Net Positive Suction Head (NPSH) and Suction Head
When a pump is operating, the liquid at the impeller inlet generates vapor under a certain vacuum pressure. The vaporized bubbles, through the impact of liquid particles, erode the metal surfaces of the impeller, causing damage. This vacuum pressure is called the vaporization pressure. NPSH refers to the excess energy per unit weight of liquid at the pump inlet, exceeding the vaporization pressure. It is expressed in meters and denoted by (NPSH)r.
Suction head, also known as the required NPSH Δh, is the vacuum level the pump can draw liquid from, i.e., the allowable installation height of the pump, also expressed in meters. Suction head = Standard Atmospheric Pressure (10.33 m) - NPSH - Safety Capacity (0.5 m). Standard atmospheric pressure can withstand a pipeline vacuum height of 10.33 meters.
For example: A pump has an NPSH of 4.0 m. Calculate the suction head Δh.
Solution: Δh = 10.33 - 4.0 - 0.5 = 5.83 m
What are the respective units of measurement and their corresponding letters?
Net Positive Suction Head (NPSH) refers to the difference between the total head of the liquid at the pump inlet and the pressure head when the liquid vaporizes. It is measured in meters (water column) and denoted by (NPSH). Specifically, it is divided into the following categories:
NPSHa – System NPSH, also called effective NPSH; the larger the value, the less prone to cavitation.
NPSHr – Pump NPSH, also called required NPSH or pump inlet dynamic pressure drop; the smaller the value, the better the cavitation resistance.
NPSHc – Critical NPSH, which refers to the NPSH corresponding to a certain decrease in pump performance.
[NPSH] – Allowable NPSH, the NPSH used to determine pump operating conditions, usually taken as [NPSH] = (1.1～1.5)NPSHc.
What is the difference between required NPSH and effective NPSH?
NPSH is divided into effective NPSHa and required NPSHr. The required net positive suction head (NPSH) of a pump is a characteristic determined by the pump's design, while the effective NPSH is determined by the process piping.
For a given pump, the required NPSH at a given speed and flow rate is called the required NPSH, commonly denoted as NPSHr. Also known as the pump's NPSH, it is a specified cavitation performance parameter that the pump must achieve.
NPSHr is related to the internal flow of the pump and is determined by the pump itself. Its physical meaning is the degree of pressure drop of the liquid at the pump inlet. In other words, to prevent cavitation, the liquid per unit weight at the pump inlet must have excess energy exceeding the vaporization pressure head.
The required NPSH is independent of the system parameters and only depends on the motion parameters (vo, wo, wk, etc.) of the pump inlet. These motion parameters are determined by geometric parameters at a given speed and flow rate. That is to say, NPSHr is determined by the pump itself (the geometric parameters of the suction chamber and impeller inlet).
For a given pump, regardless of the medium (except for highly viscous media which affect velocity distribution), at a given speed and flow rate, the pressure drop (NPSHr) is the same when the fluid flows through the pump inlet because the velocity is constant. Therefore, NPSHr is independent of the liquid properties (thermodynamic factors are not considered).
A smaller NPSHr indicates a smaller pressure drop, requiring a smaller NPSHa from the pump, thus resulting in better cavitation resistance. Therefore, 'r' represents 'required,' determined by the pump itself and specifically related to speed, impeller type, etc.
The effective net positive suction head (NPSHa) is the NPSH determined by the pump's installation conditions. Also known as the system's NPSH, it represents the excess energy per unit weight of liquid at the pump inlet provided by the suction system, exceeding the vaporization pressure head.
A larger NPSHa means the pump is less prone to cavitation. The effective NPSH is related to system parameters and liquid properties (p, pv, etc.). Because the hydraulic losses of the suction system are proportional to the square of the flow rate, NPSHa decreases with increasing flow rate.
Therefore: A represents available and effective, which is determined by the system and piping and must be rigorously calculated;
To ensure the pump does not cavitate, NPSHa must be greater than NPSHr. The specific difference varies depending on the type of pump and has empirical values; generally, an allowable NPSH is defined as the pump's required NPSH plus a margin of 0.5-1m.