5 replies to this topic

[Amit Sonpal]

Hi
I am doing max stretch capacity evaluation for SHTE. No problem thermally upto 150% capacity . But in vibration analysis (using HTRI), shell entrance and exit velocities are coming out to greater than critical velocities. Although no warning messages for bundle entrance/exit or tube bundle vibration. Also in one different case, HTRI is showing warning bcoz SHELL entrance RHO-V-SQ exceeds 5200 kg/m2-s by 10 kg/m2-s. Can nyone plz tell how bundle entrance/exit velocities are different from shell entrance/exit velocities? How vibration in this case can affect inlet/outlet nozzle? Is there anyway to reduce vibration for such cases ? I have alrdy been through TEMA guidelines ( not specific on vibration.. only tells about erosion velocitiy limit) and also exchanger already have impingement baffle...

Thank you
AS

[srfish]

In TEMA refer to the "Recommended good practice" section. The figures 4.62xx show the difference. The bundle entrance or exit area is for after the shell fluid has encountered the bundle and depends on the baffle spacing.

[Amit Sonpal]

Thank you for reply.
Bundle entrance/exit areas are fine but HTRI is showing shell exit velocity exceeds critical velocity. I am not able to find anything on exit velocity limitation for vibration. Can anyone pls recommend some literature or guidelines? Also flow induced vibrations should affect tube bundle only ? if bundle entrance/exit velocities are fine how can shell entrance/exit velocities contribute to tube vibrations?

Edited by Amit Sonpal, 07 February 2012 - 11:33 PM.

[srfish]

I can understand that bundle entrance/exit areas are fine but not that only the shell exit velocity is exceeding critical velocity. If the shell entrance velocity is OK, does it have an impingement plate?

One possibility where vibration can be created is when the outer tube limit is close to the shell ID.This packs the tubes right up under the nozzles. This can create some unusually high velocities because there are too small of bypass routes around the bundle. There are engineeng tips in the speadsheet by Art Montemayor and in www.gulleyassociates.com with titles:

Entrance and exit space for shell nozzles.
Effect of 1st tube rows on shell nozzle pressure drops.

[Amit Sonpal]

Thank you again. Let me explain in more details:
Actually it’s an existing STHE in field with impingement plate at shell entrance. I am evaluating max stretch capacity for exchanger without any hardware modifications. Upon increasing flow up to 150% of design, first warning in HTRI is, " Shell exit velocity is more than critical velocity, Vibration might occur.". All other checks like thermal adequacy (% overdesign), hydraulic adequacy (pressure drop), and vibration (Vortex shedding, fluid-elastic instability, bundle entrance/exit velocities) are fine. Now I am not sure Shell exit velocity alone (over critical velocity) can or can’t, cause trouble in plant at increased capacity.
(I have this doubt bcoz.. Critical velocity limit for vibration purposes not erosion, is for fluid-elastic instability check.. but at shell exit it should not be able to affect tube bundle ( no contact between ‘fluid leaving’ and tubes).. only concern should be erosion at outlet (nozzle) . I have checked for erosion which is not a problem at increased throughput. Should I ignore shell exit velocity which is more than critical velocity? )
I have attached vibration results. Any suggestion will be really helpful....
I have gone through above mentioned paragraph. Heat exchanger design is as per TEMA standards (Clearance etc.)

Attached Files

•  STHE vibration.xls   26.5KB   128 downloads

Edited by Amit Sonpal, 09 February 2012 - 07:25 AM.

[srfish]

HTRI says that several of the variables used in calculating the critical velocity for fluidelastic instability have uncertainity. Since the RHO-V-SQ is 3133 kg/m-s2 which is quite below the TEMA maximum I wouldn't be too concerned.

You might try backing off the 150% capacity to see about where the warning message disappears. If its just approximately 5% below the 150%, then there shoudn't be a problem.