One of the key process indicators for the performance of your dredge pump, is the capability to work with low suction pressure. The parameter involved is called ‘Required Net Positive Suction Head’. Which translates more or less to: ‘the head value at a specific point required to keep the fluid from cavitating.1’ Effectively, this is the extra pressure above the vapour pressure. From the pump inlet to the blade, there still is a pressure drop. And the geometry and the form of the blade influence this pressure drop. The operator will notice this as when the blade wears down, the pressure drop becomes greater and the required suction pressure goes up. Resulting in less performance and less production. Regular inspection of the pump will warn the operator of prospective deterioration.
Normally, the measurement of the NPSHr requires a valve in the suction pipe and a valve in the discharge pipe to control the flow. Every time you want a data point, you have to adjust both valves and iteratively return to the same flow conditions, albeit with a different suction pressure. This usually takes a lot of time and one hour per data point is not uncommon. Klaas Slager presented an alternative method at the CEDA Dredging Days2. His method is more suitable for testing the NPSHr as installed in a dredge. It does not involve the dredge valves and is quicker to execute. It is optimised to check if the NPSHr wanders off nominal and thus will yield an indication on the condition of the pump. If the internal pressure drop increases, there is less differential pressure available in the suction pipe for the dredging process. Less concentration or less capacity, or less in the combination of the two: less production.
Instead of varying the flow conditions, he proposes to vary the pump speed. This will influence both flow and suction pressure at the same time. However, by cleverly applying the affinity laws and presenting the operating conditions in a dimensionless scale, the cavition is immediately visible. A quick post processing will reveal any wandering of the NPSHr conditions. As this can be implemented in the PLC and executed during start-up every day, the operator will receive a daily update on the suction condition of his pump and can plan actions accordingly. This will prevent unnecessary delays and downtime.
This concludes my scheduled series of posts about the CEDA Dredging Days. There was much more to discover. The interactive session was fun. There were a couple of interesting presentations. And I’ve seen some innovations at the exhibition. So, I will write some more reports, although at a more leisurely pace of about once a week. Later on, the other promised topics will be covered3. I’ll keep you posted.