3 replies to this topic

[go-fish]

1. Is it always required to calculate the Adiabatic Flash Temperature (AFT) as a factor for determining Minimum Metal Design Temperature (MDMT) for material selection of lighter hydrocarbons (eg LPGs, compressed methane, propane, ethane vapors) services?
2. Is AFT calculated using initial conditions as operating or design (PSV set pressure on upstream system) pressure?
3. As AFT considers isenthalpic flashing up to atmospheric pressure. What are the possible upset conditions which justifies using final condition as atmospheric for MDMT calculation rather than 50% of design pressure or 6.9 barg as per the Emergency Depressurizing in API  521.

Edited by go-fish, 01 October 2013 - 10:46 AM.

[PaoloPemi]

Solving a single H-P flash you get an idea of the final temperature (and conditions) of the fluid at Pout for that

specific composition and condition at the inlet.  This could be useful or not; it depends upon the process.  For example for a wide boiling mixture you may find different values with initial and final vapor and liquid compositions.

 

A better approach would be in my opinion a direct integration procedure which simulates the different conditions.

[go-fish]

Solving a single H-P flash you get an idea of the final temperature (and conditions) of the fluid at Pout for that

specific composition and condition at the inlet.  This could be useful or not; it depends upon the process.  For example for a wide boiling mixture you may find different values with initial and final vapor and liquid compositions.

 

A better approach would be in my opinion a direct integration procedure which simulates the different conditions.

 

Thank you for your response. However, I question is more towards why AFT (depressurising up to atmospheric pressure) is used as a basis for material selection.

[PaoloPemi]

if you open a valve on top of a vessel under pressure the final temperature for the fluid or Lowest Possible Fluid Temperature (LPFT) is defined as adiabatic flash temperature of the fluid at atmospheric pressure,

if a manual (or automatic) depressurisation is possible

it makes sense to utilize that value as minimum design temperature