3 replies to this topic

[bernath]

Hi all,

I have a question regarding latent heat of vaporization.

Many references state that pool fire load of wetted vessel should be calculated from simple calculation of latent heat of vaporization, i.e.

W = Q / lambda
where,
W is mass load;
Q is heat absorbed by vessel from external fire;
lambda is latent heat of vaporization at relieving condition.

What I don't understand is why we neglect the sensible heat and only take into account the latent heat?

In my opinion, the possible reasons are as follow:

1. The quantity of sensible heat value is negligible compare to latent heat value

2. It is perhaps something to do with an enthalpy as a state function and not a path function

Please kindly point me towards the right direction

thank you

bernath

[ankur2061]

bernath,

Try to get a copy of API STD 521, 5th Edition, 2007. Sub-section 5.15.3.2 of Section 5.15 - External Pool Fires outlines the method for estimation of Latent Heat and also mentions that in absence of reliable calculated values of Latent Heat for hydrocarbon mixtures an approximation can be done using a value of 115 KJ/kg (50 Btu/lb). This value in my opinion should give a conservative value of the mass flow given the value of the heat absorption by the vessel seeing a pool fire.

Additionally have a look at an old post through the link below:

http://www.cheresour...h__1#entry27874

Hope this helps.

Regards,
Ankur.

[paulhorth]

Bernath,

In a fire, on a vessel containing liquid and vapour, the heat causes the liquid to boil, while the pressure rises. When the relief valve opens, the pressure stays constant, and the liquid is already at its bubble point so it will be boiling. If the liquid is a single component (eg water, or butane) the temperature does not rise. The heat added from the fire therefore all goes into latent heat and none goes into sensible heat (which is heat content due to a temperature rise).

With a mixture of hydrocarbons, the picture is slightly more complicated. The pressure is still constant but the temperature will rise over time because the light components are boiled off and the remainig liquid becomes heavier with a higher bubble point temperature. So in this situation, some of the heat from the fire does go into sensible heat, over time. But it is a small part, and the engineer is interested in the INITIAL relief rate when the PSV first opens, and this will be vapour generated by latent heat.

The latent heat chart in API RP 521 which Ankur refers to is an approximation, good enough for a conservative valve sizing. It dates from the days before we had computers to simulate the situation. Problems can arise for high pressure vessels where the latent heat is low or almost zero ( dissolved methane in HP liquid) . Then you have to resort to modelling the fire case with a simulation.it is only the initial vapour rate that defines the relief load.

Your point 2 makes no sense to me and is certainly irrelevant.

Paul

[ankit.kala2007@gmail.com]

Ankur,

how should we calculate Latent Heat for hydrocarbon mixtures. or how is value of 115 KJ/kg (50 Btu/lb) calculated

Ankit