Table of Contents

  1. Orifice Plate Calculation
  2. Restriction Orifice Calculations
  3. Appendixes:
    Appendix A, Calculation of Compressibility Factors
    Appendix B, Flow Formula
    Appendix C, Additional Specifications for Maximum Metering Accuracy
    Appendix D, Conversion Formulae for Orifice Meters
    Appendix E, Calculation of Meter Maximum Flow, When Orifice Bore Is Given

1. Orifice Plate Calculations

SCOPE : This standard gives instructions for the use of Company orifice plate calculation sheets :

  • form B.N. -K003-1 (English Units)
  • form B.N. -K003-2 (Metric Units)

GENERAL : The method of the orifice plate calculations mentioned above is based on: “Principles and practice of flow meter engineering” by
LK Spink, 8th edition, and is in accordance with AGA-ASME standards, and the AGA gas measurement committee reports numbers 2 and 3.
Moreover the draft ISO recommendation No. 532 (revised text) dated September 1964 has been used as a basis as well.
For a simplified description of the theoretical background see appendix B.

REFERENCES :

  1. Principles and practice of flow meter engineering L.K. Spink, 8th edition.
  2. Flow measurement with orifice meters R.F. Stearns, R.R. Johnson, R.M. Jackson, C.A. Larson published by D. van Nostrand Co. Inc.
  3. Gas measurement committee report No. 2 AGA, May 1935.
  4. Gas measurement committee report No. 3 AGA, April 1955, Reprint 1956).
  5. AGA flow constants (Foxboro)
  6. Draft ISO recommendation No. 532 September 1964.

INSTRUMENTATIONS : The instructions are numbered 1 to 30.

The same numbers appear on the calculation sheets for reference.

No. 1: FLUID: Fill in process fluid flowing through the orifice.
No. 2: MEASUREMENT UNITS: (see also note below)
Fillin the units of measurement in which the client requires the meter to read. These units should be agreed upon with the client and are generally specified in the applicable general specification for instrumentation. These units may be different from the units in which normal flow is given.

No. 3: BASE PRESSURE: (see also note below)
Fillin the common base pressure to which all measurements in the plant are referred. This again is to be agreed upon with the client.
Common accepted base, pressures are 14.70 psia, 760 mm Hg and 1 Kg/cm²A.

No. 4: BASE TEMPERATURES: (see also note below)
Fillin the common base temperature to which all measurements in the plant are referred.
Agreement with client necessary. Some accepted European base temperatures are:

  • Liquid : 15°C or 60°F
  • Gas : 0°C or 60°F

Note: See BN-SP-K1 Specification for Instrumentation.

No. 5: BASE SPECIFIC GRAVITY:
Fillin the specific gravity at base conditions as specified under Nos. 3 and 4. For liquids, the specific gravity is related to water, for gases to air. When working in metric units, liquid specific gravity is related to water at 15°C (density of water at 20°C is 0.99823 grams/cm³).
The specific gravity of gases can be obtained by dividing the molecular weight of the gas by the molecular weight of air (28.996).
For English Units the specific gravity is referred to water at 60°F.

No. 6: STEAM QUALITY:
Fillin quality of the steam, and when it is superheated or saturated, state this.

No. 7: NORMAL FLOW AT BASE CONDITIONS:
Fillin the normal flow at base conditions as specified and the units in which it is given.

No. 8: NORMAL FLOW IN WEIGHT UNITS:
Fillin the normal flow rate and the units in which this is given.

No. 9 and 10: METER MAXIMUM FLOW:
Fillin meter maximum flow after calculations as outlined under 27.

No. 11: MINIMUM FLOW:

If the minimum flow falls below 20% of the meter maximum flow, consideration should be given to the use of a second lower differential range instrument or a second orifice run.

No. 12: COMPRESSIBILITY:

Fillin the compressibility ratio’s Zb, Zf for gas flow which are defined as:

Zf = deviation from true gas law at operating conditions.
Zb = deviation from true gas law at base conditions.

The compressibility ratio’s can be determined from reduced pressure and reduced temperature as outlined in Spink, pages 351 and 352. See also appendix A.

No. 13: MOLECULAR WEIGHT:

Fillin molecular weight for gas flow only.

No. 14:OPERATING PRESSURE:

Fillin upstream pressure in psia or Kg/cm² Abs.

(Kg/cm²A = Kg/cm² gauge + 1.0336).
(PSIA = 14.70 + psig).

No. 15:OPERATING TEMPERATURE:

Fillin the operating temperature at the orifice in degrees Rankine for the English calculation sheet and degrees Kelvin for the metric calculation sheet (°R = °F + 459.6, °K = °C + 273.2).

No. 16: OPERATING SPECIFIC GRAVITY:

Fillin specific gravity at operating conditions. For liquids this specific gravity is referred to water of 60°F, when working in English units and 15°C when working in metric units.

No. 17: OPERATING DENSITY:

The operating density is in some cases to be found in tables such as the steam table but in other cases must be calculated as out-lined under 27.

No. 18 ABSOLUTE VISCOSITY:

The operating viscosity shall be given in centipoises. Viscosities of several liquids are given on pages 325 thru 331 of “Flow measurement with orifice meters” published by D. van Nostrand Co. Inc. or page 184 of “Principles and practice of Flow Meter Engineering” by L.K. Spink.

To convert from centistokes to centipoises multiply by the specific gravity. For other conversions see page 311 thru 324 of the Van Nostrand book.

No. 19: METER DIFFERENTIAL: (h)

A meter differential of 100 ins W.G., or its metric equivalent is preferable. To avoid swaging up higher ranges 150” W.G., 200” W.G., etc., may be used.

For compressible fluids, the range in inches water should be lower than the absolute static pressure, (at upstream side), this to keep the Y factor (see 29 III) within reasonable limits.

For computer calculations these values are:

  • gas : (in H2O) = 1.3 upstream pressure
  • steam : (in H2O) = 2 upstream pressure

No. 20: NOMINAL PIPE SIZE AND SCHEDULE:

Fill in the nominal size of the pipe, and the schedule (see piping specification).
If the pipe size is given to DIN standards, fill in outside diameter and the wall thickness. (When the form English units is used, state also the inside diameters in mm).

No. 21: ACTUAL I.D.:

The internal diameter as given in line tables and the square of this figure shall be stated in units as indicated.
When the form for English units is used, and the pipe is to DIN standards, fill in I.D. in inches, converted from I.D. in mm. stated in 20.

No. 22: CHART GRADUATION AND CHART FACTOR:

A chart graduation shall be specified preferably 0-10 or 0-100 square root.
This should be agreed upon with client. In special cases, a direct reading graduation may be used.
The chart factor is the rounded off meter maximum flow divided by the maximum chart reading.

No. 23: PRESSURE TAKE-OFF LOCATION:

When the static pressure measuring point has been specified, state if pressure is taken-off from upstream or downstream tapping. This in connection with the use of diagrams for expansion coefficient. (See 29, correction III).
It is Company’s standard practice to take off the pressure from the upstream tapping, as specified in the ISO standard no. 532 (Draft recommendation). Pressure measured at downstream tapping may be considered, when the static pressure is very low, so that the rule of thumb as mentioned is no. 19 page 7 can not be followed.

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