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In our sea water intake unit, the high flow sump pumps are designed and installed 10,000 m3/hr capacity each of pumps 6 numbers ( 5 running +1 standby) at 4.60 Kg/cm2g pressure. But after commissioning in 2009 we found the real demand is only 37,000m3/hr instead of 50,000 m3/hr of design. So we proposed to run only 4 pumps for the actual demand. We run only 4 pumps with throttle the discharge MOV to maintain the header pressure of 3.50 kg/cm2g and that situation, the pump discharge pressure is around 4.0 to 4.2 kg/cm2g. These pumps are not having any online monitoring instruments. The motor vibration taken for analysis with portable hand held vibration meters. Pump bearing temperature being measured through RTD and monitored in DCS. but for the suction bell area suction conditions, recirculation or cavitation are unable to monitor because of lack of technology ( as per vendor)
After 8 years of operation, we start do major overhauling the pump one by one. During first pump overhauling, we noticed severe erosion in suction bell and diffuser area and not much pettings or erosion in the impeller except discharge erosion marks ( Impeller made out of Duplex stainless steel). After overhaul of first two pumps we noticed huge variation in power consumption between overhauled pumps and other pumps.
We recently had two major breakdown with thrust bearing failure during pump scheduled change over period with different manner. During pump switching over 4 pumps are running and operator starts fifth pump and try to stop one pump. During this period, one of the pump which is running normal power and bearing temperature suddenly bearing temperature increased and touches the thrust pad and pump stopped. In the last two months there are two consequent failure of same nature and the operator unable to take any action to avoid this problem because no any instruments installed in the system.
We get confused that during switching over, respective two switching over pumps conditions are normal but the same time an another pump thrust bearing failed within no time before take any decision by operator. During the failure, we observed the power consumption and flow rates of all the pumps were fluctuating of and suddenly the third pump thrust bearing temperature increased from 60 to 125 within few seconds. The bearings are tilting pad babitted lined. The oil condition found normal ( through oil analysis).
If any one have similar problem on high flow open sump sea water intake system, please share your experiences and is there any instruments developed for the suction bell and suction well with detailed information of such instruments.