Pump killers: How to fight the 13 most common centrifugal pump failures? Number 1.
In the series “Pump killers” we investigate the 13 most common causes of centrifugal-pump failure and take a look at the measures that can be taken to fight these ‘killers’. Every week we will share a new, even bigger, killer. Our goal: to reduce the number of victims by sharing our knowledge.
We are already ready for pump killer number 1. And as you might expect, this pump killer lives up to its title. Let's announce him: Pit-shaped craters at the end of the impeller blades? Excessive pumping noise that sounds like “marbles in the pump housing”? Pressure fluctuations visible on the pressure gauge? Unexplainable pressure fluctuations in the discharge line? Pump shafts that break? Yes, there he is, the greatest pump killer of all time: A pump running too far right in its curve.
Source: "Broken shafts" - credit: Oveducon.
Each centrifugal pump is designed for a specific capacity and head. This design point can easily be found in the pump curve by looking at the efficiency in the pump curve. Where efficiency has the highest value is the design point of the pump.
If the pump can be selected at the highest efficiency point, the pump will achieve an optimal service life with the lowest possible maintenance and energy costs. You will wonder, "Who wouldn't want that?" In practice we see that the pump is often not selected at this point or eventually turns out not to be running at it.
How does this phenomenon arise? Operating a pump too right in its curve can be caused by one of, or a combination of, several factors. For example, we often see that a pump is chosen too small to save costs. The response we get is often: “But why? Doesn't it just pump the capacity and head that I want? " Yes, that's right, but the question is how long the pump will continue to do that, because before you know it you will have to buy a new one. The lifespan is seriously shortened.
Source: "Disc Calculator" - credit: Oveducon.
In addition, it may be that the calculated pipe losses are incorrect. If the calculated pipe losses turn out to be lower in practice than in theory, then the duty point at which the pump will operate will shift to the right in the graph. This is a zone where you don't want to sit with your pump!
What are the consequences? The consequences, as you can expect from this "killer", are huge. A pump that runs too far to the right in its curve will give considerably less efficiency in a short time. In addition, the NPSHr will be higher and increased forces will occur on the impeller and the shaft. Also, a pump that operates too far to the right in its curve has an increased risk of cavitation; more about this phenomenon can be read in Pump Killer #4.
Source: "Crater pattern created by cavitation" - Credit Oveducon.
How can this be controlled? Of course you can also combat this pump killer. If a pump is deliberately chosen too small, the major advantage is that the purchase price is lower, but the result is a pump with a greatly reduced life and higher maintenance costs. In this regard, the calculation was made very quickly.
As a seller you can play a crucial role in combating this cause. Back in the day, when I was still a commercial technical advisor, I regularly had clients who just wanted the cheapest option. Sometimes they just left the competitor's quote on the table, with the text my way of course. In situations like that, my knowledge of pumps has repeatedly proven its added value for the customer. If you can demonstrate in such a situation what the consequences are of choosing one (too small) pump over another, you can help the customer make the right decision.
In the slightly longer term, a well-selected pump will always turn out to be cheaper than the slightly higher investment at the time of purchase suggested. Perhaps you have heard the saying: “The joy of a low price lasts only a moment, the joy of good quality lasts a lifetime”. This certainly also applies to a pump selection. In the worst case, the customer will still go for the quotation with the pump chosen too small, will cut himself in the fingers and will ultimately choose a pump that has been selected within the allowable operating range.
The second cause: the calculated pipe losses are incorrect, can in many cases be prevented. In practice, for example, I sometimes come across that a pump has already been purchased before the pipe losses have been calculated. This is not possible at all and this way of selecting is asking for misery. The choice of available pipe diameters is much smaller than the range of pump charts. Therefore, calculate the pipe losses first and only then start looking for a suitable pump. When calculating the pipe losses, always try to approximate the practical situation as closely as possible to allow the installation to function optimally.
Source: "Pipeline network" - credit: Oveducon.
Sometimes it takes some effort to get the right data, but it pays off to act decisively. The pipe losses determine the duty point at which the pump will operate.
Source: "Pipeline losses are higher than calculated" - Oveducon.
If the pipe losses become higher in practice, the operating point in the pump curve shifts to the left, causing the capacity to decrease and the head to be increased.
Source: "Pipeline losses are lower than calculated" - Oveducon.
When the pipe losses become lower, the duty point in the pump curve moves up to the right, which increases the capacity and the head lower.
That's it: Pump Killer number 1. We hope you have enjoyed the series and learned a thing or two. For now, we would like to thank everyone for reading, commenting and sharing the articles! It is great to see so many people passionate about pump technology.
Keep spreading the pump gospel and raise that bar!
Un hombre de muchas cualidades.
