Flow Bases

FLOW METER GROUPS:

1.Quantity meter- POSITVE DISPLACEMENT METERS- WEIGHERS- REPROCICATING PISTON- ROTATING VANES

2.Rate of flow meters- ORIFICE PLATES- VENTURI TUBES- TURBINE METERS- VORTEX METERS

FLOW MEASURING PARTS:

1. Primary element - in contact with fluid

2. Transducer - convert flow to a readable signal

ORIFICE METER IS:Restriction to create pressure drop

MAIN CHARACTERISTICS:- Simple construction - Easy duplicating- Extensive calibration work done- Adopted as standard

HOW DOES IT WORK?

-PRESSURE PROFILE UPSTREAM/DOWNSTERAM THE ORIFICE

-INCREASE OF FLUID VELOCITY IN VENA CONTARCTA

-PRESSURE RECOVERY 5 TO 8 D DOWNSTREAM

-PRESSURE LOSS

-DIFFERENTIAL PRESSURE PROPORTIONAL TO SQUARE OF THE RATE OF FLOW

HISTORY

- FIRST USE BY GIOVANNI VENTURI IN 1797

- MODERN VENTURI BY HERSCHEL 1886

- EXTENSIVE EXPERIMENTAL WORK BY AGA IN 1935

- MPMS 14-3 IS UPDATED VERSION OF AGA

- ISO 5167 IS INTERNATIONALLY ADDOPTED

GAS LAWS

- MOLECULA IS THE SMALLEST PARTICLE IN FREE AND UNDECOMPOSED STATE

- MOLECULES MOTION IMPACT IS MEASURED AS PRESSURE

- TEMPERATURE INFLENCE ON THE MOTION OF MOLECULAS

- AT ABSOLUTE ZERO PRESSURE AND TEMPERATURE THERE IS NO ACTIVITY

THE VOLUME OF IDEAL GAS IS:

- INVERSELY PROPORTIONAL TO THE ABSOLUTE PRESSURE (BOYLS LAW)

- DIRECTLY PROPORTIONAL TO THE ABSOLUTE TEMPERATURE (CHARLES LAW)

- THIS IS COMMONLY WRITTEN AS THE FOLLOWING EQUATION:

P1 x V1 / T1 = P2 x V2 / T2

ORIFICE GAS EQUATION:

QV = K x Cd x Ev x Y1 x d2 x (Tb / Pb) x

[(Pf1 x Zb x hw) / (Gr x Zf1 x Tf)]0.5

WHERE IS:

QV - STANDARD VOLUME FLOWRATECd - COEFFICIENT OF DISCHARGEEv - VELOCITY OF APPROACH FACTORY1 - EXPANSION FACTORd - ORIFICE PLATE BORE Tb - BASE TEMPERATUREPb - BASE PRESSUREPf1 - FLOWING PRESSURE UPSTREAMTf - FLOWING TEMPERATUREZb - COMPRESSIBILITY AT BASE CONDITIONSZf1 - COMPRESSIBILITY AT FLOWING (UP.) CONDITIONSHw - ORIFICE DIFFERENTIAL PRESSUREGr - REAL GAS RELATIVE DENSITY

ORIFICE PLATE COEFFICIENT OF DISCHARGE (Cd)

- EMPIRICALLY DETERMINED

- TO BE VALID THE ORIFICE MUST BE MANUFACTURED ACCORDING TO M.P.M.S

- DEPENDS ON THE REYNOLDS NUMBER, TAP LOCATION, METER TUBE DIAMETER AND ORIFICE DIAMETER

VELOCITY OF APPROACH FACTOR (EV)

- CORRECTS THE CHANGE IN VELOCITY BETWEEN THE UPSTREAM CONDITION AND THE VELOCITY IN THE ORIFICE BORE.

- DEPENDS ON BETA RATIO

EXPANSION FACTOR (Y1 OR Y2)

- CORRECTS THE DENSITY CHANGE BETWEEN THE MEASURING TAPS AND DENSITY AT THE ORIFICE PLANE

- Y1 OR Y2 DEPENDS IF THE STATIC PRESSURE IS MEASURED ON THE UPPSTRAEAM OR DOWNSTREAM TAP.

