1. Custody transfer metering measures product supplied to customers for billing purposes using flow meters, pressure/temperature sensors, and flow computers. 2. Common flow meters are Coriolis, ultrasonic, and turbine meters which use different principles like density changes or sound/rotation to measure flow. 3. Meter proving tests flow meter accuracy under operating conditions by comparing measured volumes to a prover of known volume, establishing correction factors like K-factors and M-factors.

1. CTM : PROVING
By: Vinay Varghese

2. INDEX
• CTM
• FLOW METERS
– Coriolis flow meter
– Ultrasonic Flow meter
– Turbine Flow meter
• Proving
– Why proving ?
– Basic principle
– M-Factor
– K-Factor

3. INDEX
• Pulse Interpolation (Double Chronometry method)
• Topics that can be discussed.
– Water draw.
– Leak test.
– Working principle of the Compact prover.
– Daily interpolation.

4. Custody Transfer Metering
• We use custody transfer metering to measure how much
product do we supply to the customers and in the end to bill
our clients for the same.
• There are three major components of the CTM.
1. Primary Devices: Flow measurement devices
2. Secondary Devices: Pressure, Temperature, BS&W, Density
measuring devices
3. Tertiary Devices: Flow Computers
• The Primary devices in the CTM are Flow meters and it is
important to have an understanding of the working of the
various type of flow meters normally used in Custody
Transfer.

5. Flow Meters
• There are different types of flow meters that are used in the
CTM.
• Mainly there are three:
1. Coriolis Flow meters
2. Ultrasonic (Transit Time Based) Flow meters
3. Turbine meters
• All three type of flow meters work on different principles.

6. CORIOLIS FLOW METER
Flow is measured by using velocity sensors to detect the twist in the
tube and transmit electrical signals having a relative phase shift that is
proportional to mass flow.
Coriolis meters also measure density, whereby the resonant frequency
of the forced rotation as a function of fluid density.
When the fluid is flowing, it is led through two parallel tubes. An actuator
(not shown) induces a vibration of the tubes. The two parallel tubes are
counter-vibrating, to make the measuring device less sensitive to outside
vibrations. The actual frequency of the vibration depends on the size of
the mass flow meter, and ranges from 80 to 1000 vibrations per second.
When no fluid is flowing, the vibration of the two tubes is symmetrical.

7. ULTRASONIC FLOW METERS
• Types of ultrasonic flowmeters
1. Transit time method
2. Doppler method
3. Pulse-Doppler Method
4. Phase difference method
• Transit times are measured from sound pluses travelling diagonally across
the pipe, downstream with the flow and upstream against the liquid flow
• The difference in these transit times is related to the average liquid flow
velocity along multiple acoustic paths.
• Numerical calculation techniques are then used to compute the average
axial liquid flow velocity and the liquid volume flow rate at line conditions
through the meter.

8. MEASURING FLOW USING USM
t21,12 = Lw and Lw
c + va cos(ᶲ) c - va cos(ᶲ)
• t21 is the time taken for the sound pulse to travel in the upstream
direction.
• t12 is the time taken for the sound pulse to travel in the downstream
direction.
• C is the velocity of sound.
• V is the flow velocity.
• ᶲ is the angle at which the transducer and receiver are positioned with
respect to the pipe.
• L is the distance between the two transducers.
• Since we have the time, we can compute the velocity of the fluid.

9. • There are 4 sets of transducers and receivers in the LUSM.
• Each set is covering a particular region of the pipe and so, we cannot give
equal weightage to the flow measured by each set.

10. DATA CALCULATION SUMMARY

11. TURBINE FLOW METERS
• When fluid flows past a turbine meter, the meter's vaned
rotor rotates
• The angular velocity of the rotor is proportional to the
velocity of the fluid.
• A magnetic pickup coil is mounted outside the pipe. The
turbine or paddlewheel blades are made of or contain
magnetic material. As the fluid flows and each blade passes
by the coil, the magnetic flux through the coil changes,
causing a voltage pulse. A sensor measures the pulse rate and
from that determines the flowrate.

12. WHY DO WE NEEED PROVING ?
• What is a Meter Proving
– Meter proving is the physical test used to determine the accuracy and
performance of a liquid meter
• Why is Proving performed
– The need for proving arises because operating conditions differ
significantly from the conditions under which the meter is calibrated
• Why Prove
– Establish reference point under operating conditions.
– Account for changes in meter accuracy
– Verify repeatability / linearity of meter
– Contractual requirement for custody transfer

13. BASIC PROVING PRINCIPLE
• Regardless of the prover type, the same
• Basic proving principle applies
• Meter Factor = Known_Volume
Measured Volume

14. SMALL VOLUME METER FACTOR
• API Definition:
• Small volume provers are provers that will collect less than
10,000 pulses in a single pass.
• We need 10,000 pulses per pass is necessary to
mathematically generate a meter factor accurately up to 4
decimal points.
• The small volume prover method is possible because of the
precision of optical switches and the method of double
chronometry pulse interpolation that is used in small volume
provers.

15. K-FACTOR AND M-FACTOR
• The operational point of view for proving is simple enough:
– We take the flow in a desired stream through the prover take-off valve
into the prover.
– See to it that the flow remains steady with minimal deviation from the
flow at which we want to check the accuracy of the flow meter.
– Get the repeatability of the flow meter within the desired range and
feed the K and M factors computed by the software to the system and
enjoy another three months of peaceful operations.
• What really is important while understanding proving is how
this K and M factor is derived for the flow meters.

16. M- FACTOR
• M- Factor is pretty straightforward.
• In this process the volume registered by the flow meter being
tested is compared to the volume registered with the flow
meter which we consider as a reference(Master Meter), giving
us the formula of
Multiplied by
To correct for the pressure and temperature variation from the base
temperature and pressure ( 15oC and 1 bar)

17. • How do we know that the flow measured by the reference
meter is accurate ?
• For that we need to prove the master meter with reference to
the compact prover which is essentially the most complicated
part, both operation wise and in terms of understanding.
• Compact prover is of a known volume established by the
water draw test.
• Essentially water draw test is a physical measurement of the
volume of water displaced in one pass of the prover in the
downstream stroke.
• We get to know that one pass of the prover is complete
because of the optical switches installed along the piston
movement of the prover.

18. K-FACTOR
• These optical switches are going to come in use soon, when
we establish the K-factor for the reference meter.(Master
Meter)
• As we have seen earlier that the turbine type of flow meter
works on the principle of cutting the magnetic field with each
rotation of the blades, w.r.t the flow through it.
• The master meter is also a turbine meter, when the magnetic
field is cut it generates a pulse as a signal of a unit volume
that has passed through the flow meter.

19. Contd.
• To set the master meter as a reference for the other flow
meters to be tested we need to know the no. of pulses it
generates for a particular volume displaced.
• This we achieve by double chronometry pulse interpolation
while proving the master meter w.r.t the compact prover.
• For this pulse interpolation method we need:
– Time taken to displace voume in the compact prover.
– No. of pulses generated by the flow meter during the pass.
– Time taken for the generation of pulses in the flow meter.
– Base volume of the compact prover.

23. Since we need to find out the exact no. of pulses generated in
that pass, we use double chronometry pulse interpolation.

26. TOPICS THAT WE CAN DISCUSS
• Water draw.
• Leak test.
• Working principle of the Compact prover.
• Daily interpolation.

27. • Proving is a very elaborate topic. What I have
covered is something that I found interesting
in the process of proving, and thought to be
apt for this short session.