3 年Yes indeed this article is about most common failures. I would like to share some my aspects as well i have seen past 15y in Pumping system business. First yes... very sad that Money Makes roles. Till there is sales persons who offer whatever ....to have a deal... till nothing changes. Financial managers do not understand any those basic theory or drawings. Those guys looks costs. IF they look only pc vs pc then for sure selection will be made by cost. If you no have more money then you need find balance between. Not all companies can afford good products and they must handle best they have for this money. Pumping systems is heavily related all what is installed: pipes, valves, consumers etc. Most pump sales persons do not never re check complete system. So called Catalogue or web app salesmans. I have seen Several "big" pump company sales directors, who has told to me that we are not system engineers, we produce pumps and we do not take responsibilities if selected pump is wrong. That is true and why they should. Would be good if they Could. So pump killers are as well Sales persons who are interested to have only a deal. One more aspect i have noticed is that We have in every country quite a different legislation ( even in EU) for design. Example water systems have regulated per person consumption of water by time period. All consumptions has summarized and network characteristics has been used to estimate different system loads. More better theoretical (so called mat. model) soft you use more accurate results you will get. According this you get load points and according this (example revit, pipe flow etc.) suggestion of "newton fluid" pump will be selected. Problem is that most of pump suppliers has standard water pump products and they do not fit 100% , so designer must select best possible option, not supplier they know. Does they do this_? Mostly no, they select supplier whos soft they has or whos sales guy responds quick and sends data sheets what they connect to the documents. So designer is also Pump killer. Bigger Problem is that those piping system calculation programs is very much related how you build up your system (it's like an architect building a house) and how much you really have know how about pumps, valves or any related systems( Very different, in different countries). Most load problems are related that consumer (consumer can be heat exchanger, steam generator, person who opens tap etc) is not using system as was planned (50% cases). Financial manager did not buy valves as described KV values. Welder did not weld pipes as planned. Elbows was 3D, not 5D length. Designer used BIM and did not added detailed info how for example should be heating set to build, No have 3D CAD and project manager organized from different suppliers (most cost effective products) all is needed, so called complete set. Set with products what all cause different loads compared what was planned. Also installation company could not fit system this way to planned space. So they did rework up to 30% all system. More and more i see that is so common that No One re checked or re calculated after that real system. What was changed and are those pumps able at the end.... at all work.... without cavitation, near BEP. Nowadays pumps mostly run with VFD. So i have more and more noticed that people thinks that if I use VDF then ... no problem this small system change..pump can handle.... That is not correct way to think (especially when you have multi changing system). Must always recheck .... .because If you open pipe system calculation programs (not simple pump suppliers soft) then most those do not very well consider that you operate your given loads with pump VFD. They show load points in pipings according the values you giving (speed, flow, Friction, pressure, density and some cases viscosity etc) and your selected prefered loads, not how pump or regulation valves works in different loads (Changes of KV, NPSH etc). What happens when you manually recalculate and force the program to use your prefered set points. Those softs giving so many cases that your piping system is wrong (all is RED). Reason they do not accept mathematical model what you prefer or they consider some ISO standard or "US" regulations. You can do inside soft also corrections, but then you need to know exactly how this soft thinks and works. Or what modelling standard it is using... We do not see this. So Pump Killer is as Well Soft we use. I hope those thoughts helped a bit more to understand that design piping systems is the key to select correct pumps.
End User Sales Engineer at Ruhrpumpen
3 年Jos, do you have these 13 articles in PDF? Can they be downloaded?
Pump Technical Manager at Andrews Sykes Group plc
4 年Nice article
Passionate about unleashing hidden potential for sustainable and cost-effective reliability in process industries!
4 年Totally agree. But practice is also that many operators have to deal with off BEP operation to some extent. An upgrade to a solid shaft may than be a simple way to improve the mechanical robustness and improve seal and bearing life.
Sales Engineer at GRUNDFOS
4 年Yes, indeed! More ideas;) 10 Ways to Kill Your Pump 1. Overwork it Work the pump continuously at higher capacities, flows, heads, or speeds than originally specified. 2. Starve it Never grease or oil the pump. 3. Choke it ? Lower the water level in the sump. ? Let the suction strainer clog and never clean it. ? Let the temperature of fluid rise without raising the suction pressure. 4. Fry it Operate at shutoff for a long time with the bypass line closed tight will convert your power to heat. 5. Poison it Change the pumped fluid without checking with the manufacturer (for example adding chemicals). 6. Stab it Remove the suction strainers which will introduce grit, sand, and scale into the fluid. 7. Break its limbs Impose heavy piping loads on the suction and discharge nozzle, either through initial misalignment or through thermal expansion. 8. Shake it Don’t align at installation or install on a flimsy foundation. 9. Drown it For a packed pump with a drain for the gland leakage: ? Plug the drain with a cigarette butt, gum or paper. ? Remove the water shield. 10. Neglect check-ups ? Ignore the manufacturer’s recommendations for “check-ups” ? Don’t check vibration.