Avoiding Pump Cavitation in Open Systems: The Dynamic Relationship Between NPSH, Vapor Pressure and Temperature
/By Chad Edmondson
Anyone involved in the design or application of fluid pumping systems is familiar with NPSH or Net Positive Suction Head. We typically equate NPSH with one singular question: Is there enough pressure at the suction of a pump to ensure that the pump doesn’t cavitate? We associate cavitation with noise, inefficiency, and broken pumps. While all that seems pretty straightforward, it’s actually a bit more complicated than many understand or remember from their last exposure to the subject. This short series fills in all the blanks about NPSH and its relevance to pump selection so that mechanical professionals truly have the tools they need for selecting a pump that will not cavitate.
What is Cavitation?
Before we even begin to talk about what NPSH is, it’s important to understand what cavitation is. A firm grip on this noisy, costly phenomenon will help you better understand NPSH and why it is such a crucial part of pumping design, particularly in open systems. Why open systems? Because open systems only have atmospheric pressure to work with.
If the pressure of the pumping water drops below its vapor pressure, vapor pockets form. You are basically making pockets of steam in what is supposed to be a hydronic system. When the pressure of the water is subsequently increased above its vapor pressure, these vapor pockets will collapse, creating a powerful implosion of up to 100,000 PSI. These powerful implosions create any number of unwanted circumstances, including:
Excessive noise
Pump impeller damage due to vapor bubble formation and collapse
Drastic changes to the pump curve because pumps cannot deliver both liquid and vapor
Broken pump shaft due to slugging of the impeller against alternate bodies of liquid and vapor
Pump seal failure because the vapor flash causes “dry” seal operation and rapid wear
Required NPSH: A Moving Target
What would cause the vapor pressure (the pressure at which water begins to boil) of the system water to drop?
Many do not realize that all centrifugal pumps have an internal pressure drop that occurs between the suction and the discharge. That is why all pump manufacturers publish required NPSH for their pumps at specific flow rates. For a given pump curve the required NPSH increases with flow. So, the amount of pressure at the suction of the pump (which is what NPSH is) has to be higher than the pressure at the pump suction (Ps) flange MINUS the minimum pressure that may occur inside the pump at the impeller vane inlet (Pv):
Required NPSH = Ps - Pv
Cavitation occurs when the pressure inside the pump drops below the vapor pressure, causing vapor pockets inside the pump to form and then implode when the pressure suddenly increases.
The internal pressure drop of the pump is one consideration. Another has to do with the dynamic relationship between temperature and vapor pressure of water.
Did you know that 85°F water can boil? At any given temperature, if the pressure imposed on the water drops below its vapor pressure, it will begin to flash into steam. For 85°F water the water vapor pressure is -14 PSIG. This means if you drop the pressure on 85°F water to -14 PSIG it will turn into steam! You can even turn 33°F water into steam if the pressure is low enough. The following chart shows the vapor pressure of water at various temperatures.
The moral of the story is that available NPSH (NPSHA) is to a certain extent a moving target. Engineers have to be very careful with open systems to make sure they select pumps that can safely operate within the limits of its required NPSH (NPSHR).
Over the next few weeks we’ll show you how to master all the variables that impact both NPSHA and NPSHR so that your pump selections are quiet, efficient and long-lasting.
