CTM : PROVING

By: Vinay Varghese

INDEX• CTM• FLOW METERS– Coriolis flow meter– Ultrasonic Flow meter– Turbine Flow meter

• Proving– Why proving ?– Basic principle– M-Factor– K-Factor

INDEX• Pulse Interpolation (Double Chronometry method)• Topics that can be discussed.

– Water draw.– Leak test.– Working principle of the Compact prover.– Daily interpolation.

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 devices2. Secondary Devices: Pressure, Temperature, BS&W, Density

measuring devices3. 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.

Flow Meters• There are different types of flow meters that are used in the

CTM.• Mainly there are three:

1. Coriolis Flow meters2. Ultrasonic (Transit Time Based) Flow meters3. Turbine meters

• All three type of flow meters work on different principles.

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.

ULTRASONIC FLOW METERS• Types of ultrasonic flowmeters

1. Transit time method2. Doppler method3. Pulse-Doppler Method4. 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.

MEASURING FLOW USING USMt21,12 = 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.

• 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.

DATA CALCULATION SUMMARY

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.

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

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

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.

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.

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 basetemperature and pressure ( 15oC and 1 bar)

• 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.

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.

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.

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

TOPICS THAT WE CAN DISCUSS

• Water draw.• Leak test.• Working principle of the Compact prover.• Daily interpolation.

• 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.

THANK YOU FOR YOUR PATIENCE