my question is with regards to checking the loads of the nozzles of a compressor. the data from the vendor hasn't been in yet, so I have to sort of "fumble" my way out.
the situation is, there were 2 nozzles on the LP stage of the compressor, now they are going to merge it with an extra stage (LLP-low low pressure) and replace the old barrel with a new one housing 4 nozzles (2 for LP and other 2 for LLP). I have got the old stress calculation and I see that the loads used for nozzle load checking Nema style are derived from EXP. case. I know that the SUS loads on the nozzles of a compressor should be almost zero but, I wonder if anyone could confirm this method?
my other question is if anyone knows what the base point of nozzle displacement calculation is. can I simply calculate the expansion based on the resolution point and used as nozzle displacement?

First of all, it is good engineering practice that the compressor nozzle loadings should fall under NEMA or API limits for BOTH the cold sustained case and the operating case. I've seen a lot of engineers neglect the cold sustained case because the code does not explicitly say if it should be checked for both sustained and operating. But think about it, a lot of weight in the sustained case could cause deformation of the casing just as easily as the operating case. Compressor don't heat up instantaneously.

Second, whoever checked the nozzle loads in the EXP case made a severe mistake. This is WRONG. You should check it for the OPE case. The EXP case is just substracting the effects of temperature to check against the code's allowable expansion stress range. So when you are checking for support loadings and equipment loadings you should always refer to the OPE case and the SUS case.

Third, as far as the nozzle displacements go. I would look at the compressor itself and determine how it is attached to the foundation/skid. I can provide some guidance but it really depends what type of compressor you have.

Hope this helps.

Dear Ltorrado,

Thanks for your reply, I agree with you when you said that the loads should be checked in OPE case but, considering the fact that the nozzles should be load-free during installation, the only effective element remaining is temperature. So, although it was the first time I saw such a thing, it did not seem completely wrong to me.
Unfortunately there is no final drawings of the compressor available however, your third note brought up one huge doubt for me; if we could calculate the nozzle displacements of a compressor, why would we bother keep asking it from the vendor at all? the compressor is a centrifugal and the nozzle sizes are 8,10,8 and 6.Could you provide me with more guidance in this regard?

Thanks

You will never have a nozzle "load-free" during installation. A lot of vendors that don't know what they are doing put a note in their equipment drawings/documents that states: "Zero loads should be applied to the equipment nozzles". That's just their way of covering their ass. That is ignorant since it is impossible to provide absolutely zero loads at the nozzle connections.

And really your argument is irrelevant because, like I said, EXP case loads are not real (so it is completely wrong). Again, the loads should pass for both the SUS and OPE cases, individually.

I said you can calculate the displacements of the compressor because I thought it was an existing unit. If you are communicating with the vendor on a new unit, then you could request the displacements although some people will probably not be able to do this. You can try though.

You can also request they provide allowable nozzle loads because sometimes some vendors allow increases from NEMA or API limits.

Oh, but this forum so needs a way to add upvotes for peoples posts. +1 for Ltorrado.

Hi sir,

For conservatism you may model compressor,like pump modeling,placing anchor towards the motor end and connect your nozzles to it as rigid elements along cg.

You will get the thermal displacement about your nozzles.But all the nozzles must be modelled in single Package.

and then start your analysis checking all OPE and SUS cases.
Design condition may not be checked for loads but for stress range need to be.

Comparing the load cases as per 4 times NEMA or 2 Times API617 as standard practice.

for load free during installation you need to check WNC case of nozzle, during that condition nozzle shall not exceed movement in any direction beyong 1 mm.