Cavitation at the pump (2)

In the first part of the paper earlier, we have to know what it is cavitation, its effect and the classification of cavitation, namely:
1 Vaporisation - Evaporation.
Next we examine briefly the second classification
2 Water ingestion - The entry of outside air into the System
Centrifugal pumps are able to handle only 0.5% of the total volume of air. More than 6% air, the consequences could be very dangerous, it can damage the pump components.
Air can enter the system through several causes, including:

     Of the stuffing box packing (Section A - See Figure). It happens, if the pump from the condenser, evaporator or other equipment working in a vacuum.
     Location of valve in the line above the water surface (water line).
     Flange (pipe joints) are leaking.
     The pull air through the vortex fluid (fluid vortexing).
     If 'bypass the line' it is too close to the suction side, this will increase the air temperature on the suction side.
     Reduced fluid on the suction side, this can occur if the fluid level is too low.

vortexing Fluid

Both evaporation and entry of air into the system greatly affect the performance of the pump when the air bubbles that burst when passing through the 'eye of the impeller' (Part G - See Picture) until the exit side (side with higher pressure). Sometimes, in some cases can damage the impeller or casing. The greatest effects of the air trap is the reduced capacity of the pump.
3 Internal Recirculation - Turn on the Circulation System
This condition can be seen on the outer corner (leading edge) of the impeller, close to the outer diameter, rotates back into the center of the fan. He can also occur at the beginning of the suction side of the pump.
This behind-round effects can increase the speed until it evaporates and then 'broke' when through a higher pressure. It always happens at the pump with a low NPSHA. To overcome this, we must know the value of Suction Specific Speed, which can be used to control the pump during operation, what is the safest nearby on the value of BEP (Best Efficiency Point) pump that must be taken to prevent the occurrence of problems.
Suction Specific Speed ​​value allowed is between 3,000 to 20,000. The formula used is:

where:
  rpm = Speed ​​Pump
Capacity = Gallons per minute, or liters per second from the largest impeller on the value of BEP (Best Efficiency Point) it.
Net Positive Suction Head = Head is Required (feet or meters) on the rpm value.
Important note:

     For a double suction pump, capacity divided by 2 because there are 2 impeller eyes.
     Ideal to 'buy' pump with Suction Specific Speed ​​value is less than 8500 (5200 metric) except for extreme conditions.
     Mixed Hydrocarbon and hot water ideally at 9000 ÷ 12000 (5500 ÷ 7300 metric) or higher, the better.
     Suction Specific Speed ​​value that indicates high impeller eye is larger than usual and efficiency values ​​are usually adjusted with a low NPSHR value.
     Higher value of Suction Specific Speed ​​requires special design, operation allows for cavitation.
     Typically, the pump is operating below 50% of its BEP value is not reliable.

If we use an open impeller, we can correct by adjusting the adjustment of internal recirculation (clearance) impeller according to manufacturers' specifications.

type of impeller

For Closed Impeller type of problem and many more in practice be returned to the manufacturer for evaluation or maybe redesigned impellernya or change in size adjustment (clearance) on wearing the ring.