Design Temperature -In service repair /Modification

[deepu78]

In what circumstances design temperature of a piping system can be changed?
-In a operating plant ,the operating temperature of a particular pipe system (where alteration/Modification required) is max 300 deg C,above which there will be alarm/Trip.But the piping is designed to a higher temperature of 450 deg C.Can the design temperature changed to 300 deg c or less than 450 deg C for piping modification/alteration etc.This is for pipe stress analysis compliance.
DV

 

[LittleInch]

If this is to B 31.3, it uses the term "maximum metal temperature" for such analysis.

Therefore I believe that if there are alarms and trips to prevent the MMT exceeding 300C, then you could use that in the stress design and other temperature related issues such as material strength.

This needs to be documented in your MOC or design basis or similar.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[GD2]

To expand on LI, piping stress analysis is always based on the minimum and maximum metal temperature of the thermal cycle.
These temperatures are not related to operating/design pressures.
If the piping trips at 300C, them 300C is the max metal temperature.
In piping industry, it is also called 'Expansion Temperature'.

As the piping system was designed for 450C and now modify to 300C, you need to revalidate the piping stress for the new condition as parameters like S[sub]h[/sub], S[sub]E[/sub], S[sub]A[/sub] will all change.

GDD
Canada

 

[Heaviside1925]

OP,
If, there is an alarm to indicate this condition, it may not have anything to do with the piping design temperature. There may be an instrument, a gasket, a seal, a piece of equipment that is the limiting factor and not the piping itself. The reason for this condition should be documented somewhere, if not in the piping specification, it may be indicated through an MOC on the P&ID. Consider there is a reason that there is an alarm/trip on the piping, and you will need to determine that reason before proceeding with any modifications.

 

[Snickster]

Where did you see what the maximum system design temperature was? Was it the line list or project design basis? Or project specification? For large clients project specifications are standard company specification used on multiple projects so what is shown on the piping material specification may not be what the original piping was designed to and stress analysis done for. Usually the actual design pressure and temperature of the system is shown on the line list.

 

[r6155]

And the design pressure doesn't change?

Regards

 

[deepu78]

Design pr 54 bar
Operating pr 35 bar

 

[georgeverghese]

Design temp, strictly speaking, has 2 values; LDT(lower) and UDT(upper). Though you may find, in this case, that stress analysis requirements pass at some new higher operating temp less than UDT of 450degC, you've still got the LDT to accommodate.

 

[r6155]

AGAIN: And the design pressure doesn't change?
@ 450 °C operating pressure= ???
@ 300°C operating pressure= ???

Please be clear with the information you send us.

Regards

 

[deepu78]

Operating pressure is same for both temp

 

[NovaStark]

@deepu78

The design temperature is most likely based on a factor of the maximum operating temperature of the system. The piping specification would thus likely be based on components with thicknesses/materials to withstand at least 450 degC.

I would think that if you install something that is rated for 300 degC, it would now become the limiting component and thus technically 'de-rate' your system.

Of course, I am assuming that the initial stress analysis was done at 5340 kPa @ 450 degC.

 

[deepu78]

NovaStark (Mechanical)
How do you de-rate an existing system?
What all things to be considered?

 

[LittleInch]

deepu78,

The key issue to resolve is how you prevent the fluid from exceeding your new design temperature or pressure.

So you normally need a mini HAZOP or LOPA to see what controls and mitigations you have in place to prevent this event if things fail to work properly.

So e.g. if previously your incoming fluid was at 450C, but then went through a cooler which could fail so you saw 450C instead of 300, but now your incoming fluid is only say 300C, then you can't exceed 300 even if the cooler failed.

Or you've added a high integrity temperature trip system to SIL 2 or SIL 3. Or you've now set this trip temperature or pressure lower

Basically you need to look at the whole system not just one little bit to see if you can justify de-rating the design temperature or pressure.

It does seem quite a large gap between operating and design, but we have no idea about your system, its controls and safeguards as to why this happened.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

If you have a high temp trip setting, set the new design temp to say trip set + 10 degC. Check that the temp trip loop is a SIL 1 rated loop, and not a SIL a2 loop.

 

[LittleInch]

You also need though to make sure that when the system trips, the temperature can't keep increasing beyond your new design temp due to some sort of chemical or exothermic reaction even though the system has "tripped".

Or if the cooling system fails does the temperature keep going up after the trip.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[deepu78]