CAVITATION IN A PUMP

WHAT IS A CAVITATION?

HOW TO DEFINE THAT THE PUMP IS CAVITATING?

HOW TO PREVENT THE CAVITATION?

HOW TO CALCULATE THE AVAILABLE NPSH?

HOW TO SELECT A PUMP WITH THE PROPER NPSH?

CAVITATION - Cavitation is a phenomenon in which the static pressure of the liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, called "bubbles" or "voids", collapse and can generate shock waves that may damage machinery. These shock waves are strong when they are very close to the imploded bubble, but rapidly weaken as they propagate away from the implosion.
WHY CAVITATION HAPPENS IN THE PUMP

When the bubbles go further to the area with the higher pressure It collapses. At this place, the local water hummer occurs the pressure inside this microzones can reach a very high value. 
This local water hammer causes the removing the material from the impeller which is called erosio
n.

EVIDENCE OF CAVITATION 

  • NOISE VIBRATION

  • NOT ENOUGH HEAD OF PUMP

  • ABSENCE OR UNSTABLE FLOW OF PUMP

  • EROSION OF IMPELLERS

  • VACUUM METER READINGS 

NOISE VIBRATION 

A very specific noise comes from the pump it sounds like rattling stones.

EROSION
Impeller.png
Impeller_1.png

WHY CAVITATION HAPPENS

The boiling point of a liquid is proportional to temperature and pressure.
At normal atmospheric pressure (at sea level), water boils at a temperature of 100 C.
With a drop in pressure, the boiling point also decreases.
A low pressure area is always present at the impeller inlet.
If the pressure is low enough the water will boil at room temperature.
If the pressure at the inlet to the RK is low enough, this leads to the appearance of vapor bubbles in the liquid.

NET POSITIVE SUCTION HEAD (NPSH) this is the excess of the total pressure of the liquid in the inlet of the pump over the pressure of its saturated vapor.

Scale_cavitation.png

The scheme illustrates the definition of NPSH. 

NET POSITIV SUCTION HEAD 3% (NPSH3)

NET REQUIRED SUCTION HEAD

This is the standard Q-H curve we usually use for the pump selection. We pay attention to the flow head and the efficiency. But not always pay attention to NPSH.

PUMP CAVITATION CHARACTERISTICS 

PUMP CAVITATION TEST

 

The procedure of the cavitation test.

NPSH 3 is defined for the different pump flows.

For every operating point, the inlet pressure is being lowered. We see on the graph that the pump head stays stable. But at some moment the pump head starts to drop. When the pump head drops on 3% the value of the inlet pressure and corresponding NPSH value is been fixed. This value of NPSH is NPSH3%. this procedure is repeated for several pump flow values. for example 

Q=2338 m3/h and 1458 m3/h.

2338 HD.png
1458.png

After measuring NPSH3 for different flows from the right border operating range to the minimum flow of the pump, all points are been plotted on the graph - the horizontal axis is the pump flow, the vertical axis is NPSH3. All points are connected by the spline and the NPSH3 curve is received.

NPSHR and NPSH3 what is the difference?

The NPSH3 is the moment when the pump cavitates and bubbles bibles destroy the impeller.

To prevent the cavitation which influences the pump Q-H curve we need to know the value when the pump will not cavitate. The pump manufacturer should provide the minimum NPSH value when the pump does not cavitate so that it influences the Q-H curve. 

It means that we need to have the NPSH with some safety margin. 

This parameter is called the NPSHR (NET POSITIVE SUCTION HEAD REQUIRED)  

The typical NPSHR curve from the cataloge for three different impeller diameters

NPSH_curve_300.png

What is necessery to remember about NPSH.

  1. The shape of the NPSHR rises steep at big flows. Check the NPSHR not only at the rated point but also at the maximum flow.

  2. The impeller with less diameter has bigger NPSH. It is necessary to remember when you are going to trim the impeller and check the NPSHR when you select the impeller with less diameter.

  3. NPSH depends on the speed of rotation.  

NET AVAILABLE SUCTION HEAD

NPSHA=Hatm - + Hin-Hvapour-hlosses

For the standard conditions and water, this formula can be simplified.

NPSHA=10 - or + Hin-Hvapour-hlosses
Sheme_cavitation.png
NPSHA=Hatm Hin-Hvapour-hlosses
Cavitation schem.png
NPSHA=Hatm - Hin - Hvapour - hlosses
THE CONDITION OF PUMP OPERSTION WITHOUT CAVITATION 
NPSHR>NPSHA
NPSH CHARACTERISTICS
Cavit_level.png
NPSHAA.png

If the liquid level drops in the suction tank the NPSHA lowers. At some moment the main condition NPSHR < NPSHA is not met. Cavitation can start,

HOW TO TAKE INTO  ACCOUNT CAVITATION AT PUMP SELECTION

DO NOT FORGET TO CHECK THE CAVITATION  DURING  PUMP SELECTION

THE MAIN CONDITION FOR CAVITATION FREE PUMP OPERATION

NPSHA > NPSHR

NPSHR (request form)> NPSHR (selected pump)

IF THIS CONDITION CAN NOT BE MET. 

NPSHR IS HIGHER. WHAT TO DO?

Select the pump with lower NPSHR

It can be pump with less efficiency.

SELECT A PUMP OF OTHER TYPE

SELECT A DOUBLE SUCTION PUMP NOT A END SUCTION. 

PUMPS WITH DOUBLE SUCTION IMPELLER HAVE LESS NPSHR.

SELECT A PUMP WITH LOWER SPEED

IF PUMP WITH LESS IMPELLER IS SELECTED.

USE THE CAVITAION CURVE FOR THE PROPER IMPELLER DIAMETER

RECOMMENDATIONS

IF YOU WANT TO REPLACE OLD PUMP BY NEW PUMP.
DO NOT FROGET TO COMPARE NPSHR

Probably old pump has lower NPSH.
For example. Old low speed pump was replaced with the new with higher efficiency and higher speed AND higher NPSHR.

HOW TO AVOID CAVITATION IN A PUMP?
  1. THE LEVEL OF LIQUID IN THE SUCTION TANK IS TOO LOW. INCREASE THE LEVEL.

  2. PUMP OPERATES WITH THE HIGH FLOW NPSHR IS TOO HIGH. THE PUMP CAN BE OVERSIZED, REDUSE THE FLOW BY THE THROTTLING VALVE.

  3. THE SUCTION STRAINER IS CLOGGED. CLEAN THE STAINER.

  4. THE RESISTANCE OF THE SUCTION LINE IS TOO HIGH. INCREASE THE DIAMETER OD THE SUCTION LINE. REDUCE THE NUMBER OF BENDS.

  5. THE VALVE AT THE SUCTION LINE IS NOT FULLY OPEN OR CLOGGED. CHECK THE VALVE AT THE SUCTION LINE.