Root cause analysis of vibrations and pulsations in a naphtha pipe system with centrifugal pumps

The capacity of a platform installation consisting of naphtha feed lines from centrifugal pumps to an oven has been increased by 25 % in combination with rerouting of the piping layout upstream of the oven and inside the convection zone. The operating company observed considerable increase of the vibration levels of the naphtha feed lines since the restart of the plant. The vibrations have resulted in breakage of the supports in vertical pipe sections on two out of four 6-inch naphtha feed lines. TNO has received the assignment to perform a root cause analysis of the vibration problems and provide remedies to reduce the vibrations to acceptable levels. The analyses includes on site pulsation and vibration measurements followed by a pulsation and mechanical response analysis and finally a check on pulsations and vibrations after the modification. A number of possible root causes are presented and analyzed in this paper. It is most likely that the low frequency vibrations are caused by slug flow in the naphtha feed piping or two-phase flow developed into the convection zone of the oven. A simulation of the naphtha system from the pumps up to the convection zone in the oven shows pressure pulsations up to 4% peak-to-peak of the line pressure, which causes pulsation-induced vibration forces up to 450 Newton peak-to-peak. Several modifications have been investigated to reduce pulsation levels and forces: • Omitting the sources of two-phase flow in piping to the convection zone of the oven • Install a pulsation damper at the inlet of the oven • Install orifice plates in combination with the pulsation damper • Improve the stiffness of the pipe supporting structure The paper shows that residual vibration levels are acceptable after modification of the piping support layout and that the installation can be operated over the entire operating envelope without problems.