Pump killers: How to fight the 13 most common centrifugal pump failures? Number 3.
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.
Photo: "Pump with frequency converter can continue to run" - Oveducon
It has already been discussed in two previous articles: centrifugal pumps are designed to pump liquids. So far nothing new, but what if, for example, a suction pipe is not airtight? Then this can cause many problems. A hole in the suction line the size of a pinhole can be enough to stop a pump system from functioning.
Photo: "Tape around the suction lines" - credit Oveducon.
In addition, air / gas can also enter the pipe through, for example, the outgassing bubbles from peat. These can then cause the impeller to become "clogged" with gas / air.
How does this phenomenon arise? If a connection in the suction line is not completely tight, a pump can suck in false air. This air can collect in the suction pipe or in the pump.
Photo: "Three self-priming pumps" - credit Oveducon.
Depending on the type of pump, it is able to pump air bubbles. A self-priming pump is capable of pumping out air bubbles with the medium, but a normal-priming pump is not.
Another possibility is that in a suction pipe, of a pump that has to suck the medium from under the pump, there will be an under-pressure relative to the atmospheric pressure. This allows air to be drawn into the suction line.
In practice I sometimes hear pump users declare that they have squeezed the suction pipe and that it was closed. Note: pressure tight is not the same as vacuum tight.
Photo: "... and sometimes press-tight isn't closed either!" - credit Oveducon.
What are the consequences? The consequences are that the pump can run without pumping medium, because there is an air bubble in the center of the impeller.
With a dry installed pump with suction pipe, we sometimes see that it does pump capacity when it rains, but no longer when it has been dry for a long time. If in this case connections of the suction pipe are located underground, a connection can rise above the groundwater level in prolonged dry weather and thus start sucking in air. Nothing is as difficult as this kind of malfunction, because go and find that leak.
How can this be controlled? "Simple": make sure that all connections are really 100% airtight. That is why screw couplings / couplings in a suction line are so dangerous, because especially as the gaskets get older, the chance of 'false' air increases. So avoid these links!
Photo: "Suction pipe with a coupling" - credit Oveducon.
Another way is, if the liquid level is below the pump, to mount the suction pipe of a pump slightly rising towards the pump.
In addition, if you know in advance that air or gas can be expected in the medium: consider purchasing a self-priming pump. This saves a lot of damage in the long term.
We already saw in an earlier article that air, due to dry running and air ingress, can cause many problems in the pump installation. So this week there is yet another one.
The conclusion is clear: for a centrifugal pump, air can kill!
What are your experiences with pipes that are not airtight? Have you ever been allowed to search for a pipe underground where there should be a hole somewhere or where a connection is not completely closed? We are curious about your experiences!
Next week we will look at the penultimate in this series: number 2.
Retired at home
4 年In a booster set with more than 1 pump and a single suction pipe the air will enter in the standby pumps if the pressure is lower than atmospheric pressure. One solution is to put check valves in suction side not on delivery side of pumps. In my case, vertical pumps with mechanical seals accumulated air and seals were destroyed faster than normal. Temperature of mechanical seals was higher than usual because of air.
Principal at FOG Group
4 年I once had a problem with a double suction centrifugal pump that repetitively sheared the shaft on one side of the impeller. We tried fixing the symtoms, e.g., larger shaft diameter, different shaft shoulders, different shaft material to no avail. Finally, through investigation, determined that chemical additional inserts into the suction side of the pump were introducing cavitation as the chemicals were introduced to the supply stream. viola! Not a bearing failure, even worse.
Business Development Manager - Middle East, Africa ???? Enjoying ???? 5?0?6?5?2? LinkedIn Followers
4 年You made me think of making Bearing Killers ;-)
Global Director of Engineering and Technology at Trillium Flow Technologies
4 年As a general rule of thumb, most centrifugal pumps will show signs of distress at around 2.5% air in the fluid and will likely not work at all when you get to 5% air. As Jos and other commentators noted, if the design is such that is allows air accumulation in the pump, even much lower amounts of air over time will result in your pump not behaving :\
Dredging technology expert
4 年Finding a vacuum leak can be a real pain. Even more when the leak closes when the pump is stopped for inspection. Thanks for the article.