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We are operating vacuum gas oil hydrotreater unit after shutdown at 125 KBPSD. At 125 KBPSD we checked the vibration of Combined feed heater connected lines and tubes. Same we checked in product fractionator heater. But after 2 to 3 days it was observed that product fractionator heater pass 1 convection to radiation vibration is significant. We reduced the throughput to 121 KBPSd. But still vibration persists. Can anybody guide me about sudden increase in vibration?
 
Answers
07/12/2011 A: keith bowers, B and B Consulting, kebowers47@gmail.com
This suggestion is, of necessity, quite general in nature. Considerably more specific information is needed to focus on likely causes for a specific configuration and vibration. High frequency and low frequency vibrations have greatly different causes. Vibrations can be excited by internal fluid flow and/or external combustion gas flow. Mechanical inputs from rotating equipment or other connected piping also are common. The 'general' solution is either a) remove or change the frequency of excitation source, or b) change the resonate frequency of the vibrating component (by restraints,supports, arrangement, mass, wall thickness,etc. If flow maldistribution is present, hot spots may 'coke-up' and induce turbulence and excite the vibration in the more flexible (hotter) section,etc.
Since the vibration was not happening earlier, and began a few days after start-up---it seem likely a tube support (s) has loosened, allowing the vibration amplitude to increase.
IF the piping configuration was not changed from previous good operating history, it is also likely some object has lodged in the piping and is causing excessive turbulence and causing the excessive vibration.
'Eddy shedding vibration' is also a potential cause, but usually does not suddenly 'appear' in a STABLE mechanical system. IF a vibration dampening support failed however, eddy shedding can excite large amplitude and high energy vibration at a system resonate frequency.
In summary, SOMETHING CHANGED. WHAT?
There is a multitude of potential sources for the excitation energy, and likewise numerous 'resonate frequencies' inherent in a flexible piping system such as furnace radiant and convective tubing. It is not possible to tightly constrain the piping because of the very large thermal expansion which must be accommodated.