Relate NORMAL to STANDARD Volumetric Flow

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In gas processing industry, gas flow has been referred to STANDARD (i.e. Sm3/hr, MMSCFD, etc) and NORMAL condition (i.e. Nm3/h). Vendor catalog in many event can be in Sm3/h or Nm3/h.

What is the useful factor to convert Nm3/h to Sm3/h or vice versa ?

Definition
As discussed in "Avoid Confusion In "Standard" Flow Definition", definition is one of the most important factors to avoid discrepancies. First far most important task is to provide a correct definition of STANDARD and NORMAL condition. In "general",

NORMAL condition : 1.01325 bara @ 0 degC
STANDARD condition : 1.01325 bara @ 15 degC

Conversion
From this post,

Q₂ = (z₂/z₁) x (T₂/T₁) x (P₁/P₂) x Q_{1 .....[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 (NORMAL Contractor manual)
Condition 2 : 1.01325 bara @ 15 degC (STANDARD)
Assume z₁ = z₂ = 1
Q₁ = 1 Nm3/h

From [1],
==> Q₂ = (z₂/z₁) x (T₂/T₁) x (P₁/P₂) x Q₁
==> Q₂ = (288.15 / 273.15) x 1
==> Q₂ = 1.055 Sm3/h

Therefore,

1 Nm3 = 1.055 Sm3

when
NORMAL condition : 1.01325 bara @ 0 degC
STANDARD condition : 1.01325 bara @ 15 degC

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Avoid Confusion In "Standard" Flow Definition

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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 Q_{1 .....[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.

Related Post

• Relate Normal to Actual Volumtric Flow
• Air Receiver - Doubt on SCFM & CFM
• Flow Element (FE) Upstream or Downstream of Control Valve (CV) ?
• Use Ultra-Sonic Flowmeter in FLARE Gas Header for emission monitoring
• FAYF - Flowmeter selection in brief...
• Useful Documents Related to Control Valve

Be alert when you specify UNIT

A good chemical & Process engineer should read this...

Case 1

API RP520, 7th Edition, January 2000 is commonly used in Selection & Sizing of Pressure Relief Valve (PRV).

In this code, figure 30—Back Pressure Correction Factor, Kb, for Balanced-Bellows Pressure Relief Valve (Vapors and Gases) and Figure 31—Capacity Correction Factor, Kw, Due to Back Pressure on Balanced-Bellows Pressure Relief Valves in Liquid Service, both use the ratio of gauge pressures.

However, figure 35—Constant Back Pressure Correction Factor, Kb, for Conventional Pressure Relief Valves (Vapors and Gases Only) use ratios of absolute pressures.

Although the differences between ratio of gauge pressures and ratio of absolute pressures are minimum, it still different.

Case 2

An engineer wrote in the report : “I have a control valve, the upstream pressure and downstream pressure are 1.5 bar and 1 bar respectively. Pressure drop is 0.5 bar”. If the upstream pressure is understood as 1.5 barA (mean 0.5 barg) and downstream pressure is interpreted as 1 barG, the pressure drop should -0.5 bar, not 0.5 bar.

Does it make sense ?

Case 3
One operator reported to supervisor. “I have checked the tank water temperature. It is 25 deg and the pump is running smoothly.”. If the statement mentioned in one of the plant in US, supervisor will scratch his/her head, isn’t the water already freeze at 20 deg F ?

Thus, whenever we specify any unit, it shall always be clear on the unit to avoid any confusion.

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