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Piping for Hydrogenation system

Piping for Hydrogenation system

Piping for Hydrogenation system

(OP)
As a chemical engineer, I am asking out of my area of expertise. I should add that I took a graduate course at UCLA in stress analysis.

I am a consultant at a small chemical company brought in to look over a new polymerization process.

Here's my issue.

I have a hydrogenation system operating at 250 C and 500 psig. This system consists of two carbon steel columns with catalyst heated with hot oil. The feed goes through two preheaters: a reactor (product/feed) preheater and a final electric heater prior to entering the first column. The piping is small: 1-inch (25 mm); 304SS is the material.

Oil piping consists of the following sections:

1.  Feed piping to the preheates (1, 2).
2.  Heated feed between the final electric heater and the column.
3.  Product piping from the column to the first (1) preheater.
4.  Cooled product piping from the first preheater (1).

The piping for item 1 (above) starts out at about 340 F (170 C). The piping for item 4 is about 356 F (180 C). Assuming the final temperature (for convenience), about 482 F (250 C) for items 2,3 yields some troubling conclusions.

The piping for items 1, 3,4 are schedule 10. The fittings are butt-welded, not socket-welded throughout the process. The fittings for item 2 are 600 psi ANSI. The fittings for item 1,3,4 are schedule 10.

Using an old reference, CASTI's guidebook on ASME B31.3, I ran a quick check on the pipe schedule. It is handy to use simplified methods in the field as a spot check. Here's what I got using allowable stress values from Perry's Chemical Engineering Handbook:

Pipe Schedule = 1000(P)/S(material)

P = maximum pressure, psi
S = allowable stress, psi

For 304SS, Perry's produces the following values for allowable stress:  17,000 psi (300 F); 16,000 psi (400 F); 14,000 psi (600 F).

Interpolating for 600 F and 400 F for 482 F for item 2, 3:

S = 16 - (82/200)(16-14) = 15,200 psi
P = 500 psig

Sch = 1000(500)/15,200 = 33???

Interpolating between 300, 400 F for item 1, 4:

S = 17 - (38/100)(17-16) = 16,600 psi
P = 550 psig (assuming dP for column, pipe, heaters)

Sch = 1000(550)/16,600 = 33

This means we should be using Schedule 40 for 1,2,3, 4 piping. Of course, I could be taking this generality too far --- it is 1-inch pipe (25 mm).

What do you think?

RE: Piping for Hydrogenation system

Looks more or less correct, but use a real hoop stress formula; not that sch = thing.

I would have used sch40 just for mechanical strength.  1" sch10 won't take much of any external force.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch
http://virtualpipeline.spaces.live.com/

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