When does your pump suck?

Regular pump inspection

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.

Test arrangement NPSHr

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.

NPSHr measurement processing

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.


  1. NPSH
  2. Presentation Klaas Slager
  3. Discover Dredging: A new personal website for dredging enthusiasts

See also

CEDA Dredging Days

CEDA DMC Checklist to start thinking

CEDA Checklist logo (Credit: CEDA 2017)

Oops!…I did it again. Nobody ever made money exclaiming that phrase. Unless you are a famous pop idol. If only you could be that wise and make mistakes only once. Even better: don’t make the mistake in the first place. But who would be so wise to never make a mistake? Well I am not one of those people. But I do try to prevent other people from making the same mistake again. So, that is why I participated in the CEDA Dredging Management Commission.

In the commission we are discussing how we can help people in the dredging industry to efficiently and safely manage dredging projects. One of the items that came up was a list with all the failures and problems in managing a dredging project. This could at least categorise the problems. It also provided a framework to advise people. i.e. It became the CEDA’s Checklist for Successful Dredging Management1. Today, this checklist was presented by Kathleen de Wit at the CEDA Dredging Days2.

CEDA’s checklist for successful dredging management (Credit: CEDA 2017)

The checklist tries to capture various problems, that mostly have multiple aspects. This is immediately the problem of making a comprehensive list. It was decided, to discern problems to topics involved and stages in the project, with an addition of affected parties. It is a long list and let this not intimidate you. Mostly, you will just check out the section you are working on. Just be sure, there is somebody in the project, that checks, that everybody checked their part.

The checklist is not intended to be another contract template. There are enough examples of those. e.g. FIDIC, NEC. It might be more of a starting point for a risk assessment. Or an initial resource planning estimate. Eventually you will have to do the work yourself, this list will only make you start thinking about everything that could go wrong in your project. You will have to provide the solution for it. And that is where your expertise and creativity comes in.

When I was a young innocent and inexperienced graduate student, I had an assignment on an auger dredge. The dredge was not performing well and the operator, contractor and the project owner were battling each other over the contract. It turned out, they had chosen a contract, that had no incentive for the operator to have a high production and the project owner had no way to put a penalty on the delay. No wonder the management of the project was underperforming with the dredge. If only they could have consulted such a checklist, cost and time overruns would have been minimised. On the other hand, I wouldn’t have had such a great graduation project.

Auger dredge ‘Detritus’, Schakt och Transport


  1. CEDA’s Checklist for Successful Dredging Management
  2. Presentation of the checklist at the conference

Note: The Checklist will be available to CEDA members from this page.
Remember, first you have to sign in to access the full document.

See also

CEDA Dredging Management Commission

CEDA Dredging Days 2017 Technical Visit Prinses Beatrix Locks

New and old basins Prinses Beatrix Locks

Today was a trip down memory lane. I went on the Technical Visit of the CEDA Dredging Days1 to the extension of the Prinses Beatrix Locks in Nieuwegein2. As a small boy, we sometimes drove to the locks to see the ships being raised or lowered according to the water level in the river. Mostly, the locks handle merchant traffic. However, in the summer, lots of pleasure boats come through and that is when the fun starts. Usually the crew on these small boats is not very experienced and funny mistakes and amusing near accidents happen all the time.

Actually, there is a lot more history about the locks3, than my own little stories. The locks were built in 1932 in a time, where such functional constructions were allowed to be beautiful. One of the most prominent features of the old locks are the monumental lock gate lifting constructions. They are really like big portal buildings signifying the transition from the ‘natural’ Lek River to the civilised environment of the Amsterdam-Rhine-Canal. As this is a ‘historical horizon element’ the new lock is designed to have the lowest possible impact on the scenery.

Lock gate buildings Prinses Beatrix Locks

The new doors will be rolling to stay out of sight. The doors are also double executed. One reason is for redundancy, in case the other is damaged or in maintenance. As the contractor is also responsible for the operation later on, any delays will be penalised. Therefore the most reliable solution was chosen. However, there is another reason. When both outer doors are used, the lock basin is long enough to transit two 135m long inland vessels. The so called XL operation. The construction of the lock recesses are in full swing. The floors are already poured.

Lock gate recess canal side third basin

One of the first items finished in the project is the new high water dike to protect the polders behind. The material for the construction is mostly reused from the old dikes and other parts of the approach channel. Now, the new approach channel is excavated, making the river side a 120m wider. It also provides extra waiting jetties. Although the third lock should minimise the waiting time to a bare minimum.

Finished dike river side Prinses Beatrix Lock

Most of the work in the project is related to the construction of the new lock and the dredging of the approaches. But, one part of the project is to renovate the old locks. Once the new lock is in place, the contractor has a ten week time slot to renew everything but the buildings. All the mechanical and electrical installations have to be made up to date. To my opinion this is a loss. When you see all the beautiful shafts, gears and sheaves. And they have been working already for more than 80 years. I guess they should have proven their reliability by now. Well, maybe it is just my sentimental heart.

Lifting machinery gate building Prinses Beatrix Locks canal side

One thing, that we luckily not have to be sentimental about, are the three bunkers of the Hollandic Water Line4, a military defence system, that is now defunct. The bunkers are being relocated and taken up into a display about the darker side of history. They even made a colouring page to make even children aware of what it was for and how they were transported. They were distributed during the monthly public visits, which you can do yourself also.

Colouring page bunker transport Hollandic Water Line


  1. Technical Visit of the CEDA Dredging Days
  2. Expansion of the Prinses Beatrix Locks
  3. Prinses Beatrixsluizen (Dutch)
  4. Hollandic Water Line

See also