Hydrostatic test pressure for external pressure

[FILGUEIRAS]

While designing a vessel with 2 independent chambers, which shall be tested separetely, a question arised:

How should I take in account the maximum allowable externa pressure in the intermediate head?

The MAWP for the chamber in the convex side is ( disregardig the head ) 1.81 MPa.
The EMAP for the head is 2.0 MPa.

Wich is the test pressure to be used? My question is, should I admit 1.3 * EMAP ( that means, 1.3 * MAWP ) in the convex side of a ellipsoidal head?

In other words, which is the safety factor in ASME for external pressure?

 

[vesselfab]

what i am assuming is that you have an intermediate head.

what we do is test the chamber that has pressure on the concave side first.

we then drain and dry that section and test the chamber at it's hydro pressure.

The head should have been designed to handle the external pressure (the chamber on the convex side) with no pressure on the concave side.

 

[FILGUEIRAS]

Vesselfab,

Yes, there's an intermediate head. I understand that I´ll test the chamber on the convex side with no pressure in the other.

The problem is, the vessel is already calculated ( not by me, I´m only reviewing the calculation ), and I found that the maximun pressure on convex side is less than 3 times the MAWP ( disconsidering the head ). I really don´t know if the test pressure can be defined as 3*EMAP, as it would be for internal pressure.

 

[doct9960]

FILGUEIRAS,

Read UG-99(e)

 

[vesselfab]

The test pressure should be 1.3 MAWP of the chamber being tested.

 

[CodeRef]

What is EMAP? I am not familiar with that term? I think that would clear up alot of confusion. Also you stated...

I really don´t know if the test pressure can be defined as 3*EMAP, as it would be for internal pressure

Where are you getting 3*EMAP? Which Code paragraph is that coming from? I can only guess....Does reading UG-99(e) help?


There are three kinds of people in this world; those who can count and those who can't.

 

[FILGUEIRAS]

CodeJackal:

EMAP should means extermal maximum allowed pressure, calculated per UG-33 a, for ellipsoidal heads. the * 3*EMAP * was a typo, I'd have written * 1.3*EMAP*, excuse me.

doct9960: UG-99 e) is reaaly to be used. My problem is that UG -99 a)says that the vessel shall be tested at 1.3 times the MAWP to be stamped in the vessel, and I am not confident to apply pressure greater than EMAP on the head, since I could not find in the code what is the safety allowance for external pressure.

What I'm going to do is to stamp the design pressure in vessel, as MAWP, then recalculate the head thickness so that it's maximum external pressure comes to be (design pressure/1.3), unless, of course, I learn where in the code is stated how big is the safety allowance in this case.

Thank you all.

 

[rhg]

Use one nameplate on each pressure chamber

see UG -116 (j)

regars

rhg

 

[rhg]

Use one nameplate on each pressure chamber

see UG -116 (j)

regards

rhg

 

[FILGUEIRAS]

Hello everyone,

you kindly heard my problem, so allow me to share with you my findings.

I'm using PVElite. the first point is that the design temperature is 200 C. Obviously, for hydrotest, I can lower it to 20 C, wich reduced a lot my problem, but not enough.
Then, I changed the head from ellipsoidal to torispherical, and the difference between allowable and required pressures vanished. The point is, ASME uses the required thickness for *internal pressure* ( UG-32 and 33 ). This is bigger for torispherial heads, so the allowable pressure turns to be slightly bigger, too.

Anyway, Code Case 2286-1 allows us to access what is the safety factor in a situation.

Thank you, again

 

[Shmulik]

UG-33(a)(1)(a): "For ellipsoidal and torispherical heads, the required thickness shall be the greater of the following: (a) the thickness computed by the procedure given in UG-32 for heads with pressure on the concave side using a design pressure 1.67 times the design pressure on the convex side".

To my opinion, test pressure on your concave side should be 1.81*1.67*1.3=3.93 Mpa.
Don't forget to take into your consideration the ratio (if any) between material allowable stresses at design temp. and test temp. UG-99(b).

 

[FILGUEIRAS]

Shmulik, you're right. I use this to calculate a convex head in that situation. The problem is, while reviewing calculations done by others, I found that the head was thinner than required - and the head, already formed. So, I went back to the code, to find out if the part was, or not, useable. The result is what I wrote before. I surely learned something on the subject.
Thank you for your comment

 

[Shmulik]

FILGUEIRAS,

Coming back to that former discussion…
I've found out I wasn't right……. Hmmmmm……..

It is correct that we have to calculate head's concave side with convex design press. * 1.67 in addition to the procedure given in UG-32 and produce acc. to the higher thickness we get.

BUT

Hydrostatic Test Pressure has to regard only to the MAWP to be marked on the vessel as it is stated at UG-99 (b). (1.81*1.3=2.353 Mpa)

I apologize for my misleading!

 

[FILGUEIRAS]

Shmulik,

This is a tricky problem, that refuses to go away

As I posted, my job was to review the calculations, and I made the mistake of assuming that the indicated MAWP was correct. So I took the long way, until I noticed that it was a matter of redefining the MAWP as equal to the design pressure, and applying 1.3 times that to the head, following UG-33.

The lesson I've learned is, if I'm going to review some design, I should review everything, not only do some spot calculations.

It was an interesting subject to study, but very time consuming. ( And I still don't know if I could apply 1.3 times the calculated pressure - by UG-33 - to the convex side of a head. But Code Case 2286-1 may answer this. )

Regards,

 

[Shmulik]

FILGUEIRAS,
Thanks for your comment.
I completely agree with your conclusion that checking the calculations is only the last station in your job consequence.
I assume you haven't got my point at my former post: UG-33 DOES NOT redefining your MAWP! The requirement of UG-33(a)(1)(a) is only additional requirement for HEAD (only) exposed to external press. to be designed to an assumed internal (concave) pressure that is 1.67 times the external press. Nothing is required to other pressure parts connected to that head, thus, it DOES NOT affect MAWP and test press.
Test press. should be 1.3 times MAWP marked on vessel's name plate, regardless internal head calculations.

Best regards!

 

[SnTMan]

I have designed any number of floating head heat exchangers that had a dish-only head welded to a ring. Sometimes the external (shell) pressure was the higher, sometimes not. These heads were designed as spherical heads using the external presure tables in Section II, Part D. For a number of years they were always hydro-tested at 1.5*MAWP. Then the tensile allowables were raised and further heads were designed the same way and tested at 1.3*MAWP. However, the external pressure charts WERE NOT CHANGED when the allowables were raised.

In no case was there ever a failure of any of these heads. Part of the reason is that the external pressure charts are temperature dependent from a relatively low temperature, whereas the tensiles are not, at least for CS materials. Part of the reason is the conservatism built into ASME Code. In fact as I related above, in this situation they are MORE conservative now than they were in the past.

This is not just my experience, there are thousands of these things out there from fabricators all over the world, and have been forever.

Regards,

Mike