Allowable equipment nozzle or connection loads 1

[ZHWIL28]

I would like to know how other stress engineers handle this kind of issues.

To model piping stress, being knowledge about the flexibility and maximum allowable loads on the piping connections is very important to get a real idea of pipe stresses and if reaction forces are ok. Approaches of modeling the connections on the equipments (fixed anchors, flexible anchors and integration of the structures) can largely affect the results. However, the approaches we can apply are pretty much limited to how much information we can get from equipment suppliers.

For vessels, it is a little bite easier to get the allowable loads and nozzle stiffness because there are plenty of software and publications which allow the manufacturers and us to calculate them. Even that, I often met the situations that the vessel manufacturers ask me to give them the maximum piping loads on the nozzles before I even know what the piping will look like. Some of Vessel manufactures simply can not supply these information and I have to get the vessel drawings to do my own calculations.

Fortunately, pumps, turbines and compressors typical have the allowable loads and their standards; however I know a lot of people do not bother to check.

More difficult situations are for the connections on other equipments such as heaters, coils, heat exchanges and other irregular connections on various equipments. The piping usually will be connected on the headers which have heating coils and structures tied to the headers. This has made extreme difficulty to calculate the allowable loads for the piping connections. What I did was trying to build the headers in the model and found about the details of the header supports. This means that we have to access the fabrication details which we usually do not have, and most time it did not make me confident or maybe create more difficulty about checking the integration of the header supports. Even if I found that there are large reaction forces on the connections, it is usually too late or impossible to ask the manufacturers to modify their connections (even changing to a higher class flange). Some experienced engineers just simply told me it had been done like that forever without problem OR PIPE WILL FAIL BEFORE IT DAMAGES THE EQUIPMENT since the equipment is typical seen as more rigid.

I am really looking forward to hearing about how industry or stress engineers handle these issues.

Thanks,
William Zhang, P. Eng.

 

[NozzleTwister]

There is a similar thread going on in COADE's CAESAR II forum. See the link below.

http://www.coade.com/ubbthreads/ubbthreads.php?ubb=showthreaded&Number=12279

NozzleTwister
Houston, Texas

 

[JohnBreen]

Hi Bill,

As you mention if there is a standard (API, ASME, NEMA, ANSI/Hydraulic Institute, etc., or if it is an ASME vessel, we have something to reference. If it is a piece of specialty equipment (non-standard or proprietary) the only appropriate source for allowable loadings is the OEM. Then of course we are at his mercy and we must assure that we do not exceed the maximum allowable loadings provided by him under penalty of having the guarantee voided.

Of course the problem is that in the design of the equipment at issue the design engineer will have PLANNED the path of the loading from the piping attachment to the ground or to the support structure. We will never have enough information from the OEM to properly follow the load path with our applied loadings and so objective and logical structural analysis is difficult or more likely impossible.

In this case, the only device at our disposal for assuring that the equipment will be robust enough to accommodate the loadings transferred to the equipment by the piping is to make the maximum allowable loadings part of the Request for Proposal for the procurement of the equipment. If the procurement documents set the required maximum allowable loadings, then it will be the responsibility of the OEM to assure and guarantee that the appropriate degree of robustness is built into the equipment such that the loadings will not challenge its structural integrity.

 

[ZHWIL28]

Thanks John's comments. I agree that it would be good for us to specify maximum allowable loads on the equipment. However, most equipment will be purchased by other departments or engineers even before we start to plan the piping. How could we assume the reasonable sets of forces and moments (at all directions of each connection)? I have seen some engineering companies have their internal standards for vessel nozzle allowable loads. For other type of equipments besides rotate equipments, has anyone gotten a reasonable assumption or general rules to specify these loads? We do not want to be too conservative and of course not too limited to our piping design. Just a few days ago, one supplier told me the allowable loads of 100 lbf for the equipments connected to 6" 500 deg F thermal oil pipe. It is not possible for me to meet such small and guessed values.

 

[stanier]

We have the classic real life "triangle" problem with a project. Time , cost and quality are at the points of the triangle. You are somewhere in the bounds of the triangle. Construction wants to save time and will build anything, Procurement wants to get it at the lowest cost and Design takes all the risks and wants quality so it will work.

