner, (generally a tube-bundle), 19D of straight upstream pipe should be installed upstream of the orifice
chamber. So, for example with a 6” orifice meter, this equates to a minimum of almost 10 ft of 6” pipe. Many test
separator installations have limited space so this upstream length can be compromised, reducing accuracy and
increasing uncertainty of the gas flow measurement. Flow meters which utilise differential pressure cone meter
technology have been shown to require only 6D upstream and 3D downstream, fulfilling the requirement of ISO
5167 Part V.
Swirl in Orifice Meters
The orientation of the gas outlet pipework, on most test separators, is mostly very similar - gas travels up and
out the top of the separator (or end cap) , then passes through two or three 90 degree, out of plane elbows,
before passing through the gas flow meter at the lowest level on the skid base. This pipework orientation does
several things - firstly, the out of plane elbows generate swirl in the pipe. The presence of swirl is well known
and the effect on the flow accuracy1 is (somewhat) corrected using
a tube bundle in the upstream meter tube. However, the restricted
upstream distance, means that the tube bundle may not be able to
condition the flow effectively prior to measurement in the orifice
meter. Differential pressure cone meters, such as Adjusta-Cone
are generally better able to deal with swirling flow, due to the
nature of the flow through the device. In an orifice meter, like water
draining from a bath, any remaining swirl is multiplied as the fluid
accelerates towards the centre of the orifice. A swirling fluid can
therefore be made worse by an orifice plate, which can in turn
affect the accuracy of the reading.
1. https://www.sciencedirect.com/science/article/abs/pii/0955598689900034
Swirl in Cone Meters
As fluid travels though a cone meter however, the flow is forced outwards towards the pipe edges. Like a
skater on ice, the rotational velocity (swirl) is reduced. If a skater pushes their arms out, they slow their rate of
spin. If they pull their arms in, they increase their rate of rotation 2. Differential pressure cone meters therefore
reduce the rotational speed by increasing the radius of rotation. Cone meters can therefore improve or
condition the flow, increasing accuracy in swirling conditions induced by out of plane elbows, which increases
the overall accuracy3 and shorten the required installation pipe lengths.
Reduced Pipework Requirements
This conditioning effect means that cone meters can be used to cut the amount of upstream pipework
requirements, as defined by ISO-5167 part V4, which states that 6D upstream and 3D downstream straight lengths
are sufficient to maintain the meter’s accuracy.
2. https://www.youtube.com/watch?v=0RVyhd3E9hY
3. https://nfogm.no/wp-content/uploads/2019/02/1995-22-A-Performance-Study-of-a-V-Cone-Meter-in-Swirling-Flow-Shen-Chevron.pdf
4. https://www.iso.org/standard/62568.html
Reduced Cost of Ownership
In can be concluded therefor that a DP cone meter will require less upstream (and downstream) pipework, and
that this type of meter could easily be installed or retrofitted in virtually every existing test separator. It would fit
into the same space and the meter would not be affected by the restricted upstream pipework as the existing and
likely badly affected orifice meter. The cone meter could be retrofitted into the same space with only minor
pipework modifications.
An additional benefit of the Adjusta-Cone product, is the wide turndown ratio, discussed in issue 1 of these
articles. A single meter could replace the normally used parallel 6” and 3” (or 2”) orifice meter runs without any
reduction in flow range. Customers should also consider that the Adjusta-Cone meter is supplied with a
multivariable transmitter and RTD, providing differential pressure, flowing pressure and temperature readings as
part of the package.
The following costs could therefore be justifiably removed from the cost of ownership of Adjusta-Cone:
1 x 3” Dual Chamber Orifice Meter (est $12,000)
1 x Static Pressure Transmitter (est $750 ea)
1 x Differential Pressure Transmitter (est $1500 ea)
1 x Temperature Transmitter (est $500 ea)
1 x Chart Recorder (est $1000 ea)
2 x Ball Valves (est $2000 ea)
Additional Pipework, flanges and gaskets. (est $2000)
Calibration cost of items 2 to 4 (est $250 per device x 3)
A capital and first year’s operational cost saving of approximately $23,000 can therefore be conservatively
demonstrated against any perceived higher capital cost of Adjusta-Cone.
