I've seen several specifications for the allowable loads on skid edge flanges (pipe connections not nozzles on equipment). The allowable load varies with the pipe diameter and flange rating. Usually the forces in the plane of the flange (vertical and horizontal) are the same amplitude as well as the moments about the same axes.

Does anyone know how these allowable loads have been derived and what assumptions have been made to calculate these loads?

Are these loads meant to prevent leakage at the flange or are they somehow used as a boundary condition for the piping model where the responsibility for the piping design changes from one party to another?

Dear Kelly,

I do not know if ther is standard method doing it. There might be one, we see you talk about the allowable loads.

If I understand correctly there are several think you need to consider ( My undertanding is this. There is a pipe hole on each side of the skid beam flanges. The pipe inserted in those leaving 100-150 mm extension on the ends outside the skid. The pipe is welded onto the web circumferentially and close the ends by welding blank flanges ) :
1. These pipe extensions are to be used in the lifting, therefore you need to calculate a safety factor for lifting ( in Australia they used to call it hoisting factor which limits the lifting speed and considers some dynamic actions during lifting ).
2. You need to look at the equipment to be attached to lift onto the pipe. The contact pressure stresses on the contact surfaces have to be limited. This may involve in the selection of diameter of pipe and lifting tackle attached.
3. The bending moment on the pipe has to be calculated to verify the pipe is adequate.
4. The pipe is to be checked against the shear forces introduced on the beam.
5. If the lifting is done without considering a spreader beam you need to consider an axial compression force on the beam design in addition to bending.

The other option ( you may be after this )is using the similar to the above . However you may use a slip-on or welding neck flanges attached to the end of the pipe beam.
The lifting tackle to be attached onto the flange can be a blank flange with an extension for the shackle connection.
You may use half of the the factorised lifting load on each flange connections as a shear load. You may consider that this load may be taken by the shear of the bolts or by the friction of the tensioned bolts. Therefore the number of the bolts, their diameters, materials will play role on the selection of the pipe diameter agaists the lifting load.

I hope this helps. Please do not forget applying the dynamic hoisting factor on the lifting load. Additionally, you need to consult to a local lifting equipment expert on this factor required by local rules.

Ibrahim Demir

Let me clarify. The case I am referring to is the allowable load at the pipe flange or skid edge connection. The pipe is not used for lifting the skid. The pipe is part of the equipment on the skid, such as a inlet suction line or final discharge line for a pump mounted on the skid. There is a pipe flange at the skid edge to connect the equipment on the skid to the piping at the plant or wellsite or other facitilty. Typically the pipe is either clamped or has guides or shoes to support the piping to the skid. The pipe is not welded to the skid beam.

We often see specifications of the allowable forces and moments that may be imposed at the skid edge connection due to thermal expansion, pressure, dead weight, etc. I don't understand how this allowable force and moment is derived when there is not understanding of how the piping system will respond due the thermal expansion, pressure or other loads.

Dear KellyE,

Again, there is a fundamental issue here. I believe, the allowable forces and momets came from the tests and stastistics. If you look at the NEMA SM23 for turbine casing, API610 for pumps and other pump manufacturer catalogues they specify those to protect the equipment. Some of them are trying to protect the equipment shaft against unwanted deflection, leak and failure some of them are for the foundation connection if there is no rotating equipment involved.
So, if you are a manufacturer you need to think about your equipment, load path to the foundation and the connection onto the piping as well. There is no straight answer.

If there is I would like to know.

Regards,

Ibrahim Demir

These "boundary conditions" are often uncertain. It is wise for the purchaser and seller of the equipment (here the entire skid) to agree on what may actually be arbitrary load limits - the skid "promises" to accept the load and the connecting pipe "promises" not to exceed these loads. It's more design by rule than design by analysis. The NEMA and API limits follow this approach. I have also seen a document out of Australia where vessel nozzle load limits are set based on flange size and class.

So there may be no design limit but instead an agreed limit. It saves paperwork, analysis, and the equipment too. BUT, for this approach to succeed these limits must be agreeed upon before the work begins.

Ok. If the purchaser and seller agree on a load limit that is reasonable but I don't think it is of much value or technical merit. The seller of the equipment should do a complete pipe flexibility model of his piping and the piping outside his scope of supply and then determine if the design is acceptable. Using some arbitrary forces and moments does not any technical merit.

I guess if you agree on some arbitrary forces and moments, how do you do the analysis fo the seller's equipment package. Do you assume all possible combination of postive and negative forces and moments to qualify the seller's piping? Seems like a lot of unnecessary work.

Dear Kelly,

Dave has a point on the arbitary force limitation that you can not ignore.

In case the equipment vendor and pipe designer are the same company or person, the entire system can be modeled and solved in once.

However the equipments are standard for the purpose and mostly are the weak points in the piping analyses. The frame plate heat exchangers and sootblower connection flanges are the most delicate equipment I have ever seen in my piping career. The Plate Heat Exchanger pipings are very large comparing with the size of the equipment. Most cases the foundation connections are governing the allowable forces and moments on the piping connections. Therefore, the equipment supplier has to give the allowable forces and moments at flange connections by an fundamental calculations. This may include a FEA.
In the sootblower steam piping connection on boilers case the flange acceptable loadings are almost zero. The flanges are capable to take low shear force and overturning moment in one direction only. The acceptable loads in the other directions are either zero or close to zero. Therefore the piping has to be very flexible to take the thermal loads.

I understand it is a painful process, however, mostly essential. If you analyse the similar equipment once or twice for different jobs you may get proper answer for the following jobs.

In the different applications, the equipment supplier ( which their equipment includes piping at terminal point ) and piping designer agree on the anchor at the therminal point. However the final anchor load has to be taken into consideration by the anchor designer.

Some cases the equipment is small and part of the piping. Therefore they have to provide the full stiffness of the piping essentially if there is nothing inside to protect.

In case you are the equipment vendor you need to provide the acceptable forces and moments to the piping designer.

If I was an equipment buyer I would definitely see the acceptable forces and moments on the equipment in addition to the functionality and the cost.
If you are the only supplier of the equipment, you normally drive the acceptable forces and moments similar to sootblower vendors.

We do not know what exactly you are working on. I believe, the explonation above and here are sufficient for most of piping applications.

Regards,

Ibrahim Demir

There's a discussion about this subject on Engineering Tips - take a look at

http://www.eng-tips.com/viewthread.cfm?qid=129476

"In the different applications, the equipment supplier ( which their equipment includes piping at terminal point ) and piping designer agree on the anchor at the therminal point. However the final anchor load has to be taken into consideration by the anchor designer.

Some cases the equipment is small and part of the piping. Therefore they have to provide the full stiffness of the piping essentially if there is nothing inside to protect. "

I have two questions about I Demir's comment and need help/clarification from members of our forum.
1) For a small or part of piping at skid edge, how can I calculated "the full stiffness". Can Caesar II do that?
2) For skid edge tie-in point(boundary condition). What kind of restraint should I use to get a conservative results on pipe stress and pipe support design which can be acceptable for both side of skid(client and packager). If I assume it is an anchor at tie-in point(but I don't really add an physical anchor and just treat the client piping as an anchor of my piping). I will assume it will give me the most conservative results on the piping design and restraint loading.

For tie-in point allowable load, some companies use the same vessel nozzle allowable load at these skid edge flange connections. Can anyone comment on this practice?

Thanks.

Glenn