29-12-2012, 05:50 PM
Dynamics of Pump affecting the Reliable Operation of Process Plant
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Abstract:
A Pump unit of Petrochemical Plant experienced higher vibrations
after few months’ operations from the commissioning of the plant, operating
with the Steam Turbine driven Gas Compressor, the most critical equipment of
the plant. The maintenance personnel had observed wear of the pump parts &
bearings. The clearances of pump bearings were modified by them thinking
that the rubbing of shaft was due to less clearance. On the other hand the
vibrations continued to show the same trend & did not yield any fruitful
results. The pump had to be checked after 3 ~ 4 months’ operations. However,
the other similar pump did not show such kind of a problem. The analysis
showed that both the pumps were put in operation due to generation of excess
quantity of steam condensate than the designed estimates. As, the suction of
the pump unit-A was near the baffle plate in the steam condenser, it was
causing the starvation, cavitations, inadequate dynamic suction head resulting
in the erratic vibrations. Ultimately, recirculation of the steam condensate was
recommended to maintain the constant dynamic suction head on the pumps,
and to remove the unstable vibrations. Thus frequent repairs, production loss
due to non-availability of the pumps resolved permanently.
Background
The Petrochemical plant under study uses a Syngas Compressor driven by extraction type
Steam Turbine, the most critical equipment of the plant. The Low Pressure Exhaust Steam is
cooled in the Shell & Tube type Condenser. Two vertical pit type pumps are installed for
recycling the steam condensate back in to the cycle. The capital equipment are supplied by
the Original Equipment Manufacturer (OEM) from the European countries. The plant has two
identical vertical type pit pumps “A” & “B” of same size & type from the same OEM;
Operational philosophy was to operate only one pump & to keep other one in standby mode.
The operational experience of pump unit-B was quite satisfactory since commissioning stage.
The generation of steam condensate was more than the designed estimates, causing to handle
more quantity of steam condensate, requiring operation of both the pumps A & B. Otherwise
accumulation of excess quantity of steam condensate in the condenser would cause a
shutdown of the whole plant. In such situation, restart of the plant from the cold conditions
can take many hours, loosing production and man-hours. Hence operation of these pumps
was directly affecting the reliable operation of the plant. The pump unit-A experienced high
vibrations after few months’ operation since commissioning of the plant. The maintenance
personnel had observed the wear of pump parts & centralizer bearings. As such this was a
routine kind of observation, and hence the clearances of the pump bearings were increased
thinking that the rubbing of the shaft was due to less clearance.
Operational History
a) Only the pump unit-A was experiencing high vibrations after few months’ operation
since commissioning of the plant. The maintenance personnel had seen the wear of
pump parts and centralizer bearings. The OEM never responded to the reports of the
plant personnel on such typical kind of problems experienced from commissioning.
b) Notwithstanding to the poor responses from OEM, the clearances of the bearings were
increased by the O & M personnel thinking that the rubbing of shaft was only due to
less clearance. On the contrary vibration levels continued to show the same pattern.
c) The pump had to be checked every after 3~4 months operation and the wear pattern
appeared to be of similar nature. However, the other pump unit-B had not shown such
kind of a problem. Hence, the flaw in the design of pump was straightway ruled out.
d) While studying further, it was observed that normally both pumps are required to be
operated due to generation of excess quantity of condensate than designed estimates.
e) The operators were keeping the lowest possible level of condensate in the condenser
for avoiding excess accumulation of steam condensate (generation was around 100
cubic meter/hour), and thereafter to avoid a shutdown of the plant.
f) The operation of pump unit-A, also observed that the suction of the pump was near
the baffle plate in shell and tube type steam condenser, was causing starvation and
inadequate dynamic suction head resulting in the erratic vibrations. The suction of the
pump unit-B was far off from the baffle plate, and it was on the opposite end side.
g) Trend of vibration and spectra are shown in the Figures-3 to 7 in the following pages.
Repair History
a) The pump unit-A had been exhibiting the higher vibration levels since its
commissioning, which was reported to the OEM well in time to rectify/correct.
b) The pump had undergone repairs/overhaul works before January 2006. The parts of
pump like shaft, bearings, guide bushes, wear rings etc. were replaced during the
repairs/ overhaul works. But the clearances of bush bearing were little over sized. The
vibration levels had decreased considerably after carrying out the repairs/overhaul
works, which were all well within the acceptable limit.
c) Again the pump had undergone repairs/overhaul works during first week of February
2006. The parts of pump like shaft, bearings, guide bushes, wear rings etc. were
replaced by during the repairs/overhaul works. The vibrations had decreased
considerably after the repairs/overhaul works, which were all within acceptable limit.
Operational Variations
a) According to the original operational philosophy considered during design stage it
was decided to operate only one pump & other pump would be in standby duty mode.
b) But, in fact, the operator had to operate both the pumps in parallel for handling the
excess quantity of steam condensate than the anticipated during the design stage. This
resulted in use of both the pumps in normal operation, causing real depletion of the
standby one. This was a serious problem in operating the plant in a reliable manner.
c) The OEM and Designing Agency had been informed time to time on this typical
problem, which had never come forward for studying the root cause or for suggesting
a viable solution with the necessary change in plant operations.
Operational Controls
During the test period from February 2006 to July 2006, it was observed that both the pumps
A & B were in operations. It was also observed that the suction of pump unit-A was nearer to
the baffle plate in the condenser, causing starvation or inadequate dynamic suction head,
resulting in to unstable or erratic type of vibrations. This was not same for the pump unit-B
Ultimately, recirculation of the steam condensate was recommended and had to be
established to maintain the adequate minimum level in the condenser, so that the Dynamic
Suction Head is maintained constant, for avoiding the unstable vibrations caused by hydraulic
effects. Significant reduction in the vibrations is evident in Figure-7, after overhaul &
restoration of the pump done in April 2006, and after raising the level of condensate in the
condenser / hot pot / hot well by establishing adequate recirculation on 8th July 2006.
Conclusions
Design, engineering & installation of the equipment may have some kind of variations or
differences within the similar kind of models, can exhibit quite different characteristics and
the operational problems. It is also possible due to the kind of variations or differences
existing in their associated or auxiliary components & systems.
Sometimes, the past experience on the similar kind of equipment may not be of any help or
much useful due to an inherent design flaw or the operational variations. Also, the designer or
OEM may not be of any help in resolving the problems due to lacking of their own
experience on the operations of their own products in a typical operational situation.
Therefore, the end user or operator must carry out a comprehensive study by integrating all
the available parameters & experiences, which can only provide a better solution or the most
viable solution in such kind of unusual problems.