Cavitation In Depth

Part 3 - Types of Pump Cavitation

 

There are 4 causes or types of cavitation in centrifugal pumps as follows:

 

1 - Suction Cavitation (Due to Low NPSH)

    Caused by Insufficient NPSHA.  The general concept is that NPSHA should be at least equal to, and if possible, greater than NPSHR, to avoid Suction Cavitation.  The question then arises, How much margin of NPSHA over NPSHR Is enough ?. 

    A general consensus within a specific industry, regarding specific pumps and specific fluids, is often the answer to that question.  That solution may seem unsatisfactory to many people, and it should.  But the exact nature of cavitation and pumps is very complex and knowledge is hard to come by if one is seeking definite answers.  There have been studies made that can be helpful.

    In the end, this problem highlights a little known fact about pumps, most pumps have some cavitation occurring within them at all times, incipient cavitation is almost ubiquitous.  Incipient cavitation is almost impossible to totally suppress, but that cavitation is not always very harmful, nor does that type of cavitation always limit flow through the pump significantly.

    So, the answer to the question,  How much margin of NPSHA over NPSHR Is enough, is simply, the amount of NPSHA that produces the least amount of damage to a pump.  Now for the complexity, larger margins of NPSHA over NPSHR often produce more damage in a pump than lower margins, especially when dealing with cool water (less than about 150 degrees F.).

 

The best answer may be to hire the services of a true and honest expert with extensive experience in the specific area of work.

Follow These Internal Links To Learn More About Suction Cavitation

Glossary - Net Positive Suction Head (NPSH)

Pump Intakes

Pump Characteristics - NPSHR

Glossary - Cavitation

Fluid Type

2 - Recirculation Cavitation

    Caused by low flow rate through the pump. There are two types which may occur together or separately: Suction Side and Discharge Side.  Both types of recirculation work by the same phenomena of reverse fluid flows in close proximity to each other.

    When two flow paths within a fluid are moving in opposing directions and in close proximity to each other, vortices form between the two directions of flow, causing high fluid velocities and turbulence, resulting in localized pockets of low pressure where cavitation can occur.

    How recirculation cavitation occurs within a pump at low flow rates is an inherent function of pump type and design.   In general however, pumps with lower pump specific speed (Ns) and lower suction specific speed (Nss), are more resistant to recirculation cavitation.

 

Suction Recirculation Cavitation

    Fluid entering the pump suction nozzle is reversed, resulting in high velocity vortexes either in or near the impeller eye, in the suction nozzle, or in the pipe close to the suction nozzle.  High velocities result in low localized pressures, local pressures may drop below the vapor pressure of the fluid, resulting in cavitation.

    Cavitation damage observed on the pressure side of the inlet vanes, near the impeller eye, are a sign of suction recirculation, and therefore this observation is diagnostic.  When looking into the eye of the impeller, the pressure side of the inlet vanes is on the underside of the vane, and therefore may only be observed using a mirror.

    Noise due to suction recirculation cavitation can be distinctive from other cavitation noise, and is therefore also diagnostic.  Suction recirculation cavitation noise is reported to be a loud popping, crackling, hammering, or knocking sound, with highest intensity detected at the suction nozzle.

Discharge Recirculation Cavitation

    Fluid leaving the impeller discharge side or the pump discharge nozzle, at low flow rates, may be reversed, resulting in high velocity vortexes between the two flow directions, causing localized low pressure areas.  Pressures may drop below the vapor pressure of the fluid resulting in cavitation.  Recirculation cavitation damage also occurs on the discharge side of the impeller periphery, at the cutwater(s), inside the discharge nozzle, or in the pipe close to the discharge nozzle.

    Noise due to discharge recirculation cavitation is generally less noisy than suction side recirculation.  Discharge recirculation cavitation noise is heard mostly at the pump discharge nozzle, and there will not be the loud popping or crackling noise heard when suction recirculation is occurring.

 

The two photographs above illustrate the type and location of damage caused by Discharge Recirculation Cavitation.

 

The photograph below shows Discharge Recirculation occurring inside an operating pump.

 

Click on the photograph to watch the video

 

Explanation of Video - The pump is operating at a low flow rate.  When the video begins the discharge pump discharge nozzle is not seen.  As the video progresses the camera view rotates to bring the discharge nozzle into view.  The nozzle begins to appear on the left hand side of the viewing pane.  Notice that fluid comes off the impeller, enters the discharge nozzle, and then the fluid is turned around and enters back into the pump, this is discharge recirculation.

