pressure vessel saddle supports at angle

[1253524]

Dear Sir of Madam:

I have a question regarding pressure vessel saddle supports. This vessel will be angled at 15 degrees which will impose horizontal forces on the saddles. Total vessel weight is approx. 30,000 lbs. Both saddles will be of the same design with one saddle provided with slotted holes in the baseplate. Do you have any insight into the reaction forces and horizontal forces at these locations? total length of vessel is 240", distance between saddles is 112.31".

Thanks

 

[TomBarsh]

The forces acting on the saddles can be found from elementary 'statics', draw a free body diagram to solve the reaction forces. Use the usual Zick analysis to determine stresses in the shell. Check the saddles for bending in weak axis due to the force acting in direction of the vessel's axis.

 

[Shofa]

If you use software like PVElite or Compress, both of it will perform the required calculations for saddles.

 

[TomBarsh]

COMPRESS will perform the Zick analysis for the saddle, and performs structural calculations for the saddle. Structural calculations for saddle include the effect of wind, seismic, and thermal expansion. However, there is no option provided for the saddles that are either:

1) tilted vertically (to maintain perpendicular to vessel axis), or

2) vertical but then tilted to the non-horizontal vessel axis.

There are a couple work-arounds available for this condition. It's not uncommon to tilt the vessel a very small amount for drainage, but a 15 deg tilt seems extreme and should be considered in the design.



Tom Barsh
Codeware Technical Support

 

[LSThill]

1253524 (Mechanical)

Technical Note: you may have another option using Saddle Pro.:

Horizontal Vessels
Typically, horizontal vessels are designed using the methods presented by L.P. Zick. Although widely used, Zick’s methods do not provide means for a full analysis including all expected loads, both internal and external. The following are several instances where Zick’s methods are not appropriate for analysis:

• Axial loads (Internal splashing, earthquake loads, wind loads, expansion)
• Transverse loads (Internal splashing, earthquake loads, wind loads)
• Cyclic loadings for fatigue analysis
• Wear plate geometries beyond the scope of Zick’s recommendations (b +10t, etc)
• Evaluation of stresses for non-integral wear plate geometries.
• Saddle support member stresses.

The most significant shortcoming of Zick’s methods is the lack of design methods for axial or transverse external saddle loads. Axial loads are becoming increasingly important as seismic codes increase the loadings and more vessels are built for offshore applications. Until now, there has not been a simple and direct tool for the evaluation of stresses in the vessel and saddle support caused by external loads.

NozzlePRO eliminates the need to develop approximations for stresses caused by axial or transverse loads. These subjects have never been fully addressed due to the complexity of the interaction between the saddle and vessel. Most research and theoretical work has focused on the same problem addressed by Zick: stresses caused by dead loads and pressure. With only a few entries, NozzlePRO reduces a complex problem to a simple solution. From an engineering and design aspect, NozzlePRO will save time and money by eliminating the often confusing and inconsistent methods used to analyzed external loads on horizontal vessels. Instead of focusing on how to solve the problem of external loads, engineers can now spend time optimizing designs while gaining greater confidence in the end results.

The basic concept behind the NozzlePRO saddle models is that the engineer can fully capture all load conditions while only using one-half of the symmetric vessel in the analysis. The user should include self-weight, liquid head, and external loads applied to the saddle base plate. In addition, it is important to always check the hydraulic case (full of liquid, no internal pressure) since this will usually result in the greatest stresses.

In general, NozzlePRO results for primary plus secondary stresses (Pl+Pb+Q) will correlate best with Zick’s predictions for circumferential bending stress at the horn of the saddles. For most cases, NozzlePRO results will likely predict more conservative stresses than give by Zick. A discussion on the reasons is presented later.
At PRG, we are currently implementing several new features and conducting full scale testing to verify the models produced in NozzlePRO. Look for features such as bearing stresses in non-integral wear plates, internal and external stiffening rings, and full encirclement saddles in upcoming releases.

Reference ASME MEETING and Technical Paper. Houston 2004.

Regards

Steve