Technical Library

UNDERSTANDING NPSH

Definition of NPSH, NPSHa and NPSHr

The NPSH is the total suction head in feet of liquid (or meters), minus the vapor pressure (in feet or meters) of the liquid being pumped.

Think of height as an energy level and not as a pressure force. All values ​​are absolute.

NPSHa is measured at the pump centerline or at the impeller eye. These two things can be in different locations or elevations. Think of NPSHa as the available energy level of the liquid at the pump inlet or impeller eye. The liquid will turn to vapor if there is not enough NPSHa. Do not confuse NPSHa with suction pressure. While suction pressure is somewhat a component of the mix, there is something more complex with this issue.

NPSHa is the amount of NPSH that the system has available in the eye of the pump impeller. This NPSHa value is entirely a function of the liquid, its properties, environmental conditions, and the design and geometry of the suction system. Basically, the calculation is about the suction system itself and has nothing to do with the pump. This calculation must be completed by the end user and / or his engineer or consultant. For liability reasons, manufacturers are normally instructed not to participate in customer calculations; however, as time goes on, the manufacturer is getting more involved.

The required NPSH (NPSHr) is determined by the pump manufacturer using empirical methods and using standards and specifications. NPSHr values ​​are normally reported on pump performance curves.

The NPSH margin is how much the NPSHa value exceeds the NPSHr. There are tables for recommended or suitable margins, and the higher the margin, the better.

Summarizing

(NPSH)

NPSH can be defined in two parts:

NPSH Available (NPSHA): The absolute pressure at the pump suction.

Y

NPSH Required (NPSHR): The minimum pressure required at the pump suction to prevent the pump from cavitating.

NPSHA is a function of your system and must be calculated, while NPSHR is a function of the pump and must be provided by the pump manufacturer. NPSHA must be greater than NPSHR for the pump system to operate without cavitation. In other words, you must have more pressure available on the suction side than the pump requires.

The formula

NPSHA: NPSHA = HA ± HZ – HF + HV – HVP

TERMODEFINICIONANOTACIONES
HaLa presión absoluta sobre la superficie del líquido en el tanque de suministro.  Típicamente presión atmosférica (tanque de suministro abierto), pero puede ser diferente para tanques cerrados. • No olvide que la altitud afecta la presión atmosférica • Siempre positivo (puede ser bajo, pero incluso los recipientes de vacío tienen una presión absoluta positiva)
HzLa distancia vertical entre la superficie del líquido en el tanque de suministro y la línea central de la bomba.Puede ser positivo cuando el nivel de líquido está por encima de la línea central de la bomba (llamada altura estática) • Puede ser negativo cuando el nivel de líquido está por debajo de la línea central de la bomba (lo que se denomina altura de elevación) • Asegúrese siempre de utilizar el nivel de líquido más bajo permitido en el tanque.
HfPérdidas por fricción en la tubería de succiónLas tuberías y los accesorios actúan como una restricción, trabajando contra el líquido a medida que fluye hacia la entrada de la bomba.
HvAltura debido a la velocidad en la aspiración  de la bombaA menudo no se incluye, ya que normalmente es bastante pequeño este valor.
HvpPresión de vapor absoluta del líquido a la temperatura de bombeoDebe restarse al final para asegurarse de que la presión de entrada se mantenga por encima de la presión de vapor. • Recuerde, a medida que aumenta la temperatura, también lo hace la presión de vapor.

Hopefully now you feel more informed about NPSH and its importance when selecting a pump. A basic understanding can go a long way toward identifying potential problems before they occur. Moving liquids from underground tanks or rail cars, extracting thick liquids over long distances or through hoses, handling high vapor pressure liquids like LPG or alcohol … these are just a few examples of applications that present the highest risk of failure for the engineer who does not understand the NPSH.

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