Chemical Engineering Interview Question:

How can I evaluate the thermal relief requirements for double block-in of 98% sulfuric acid?

Submitted by: Administrator

API RP520 gives equations to calculate relief requirements. For thermal relief, a very simple formula requires the heat input and the coefficient of thermal expansion of the liquid. The heat input could be a problem. If you are concerned about sulfuric in a line that is part of a heat exchanger system, then the heat is simply the design capacity of the heat exchanger. If it were a pipeline in the sun, then you would have to calculate the amount of heat that the sun can put into the pipe. You can get the coefficient of thermal expansion from your supplier or any book on sulfuric. You can also calculate it by taking the specific gravity at two different temperatures and divide the SG difference by the temperature difference. Coefficient of expansion has the units of 1/0F. Now for the easy part, if you are at all concerned, just put in a 3/4" x 1" thermal relief valve and do not worry about doing any calculations. However, I do not believe sulfuric has any problems in pipelines unless it is a very long one and directly in the sun. In addition, I would make it a standard procedure to drain the line if it will sit dead headed for any significant period. Just a small bleed will be enough.
Submitted by: Administrator

One common estimation for the coefficient of thermal expansion is a = RHO avg x (1/RHO @ T2 - 1/RHO @T1) / (T2 - T1) where RHO = density a = coefficient of thermal expansion the simplified equation I stated in my previous response was not very correct. In addition, try to get a hold of the following article: "Decide Whether to Use Thermal Relief Valves", by F. Bravo & B. D. Beatty, Chemical Engineering Progress, December 1993, pg. 35-38. Also, see the Letters to the Editor concerning that article in the April 1994 issue of Chemical Engineering Progress. The authors use sulfuric acid (concentration not mentioned) at 86 0F in a pipeline situated near Birmingham, Ala. sitting in the sun heating up to about 109 0F as an example. Again, a 3/4" x 1" relief valve is all that you would need. Better would be slightly to bleed the line as a standard procedure.
Submitted by: Administrator