 

This video is shown with permission of Steve Harrington at Flometrics.  This group of engineers are devoted to their craft.  Please visit their site:

Flometrics

 

3 - Incipient Cavitation

    First, to understand incipient cavitation one must know the definition of NPSHi (Net Positive Suction Head Inception), and NPSHR .

 

Differential Pressure (dP or ∆P) - The pressure differential produced across a pump, as measured at the intake and discharge nozzles.

 

NPSHi  - That Fluid pressure, as measured at the pump suction nozzle, at which all cavitation inside the pump is suppressed.

 

NPSHR  - The fluid pressure, as measured at the pump suction nozzle, at which a 3% drop in correct ∆P  occurs.  The 3% drop in ∆P  is the result of flow restriction and inefficiency caused by cavitation.

 

    Incipient Cavitation is then defined as: that cavitation occurring inside a pump, from the NPSHR  3% value, up to the incipient point.

    Incipient cavitation occurs in most pumps at all times.  The cause is turbulence created by the impeller, resulting in localized pressure below the vapor pressure of the pumpage.  In the general pump market, the ubiquitous presence of incipient cavitation appears to cause little damage and little loss of performance, therefore the concept is not commonly discussed.  Although this fact may partially be due to under-reporting, the fact remains that incipient cavitation damage is not a common topic except in specific markets.  The topic is interesting to those markets where high energy suction pumps are used.  HVAC cooling towers, chilled water systems, and boiler feed pumps are well known to have serious incipient cavitation problems.

    High margins of NPSHA over NPSHR can result in increasingly severe incipient cavitation damage, the higher the margin, the more damage that will occur, until the NPSHi value is reached, which is usually unachievable.  The cooler the water, the more damaging the cavitation.  The Hydraulic Institute and others have established general recommended margins of NPSHA for specific markets.

 

Factors indicating incipient cavitation may be a problem are:

  1. Heavy weight liquids such as water, and especially when these liquids are at cooler temperatures, for water this would be 1500 F. or less.  Actually water is one of the worst actors in regards to cavitation damage in general.

  2. Certain ranges of Pump Specific Speed.

  3. High Suction Specific Speeds (Nss > 9500), and what the HI calls "High Energy" pumps, which are determined by a chart published by HI.

  4. Systems with high ∆P  values.

  5. Systems with high margins of NPSHA over NPSHR .

 

    Incipient cavitation is strongly linked to the Suction Specific Speed of a pump, the higher the suction specific speed, the more likely that incipient cavitation may become a problem.  High Suction Energy pumps require larger margins of NPSHA over NPSHR, published opinions report this margin as 2 to 20 times NPSHA over NPSHR .  Confused?  There are no well defined simple ways to understand and know how to apply these pumps except by experience.  You need extra margin of NPSH, and yet if you supply too much NPSH then incipient cavitation damage becomes a problem.  For some pumps, a small margin works well, for other pumps higher margins are required.

    The reason for this confusion involves the way in which NPSHR values are determined.  The standard Hydraulic Institute test method for NPSHR sets NPSHR at a point where a 3% drop in correct  dP across the pump occurs as pump inlet pressure is reduced.  For low suction energy pumps and low suction specific speed pumps, that 3% drop in dP represents a small but detectable amount of cavitation.  But high suction energy and high suction specific speed pumps are much more efficient at moving water through the impeller, so that a 3% dP drop represents a large amount of cavitation that can damage the pump severely and quickly. 

 

In conclusion, NPSHR 3% does not mean the same thing for all pumps.

 

4 - Vane Passing Syndrome Cavitation

    Cavitation resulting when the impeller vane tip to cutwater clearance is too small, resulting in excessive turbulence each time a vane passes the cutwater, resulting in cavitation and also pulsation. 

 

    The location of cavitation damage is diagnostic.  Typical cavitation type damage may be observed on the center of the cutwater, impeller vane tips, discharge edge of the impeller shroud, and possibly to the pump casing downstream and directly behind the cutwater.

    Engineering specifications may attempt to preclude this problem by not allowing pump manufacturers to supply pumps with the largest impeller diameter available for a given pump family.  This is not a recommended practice for engineers because it presumes that a pump manufacturer will provide a pump with vane passing syndrome without the manufacturer knowing of the problem, or if they know, they are not telling the customer.  The practice of not allowing a pump manufacturer to provide the largest impeller diameter, is understandable in view of the behavior of some pump manufacturers.  However, if a pump manufacturer is honest and trustworthy, they should be allowed to provide the pump with the largest trim diameter, which also may be a more efficient pump.

 

End of Cavitation - Part 3

 

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