If a designer wants acceptable nozzle loads on equipment make sure that the mechanical engineers specify minimum requirements in data sheets or specifications. You may need to provide them with some "typicals" loads. The suppliers will be culled to these who can meet the specification. If they are not it is at Procurement's peril!

The engineering supply market is suffering as much as design from inadequately trained and insufficient numbers of engineers. Therefore you need to be extremely diligent in this requirement. Reject data sheets, specifications or drawings without adequate nozzle loading information. If you don't it is your problem. Purchasing will soon get the message, especially if you get Construction to understand that the equipment will be late as the design isn't approved.

Many engineers bring problems on themselves by buckling under the pressure from project managers. Remember they are all bullies and NO, I cant design it as there is not enough information! The bully will retreat and pick on Procurement.

API 650 has an appendix with a useful for determining tank nozzle loads.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

http://www.waterhammer.bigblog.com.au

 

[JohnBreen]

Hello again Bill,

Read Geoff’s reply (above) again – he is right on target!

An engineering company usually gets themselves into this dilemma but not writing a “tight” enough specification and work scope. The specification is a legal document that must absolutely be part of all contracts awarded. Any bid by a manufacturer/vendor of equipment must (by force of the bid specification) include reasonable information setting the maximum allowable nozzle loadings. Any bid that does not include such information is (and should be evaluated as) unresponsive. Someone who is willing to “assume” in the design process is necessarily NOT an engineer.

It is an old and tired cliché, but there is some truth in it – “Any time you “ASSUME” there is a potential to make an “ASS out of “U” and “ME”. It is the responsibility of the engineer to base all his/her decisions on accurate information using the logic he/she was taught in university. Anything less than this is cheating the client/owner and it potentially undermines the integrity of the engineering profession.

The very fact that an engineering company has a STANDARD that sets acceptable piping loadings on vessels (and/or other strain sensitive equipment) implies that the engineering company has previously taken a stand on this issue. The management of that company MUST enforce their Standards or there is no purpose for having either Standards or management. This Standard for acceptable loadings on vessels must have (as a minimum) a table of nozzles sizes compared to vessel OD for various standard wall thicknesses and pressure/temperature classes. The Standard should be based upon the Codes required by the jurisdiction in which the system is to be built and there should be some design margin included (above the margin of the Code) to address the vagaries of construction. The engineering company MUST be willing to share with the equipment manufacturer/vendor and the client (owner) the basis of their rationally developed Standard. For the engineer to apply such a rational Standard would not be “an assumption”.

Engineering management MUST stand behind the design engineers. The client (owner) must stand behind the engineering management. If a manufacturer/vendor of equipment quotes an absurd maximum allowable nozzle loading for an item of equipment, engineering management must expediently contact the manufacturer/vendor and demand to see the calculations that support the loading limit. The client has hired the engineer to perform a design service and if the manufacturer/vendor impedes the engineer in delivering that service the client must immediately communicate this issue to the client for resolution. The laws of physics cannot be repealed and within a given space the engineer can only optimize the design within the laws of physics. If the equipment manufacturer/vendor cannot or will not provide rational design information (including reasonable maximum nozzle loadings), it is an indication that the equipment manufacturer/vendor has no confidence in the design of the equipment that they are proposing. In this case the client (owner) must look elsewhere for an equipment supplier.

With all due respect Bill, accept the fact that your company is being compensated by the client (owner) to provide a professional design service. You must immediately stop looking for “rules of thumb” to resolve this issue. “Rules of thumb” are just avenues to escape making rational engineering decisions (engineering decisions, calculations and analyses are ALWAYS based upon facts and credible information (data)). Such behavior cannot be judged by any objective client (owner) as professional engineering. You should get all your facts and design data together (review your designs and analyses and be SURE that the loadings are the lowest that you can reasonably obtain) and take these data to your engineering management and ask them to assist you to get the client/owner involved in forcing the equipment manufacturer/vendor to provide the required information (as an alternative, the client (owner) can submit your calculated loadings to the equipment manufacturer/vendor for review and approval). Again, if the equipment manufacturer/vendor balks, it indicates that he does not have confidence in his equipment design and the client (owner) should look elsewhere to procure the required equipment.

Regards, John.