Your pump is making a bunch of noise and isn’t performing as expected. You reach out to a technical support resource and they ask, “Is your pump cavitating?”. But what is cavitation? In this post, we’ll do a deep dive on what cavitation is, what causes it, and what you can do to fix it. So let’s get started.
The term cavitation is derived from the word cavity, or hollow space. In a pumping system, the lowest pressure point of the system is at the pump inlet. This is case for all types of pumps- centrifugal, rotary, ECP, AODD, ect. When the process fluid hits the low-pressure area of the inlet, if that pressure is below the vapor pressure of the fluid, we’ll start to see bubbles in the fluid. This is actually the fluid boiling.
While people mostly think about boiling as something that requires high heat, boiling is actually a term that just means a fluid changing from a liquid to a gas. All fluids have a vapor pressure. This is the pressure required to keep something liquid at a given temperature. Water, for instance, has a vapor pressure of 20 torr or 0.4 psia at room temperature. That means if we can pull a strong enough vacuum, we can actually boil water at room temperature.
As it relates to our pump system, if that inlet pressure we create in our pump system is below the vapor pressure of the fluid, the fluid will boil, creating entrained bubbles or gas in our product. As these bubbles travel through the pump, they hit the higher-pressure discharge, which increases the pressure on the product above it’s vapor pressure, causing those gas bubbles to implode and become liquid again. And every time there is a phase change, that means there is energy released- basically sending shock waves through our pump and system.
In a pumping system, this energy release, which we call cavitation, manifests itself in a few different ways. Most notably, our pumps start operating quite loudly. To use a riff on Justice Potter Stewart’s, “I know it when I see it”, cavitation is something you know when you hear it. We often describe it as akin to marbles or rocks going through the pump, but it also can sound like loud banging accompanied with pipe vibration and reduced pumping efficiency (i.e. less flow than expected).
And sometimes, we don’t always need to hear it- we can see it too. Pitting and erosion will often occur on the pump impeller or on the body of a rotary lobe or ECP pump.
So what causes this low pressure situation and what can you do to avoid it? Well as we touched on in this post as well as our last post on suction head, it’s normally caused by poor suction conditions. Briefly, let’s review the things that quantitatively contribute to suction head:
NPSHa= Pa + hs – hfs – Pvp
Where:
Pa = Pressure above fluid level (psi or ft.)
hs= Static Suction Head (ft.)
hfs= Pressure drop in suction line (ft.)
Pvp= Vapor Pressure of the fluid (psia)
The biggest thing we can control here is hfs or our pressure loss in the suction line. What causes this? Think things like long suction headers, sharp turns, reduced tube diameters or other flow restrictions. If we’re seeing inlet side cavitation, we want to minimize these losses before trying to increase the static suction head available (elevating the supply). Since we’re probably not going to be able to do much about the pressure in the atmosphere or the vapor pressure of the fluid, these are the tools we have- run the pump slower, reduce suction side loss, and increase static head.
We also want to be mindful of discharge cavitation. While we’ve spent most of this post talking about suction side cavitation, it is also possible to see cavitation in the discharge line of the pump. This will occur when the discharge head of the system is so high that we see slip back of fluid through the pump. When high discharge pressures cause fluid to slip back through tight spaces- like that the gaps between rotor and pump casing or centrifugal pump impeller and volute, we get a classic venturi effect- an increase in velocity and decrease in pressure. If that pressure decreases below the vapor pressure of the fluid, we will again see cavitation. To correct this, we need to make sure we’re selecting the proper pump, check for clogged filters or blockages downstream and revisit piping layout.
So to wrap this post up, cavitation occurs any time the fluid we’re pumping sees a pressure below it’s vapor pressure. When this happens, our liquid undergoes a phase change to a gas, creating bubbles, which when re-pressurized, collapse back into a liquid, creating mini-implosions which propagate shock waves through the system. This usually occurs on the inlet side of the pump, but can also happen on the discharge side. To correct this, focus on your inlet and discharge piping. On the inlet side, we want to make it as easy as possible for product to flow into the pump. On the discharge side, we want to make sure we have a pump that can handle to discharge head of the system and also eliminate any unnecessary downstream restrictions. As always, if you’re pump is cavitating or you have any questions about best inlet and discharge piping practices, contact a Triplex Sales Engineer today!
