Cavitation – Discover Dredging

Graduation Of Alex De Rooij: Pumps Actually Fly Like An Unusual Airplane

Alex de Rooij receiving flowers from Suman Sapkota for his graduation

When you hire a carpenter, he repairs everything with a hammer. So, what happens when you ask an aeronautical student to solve some issues in a dredge pump? He models the pump as a badly behaving airplane. And with success, Alex de Rooij joined our company as a graduation student and recently graduated on the topic of ‘Numerical Study on NACA Profiles Sensitivity in Dredge Pump Impellers’.

The normal procedure for designing pumps is relatively straight forward. Set the performance specifications and try to hit that mark with the simulated behaviour from an iteratively improved design. This is well documented and I’ve been writing about this process before.1

Pump design workflow (inspired by Suman Sapkota)

One of the design parameters is the NPSHr. This is basically the amount of absolute pre-pressure the pump requires to operate.2 The system and operating conditions will result in a certain available pre-pressure: NPSHa. When the NPSHa drops below the NPSHr, the pump will experience cavitation at the low pressure side of the blade. The flow of the medium will be disturbed and the performance of the pump will abruptly crash. There is some warning. Operating near the NPSHr, there will be an incipient cavitation where the vapour bubbles start to form, but do not cause any issue. The imploding vapour bubbles may be detected audibly for the trained listener.3 Next will be a stage on the NPSHr, where the bubbles get larger and they loudly implode. At this stage, the pump sounds like it is tumbling nuts and bolts inside. These imploding bubbles will definitely cause damage to the impeller. At last, working below the NPSHr, the bubbles will become so large, they will block the passage between the blades. The result is an immediate drop in delivered head.

Explanation of NSPHr, NPSHa and cavitation

The flow of the medium through the impeller can be simulated in a CFD program. Specifically for impellers, you will need to model a rotating frame of reference. And that is the usual representation of the results. However, with mathematics being one of the most powerful tools invented by humanity, we can have an alternative view on these results. We can cut the impeller along a radial and stretch open the meridional passage and the blades to a row of foils. And that is exactly where our young aspiring engineer comes in. In literature, the blades in the impeller are modelled having a constant thickness. But, Alex has been investigating what the influence will be when we model the blades as foils. Selecting a proper profile makes the blade less sensitive to stalling.

Conversion of axial view to blade to blade view

Alex, thank you very much for your work here at Damen. We’ve learned the influence of certain profiles on the performance and geometry of the pumps. You have the right mindset to pass your time at the TU Delft and graduate successfully over there also. And whenever you have some days of the month left after you spend your allowance, know that we can give you a warm reception at our office.

Alex working at the reception desk at Damen Dredging Equipment

References

Sniffing Out The Details Of Dredge System Fittings

The mixture carrying system of a cutter suction dredge, is more than just a cutter and a pump in a pontoon. In the dredge system, there are many valves and fittings, that make the system work. One question I was asked, what these extra valves do. Actually, there are several valves, that are worth mentioning. The sniffer valve, the vacuum relief valve and the non-return valve. For working in the designed operating point, you don’t need them. But, to get there and back, they can be quite useful.

Dredge system layout and fitting locations

The first valve is affectionally called a sniffer valve. A more descriptive name would be ‘discharge line de aerator valve’. Usually, it consists of a floating ball in a cage with a seat at the top, that can be closed by the ball. Provided the ladder is already under water and starting up the dredge pump from a fresh situation, probably air is in the high onboard discharge pipe sections on deck. Behind the dredge, the line goes down again and the air is basically trapped, preventing the dredge pump from properly priming. The sniffer valve allows the air to escape and the water to enter the floating discharge pipe at the water line. Problem solved.

Arrangement and operation of a sniffer valve

On the other hand, when the discharge pressure falls, the ball floats down with the receding water level. This opens the top and allows air to enter the pipe again. Which is no problem as it can be expelled again through the same sniffer valve. When the water can flow away from the high section on board, this will break the water volume in the system. When opening the pump, only the small section between pump outlet and bulk head passage will fall in the pump well. Or, if properly executed: need to be drained and discharged.

A last function of the sniffer valve is in case there is a blockage of the suction pipe. There will be no new mixture flowing in, but the mixture in the discharge line still has a lot of momentum. For a 1 km, 500ø mm pipe, the mixture has the equivalent momentum of a 75 ton truck barrelling down the pipe at 80 km/h. You don’t stop that in an instant either. The mixture keeps flowing and draws a vacuum. The sniffer ball drops and allows air to enter the pipe.

A better way to prevent the vacuum, is to install a vacuum valve in the suction line. That will allow water in and enable you to clean the discharge line without a cavitating pump. Once the suction block is removed, the relief valve opens again and mixture can be inserted in the dredge line system.

In case there is a high discharge height, the mixture mass will not be broken by the sniffer valve. The geodetic pressure will close the sniffer and all of the mixture volume wants to return through the system out of the suction mouth; or open pump, swamping the dredge. To prevent this from happening, a non-return valve can be mounted in the onboard discharge line.

These fittings will cover most operational situations. There might be even more for exceptional situations, depending on the design choices by the manufacturer1 and to the taste of the owner.2 e.g. We provide a suction deaerating valve. Any ideas about such a provision?

References

Comment

04/03/2020, Mark:

I did receive the right comment about to the purpose of the suction deaerator valve. It is indeed for letting out the air trapped in the suction pipe when the ladder is being lowered. It could be argued that the air will also leave through the sniffer valve at the back of the dredge. If somehow, the air would have trouble escaping all the way to the back, the pump will be very slow in priming itself. Providing a deaerator on the local high at the bulkhead passage, the suction line can purge the air there and the pump starts quicker.

When does your pump suck?

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.

Worn down suction side of a dredge pump deteriorates NPSHr

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.

Tag: Cavitation

Graduation Of Alex De Rooij: Pumps Actually Fly Like An Unusual Airplane

References

See also

Sniffing Out The Details Of Dredge System Fittings

References

See also

Comment

When does your pump suck?

References

See also