Measurement of Oil and Gas

MEASUREMENT OF OIL AND GAS

Accuracy: difference from the actual measurement and the meter reading, accuracy is stated in following terms:

Repeatability :meter’s ability to reproduce same measurement for a set of constant conditions of flow rate, temperature, viscosity, density, pressure. Repeatability of a custody transfer meter should be within +/-.025% in 3 runs or +/- 0.05% of each other in 5 consecutive prove runs.

Linearity :ability to maintain a meter factor through-out the stated turndown. Depending on meter size and application this is typically +/-0.15% or +/-0.25% for custody transfer use.

Resolution. is a measure of the smallest increment of total flow that can be individually recognized by the meter.

Turndown. Turndown is the meter’s flow range capability. The flow range of the meter is the ratio of maximum flow to minimum flow over which the specified accuracy or linearity is maintained. a meter with a minimum flow rate of 100 bbl/hr and a maximum flow of 1,000 bbl/hr is said to have a 10:1 turndown.

SELECTION CONSIDERATIONS

• FLOW RATE• PRESSURE• TEMPERATURE• VISCOSITY• ACCURACY

TYPICAL POSITIVE DISPLACEMENT METER

MEASURING ELEMENT

low pressure drop low mechanical friction non jamming rotation high driving torque

POSITIVE DISPLACEMENT METERS

FLOW OUTPUT mechanical counters electrical ACCURACYFactors affecting accuracy:1.Displacement,Temperature,Wear,Viscosity,coating2.Slippage Hydraulically induced : flow, viscosity Mechanically induced : friction, wear

PROS AND CONS

• Low cost• Good for viscous liquids

• High maintenance cost• High pressure drop• Not suitable for low flow rates• Clean fluid with no solids

INDIRECT METERS INFERENCE METERS

• TURBINE METERS• CORIOLIS METERS PRINCIPLE : INFERS FLOW BY MEASURING SOME DYNAMIC

PROPERTY OF THE FLOW STREAM

TURBINE METERS

• Principle: Angular velocity of rotor is proportional to linear velocity of liquid

• DESIGN CONSIDERATIOS: Housing• Measuring Element : Rotor with multiple blades on a spindle

mounted on a free running bearing assembly FLOW OUTPUT: Voltage pulse

Accuracy based on two assumptions: (1) the flow area remains constant and; (2) the rotor velocity accurately represents the stream velocity

“K” factor (pulses / unit volume), Erosion, Corrosion, Deposits, Boundary Layer

Thickness, Cavitation , Obstructions Rotor velocity Rotor Blade Angle, Rotor

Stability ,Bearing Friction, Velocity Profile,Swirl, Fluid Density

TYPICAL TURBINE METER

Typical turbine meter

OSCILLATION

TWIST

FLOW RATE AND VISCOSITY FACTORCHOICE OF FLOWMETER FOR LIQUIDS

Meter Proving. Meter proving is the physical testing of the performance of a liquid meter in a liquid service. The main purpose of the test is to assure accuracy.

Meter factor = prover known volume meter reading

While proving a meter, the process-fluid conditions must be as stable, includes temperature, pressure, flow rate, and density. Before starting a meter proving, let the liquid flow through the meter and prover long enough so that the conditions stabilize. Check for leaks or fluid bypassing around the prover or meter. The only way to obtain a reliable meter factor is to have all the liquid that is measured by the meter also measured by the prover.

LACT Units. LACT units are designed for unattended custody transfer of crude oil from a seller to a buyer. Flow rate, operating pressure, gravity, and temperature of the oil determine the LACT design. Minimum pressure drop through the piping and components is desirable.

TYPICAL LACT SYSTEM

TURBINE METER

CORIOLIS METER

ORIFICE METER

• Principle: differential pressure proportional to squire of

flow rate Standards: AGA 3 ISO 5167

BASIC EQUATION: qh =C’ (hw pf )1/2

where: qh - quantity rate of flow at base

conditions, cfh C’- orifice flow constant, hw differential pressure in inches of water at

600F, pf absolute static pressure, psia

C’=(Fb)(Fr)(Y)(Fpb)(Ftb)(Ftf)(Fg)(Fpv)(Fm)(Fl)(Fα)

Where: (Fb) basic orifice factor, cfh(Fr) Reynolds number factor (Y) expansion factor (Fpb) pressure base factor (Ftb) temperature base factor (Ftf) flowing temperature factor (Fg) specific gravity factor(Fpv) supercompressibility factor(Fm) manometer factor for mercury meter(Fl)gauge location factor,(Fα) orifice thermal expansion factor

Typical orifice meter installation details

GAS TURBINE METERS

• Principle: flow rate is proportional to velocity of gas Standards: ISO 9951 OIML R32

Turbine meter installation

ULTRASONIC METERS

Principle: Difference in transit time of high frequency

sound waves travelling between a pair of fixed sound transducers with the flow and against the flow determines the flow

TYPICAL ULTRASONIC METER

Ultrasonic meter installation