Simple Techniques to improve performance of Variable Area Flowmeters in Chem Inj System Peria G Mohamed, Abbas Al-Shawwaf, Mahdi Al-Qazweeni & Nasser Bu-Arki Kuwait Oil Company
Dr. PERIA GULAM MOHAMED PhD in Chem Engg ( Fluid Dynamics) Univ Of Newcastle, Australia > 30 Years Experience in Process Development, Production Operations, Proj Engg &Process Engineering Current Title : Sr Process Engineer, Operations Technical Services (NK) Kuwait Oil Company
INTRODUCTION Variable Area Flowmeters ( Rotameters) installed in Chem. Injection Skids for Well Fluids (Oil & Water +gas) in upstream Oil & Gas Installations are often considered unsuitable and are left out of service! Reasons for this problem are evaluated and remedial measures identified with a case study.
SCOPE & COVERAGE 1. Introduction to Variable Area Flowmeters 2. Operating Principle,Advantages &Limitations 3. Design Issues-Chem Inj Application 4. Case Study - 5. Conclusions 6. Applications for Oil & Gas Industry
Some Facts on Variable Area Flowmeters Called Rotameters or Variable Area Flowmeters. A float moves inside a tapered tube. Cross sectional area of the fluid path is allowed to vary causing a measurable effect. By far the most common specified, purchased, and installed flowmeter in the world.
Variable Area Flowmeters Fluid flow moves the float upward against gravity Float in equilibrium when area around float generates enough drag equal to (weight buoyancy) Low Cost Simple Reliable Design Can Measure Liquid or Gas Flows Tolerates Dirty Liquids or Solids in Liquid
Measuring Principle Variable Area Flowmeters The forces acting on the float lead to equilibrium between: the weight of the bob r b gv b acting downwards the buoyancy force rg V b acting upwards and the drag force F d acting upwards. gv b b gv Where V b = volume & r b = density of the float(bob), r = the density of the fluid, and g is the gravitational acceleration b F d
Measuring Principle Variable Area Flowmeters The drag force results from the flow field surrounding the float and from the wake of the float Laminar Flow, Turbulent Flow, F d F d C C L T D U D b 2 b U 2 The drag force is dependent on viscosity in laminar flow and is independent of viscosity in turbulent flow Significant change in viscosity may change flow regime
Design Issues with Chemical Injection During Design,important fluid properties to be specified are Density & Viscosity Oil Field Chemicals (e.g., Demulsifier) are performance based,as against commodity type Different Manufacturers offer different chemicals with different properties for same function During design, usually conservative ( highest possible) values are specified. Change in chemicals used can lead to deviation from Flowmeter Vendor Calibration Chart
Case Study Bench Mark Observations Application : Injection of Demulsifier to Well Fluids upstream of an LP Separator in a GC Flow Meter : 1 Metal tube Rotameter with local dial & Remote Indication at DCS. Field Observations (Flow rate): Field indicator : Not readable due to severe oscillation DCS reading : Stable, but significantly lower than actual The Flowmeter was concluded as Not working
COMPARISON OF PARAMETERS : DESIGN Vs ACTUAL Parameter Design Actual Field Value Flow Rate l/h 8 to 80 8 to 18 Inlet Pressure bar-g (Note-1) 18 to 34.5 3 to 3.5 Liquid Specific Gravity 0.945 0.95 Viscosity cp 50 15 Accuracy % of Full Scale 1.6 Very high! Note-1 : Pressure drop across the flowmeter was low ( 0.026 bar). So Pressure downstream was monitored.
Elimination of Excessive Oscillations Action Taken : Valve downstream of the Flowmeter was partially closed to raise the Back pressure on the flowmeter (from 4 barg) to that approaching Design value. Result : Oscillations practically Eliminated! Local Flow Indication was readable!! The issue is resolved!!!
Flowrate correction for Fluid Properties Actual Density is close to Design Value Actual Viscosity is about 30% of Design Value Recommended solution : Conduct physical calibration of the Flowmetwer Incorporate the calibration curve in the Output Range of flow transmitter in DCS
Back Pressr Barg Calibrn Actual Flow l/h CASE STUDY RESULTS Field Dial Flow l/h Original DCS Flow l/h Revised DCS Flow l/h Original DCS Error % FS Revised DCS Error % FS 5 to 7 8.8 10 10.6 9.9 2.3 1.4 19 to 22 31.8 22 24.3 33.1-9.4 1.6 24 to 27 47.2 29 30.7 46.4-20.6-0.9
Conclusions & Application to Oil & Gas Industry Oscillations of Rotameters can be mitigated through imposing back pressure up to design pressure FT Range based on Field Calibration can account for Density & Viscosity variations from design figures Existing Rotameters so far ignored can be made Operational These results are applicable to most of the rotameter applications in the Oil & Gas Industry
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