1 reply to this topic

[dattatray kolte]

Vapour flow in relief-discharge piping is characterized by rapid changes in density and velocity; consequently, the
flow should be rated as compressible. Several methods for calculating the size of discharge piping have been
developed using isothermal or adiabatic flow equations. Actual flow conditions in relief systems are normally
somewhere between isothermal and adiabatic conditions. For most cases, the slightly more conservative
isothermal equations are recommended; however, the adiabatic flow equations can be preferable for some less common
applications (e.g. cryogenic conditions).

Questions
1- Why isothermal equation are more conservative.
2- Adiabatic equations are preferred for cryogenic conditions.
& how
3- Actual flow condition in relief system falls in between Isothermal & Adiabatic conditions

[sheiko]

Hi,

1- My understanding is the following:

The main difference between both flow types is the effect of temperature on the gas density (gas density being proportionnal to absolute pressure times molecular weight divided by absolute temperature, P*MW/T).
- Isothermal flow: Temperature is constant, so density decreases only due to the drop in pressure along the line.
- Adiabatic flow: Same effect of pressure on density but If your gas cools upon expansion (expansion is when gas volume goes up because pressure goes down), as it is the case for most gases, temperature decreases and tend to increase gas density. So the gas density will either go up or down depending on the net effect of pressure and temperature on it. So, unless the gas warms upon expansion, the density in adiabatic flow condition is usually higher than during isothermal flow.

Now, why is isothermal flow considered conservative?
This is because, as mass flow = velocity*density*cross sectionnal area, so the lower the density, the higher the velocity and, thus, the higher the pressure drop (given a mass flowrate and a pipe diameter). As a result, isothermal equation generally provides more conservative results regarding pressure drop (it gives higher pressure drop), than does adiabatic flow.

Download the following article for more knowledge:
http://www.aft.com/n...ons-donat-choke

Edited by sheiko, 22 January 2012 - 07:21 AM.