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There are many factors that go into calculating the NPSHa and selecting the proper pump depending on the fluid temperature in your application. Before we dive into details, let’s go over the basics. It’s important to understand what NPSH is and why it matters.
NPSHa, or NPSH available, is the absolute pressure at the suction port of the pump. NPSHr, or NPSH required, is the minimum pressure required at the suction port of the pump to keep the pump from cavitating.
NPSHa is a function of your system and must be calculated, whereas NPSHr is a function of the pump and will be provided by the pump manufacturer. NPSHr fluctuates and will increase as you move to the right along the pump's curve. If you’re trying to pump more liquid through the pump, the pressure required increases.
Your NPSHa must be greater than NPSHr for the pump system to operate without cavitating. Put another way, you must have more suction side pressure available than the pump requires. This simply provides a safe margin for your system to operate efficiently. When NPSHr exceeds the NPSHa, your pump will start to cavitate.
Cavitation occurs when vapor bubbles form on the suction side of the pump, and then collapse on the discharge side of the pump causing damage. This might not sound like a big deal, but its equivalent to a tiny explosion going off inside the pump every time a bubble implodes. When a pump is cavitating, these vapor bubbles are constantly forming and collapsing, over and over again – causing a lot of stress. The seals and bearings will wear prematurely because the pump is constantly being shocked from different angles.
These vapor bubbles form when the liquid being pumped starts to boil. Be careful not to associate boiling with being hot to the touch. Liquid oxygen will boil and no one would ever call that hot. Fluids boil when the temperature of the fluid gets too hot or the pressure on the fluid gets too low. At ambient sea level pressure of 14.7psi, water will boil at 212°F. If you lower the pressure on the water it will boil at a much lower temperature and conversely if you raise the pressure the water will not boil until it gets to a higher temperature.
Now in order to fully understand cavitation and how it relates to NPSH and temperature, you must understand what vapor pressure is. Vapor pressure is the pressure required to boil a liquid at a given temperature. This value is strongly dependent on temperature and thus so will both NPSHr and NPSHa.
Take a look at this chart which shows the correlation of temperature to vapor pressure. Click for full resolution.
As temperature increases, so does the vapor pressure, but not at the same rate. As you work your way up from 32°F (just above freezing at sea level), the vapor pressure rises very slowly. However, as you approach and surpass temperatures of 100°F, the vapor pressure begins to exponentially increase. This concept is crucial to understand because the temperature of the liquid impacts vapor pressure, which affects the NPSH of your pump. This is especially important if you’re dealing with high temperature steam or boiler applications because the vapor pressure.
To calculate NPSHa, you must consider your atmospheric pressure, static head, friction losses, and vapor pressure. Usually water temperature is a pretty small factor when calculating NPSHa. However, when pumping fluids of high temperature, the vapor pressure will have a very large effect on your NPSHa.
To illustrate this point, let’s look at an example. We will calculate NPSHa for the same application twice, with the only difference being a change in water temperature. Everything else will remain the same and we will assume the tank is vented to the atmosphere.
Now; let's solve the equations with the two different water temperatures:
NPSHa at 80°F = 33.95 – 3 + 4 – 1.172 = 33.778 ft
NPSHa at 180°F = 33.95 – 3 + 4 – 17.825 = 17.125 ft
As you can see, the NPSH available is halfed when the water temperature was increased by 100°F. The fluid vapor pressure has a significant impact on the equation as the fluid temperature rises.
Be sure to take this into consideration when sizing a pump. If you're in Wisconsin or Upper Michigan and have any questions, please reach out to us - our engineers would love to help!