Mandar,
The use of operating or design ratings is a philosophical thing. It all
depends on how you believe the system will be treated when you leave it. I
have experience on the oil patch where when things went wrong, the
operators raised pressures all the way to DESIGN because as they saw it ,
THAT is what the system was designed for. Wouldn't you?. My opinion is
that the design rating is just that. It is the rating you design the
system for so I always plug for using design and not operating conditions
in design. I don't always win out mind you. I know this is a hard one
and you are not the first to face the problem. It is a philosophical
problem you have to sort out internally. Ask yourself, why do you have
two figures, "design" and "operating". Is the design figure there for
"occasional "loads in which case a higher stress value can be used. If
the system is such that it can never operate higher than the operating
pressure why not rate the design pressure the same as the operating
pressure? I honestly don't have the answer of this one for you because
the choice of these ratings and the terms is something you have to sort
out with your company and I know only too well how much of an agony that
is.
In the end you MUST ere on the side of SAFETY and if necessary install
flanges with a higher pressure rating. If I can not prove the strength of
the flanges under external loads under the worst operating conditions
using respectable techniques I consider it necessary to bite the bullet
and insist on either loosening the pipework to reduce external loads or
installing a higher rated flange.
Regards,
Gordon Reddek
Gordon Reddek
Specialist Mechanical Engineer
Alcan Engineering, Level 3, 443 Queen St, Brisbane, Qld 4001, Australia.
Tel: +61 7 3328 6424
Fax: +61 7 3328 6990
Email: gordon.reddek@alcan.com
mapte@technip.com
26/07/2004 12:19 AM
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Subject
[PipingDesign] Allowable external forces on Wellhead christmas tree
equipment.
Gordon,
Thanx for the reply.
Just one doubt,
What pressure to use to calculate the external loads? Design or
operarting?
We use design pressure/temperature to do stress analysis. The bending moment using design pressure is high and obiviously Peq is also high. As a result in almost all cases the Popr+Peq comes out to be higher than rated pressure.
Have a nice day.
-Mandar.
This message comes from Mandar's desk.
Gordon.Reddek@Alcan.com on 21-07-2004 02:46:22 AM
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Subject: Re: [PipingDesign] Allowable external forces on Wellhead
christmas treeequipment.
Filing Date:25-07-2004
Category: Sangrahya
Maple,
This is a standard problem and not well resolved in the industry. A very
safe way to handle this one is to assume that the flange can take only the
rated pressure. If you are operating below the rated pressure of the
flange then you can use the approach I have listed below where the excess
pressure between the design pressure and the flange rated pressure is
converted into acceptable loading on the flange. If you are operating the
flange a the full rated pressure then I am afraid you are in deep dudu
because you do not have excess pressure to convert to applied loads. You
then have the following messy options:
1) calculate the flange loads using CAESAR, AutoPipe or another
respectable program, then check the design of the flange to one of the
pressure vessel codes like ASME VIII Appendix 2.
2) Carry out a finite element analysis.
Now I will put in a free plug. The easy way out of 1) and 2) above is to purchase the program called AxiPro at www.paulin.com. Check it out on the web.
Quote:
The flange external loads are converted to an equivalent pressure and
added to the operating pressure to calculate the total flange pressure.
The equivalent pressure for external moment and force can be calculated
according to B31.7, Nuclear piping code Par 1-704.5(a), as follows:
Peq=(16M/(pi,G^3)) + (4F/(pi.G^2))
where:
M = External bending moment F = External axial force (tension only) G = Diameter at location of gasket load reactionEffective flange pressure = Poperating + Peq
Unquote.
Regards,
Gordon Reddek
Gordon Reddek
Specialist Mechanical Engineer
Alcan Engineering, Level 3, 443 Queen St, Brisbane, Qld 4001, Australia.
Tel: +61 7 3328 6424
Fax: +61 7 3328 6990
Email: gordon.reddek@alcan.com
mapte@technip.com
20/07/2004 04:55 PM
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pipingdesign@yahoogroups.com
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Subject
[PipingDesign] Allowable external forces on Wellhead christmas tree
equipment.
Hi,
We are working for an onshore oil extraction project, including a number
of
well head christmas trees and associated piping.
The question is as follows:
What are the allowable external forces (due to connected piping) on the
flowing wing valve of the well head christmas tree equipment.
We looked in to API 6A, but it clearly states that "The effects of
external
loads (i.e. bending moments, tensions, etc.) on the assembly of components
are not within the scope of this document".
API 6AF (Technical report on the capabilities of API flanges under
combinations of load) and API 6AF1( Technical report on temperature
derating on API flanges under combinations of loading) talk about load
capacity of API flanges in relation with the bore pressure. (No mention of
external loading.)
Anybody who has faced a similar problem? What values are used for
allowable
loads? Is there any other standard / code that covers this topic?
Have a nice day.
-Mandar.
This message comes from Mandar's desk.
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[Non-text portions of this message have been removed] Received on Sun Jul 25 19:58:00 2004
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