- Y2 IS SMALLER THAN Y1

ORIFICE PLATE BORE DIAMETER (d)

- ACTUAL DIAMETER AT FLOWING CONDITIONS

- CORRECTION MUST BE DONE IF THE TEMPERATURE AT WHICH THE PLATE WAS BORED IS DIFFERENT FROM THE OPERATING

GAS SPECIFIC GRAVITY (Gr)

- REPRESENTS A PHYSICAL CHARACTERISTICS OF THE GAS

- REPRESENTS THE RATIO BETWEEN THE DENSITY OF GAS DIVIDED BY AIR AT SAME CONDITIONS

- WITH GIVE FORCE MORE GAS WITH LOWER Gr IS PASING THROUGH AN ORIFICE

COMPRESSIBILITY (Zf)

- THE REAL GAS COMPRESS MORE THAN THE IDEAL GAS PREDICTS

- THE ABOVE RATIO IS CALLED COMPRESSIBILITY

- CALCULATED BY AGA 8

- ROUGHLY THE COMPRESSIBILITY AFFECTS THE VOLUME BY 0,5% EACH 100 PSIG

BETA RATIO ()

- IS THE RATIO OF ORIFICE BORE DIVIDED BY PIPE INTERNAL BORE

- THE LIMITS ARE 0.2 AND 0.7

CRITICAL FLOW

- OCCURS WHEN THE VELOCITY OF THE GAS OR VAPOR REACHES THE SPEED OF SOUND

- GAS CAN NOT TRAVEL ANY FASTER

- APPLICATION IN FLOW LIMITING DEVICES (RO)

- A RULE OF THUMB SAYS THAT CRITICAL FLOW OCCURS WHEN P2 IS LES THAN 50% OF P1

ADVANTAGES OF ORIFICE METERS

- FLOW CAN BE ACCURATELLY DETERMINED WITHOUT FLOW CALIBRATION

- ALL OTHER METERS REQUIRE FLOW CALIBRATION AT OPERATING CONDITIONS

- WELL ESTABLISHED PROCEDURES CONVERTS DIFFERENTIAL PRESSURE INTO FLOW RATE

- THE CALCULATION COEFFICIENTS ARE BASED ON ACCURATELY MEASURABLE DIMENSIONS

- SIMPLE, RUGGED, WIDELLY ACCEPTED, RELIABLE AND INEXPENSIVE

RELIABILITY

- THE COEFFICIENTS CALCULATED BY AGA 3 ARE SUBJECT TO UNCERTAINITY ON APPROX. 0.5%

- THIS IS FOR BETA FROM 0.2 TO 0.7

- FOR BETA BETWEEN 0.1 - 0.2 AND 0.7 - 0.75 THE ADDITIONAL UNCERTAINITY SHOULD BE ADDED.

- THE REYNOLDS NUMBER BELOW 1,000,000 WILL INCREASE UNCERTAINITY

- REYNOLDS NUMBER PRACTICAL LIMIT IS 4,000

RANGEABILITY

- CALLED ALSO "TURN DOWN RATIO" IS THE RATIO BETWEEN MAXIMUM AND MINIMUM MEASURABLE FLOW WITH DECLARED ACCURACY

- COMMON RANGEABILITY FROM 3 TO 1

- ADOPTING MULTIPLE DIFFERENTIAL PRESSURE TRANSMITTER (STACK) THE RANGEABILITY CAN BE INCREASED

REPEATABILITY

- THE METER CAPABILITY TO INDICATE THE SAME READING AT THE SAME FLOW

- THE READINGS MAY OR MAY NOT BE ACCURATE BUT CAN BE REPEATABLE

- THIS CAPABILITY IS IMPORTANT WHEN FLOW METER IS USED FOR FLOW CONTROL

PRESSURE LOSS CHARACTERISTICS

1. ORIFICE - HIGHEST

2. FLOW NOZZLE

3. SHORT CONE VENTURI

4. STANDARD VENTURI

5. LO-LOSS TUBE

ORIFICE PLATE TYPES

- CONCENTRIC ORIFICE, SQUARE EDGE - STANDARD

- ECCENTRIC ORIFICE PLATE - FOR GASES WITH LIQUIDS OR LIQUIDS WITH SEDIMENTS

- QUADRANT EDGE - HIGH VISCOSITY FLUIDS

- CONIC EDGE - AT REYNOLDS NUMBERS LOWER THAN FOR QUADRANT EDGE

PRESSURE TAP LOCATION

- FLANGE TAPS - ONE INCH UP AND DOWNSTREAM FROM THE ORIFICE FACE

- PIPE TAPS - 2,5D UPSTREAM AND 8D DOWNSTREAM AT POINT OF MAXIMUM PRESSURE RECOVERY

- VENA CONTRACTA TAPS - 1D UPSTREAM AND AT THE POINT OF MINIMUM PRESSURE DOWNSTREAM

- CORNER TAPS - ADJACENT TO THE PLATE FACES

ORIFICE PLATE HOLDERS

- ORIFICE FLANGES

- SENIOR ORIFICE FITTING

- JUNIOR ORIFICE FITTING

- SIMPLEX ORIFICE HOLDER

- METER TUBES