Cone Flow Calculation
ISO 5167 Part V states that a cone meter equation can be written as below in Equation 1.
Qva (SI) = π * 2ρf * (D2β2) * 1 * ΔP * Cd * Ɛ
4 ρb (1-β4)
Equation 1
Where:
ρf (“Rho sub f”) = Flowing Density in Kg/m3
ρb (“Rho sub b”) = Base Density in Kg/m3
D = Meter inside Diameter in m
β = Beta Ratio
ΔP (Delta P) = Differential Pressure Across the Meter in Pa
Cd = Calibrated Coefficient of Discharge
Ɛ (Epsilon) = Expansibility Factor
Orifice Flow Calculation
The cone meter calculation is not significantly different from an orifice meter as shown in equation 2.
Equation 2
Where:
d = Orifice Diameter
The third term containing the diameter, is different in a the Adjusta-Cone equation due to the variance in the beta
ratio calculation, which in Adjusta-Cone has two separate calculations: one for the high range and for the low range.
In the high range the pipe diameter Dmeter is used, while in the low range, when the sliding sleeve partially covers the
cone, the smaller Dsleeve is used. The same cone diameter dcone is used in both calculations.
We will discuss the influence of these diameters in the next issue, since this is very important for maintaining high
levels of accuracy.
Calibration
One of the main advantages of orifice meters is the lack of need to calibrate the device, when it is in new condition.
HOWEVER, this relies on several requirements:
• The honed meter tube always remains smooth, round and the same size as it was when brand new.
• The orifice plate is unworn, is the correct size and is installed in the correct orientation.
• The seal between the plate carrier and the meter body is good and no leaks are present.
• The plate is properly aligned in the meter body.
• The tube bundle is still in good condition (and still physically present).
Flow Calibration
If the aspects mentioned above are no longer valid, the orifice
calculation WILL have errors of unknown dimension. A manual
calculation can still be carried out to verify that the reading is within
the expected range, but an annual inspection and checking of the dual
chamber fitting is the only way to verify that the meter still meets AGA
standards. If the upstream pipework is not regularly checked, the pipe
diameter (D) will affect the beta ratio = D/d which will affect the
calculated flow rate. The pipe diameter could be larger (due to erosion
or corrosion) or it could be smaller, due to rust build-up within the
bore. A typical corroded dual chamber fitting is shown. This particular
meter had corrosion in the meter bore, withing the orifice carrier
recess and on the flange seal face, so as well as being inaccurate, it
would have presented a much higher leak risk both at the flange face
(safety), but also around the orifice carrier (accuracy). In comparison
with (say) a coriolis meter, corrosion loss or scale in the vibrating
tubes might be difficult to observe and correct, due to the all-welded
nature of such devices. Safety and measurement accuracy may be
compromised in either case.
As an example, a standard 6” Schedule 80 pipe is 5.761” diameter. If this diameter is only 0.05” out of spec, the
error in the orifice flow calculation will be 5.4%. This could easily be caused by only 0.025” of rust build up, or
corrosion loss in the wall thickness.
In comparison, each new Adjusta-Cone size is dry
gas calibrated to ensure that the best possible
accuracy is guaranteed from day 1 and more
importantly, throughout its entire life, using
erosion studies and careful selection of materials.
The meter can be dismantled completely into its
component parts so each can be inspected for
wear and/or corrosion. Any defects can be
repaired by replacing the affected part, with an
identical replacement, without affecting the
meter’s accuracy. Like orifice meters, Adjusta-
Cone meters should not require regular fluid
calibration, since each part can be compared both
visually and by dimensional checking, to verify
against its original size. if the size remains within
tolerance, the original calibration is still valid.
Some types of flow meters do not lend
themselves easily to this practice and regular and
expensive fluid calibration is required to reduce
electronics drift, and compensate for metallurgical
and dimensional changes over time. Normal calibration the differential pressure and static pressure instruments
are the only calibrations required, which are no different to regular checks already carried out during annual
checks.
