HOW CAV­I­TA­TION MUR­DERS PUMPS

Successful Farming - - SERVICE TEAM - By Ray Bo­hacz

It dif­fers lit­tle if it is a pump on an en­gine, a hy­draulic sys­tem, or any other ap­pli­ca­tion. Fact is, a bro­ken pump can bring your en­tire op­er­a­tion to a stand­still. Though pumps can fail due to age and use, the re­al­ity is that most are mur­dered, snatched from their prime with much life left in them. The cul­prit is cav­i­ta­tion, and it sends warn­ing signs (ex­ces­sive vi­bra­tion, ham­mer­ing, groan­ing, and whistling) but most are ig­nored.

Pump cav­i­ta­tion de­scribes the for­ma­tion of bub­bles or cav­i­ties in the bulk fluid be­ing moved that usu­ally de­vel­ops around a low-pres­sure area. This is the re­sult of ei­ther an en­trained gas in the liq­uid from the va­por pres­sure be­ing ex­ceeded or from a lack of flow. When the va­por bub­bles col­lapse or im­plode, they strike at the speed of sound, cre­at­ing the noise and vi­bra­tion. This col­li­sion will erode the sur­faces of the pump and im­peller, and will at­tack the bear­ing, shaft align­ment, and seal.

When you ex­am­ine a failed pump, you will no­tice an ap­pear­ance that re­sem­bles a sponge-like tex­ture or miss­ing ma­te­rial. De­pend­ing on the pump de­sign and op­er­at­ing char­ac­ter­is­tics, the bear­ing may fall vic­tim first, al­low­ing ex­ces­sive shaft move­ment and a col­li­sion of the im­peller with the hous­ing. Mi­nor cav­i­ta­tion will re­sult in de­creased out­put or pres­sure. It is im­per­a­tive that you are cog­nizant of the sound and pres­sure/flow char­ac­ter­is­tic of your pumps. Cav­i­ta­tion caught in the early stages will have min­i­mal to no im­pact on pump life.

types of cav­i­ta­tion

Suc­tion side cav­i­ta­tion oc­curs when a pump is un­der low pres­sure or ex­ces­sive vac­uum. The pump is be­ing starved of liq­uid and is not be­ing fed enough flow. At that time, bub­bles form at the eye of the im­peller (where it con­nects to the shaft). As these bub­bles move over to the dis­charge re­gion, the fluid con­di­tion is al­tered and the bub­bles are com­pressed into a liq­uid, caus­ing them to im­plode against the face of the im­peller. An im- peller sub­jected to suc­tion side cav­i­ta­tion will have pieces of ma­te­rial miss­ing.

Dis­charge cav­i­ta­tion is the re­sult of the dis­charge pres­sure be­ing ex­tremely high, so that it is dif­fi­cult for the liq­uid to va­cate the pump. It then cir­cu­lates around the im­peller and hous­ing caus­ing a very high vac­uum at the wall and the for­ma­tion of bub­bles. Dis­charge cav­i­ta­tion al­lows the im­plod­ing bub­bles to cre­ate in­tense shock waves, re­mov­ing ma­te­rial from the hous­ing and im­peller. In ex­treme dis­charge cav­i­ta­tion cases, the im­peller shaft may even break.

The most com­mon cause is a flow re­stric­tion or run­ning the pump at a speed that is out of its op­er­at­ing range. The flow is­sue can be ei­ther on the suc­tion or pres­sure side, or a cu­mu­la­tive ef­fect of both. Proper and timely main­te­nance of fil­ters and screens goes a long way in pre­vent­ing cav­i­ta­tion. Keep in mind that on a sprayer or other ap­pli­ca­tion with aged rub­ber hoses, they can be col­laps­ing slightly and lim­it­ing suc­tion per­for­mance and evok­ing cav­i­ta­tion.

Plumb­ing de­sign such as pipe di­am­e­ter and the amount of turns and the sharp­ness of them will po­ten­tially cre­ate ei­ther suc­tion or dis­charge cav­i­ta­tion. You may have up­graded to a larger pump and now the fac­tory pip­ing can­not sup­port it. No fluid likes to make turns; this will re­sult in a flow re­stric­tion, both on the feed and dis­charge sides.

If a pump does fail, you need to take it apart and de­ter­mine if it was the re­sult of cav­i­ta­tion. On an en­gine, the coolant (wa­ter) pump seal can fail pre­ma­turely if the rpm is brought too high while the ther­mo­stat is closed. Dur­ing that time, the coolant is be­ing forced through a small by­pass hose or pas­sage. Ex­ces­sive en­gine speed even un­der no load will cause the suc­tion side of the pump to ex­pe­ri­ence a very high vac­uum and, over time, vi­o­late the pump seal and leak from the weep hole. On any en­gine, the rpm should be mod­er­ated when the coolant is be­low the tem­per­a­ture of the ther­mo­stat open­ing point.

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