Reboilers may be supported from column or by making a independent structure. In both of the cases there is possibility to support the reboiler lugs directly on the structure or there may be requirement of spring hangers (bottom mounted). Sometimes Slot holes and PTFE/graphite sliding plates are required to reduce frictional effects. So this information need to be informed to equipment vendor. Location of the lugs to be fixed in such a manner with respect to nozzles, bellows and other accessories to avoid fouling etc.
If the reboiler is supported on spring supports (See attached figure Fig. 1) then following points need to be considered:
1)Spring should be designed for cold load condition
2)Cold/Hot Load Conditions should be same for all the springs
3)Spring should be designed keeping in consideration the empty weight of reboiler in case of standby and steam out case
4)Slotted Holes are not required on lug of reboiler in case of spring support
Fig. 1: Typical Reboiler arrangement supported on a Spring
5)Mirror polished SS plates/PTFE plates if required are to be provided by vendor
6)If the loads on nozzle are quite large because of the back force due to spring in WNC Case, the Lock Nut (Refer above figure) can be provided on spring to act as limit stop.
7)The required gap shall be input in Caesar as a gap on restraint applied at spring in (negetive Y) Direction .
8)Stress Engineer has to provide details of Lock Nut (See the figure attached below, Fig 2) and the loads on nut in Spring Datasheet.
Fig. 2: Typical lock nut arrangement
Load case preparation:
Load cases are to be prepared based on operation possibility. So one must consult with the process engineer to know all the possible load cases. Normal possibilities are mentioned below:
1. Operating temperature load cases (W+P1+T1)
2. Design temperature load cases (W+P1+T2)
3. Steam out temperature load cases (W+P1+T3)
4. Upset Condition load cases (W+P1+T4) : This condition may arise if fluid is flowing in one part of the reboiler and no fluid is flowing through the other. Normally during start up this type cases may appear. Sometimes it may happen that you have to first heat the reboiler shell before you can start the flow of process fluid through the tubes. Consultation must be done with process engineer while creating all such load cases.
5. Sustained load cases (W+P1)
6. Prepare all occasional and expansion load cases as per normal project procedure.
It is sometimes required that two or three reboilers of similar type are connected to the same column. In this case some reboiler may be in stand-by condition. So that situation is also required to be interpreted while making load cases. So additional load cases need to be added accordingly.
Few Important Notes:
Reboiler column circuit being one of the critical systems from stress analysis point of view one should take extreme care while analysing such systems. Following notes will help the stress engineer to take an engineering judgement:
1)All of the junctions between shell & nozzle can be considered as reinforced after confirming from Mechanical Department
2)If nozzle loads exceed the allowable, loads should be forwarded to mechanical for further evaluation. Alternatively stress engineer has to perform WRC or FEA to qualify the nozzle loads. It is better to forward the WRC/FEA output results to Mechanical for their review.
3)Nozzle loads should be checked thoroughly with spring setting when reboiler is standby during steam out.
4)Flange leakage evaluation should be performed as per client requirement and rating of flanges should be changed if required, and the same information should be forwarded to Mechanical and Process Team.
5)Initially the Nozzle loads to be forwarded to mechanical as three times the project specific pressure vessel loads in Caesar to take care the the upset conditions.
6) The requirement of Spring supports, PTFE/SS plates, Slot holes, Lock nut arrangement etc are to be marked in Engineering drawing for incorporation to final vendor drawing.
7) Location of lug to be fixed in such a manner with respect to nozzles, bellows and other accessories to avoid fouling etc.
8) Fouling of Reboiler Nozzle (Shell Outlet) with structure should be checked.