Design Pressure- MAWP & limit load analysis

[KevinNZ]

We are working on a vessels design project where;
1.The MAWP has been calculated from body plate wall thickness (much higher than the design pressure.)
2. Nozzle design is not allowed to reduce the MAWP
3. Nozzle N1 external loads (at design pressure) have been provided.
4. N1 is complex shape and Div II FEA is being used

We proposed the design of N1 use MAWP but the others say use the design pressure, with the reasoning,

If you calculate by software analysis, Through the limit load analysis, the internal pressure at the maximum load step is calculated . At this time, the load multiplied by 2 / 3 can be used as the maximum allowable internal pressure(MAWP for nozzle N1) of the structure, you can calculate for MAWP N1, and N1 MAWP should be slightly larger than the MAWP on the drawings​

How does this work?

 

[LittleInch]

How does load x 2/3 = MAWP??

I just can't follow the logic here. The MAWP of the PV is one thing, but what's the design pressure of the rest of the system?



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

 

[DriveMeNuts]

If you have decided that the body plate wall shall govern the MAWP of the equipment, then set the required design pressure of all other components to the MAWP of the body plate wall, and then begin your design of those components from there.
From what I read, it looks like you are trying to design your equipment in reverse.

 

[KevinNZ]

I agree, what's the point of a vessel MAWP higher than the system design pressure. Already a few debates on this in this forum.

How could "limit-load analysis" (ASME VIII- 2 5.2.3) give a high MAWP when the design process starts with the Design Pressure? Does not seem logical.

 

[mk3223]

IMO, this is an arguable subject and one may define the MAWP of the equipment as wanted once acceptable by the parties.

 

[DriveMeNuts]

mk3223, did you study Shakespeare by any chance? Because I don't understand your comment. As far as I am aware, the definition of MAWP doesn't have anything to do with 'desire' or 'want'.

The maximum allowable working pressure may be assumed to be the same as the design pressure when calculations are not made to determine the maximum allowable working pressure.

Sometimes, a design based on the actual MAWP is done, so that if there are issues with the equipment, the additional margin provided by the actual MAWP may prove to be helpful in remedying the issue. However, the paper work and Engineering hours required to do this is often not considered worth the effort. After the placement of PO's, sometimes a customer will kindly ask for MAWP to be calculated in addition to the standard design pressure bases design. My company always pushes back on such a request, due to the additional time required.

Having the MAWP of the wall be at some number, and the Nozzle N1 having a slightly higher number, while the equipment design pressure is much lower just sounds like a confusing mess. It is as thought the equipment has two design pressures. I bet that the people behind all this, are nervous, indecisive types.

 

[KevinNZ]

Plot thickens

We have started FEA now. The MAWP and that has been calculated (by the client) from the agreed shell thickness with no margin for the hole we are cutting for N1 or nozzle loads. Even with large repads there is stress spill out in the main shell and with no margin we can not get the vessel to pass at MAWP even without the nozzle loads.

 

[DriveMeNuts]

You just add nozzle reinforcement until the MAWP of the combined cylinder-nozzle assembly is greater/equal to the MAWP of the cylinder alone.
You can get an estimate of the amount of material required by doing a Div 2 design by rule reinforcing calc.
Stress flow shouldn't have anything to do with it.
The only way that this can fail, is if the nozzle is massive d/D > 0.25.

 

[KevinNZ]

DriveMeNuts, thinks for your input
The nozzles on these vessels are large d/D >.30 and not round. Design by rule reinforcing calc is not appliable, hence the FEM modelling.
The large nozzle size and local reinforcing is stiffening one side of the vessel. Under pressure the rest of vessel is not stressing/deforming evenly.
The client is going to have to compromise on MAWP or shell plate thickness.