bowed horizontal vessel saddle baseplate

huangyhg

bowed horizontal vessel saddle baseplate
i have been asked to design a fix for saddles on a large horizontal vessel. the vessel capacity is 400m3 and it operates at approx 130c.
the saddles have plastic deformation and have bowed upwards in the middle such the saddle base plate at the centreline is at least 20mm clear of the foundation.
this obviously means the load from the vessel is now concentrated at the extreme ends of the saddle and the concrete foundation is being damaged at these locations.
there is also a loss of thermal compensation in the system as the bolts on the 'sliding' ends are also being damaged.
anyone have any thoughts on how the saddle base plate can be returned to specification?
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i would concentrate initially on how the plastic deformation has occurred, since it seems rare for a cyclical movement. as well, a plastic deformation  so apparent points either to the loads being passed in different way than thought to the foundations (or something was neglected at the time) or to some specific thermal incidence where things went wrong.
i think the more likely scenario is:
the upper part of the saddle becomes too hot (or simply takes unconsidered thermal load), tries to make longer the outfit but restrained by the extreme bolts, a rotation is induced. it the temperature remains constant this plastic or elastic deformation may appear to be stable.
be it stable or inestable, you will have to bracket the extreme thermal conditions to ensure you have reinforced foundations, anchors and steel structure to move between wanted parameters not passing more loads anywhere than wanted.
even though you correct the saddle it will happen again. i sense an incorrect design of saddles. at the lowest location of the cylindrical shell there is a bending moment. you need certain amount of metal within a limited distance close to the cylindrical shell. most cases a plate stiffener added along the saddle on each side within the weldability distance (as close as possible to the shell) to take the tension force. however you need to go through the calculation and the geometry to correct this.
hope it helps.
ibrahim demir
in the standard zick saddle design, there is a calculated outward force applied to the horns of the saddle.  you might check this force and see how it compares to the saddle structure that is present.
it would be informative to note if that 20mm clearance was at its maximum when the vessel was empty or full.
edsun10...a photo or sketch would be helpful as well.
hi guys, thank you all for your interest.
i believe the vessel was full and operational when this happened, i strongly suspect the vessel design temperature has been exceeded during operation.
i have attempted to upload a sketch, hope it works as i am just getting used to this site.   
bear in mind that by stiffening up the saddle then that will increase the restraint on the vessel thermal expansion and increase stresses in the vessel wall.   
corus
can you add anchors to connect the center of the saddle to the floor, and then slot the existing mounting points to keep the center located and allow the ends to expand?
-- mecheng2005
my thoughts too, me2005.
ba
i still believe that the bowing action comes with the internal pressure rather than the differantial thermal expansion. there are many sample applications, including water heated boiler drums with saddle which the differantial thermal expansion is a lot higher than this, does not have this kind of problem. i can see from your sketch that there is not a welded wear plate either. this is another lack of material in that area to take the tension forces. depending on the shell thickness and internal pressure saddle must have certain amount of metal within very limited distance to the shell, otherwise this will happen.
what do you think about the saddles with excessive heights? in this case the thermal expansion has to be the major problem area and every high pressure high temperature heat exchanger should fail, but i could not find any problem with excessive height of the saddles under the gravitational loads, internal pressure and thermal loads (in circumferantial direction only). i do not say there is no thermal effect, however it is negligable against the effect of the internal pressure.
therefore, i would suggest to check the saddle calculation against tension force created in that area.
regards,
ibrahim demir
i must correct my previous post not to mislead you. please read the following instead og the previous post:
i still believe that the bowing action comes with the total saddle vertical load rather than the differantial thermal expansion. there are many sample applications, including water heated boiler drums with saddle which the differantial thermal expansion is a lot higher than this, does not have this kind of problem. i can see from your sketch that there is not a welded wear plate either. this is another lack of material in that area to take the tension forces. depending on the shell thickness and total saddle vertical load saddle must have certain amount of metal within very limited distance to the shell, otherwise this will happen.
what do you think about the saddles with excessive heights? in this case the thermal expansion has to be the major problem area and every high pressure high temperature heat exchanger should fail, but i could not find any problem with excessive height of the saddles under the gravitational loads, internal pressure and thermal loads (in circumferantial direction only). i do not say there is no thermal effect, however it is negligable against the effect of the total saddle vertical load.
therefore, i would suggest to check the saddle calculation  and the required metal area against tension splitting force created in that area.
regards,
ibrahim demir