#6 Fuel Oil PSV sizing

[Bourbon103]

The relief valves are on an oil pump and an oil heat exchanger (steam in the tube). The area the relief valve is in is a PSM (process safety management) area that is considered for external fire.

Are hydrocarbon relieving valves exposed to fire sized the same way as the "vaporizing liquids" method in API RP 520 Part 1? The reason I ask is because information on the latent heat of vaporization of hydrocarbons is difficult to find and the only thing in API RP 520 Part 1 mentioning this is that it can reasonabliy be approximated to be 50 BTU/hr if not known.

 

[rbcoulter]

There is the Vetere equation for estimating heat of vaporization from hydrocarbons. You will need to know the average molecular weight of the fluid and its boiling point.
If you know the critical pressure and critical temperature, there are other equations for calculating heat of vaporization that can be used.

 

[TangoCleveland]

Get yourself a copy of "Data Book on Hydrocarbons" by J.B. Maxwell. Excellent inexpensive source of properties for hydrocarbon fractions of all weights, from CH4 to vacuum bottoms. There are copies available at

http://www.amazon.com/gp/product/08...ref=sr_1_1/104-4315266-3904765?_encoding=UTF8

Larry

 

[Bourbon103]

Thanks for the reference, I went ahead and ordered the book.

I'm familiar with latent heat estimation using Watson's correlation, Clausius-Clapeyron Equation, and Chen's Rule but of course variables in those equations, e.g. critical properties and molecular weights, are typically difficult to find elsewhere other than specailized texts.

Thanks again, hopefully this text pans out.

 

[TangoCleveland]

Bourbon,
A wise purchase. The chapter on Thermal Properties has enthalpy charts for C1 through C8 alkanes, C2 through C4 alkenes, and petroluem fractions with mean av. boiling points from 200F through 800F at 100F intervals. Also has a chart of latent heat vs. vapor pressure for paraffin hydrocarbons with normal boiling points from 150F to 1200F.

Larry

 

[Latexman]

This reminds me of something an old boss used to say, "In God we trust, all others bring data".

Good luck,
Latexman

 

[rbcoulter]

Getting back to the original question.

1. #6 Fuel Oil would not normally be classified as a flammable liquid so wouldn't be under OSHA's PSM standard.

2. It isn't usually stored in a pressure vessel so API 520 wouldn't apply either.

 

[Bourbon103]

rbcoulter,

1.) The issue of the #6 fuel oil being in a PSM area isn't for the flammbility of the #6 oil, but for the other chemicals in the area. The concern would be for those other chemcials to cause an external fire and lead to the heating and subsiquent vaporization of the oil in the external oil heaters ("heat exchangers").

2.) You are correct. The oil is stored in an atmospheric vessel, but the relief valves in question are on (2) external oil pumps and the (2) heat exchangers that they lead to.

 

[rbcoulter]

I see. It seems to me that this would be treated as a liquid relief because the heat exchanger and pumps are completed flooded. Are the existing relief valves on these devices liquid relief valves? In that case, you would not need the heat of vaporization unless you are assuming two-phase flow per D.2.2. of API RP 520.

 

[Bourbon103]

The existing valves are rated for liquid relief. I was going to size the pump relief valves for their maximum capacity (as a function of maximum operating viscosity) and the exchanger relief valves for vaporizing liquid due to external fire. I assume that if a flooded vessel relieved during a fire it would first relieve liquid but then proceed to relieve mostly vapor so it would be safe to conservatively estimate relief to be 100% vaporizaed liquid (based on vaporized liquid relief, per API RP 520 Part 1).