Hand Calculation Method for Orifice Design

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Chevron Corporation B-1 July 1999

Abstract

This appendix explains how to perform the calculations for orifice plate design by hand. For manual look-up of the orifice coefficient, S (given beta) or the diameter ratio, beta (given S), the following eight figures are provided:

Orifice calculation sheets have been prepared for guidance through the orifice calcu-lation steps. Calcucalcu-lation sheets filled in with appropriate examples are attached. The four orifice calculation sheets are as follows:

• LIQUID, square-edge orifice, Form ICM-EF-59B (Figure B-9) • LIQUID, quadrant-edge orifice, Form ICM-EF-59C (Figure B-10) • GAS, square-edge orifice, Form ICM-EF-59D (Figure B-11)

• VAPOR/STEAM, square-edge orifice, Form ICM-EF-59E (Figure B-12) Note that in these calculation sheets, the term "sharp edge" is used instead of "square-edge."

Results of the hand calculations agree closely with the computer program ORIFICE. The two types of calculations that can be performed are orifice sizing and orifice re-ranging, except for quadrant-edge.

Figure B-1 Flange Taps - 6 Inch Pipe Size (6.056 in.); S vs. Beta for Different R_D Values (Based on Data from ISO 5167) Figure B-2 Flange Taps; Pipe Size Corrections for S Values (Based on

Data from ISO 5167)

Figure B-3 Radius Taps; S vs. Beta for Different R_D Values (Based on Data from ISO 5167)

Figure B-4 Corner Taps; S vs. Beta for Different R_D Values (Based on Data from ISO 5167)

Figure B-5 Pipe Taps; S vs. Beta for Different R_D Values (Based on Data from ISO 5167)

Figure B-6 ASME Small Bore with Flange Taps; S vs. Beta for Different R_D Values—1 in. to 1-1/2 in. Pipe Size (Based on Data from "Fluid Meters: Their Theory and Applica-tion", 6th ed., 1971. Courtesy of ASME)

Figure B-7 ASME Small Bore with Corner Taps (.546 in.); S vs. Beta for Different R_D Values—1/2 in. Schedule 80 Pipe Size (Based on Data from "Fluid Meters: Their Theory and Application", 6th ed., 1971. Courtesy of ASME) Figure B-8 Quadrant — Edge Data; S and Thickness Ratio vs. Beta

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July 1999 B-2 Chevron Corporation

B1.0

Calculation Procedure

1. Fill in the STREAM PROPERTY input blocks.

2. Fill in the FLOW RATE input blocks.

3. Fill in the ORIFICE DATA input blocks.

TEMPERAURE -- degrees Fahrenheit for liquid and vapor/steam -- degrees Rankine = °F + 460 for gas

PRESSURE -- PSIA for gas and vapor

LIQUID SPECIFIC GRAVITY -- Gb at 60 F (Gb for water =

1.00)

VAPOR/STEAM SPECIFIC GRAVITY -- cu.ft./lb.

-- for steam, use steam tables

GAS SPECIFIC GRAVITY -- Ideal sp. gr. is used

= M.W. gas/M.W. dry air = Z_b× Real sp. gr.

LIQUID KINEMATIC VISCOSITY -- use centistokes

GAS AND VAPOR ABSOLUTE VISCOSITY

-- use centipoise

UNITS: For liquid use BPD, BPH, GPM, or GPH. For vapor use lbs/hour; for gas use SCFH.

FULL SCALE: Flow that creates a dp across the orifice taps equal to the maximum value of the dp transmitter range.

NORMAL: For square-edge and ASME small-bore this should be the mid-range dp value, which equals .707 × (full-scale flow rate)

1/3 FS: For quadrant-edge, equal to .333 × (full-scale flow rate)

TYPE OF ORIFICE PLATE -- Square-edge

-- ASME small-bore

-- Quadrant-edge

TYPE OF TAPS -- Flange, radius, corner or pipe for

square-edge

-- Flange or corner only for ASME small-bore

-- Flange for quadrant-edge

PIPE INSIDE DIAMETER -- Use 3 decimal places

ORIFICE DIAMETER -- Entered for orifice re-ranging

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Chevron Corporation B-3 July 1999 4. Fill in the CORRECTIONS FACTORS.

5. Calculate Pipe Reynold’s Number. Space is provided for values in equations. See examples on sheets provided.

6. Calculate orifice size or dp range for square-edge or ASME small-bore orifice. Given h, Find d

Fill in equations and solve for S. Look up beta ratio in table. Interpolation is necessary.

Use Table in Figure B-2 to correct for pipe size other than 6-inches (2-12 inches).

Use Reynold’s number column that is closest to value calculated in step 5. Given d, Find h

Calculate beta ratio. Look up S in table. Interpolation is necessary.

Use Reynold’s number that is closest to value calculated in Step 5. See examples on sheets provided.

7. Calculation for quadrant-edge orifice size.

-- Use 3 decimal places

FULL SCALE DP -- Entered for orifice sizing

calcula-tion

-- Standard value = 100 inches of water

Fa -- Correction for thermal expansion of orifice meter at flowing temperature found in ASME MFC-3M Tables or API MPMS. Chapter 14.3/AGA-3.

Y -- Gas expansion factor for mid-scale flow Y₁ = correction based on upstream pressure Y₂ = correction based on downstream pressure Look up in table or chart - use mid-range differential pressure (dp)

Fpv -- Supercompressibility correction factor

Rarely used in refinery calculations (usually set = 1.0) Look up in table or chart when used.

Zb -- Compressibility factor at 60°F, 1 Atm

Rarely used in refinery calculations (usually set = 1.0) Look up in table or chart when used.

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July 1999 B-4 Chevron Corporation Select a plate thickness, look up values from Figure B-8, and calculate the differential pressure h.

If h is too large, make plate thicker; if too small, make plate thinner, and repeat calculation. h should equal about 100 inches of water.

The permissible range of Reynold’s numbers (at 1/3 full-scale flow) for a quad-rant-edge orifice is a function of the beta ratio:

See example on calculation sheet provided.

beta ratio 0.2 0.3 0.4 0.5 0.6

min. R_D 670.0 770.0 630.0 450.0 320.0

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Fig. B-6 ASME Small Bore with Flange Taps; S vs. Beta for Different R_D Values—1 in. to 1-1/2 in. Pipe Size (Based on Data from "Fluid Meters: Their Theory and Application", 6th ed., 1971. Courtesy of ASME)

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Fig. B-7 ASME Small Bore with Corner Taps (.546 in.); S vs. Beta for Different R_D Values—1/2 in. Schedule 80 Pipe Size (Based on Data from "Fluid Meters: Their Theory and Application", 6th ed., 1971. Courtesy of ASME)

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