PetroStreet

Can anybody explain how "blow down" calculations are done to evaluate the MDMT of reactor/vessels??

The method used to evaluate the depressuring or blowdown rate shall be as given in API RP 521. The effort required may be greatly reduced by the use of a suitable computer program, e.g. PROII or HYSYS. In general ,depressuring valve shall be designed to achieve 50% of the design pressure in 15 minutes. If the depressuring valve are provided on all equipment that processes light hydrocarbons such as butane or lighter ones , the depressuring valves may be designed to achieve 100 psig (7.0 kg/cm2g) or 50% of design pressure, whichever is lower ,in 15 minutes.

As far as MDMT is concerned, many light hydrocarbons will chill to low temperature as pressure is reduced. The depressuring pressure may reach atmosphere. Special considerations must be taken for referring MDMT thru depressurization. In some cases, MDMT is referred as the minimum design temperature after depressuring operation at the design pressure, not at depressurized pressure. This condition may be expected if the depressuring operation is performed within a short period, say within 20 minutes, so that there is no significant ambient heat input into the vessel.

As a thumb rule, for a single component such as propane, any coolant media, etc., whose pressure and temperature relationships are known, the design temperature shall be equal to its saturated temperature at the design pressure (the lower one).

Hope this will help you.

MDMT always corresponds to MAWP (Maximum allowable working pressure) of any component of vessel.
And during blow down situations, pressure is always lesser than MAWP.
You can only reach a temperature that is the saturation point corresponding to the applied pressure, so you do not need blow down calculations to determine MDMT.