Burst Pressure of a Class 125 and a Class 250 cast iron fitting?

[smyth13]

I would like to know a "general" guideline for the burst pressure of a Class 125 and a Class 250 cast iron fitting? I have looked into ASME B16.4 and it does not mention what the burst pressure is, or what the assumed safety factor is. I am just looking into obtaining some information on the actual test pressure that these cast iron fittings fail. Also, any reference or resource would be appreciated. Thanks.

 

[MJCronin]

When a real piping system with ASME/ANSI flanges and flanged valves is pressurized to destruction, several things can happen:

--Most often, the flanges will deform and leak, preventing further increases in pressure. Bolts can break

--Someimes a piping weld or threaded joint can fail. This can be the weld to the flange

--Sometimes a valve can leak through the stem.

The burst failure of the flange itself is hard to make happen....

 

[smyth13]

MJCronin: Thanks for the reply. The reason I am asking for this specific information is that a cast iron fitting has failed in service for one of our clients. I have performed some "rough" hand calculations and am currently creating an FEA analysis in an effort to determine the "burst" pressure. Ideally I would like some specification indicating the safety factor utilized to determine the "Class" so that I can back-calculate an approximate bursting pressure or some testing data that displays the actual failure pressures.

We are currently performing a metallurgical analysis on the cast iron fitting to determine its material properties, etc. If no material issues/defects are determined to be the cause of failure then our client would like to obtain the possible failure pressure of a typical fitting so that they can diagnose any system issues that may be causing the over-pressure. Thanks.

 

[StoneCold]

I would guess that pipe stress or water hammer is more likely to break the flange than straight up clean over pressure. Just my two cents.

Regards
Stonecold

 

[VerneE]

Non-shock working pressures for Class 125 is 125 psi @ 350F & for Class 250 is 250 psi @ 400F. At 150F the pressures are 175 & 400 psi repectively.

Water hammer or other shock loads will shatter cast iron. My previous employer stopped using cast iron in all water and steam lines for that reason, only malleable iron or steel. I have seen a pipe fitter shatter a cast iron fitting with a hammer that he was removing/demolishing.

 

[smyth13]

Thanks for the input. I have been considering water hammer, etc. and for me the issue is not what is causing the failure pressure, but what the actual failure pressure is. I understand the working pressures for these fittings as you have described and perhaps this particular fitting did fail at a pressure in the vicinity of the working pressure.
Either way it looks like there is no specific standard describing what the actual cast iron fitting burst pressure is. I would assume that someone has done some testing to determine the actual pressure?

VerneE - Do you have any idea what the water hammer or shock failure pressures were for those cast iron fittings? Are there any papers or articles describing this type of testing on cast iron fittings?

 

[osoloco]

I suppose that there are lawyers involved, as in most instances we tend to discover the cause and prevent the incident from reoccurring either by curing the cause or "beefing up" the system. If there are law suits you will most likely be better off testing flanges in a similar environment. I am assuming that you do not know that exactly transpired prior to the failure.
good luck!!

 

[rconner]

What end type and size of fitting is involved. Is there a standard reference and date cast or otherwise marked on the fitting? What is the service etc.?

 

[stanweld]

Have you determined the cause of failure? Undetected casting flaws are often involved and we have had a number of such failures in service.

 

[btrueblood]

The current ASME B31.3 piping code would push you to ASME B&PV for a new design, and that would tell you to use a factor of safety of 5 or more on the burst pressure (hydrotest) for cast iron...but that may not have any bearing on the grandfathered iron fittings and what their real burst pressure limits might be.

 

[smyth13]

All of your assumptions are correct. Lawyers are involved and the exact circumstances that may have caused the failure are unknown. In addition, it is not known when this fitting was installed, etc. We have not yet determined the cause of failure from a metallurgical perspective, but from a cursory visual examination there does not appear to be any macroscopic porosity or other casting defect. Either way, it looks like lab testing may be in my future (assuming we can find a similar fitting)!

rconnor - The fitting is a 1 1/2" x 1 1/4" x 1" reducing tee. The fitting has no markings except for a "G" cast on the outside. This fitting is used in the water system of a commercial building.

 

[MJCronin]

I smell defective Chinese cast iron fittings !!!!

 

[rconner]

alas I am not familiar with practice, "The fitting has no markings except for a "G" cast on the outside"(any chance however the mark at least might be "GI", for gray cast iron??)