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# Api 521 Isothermal Compressibility Coefficient

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### #1 ykkim

ykkim

Brand New Member

• Members
• 1 posts

Posted 12 November 2017 - 02:36 AM

Dear experts,

As I calculated isothermal compressibility coefficient of thermal expansion in pipe, the result seems negative (minus) number. As API 521, isothermal compressibility coefficient is

X = 1/v1 (v1-v2)/(p2-p1)

X = the isothermal compressibility coefficient, expressed in 1/kPa (1/psi)

v1 = the specific volume of liquid at the first pressure, expressed in m3/kg (ft3/lb)

v2 = the specific volume of liquid at the second pressure, expressed in m3/kg (ft3/lb)

p1 = the first absolute pressure, expressed in kPa (psi)

p2 = the second absolute pressure, expressed in kPa (psi)

The conditions I've calculated are showen below.

roh at T1 = 1046.8 kg/m3

roh at T2 = 1045.5 kg/m3

DeltaT = 1 degC (assume)

v1 at T1 =0.00095530 m3/kg

v2 at T2 =0.00095649 m3/kg

p1 = 70 kPa (Actual)

p2 = 653 kPa (Actual)

x = -0.000021

It means T2 is lower than T1. But P2 is higher than P1. What am I wrong? I am still struggling to understand. Please correct me where I am wrong. Thank you!

### #2 sukanta87

sukanta87

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Posted 13 November 2017 - 11:34 PM

It's isothermal. Why your temperature changed? v2 should have the lower value than the v1, as an  increase in fluid pressure (p2>p1) causes the fluid volume to decrease (v1>v2). Always x > 0.