In the field of fluid transportation, cavitation is a critical factor affecting the performance and longevity of In fluid transportation, cavitation significantly impacts pump performance and lifespan. Understanding Net Positive Suction Head Available (NPSHa) and Net Positive Suction Head Required (NPSHr) is crucial, especially for screw pumps.
I. Net Positive Suction Head Available (NPSHa)
(1) Definition
NPSHa is the extra energy per unit weight of liquid at the pump’s suction port above the vaporization pressure, measured in meters of liquid column (m). It shows the liquid’s actual energy reserve at the inlet, preventing vaporization and cavitation.
(2) Calculation and Influencing Factors
The formula for NPSHa is: NPSHa = P0/(ρg) + h0 – Pv/(ρg). Here, P0 is the absolute pressure at the suction port, ρ is liquid density, g is gravitational acceleration, h0 is the height difference between the suction port and the liquid level (positive if the level is above, negative otherwise), and Pv is the liquid’s vaporization pressure at the operating temperature.
- Liquid Properties: Different liquids have different Pv. For example, hot water has a higher Pv than cold water. When transporting hot water, NPSHa is lower, increasing cavitation risk.
- Installation Height: Higher installation means more negative h0, reducing NPSHa. If too high, it may cause cavitation.
- Suction Pipeline Resistance: Valves, elbows, etc. in the suction pipeline increase resistance, lower P0, and reduce NPSHa.
II. Net Positive Suction Head Required (NPSHr)
(1) Definition
NPSHr is the minimum extra energy per unit weight of liquid at the pump inlet to avoid cavitation, also in meters of liquid column (m). It depends on the pump’s structure and performance, reflecting its cavitation sensitivity.
(2) Determining Factors
- Pump Design: Different pump types have different NPSHr due to impeller shape, blade number, etc. Low – specific – speed centrifugal pumps need more NPSHr.
- Pump Speed: Higher speed leads to faster liquid flow, more pressure change, and larger NPSHr as the liquid nears vaporization at the impeller inlet.
III. Relationship between the Two and Cavitation Judgment
(1) Relationship
For no cavitation, NPSHa must be greater than NPSHr. A larger difference means better cavitation – resistance and more reliable operation. When NPSHa approaches or is less than NPSHr, cavitation risk rises.
(2) Cavitation Judgment
When NPSHa < NPSHr, liquid at the inlet may vaporize into bubbles. In the high – pressure zone, these bubbles burst, damaging components like the impeller. The pump’s performance drops, with lower flow, head, and efficiency, plus vibration and noise.
IV. NPSHa and NPSHr of Screw Pumps
(1) NPSHa of Screw Pumps
Screw pumps’ NPSHa is affected by the above factors. They’re for high – viscosity liquids, which increase suction pipeline resistance, lower P0, and reduce NPSHa. So, choose and layout the pipeline carefully, like using a larger – diameter pipe and fewer elbows to increase NPSHa.
(2) NPSHr of Screw Pumps
Screw pumps’ structure gives them relatively stable liquid flow, so their NPSHr may be lower than some centrifugal pumps. But when handling high – viscosity liquids or at high speeds, NPSHr increases due to higher resistance and centrifugal force. Thus, accurately assess NPSHr when choosing a screw pump to ensure NPSHa > NPSHr for stable operation.
