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Water control

What is the most reliable gate?

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By Ken Grubb on 13th Sep 2016

At this stage in my engineering career, I have more recently been working on numerous dam protection and flood defence projects.  In this context, I am often asked “what is the most reliable water control gate” and I thought that it would make a good subject for a blog.  So here goes…

First a bit of background about me. I joined a design-and-build water gate contractor some 35 years ago and eventually became its technical director. For much of that time I was responsible for all technical output and also for on-site commissioning. This gave me a great insight into manufacturing difficulties as well as construction experience. You need to be a quick learner when things are not working on site and everybody is looking at you to sort it out!

Subsequently I started KGAL Consulting Engineers and crossed the great divide into consultancy, where I have spent many happy hours surveying mine and other people’s gates that our decades old and may, or may not, still be working. It has been interesting seeing what stands the test of time (and vandalism!).

Now in the latter stages of a gate career I find myself working on many high-integrity schemes for which formal reliability estimates are required. Whilst there are lots of available statistics in terms of the life of an electric motor, I know of no valid studies in terms of the reliability of gate types. These days, I am often sitting on expert committees where people are trying to collect such information and endlessly want to debate their own experiences in terms of what is reliable and what is not.

The thing about a water control gate is that it is a mixture of science and art.  You can argue about what the proportions are, but I would guess 80:20. The 80% of science can be looked up in the like of structural codes and can be reasonably quickly learnt by a competent engineer. The difficult bit is the “art” which is essentially knowledge of what works and what lasts. This comes from experience and unfortunately has a disproportionate effect on the future reliability of the gate in question.

I have always liked the definition of an expert as “somebody who has made every conceivable mistake possible in a very narrow field”. We all learn more from mistakes, they represent the “scar tissue” that hopefully turns us into good engineers. Sadly these mistakes are generally at somebody else’s expense.

By way of an example let us consider a vertical wheeled gate. There are many examples of this gate around providing sterling service as river sluices and various other duties. In the earlier part of my career I was frequently surveying gate after gate where the gate rollers had seized; so much so that it would be easy to conclude that this type of design was intrinsically unreliable. But when I considered the wider picture - why had those rollers seized - there was nearly always a clearer reason, for instance:

  • The pin material was inappropriate and had corroded
  • The bushes used were inappropriate for long term use and lost their lubrication properties over time
  • There was no sealing system in an otherwise silt laden zone
  • The wheel had inadequate lubrication facilities or the owner had failed to maintain them
  • Etc…

The more gates I surveyed the more frequently I found the same situation arising; perhaps an unsurprising situation in view of the market size and the few opportunities available to engineers to develop the experience they need.

Also, when you look closely at the procurement methods used by purchasers you can see the seeds of future problems:

  • A design and build specification written functionally by a non-gate expert
  • Civil contractors on the tendering list (who will subsequently procure the gate substantially on price).
  • A heavily skewed price: quality tender decision matrix.

It is tough being a contractor: I know – I used to be one. It is difficult to think long-term; survival is generally based on short-term decision-making such as the cost of materials and construction. There is no big incentive for them to design a gate that lasts longer than their competitors’ as there is no benefit in the next tender race. I guess in gates (just like in life) you often end up not with what you wanted but with what you deserve!

Of course there are still natural factors that affect reliability. How complex is the machine (gate) you are designing? What are the special environmental factors it will be subjected to? However, the influence of design would make any statistical analysis of gate types invalid in my view.

Hence, my answer to the question “what is the most reliable gate?” is ‘the one that has been designed by an experienced gate designer who has been given adequate time and been incentivised/required to provide quality solutions’. 

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