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Saturday, November 22, 2008

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Compression system shutdown, block-in, settle out follow by system blowdown is common in any oil and gas, refinery and LNG production facilities. A methodology using HYSYS simulation software has been presented in "Simple Method For Compressor Settle Out (Vapor Only) Using HYSYS" and this method is particularly applicable to vapor only system. In order to handle system contains vapor and liquid, the method has been extended "Adjusted Method For Compressor Settle Out (with Vapor & Liquid) Using HYSYS". Some engineers asked a question, this method is applicable during detailed design. Nevertheless, is there any simpler and manual method that may be used for quick estimation during proposal and/or conceptual stage ? Yes... Following is a simple manual method may be considered.

This method is utilising universal gas law (PV=znRT) with the following basis and assumption :
  • Vapor only system
  • No condensation during the process
  • Compressibility factor assumed same for condition before and after settle-out and assumed (z=1)
  • Limited fluid with molecular weight is similar range. Higher the different, higher the deviation
A system consists of n-section with pressure (Pi, kPag), temperature (Ti, K), Molecular weight (MWi), physical volume (Vi, m3) for i-section.

Number of mole in each i-section (before settle-out),

ni = (Pi x Vi) / (zi x R x Ti).....[1]

Total mole (after settle-out),

ns = Sum (n1 + n2 + n3...).....[2]

Total volume (after settle-out),

Vs = Sum (V1 + V2 + V3...).....[3]

Volume at normal condition (Pi,n = 1.01325 bar & Ti,n = 273.15 K) for each i-section (before settle-out)

Vi,n = (Pi x Vi / Ti) /(Pi,n / Ti,n)......[4]

Total volume at normal condition (after settle-out)

Vs,n = Sum (V1,n + V2,n + V3,n...).....[5]

ni x Ti for each i-section (before settle-out),

ni x Ti = (Pi x Vi) / (zi x R).....[6]

Total ns x Ts (after settle-out),

ns x Ts = Sum (n1 x T1 + n2 x T2 + n3 x T3 +...).....[7]

Thus, From [7] and [2],
Settle-out temperature (Ts),

Ts = Sum (n1 x T1 + n2 x T2 + n3 x T3 +...) / ns .....[8]

Settle-out pressure (Ps),

Ps = (1.01325) x (Vs,n / Vs) x (Ts / 273.15)......[9]

Case Study
A methane compression system with the following conditions.

Suction :
Pressure, P1 = 5 barg
Temperature, T1 = 50 degC
Molecular weight, MW = 16.0429
Physical Volume, V1 = 1 m3

Compressor discharge (Hot) :
Pressure, P2 = 15 barg
Temperature, T2 = 150 degC
Molecular weight, MW = 16.0429
Physical Volume, V2 = 1 m3

Cooler discharge (Cool) :
Pressure, P3 = 15 barg
Temperature, T3 = 50 degC
Molecular weight, MW = 16.0429
Physical Volume, V3 = 1 m3

Using method as proposed in "Simple Method For Compressor Settle Out (Vapor Only) Using HYSYS", the settle out pressure and temperature are 11.73 barg and 86.3 degC. (see below image).

Using manual method as proposed above (program in Excel), the settle out pressure and temperature are 11.67 barg and 85.7 degC. (see below image).

The percentage error are 0.5% and 0.7% for pressure and temperature respectively.
This shown the method is reasonable.

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posted by Webworm, 1:57 PM


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