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Proce ss Design Practices Flo w Meters and Orifi ces Page : By : CK Date : Dec 09 Copyright c This data is condential and shall not be disclosed or reproduced in any manner without written permission 6.7 Flow Orice Sizing Assume ange-to-ange maximum pressure drop (h w max , Miller p.9-21). Ty pi cal inst allat ion = 2,500 mmH 2 O [100 inH 2 O] Bubble poi nt liq a nd venturi’s = 1,250 mmH 2 O [50 inH 2 O] Gases 1.0 inH 2 O × Operating pres (psia) Calculate required maximum ow (meter max) at standard conditions: Q metermax Q rated 0.8 Round meter max to even 5-10 m 3 /h (at standard or normal con ditions ). Chec k that all cases are 30-90% of meter max (3:1 ow ratio, Miller p.9-106). Calculate β and orice bore at meter max ow and dP. 6.8 Beta Ratio Limits Beta ratio β = d D = Bore PipeID Change pipe size or pressure drop to stay within beta ratio range for ow meters. Do not change pipe more than one standard pipe size. Beta ratio limits for ow meters are (Miller Table 9.54): Flow meter β Range Square edge or i ce 0.2-0.75 (des ign 0.7 max) Quadrant edg e o rice 0.24 -0.6 V enturi 0.4-0.7 Restr iction orices can hav e smalle r/larg er beta ratios. Small restriction orices normally have a standard drill size hole. 6-5

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Process Design Practices

Flow Meters and Orifices

Page :

By : CK

Date : Dec 09

Copyright c This data is confidential and shall not be disclosed or reproduced in any manner without written permission

6.7 Flow Orifice Sizing

• Assume flange-to-flange maximum pressure drop (hwmax, Miller p.9-21).Typical installation = 2,500 mmH2O [100 inH2O]

Bubble point liq and venturi’s = 1,250 mmH2O [50 inH2O]Gases ≤ 1.0 inH2O × Operating pres (psia)

• Calculate required maximum flow (meter max) at standard conditions:

Qmetermax Qrated

0.8

• Round meter max to even 5-10 m3/h (at standard or normal conditions). Check

that all cases are 30-90% of meter max (3:1 flow ratio, Miller p.9-106).• Calculate β  and orifice bore at meter max flow and dP.

6.8 Beta Ratio Limits

• Beta ratio β = dD = Bore

PipeID

• Change pipe size or pressure drop to stay within beta ratio range for flow meters.

• Do not change pipe more than one standard pipe size.

• Beta ratio limits for flow meters are (Miller Table 9.54):

Flow meter β  RangeSquare edge orifice 0.2-0.75 (design 0.7 max)Quadrant edge orifice 0.24-0.6Venturi 0.4-0.7

• Restriction orifices can have smaller/larger beta ratios.

• Small restriction orifices normally have a standard drill size hole.

6-5

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Process Design Practices

Flow Meters and Orifices

Page :

By : CK

Date : Dec 09

Copyright c This data is confidential and shall not be disclosed or reproduced in any manner without written permission

6.17 Two-Phase Expansion Factor - Close-up Taps

• The two-phase expansion factor is based on the Omega equation of state proposedby Epstein et al (1983) and refined by Leung (1992).

ρref 

ρ= ω

P ref 

P − 1

+ 1

Any two-phase point (including bubble point liquid) can be used as reference.

• The equations below are modifications of Leung’s equations to express it in termsof the upstream pressure (P 1) instead of stagnation pressure (P 0).

Y ω1 =

 (1− β 4)

(1− rs) + ωrs lnrsr

+ (1 − ω)(rs − r)

(1− r)

ω( rsr − 1) + 1

2− β 4

ω = Compressibility parameterr = Larger of P 2P 1 or rc

rs = Larger of P 1sP 1or P 2

P 1

rc = Smaller of P cP 1 or P sP 1

P 1 = Upstream pressure (kPa abs)[psia]P 2 = Downstream pressure (kPa abs)[psia], at close-up tapsP s = Bubble point pressure at T 1 (kPa abs)[psia]

• Several equations have been proposed for ω causing a fair bit of confusion. It is themost accurate and easiest to calculate ω directly from its definition (ISO 4126).

ρi =

x1

ρiG+

(1− x1)

ρL+ N (xi − x1)

1

ρiG−

1

ρL

−1

ω =

ρ1sρi− 1

P 1sP i− 1

with ω ≥ 0

=(xi − x1s)

1− P iP 1s

P 1s, ρ1s, x1s = Pressure, density and vapour mass fraction based onthe smaller of P 1 or P s

P i, ρi, xi = Pressure, densities and vap mass frac at intermediate point.Based on isentropic flash from P 1 to P , but isenthalpic flashis acceptable. API 520 uses P  = 0.9P 1, Diers uses P  = 0.7P 1.

N = Boiling delay factor, 1=HEM, 0=Frozen model

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