Cavitation… What you need to know

Cavitation is the 2nd leading cause of a pump system failure...

It’s for this reason that a full understanding of what cavitation is, how it is caused and how it can be prevented is critical in the fluid power industry.  Cavitation by definition is the formation of vapour cavities in a liquid, and it commonly happens when a pump is starved for oil.  When any liquid has a vacuum applied to it, trapped air is pulled out of the fluid and the result is tiny bubbles.  These tiny bubbles can have critical consequences on the performance of a pump.  When the fluid suffering from cavitation enters a pump and is compressed, these tiny bubbles explode.  In turn, these little but very powerful explosions can remove material from the inside of the pump and possibly destroy the system.  An operator will know if this happens to a pump as obvious signs of cavitation damage can be detected along with a very noticeable audible sound.  In order to prevent cavitation from happening, we must first analyze how it is caused.

The number one cause of cavitation is an improper pump suction line configuration or design.  Sometimes when these lines are configured, there is too much pressure drop between pumps.  It’s critical to design hoses as short as possible with the very least amount of fittings.  This has often been an issue as designers generally will not think about the length of hoses between pumps.  Additionally, a low amount of fittings is desirable because each fitting that is present adds to the pressure drop.  And as discussed earlier, the pressure drops are the leading cause of cavitation in pumps.  The suction line and the ball valve must be properly sized in order to handle the specified amount of flow.

The second most common cause of cavitation is the suction filter/strainer.  Often times a filter is installed into a system but does not get maintained properly.  If ignored for too long, the filter will get filled up with debris and this strain on the pump can lead to cavitation.

Here are some quick tips to prevent cavitation in a system:

1. Never put a suction line filter on a system.  The problem is that these suction line filters are housed down under the oil and in order to maintain it, the tank must be drained and disassembled.  As the old saying goes, “out of sight, out of mind.”  It’s best to just not have one installed at all.  Instead, install extra filtration in the actual system.  With this strategy, all of the oil entering the tank is filtered instead of when it leaves the system.  In this scenario, you’ll have one pump dedicated to pulling oil out of the tank, filtering and putting it back in.
2. Have a proper reservoir design.  A good design has a vessel in the center that divides the incoming oil with the outgoing.  This allows the oil to remain in the tank long enough for solids to drop to the bottom and the bubbles to rise to the top.  Part of the proper design of the reservoir has to do with the flow rate.  The tank capacity should be four times the amount of the flow rate.  Four times more oil should be available in the reservoir than is needed to be pulled out.  This will ensure the oil spends enough time in the reservoir for all of the oil bubbles to work their way out. 
3. Make sure your pump is below the minimum oil level of the tank.  This is known as a “flooded inlet” design.  This design ensures positive pressure on the pump inlet which will minimize the chance for cavitation.  This is in contrast to the tank top mounted pumps.  In this design, the pump must draw fluid up through the suction line, and in doing so typically must process some air before the fluid reaches the pump.  If the system sits for long periods of time, the fluid in the suction line can be pulled back to the tank by gravity, creating an air pocket in the line.
4. Install a lock on the suction line ball valve.  This is done so there isn’t any mistake where someone accidentally closes it or leaves it partially closed.
5. Finally, it’s critical to use the correct fluid for a pump.  If it has too much viscosity, the fluid will have a difficult time making its way to the pump quick enough.  Oil needs to be supplied readily to the pump.

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