H2O Pump Troubleshooting

Hi I am Taif ( Mechanical graduate engineer)

I am performing a troubleshooting for an overhung type pump (water booster pumps), 3 pumps in parallel two running and one standby lets name them (A, B ,C)

Data Sheet Data:

pump A is old and attached to low energy motor (voltage = 415v, current = 293A at 100% load, motor power= 180KW ) , rated capacity = 187.5 m3/h, Differential head= 170.5 m, speed=2980 rpm

pump B & C are new with high energy pump for each (V= 6.6Kv, I= 18.6 A at 100% load, motor power= 180Kw), rated capacity =187.5 m3/h, Differential head= 170.5 m, speed= 2975 rpm

The water goes from surge tanks to those water boosters then to water injection pumps

pump A is facing an excessive vibration with mechanical seal leak and the lock nut of the impeller keeps breaking everytime it fails, we are suspecting a high back pressure, the problem is that I don't have the disharge head or pressure to validate this assumption but I found

Calculated or actual data:

for pump B the actual head is 213 m at flow rate of 175 m3/h, but from manual pump performance curve (at flow of 175 m3/h, H= 163.2m)

for pump A the actual head is 213 m(assumed to be like B because of common head in parallel) at flow rate of 128.3 m3/h, but from manual pump performance curve (at flow of 128.3 m3/h, H= 190m)

this shows an estimation that both pumps are operating at higher heads than their predicts points to a system restriction.

What do you think ? what else I should do to look into all perspectives ?