Understand centrifugal impeller design, reduce water pump failure.

Understanding centrifugal impeller design in relationship to how does a pump work can reduce your water pump failure. This knowledge of how does a water pump work will not only enable you to increase well pump life expectancy, but all other applications of the centrifugal impeller design. Understanding how does a centrifugal pump work in general can be applied to increasing sump pump life expectancy, sewage pump life expectancy, pond pump, etc.

Mike Sondalini's (Equipment Longevity Engineer) article "Increase pump life expectancy by understanding centrifugal impeller design" in PDF format, simplifies the subject with real world knowledge and experience. Excerpts below...

What is an impeller?

A standard centrifugal pump impeller is constructed of a group of elongated, solid-walled chambers attached together in the shape of a circular ring. The ring is spun quickly and the liquid that enters the inside end of the chambers is flung out at high speed from the other end. Figure No. 1 above shows a section through centrifugal pump wet-end showing regions of low and high pressures.

The longer the chamber (i.e. a bigger diameter), the faster the liquid leaves the ring for a given speed. Alternately the faster the ring is spun the faster the liquid is flung out. High tip speeds produce higher pressures. The pressure generated in a centrifugal pump is the result of the high-speed liquid hitting and stopping at the volute wall and so changing its energy of motion (momentum) into pressure energy. The pressure then forces the liquid out the discharge nozzle.

To stop the pressure from returning to the suction inlet and back through the chambers, the gap between the impeller and the volute is made very small at the suction nozzle inlet. Usually wear rings are fitted to the impeller. When the wear from the small amount of leakage that does occur gets too bad they are changed over for new.

What happens inside the impeller?

As liquid is flung out from an impeller it leaves a space behind. This space is taken up by the particle of liquid immediately following. By flinging one lot of liquid out, it forces new liquid to be drawn in to take its place. Spinning the impeller creates a low pressure at the center inlet and the pump seems to ‘suck’ liquid from a distance.

In fact the pump does not ‘suck’. Rather the pump inlet pressure is lower than the pressure at the supply point and the higher supply pressure tries to equalize the pressure imbalance. The high pressure flows to the low pressure. As long as the pump keeps creating the low-pressure region at its impeller inlet the liquid will flow into it. The pressure does not even need to be a vacuum (i.e. less than atmosphere pressure); it only needs to be at a lower pressure than the supply point pressure.

Properties of the liquid.

Successful operation of the impeller is also dependent on the properties of the liquid being pumped. The three main liquid properties affecting the impeller are the viscosity (slipperiness), the corrosiveness and the pressure at which the liquid becomes a gas (vapor pressure). The PDF copy of this article https://bin95.com/centrifugal-impeller-design.pdf goes into more detail. It is important to have a clear understanding of those 3 too.

Excerpt from the full PDF article:

"If the suction pressure at a pump impeller inlet were so low that the liquid ‘boiled’ into a gas/vapor then bubbles would appear in the liquid stream (See article 232 Centrifugal Pump Cavitation). The bubbles at low pressure would then move into the spinning impeller chambers and suddenly experience high pressure (see Figure No. 1) as it moves through the impeller. The bubbles implode and eject a shot of high velocity liquid at the impeller and gouge a bit of it out. Cavitation destroys impellers."

Modes of centrifugal pump impeller failure.

There are numerous ways an impeller’s performance can be affected. Listed below are some of the more common ones.

?? Wear – Abrasion and erosion can occur when solid particles (as in slurry) are present. It may be necessary or select wear resistant materials. If solid bits of metal, stone or rubbish from the process are present it is critical to fit a strainer before the pump AND clean it out regularly before it blocks up and the pump starves and cavitates.

?? Cavitation/vapor lock – As noted above keep the pressure at the impeller inlet above the vapour pressure. Keep the suction flow velocity low – use big bore pipes; keep suction pipe insides clear of obstructions; clean strainers often. Pressurise the suction supply - put the supply tank way above the pump; put additional pressure onto the surface of the liquid in the supply tank (but don’t split the tank) and slow the pump. Cool the liquid so it will not be hot enough to ‘boil’ at the impeller inlet pressure.

?? Blockages/objects – Solids can block the impeller vanes. Chunks of wood can get stuck in the vane passage, gloves, foam cups, dropped tools, nut and bolts, tank floats, parts of instruments, etc may not pass through the impeller. An in-line strainer is a must in such cases. An alternative is to relocate the supply tank outlet nozzle higher up the tank and let the solids settle to the bottom, and draw liquid from the cleaner level. The rubbish at the tank bottom can be drained out a lower nozzle or the tank manually cleaned.

?? Loss of running clearances/re-circulation – If the wear ring clearance between impeller and volute becomes too great the liquid will recirculate from the high pressure at the impeller outlet to the low pressure at the inlet. The flow from the pump falls because liquid is instead drawn back into the suction inlet and just goes in circles.

?? Material of construction/corrosion – The impeller must be impervious (unaffected) to chemical attack.

?? Change of service duty – If a pump is moved from one position to another or from one service to a different service, it may not pump. Each impeller is sized to a specific flow and pressure. If the pump is move from its original duty it must be engineered for the new duty (See article 112 Changing the Service Duty of a Pump).

?? Direction of rotation – The vanes on centrifugal pump impellers are designed to rotate in only one direction. They can still generate pressure if turned the wrong way but the pressure and flow are very much less that if they were going the right way. Changing any two phases on a 3-phase electric motor reverses its rotation.

?? Air locks – Trapped gas and air around an impeller will stop it from pumping. If the impeller is spinning inside a volute with the top part of the volute full of gas the ejected liquid is not compressed against the volute wall and cannot convert its velocity into pressure. The pump will run but nothing will come out. The air, gas or vapour in the top of the volute must be removed.

If the air lock is in the suction pipe the pump will try to draw liquid and pull the air with it. However the air is always at the top of the pipe and fills part of the pipe. The liquid must then flow in the remaining part of the pipe. If this area of pipe is to small then not enough liquid will get to the pump and it will stop pumping till more arrives. Slug flow develops. In the worst air lock case no liquid gets to the pump. Air, gas or vapour must be removed.

Best practice is to slope the suction pipe so that any gas caught inside it finds a way to naturally flow through to the pump and out the pump discharge line. Or else find ways to flow back into the supply tank and bubble up to the surface of the liquid. Failing that it is necessary to install valves along the pipeline that can be opened to vent out the trapped gases until liquid appears.

Buy pumps with vertical center discharges instead of side discharges as gases can get trapped in the top circular portion of the volute. Or rotate the volute and piping to have the outlet at the high point.

Download the PDF "Increase pump life expectancy by understanding centrifugal impeller design" to read little bit more. I hope this was helpful to some.