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Friday, January 30, 2009

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A contractor engineer has sized an air receiver with 1000 Sm3/h and client engineer has rechecked the air receiver size. It was found that the basic parameters such as flowrate, operating pressure and temperature, etc are same, however air receiver size calculated by contractor engineer is different than client engineer. After several round of discussion, they found both engineers calculation method are same.

What is the factor cause the difference ?

Different Defintion
A detail analysis on both calculations found that the definition of "Standard" condition are different between the contractor engineer and client engineer. Contractor engineer has used 1.01325 bara @ 0 degC (stated in contractor design manual) whilst client engineer has used 1.01325 @ 25 degC (stated in client common requirement manual). Above situation is pretty common discrepancies in design and engineering. Although both engineers talk about the same thing, "Normal" condition, the results may not be the same due the differences in definition. Thus, it is a good engineering practice to write down the defintion clearly in the calculation note or in a common design basis document.

Following are a list of "Standard" condition for several organizations :

In SI Unit :
  • EPA - 1.01325 bara @ 25 degC
  • NIST - 1.01325 bara @ 20 degC
  • IUPAC - 1.0 bara @ 0 degC
  • ISA - 1.01325 @ 15 degC
  • SATP - 1.0 @ 25 degC
  • CAGI - 1.0 bara @ 20 degC
  • SPE - 1.0 bara @ 15 degC
  • SHELL - 1.01325 @ 25 degC
  • EXXON - 1.01325 @ 15 degC
In US custom :
  • SPE - 14.696 psia @ 60 degF
  • OSHA - 14.696 psia @ 60 degF
  • OPEC - 14.73 psia @ 60 degF
  • ISO 2314 - 14.696 psia @ 59 degF
  • ISO 3977-2 - 14.696 psia @ 59 degF
  • U.S. Army - 14.503 psia @ 59 degF
Source : Wikipedia


SI & US Custom
Another factor may also cause the descrepancies is the reference unit. The defintion in SI unit may NOT same as US custom unit eventhough within an organization. For example, SPE defined Standard condition as
  • 1.0 bara @ 15 degC in SI unit
  • 14.696 psia @ 60 degF in US custom unit
The temperature are not identical 60 degF is equivalent to approximately 15.56 degC, not exactly same as 15 degC. Thus, it is always advisable to make correct unit reference in the calculation and/or Design basis document.

Conversion between two different "Normal" condition
Let take above example, 1000 Sm3/h as defined by contractor. What is the equivalent flow (Sm3/hr) for client engineer ?

Contractor manual : 1.01325 bara @ 0 degC
Client manual : 1.01325 @ 25 degC

Equation for conversion can be taken from discussion in "Relate Normal to Actual Volumtric Flow"

Q2 = (z2/z1) x (T2/T1) x (P1/P2) x Q1 .....[1]

where
Q1 & Q2 are Volumetric Flow in m3/h for condition 1 & 2
P1 & P2 are Pressure in bar abs for condition 1 & 2
T1 & T2 are Temperature in K for condition 1 & 2
z1 & z2 are compressibility factor for condition 1 & 2

Condition 1 : 1.01325 bara @ 0 degC (Contractor manual)
Condition 2 : 1.01325 bara @ 25 degC (Client manual)
Assume
z1 = z2 = 1
Q1 = 1000 Nm3/h @ Condition 1

From [1],
==>
Q2 = (z2/z1) x (T2/T1) x (P1/P2) x Q1
==> Q2 = (298.15 / 273.15) x 1000
==> Q2 = 1091.525 m3/h

The equivalent flow for 1000 Sm3/h @ condition 1 = 1091.525 Sm3/h @ Condition 2. Client engineer shall use 1091.525 Sm3/h in his/her calculation.

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posted by Webworm, 1:42 AM

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