WHAT IS A CAVITATION?
HOW TO DEFINE THAT THE PUMP IS CAVITATING?
HOW TO CALCULATE THE AVAILABLE NPSH?
HOW TO SELECT A PUMP WITH THE PROPER NPSH?
HOW TO PREVENT THE CAVITATION?
CAVITATION - Cavitation is a phenomenon in which the static pressure of the liquid reduces to below the liquid's vapor 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 pump wet parts.
WHY CAVITATION HAPPENS IN THE PUMP
Cavitation, illustrated in figure, is a phenomenon that occurs when vapor bubbles form and move along the vane of an impeller. (What causes the vapor bubbles to form in the first place is discussed shortly.) As these vapor bubbles move along the impeller vane, the pressure around the bubbles begins to increase.
When a point is reached where the pressure on the outside of the bubble is greater than the pressure inside the bubble, the bubble collapses. It does not explode, it implodes. This collapsing bubble is not alone, but is surrounded by hundreds of other bubbles collapsing at approximately the same point on each impeller vane.
EVIDENCES OF CAVITATION
DROP OF PUMP PERFORMANCE (NOT ENOUGH HEAD OF PUMP)
ABSENCE OR UNSTABLE FLOW OF PUMP.
EROSION OF IMPELLERS.
VACUUM METER READINGS.
PUMP OPERATION AT BIG FLOW ON THE RIGHT BORDER O BEYOND ALLOWABLE OPERATING RANGE.
When pump cavitates a very specific noise comes from the pump it sounds like rattling stones.
EROSION OF IMPELLER
Erosion of impeller is a mechanical damages of impellers.
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.
The scheme illustrates the definition of NPSH.
NPSH (net positive suction head) and cavitation are described in great detail, along with a number of examples. NPSH is one of the least-understood principles of pump hydraulics and is the cause for a great many pump problems. It is also often mistakenly blamed for other unrelated pump problems that nevertheless have similar symptoms, such as air in a pump system or misalignment.
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.
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?
NPSH3 - corresponds to the moment, when the pump head drop down on 3 %. This occurs when developed cavitation begins in the pump and the gas cavity is large enough to block part of the impeller channel. The pump blades does not work along the entire length, therefore, the pump head decreases by 3%, erosive wear of the impeller begins. In order to prevent cavitation that affects the QH characteristic of the pump, we need to know the value at which the pump will not cavitate. The pump manufacturer must specify the minimum NPSH value when the cavitation starts in the pump i.e. NPSH3.
It means that we need to have the NPSH with some safety margin.
This parameter is called the NPSHR (NET POSITIVE SUCTION HEAD REQUIRED)
NPSHR is defined relative to the critical NPSH3 with a certain margin.
The margin can be selected depending on the material of the impeller or depending on the application of the pump and its power.
Recommendations for selecting NPSH margin can be found in the standard ANSI/HI 9.6.1-2017 – Rotodynamic Pumps Guideline for NPSH Margin
The typical NPSHR curve from the catalog for three different impeller diameters
What is necessary to remember about NPSH.
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.
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.
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
NPSHA=Hatm + Hin-Hvapour-hlosses
NPSHA=Hatm - Hin - Hvapour - hlosses
THE CONDITION OF PUMP OPERSTION WITHOUT CAVITATION
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
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.
THE LEVEL OF LIQUID IN THE SUCTION TANK IS TOO LOW. INCREASE THE LEVEL.
PUMP OPERATES WITH THE HIGH FLOW NPSHR IS TOO HIGH. THE PUMP CAN BE OVERSIZED, REDUSE THE FLOW BY THE THROTTLING VALVE.
THE SUCTION STRAINER IS CLOGGED. CLEAN THE STAINER.
THE RESISTANCE OF THE SUCTION LINE IS TOO HIGH. INCREASE THE DIAMETER OD THE SUCTION LINE. REDUCE THE NUMBER OF BENDS.
THE VALVE AT THE SUCTION LINE IS NOT FULLY OPEN OR CLOGGED. CHECK THE VALVE AT THE SUCTION LINE.