Greetings... Let me give complete detail of the system. There are four multistage, backpull type, centrifugal pumps, two of which are high capacity type (35kg/cm2 pressure and 210 cu.m/hr flow) and other two are low capacity ( 18kg/cm2 pressure and 29 cu.m/hr flow). The pumps are handling short residue or Vacuum residue from bottom of vacuum column. Depending on quantity of residue generated pumps will be operational. Typically the fluid temperature is at 350°C. Never high capacity pump will be operated in parallel to small capacity pump. Whenever flow is beyond 29 cu.m/hr, high capacity pump will be operated with minimum circulation flow control valve open to safeguard the pump. The pump that has been discussed in the problem are both high capacity pumps
The problem of drop in discharge pressure is random in nature with no connection bottom temperature, level in the column, downstream process conditions.
The unit is barely two years old and the pumps have not been checked so far at full load as feed was not available.
No change in heater outlet temperature, flash zone temperature of column, stripping steam flow, column bottom temperature are observed during the start of problem
Quench is mixed in the last tray and the bottom temperature is cascaded to quench flow. Temperature is measured in the column bottom line.
Pump suction pressure is around 350mmHg when measured at 1m elevation from pump eye. NPSH available as per datasheet is 4.1m, elevation diffference between column bottom to pump is around 7m. Suction line pressure drop has been calculated and found minimum.
There has been vibration in the discharge line, seem to be originating from pump when pump is losing head.
We are yet to open the pump for checking.
Individual suction of pumps are manual operated valves. But common MOV at column bottom is open and kept locked.
Minimum circulation through the control valves.
Vacuum residue pumped from these pumps passes through a series of exchangers and through flow control valve goes to battery limit. At battery limit there are two destinations, one is with back pressure controller to tank and other is with flow control either to internal fuel oil or coker unit.
There are five level indicators at column bottom three of which are DP type and two are Remote seal type.
Hope I have answered all the querries.
Thank you
Sudheer