Changes in the viscosity of the liquid, lowered flow rates, and persistent unusual sounds are indicators of pump cavitation; although these effects often seem harmless, it is important the applications are assessed and provided with maintenance to prevent further damage to the pump.
By Chad Wunderlich, Distributor Development Manager, Viking Pump®
The Cause of Cavitation
Cavitation occurs when bubbles or voids form within a fluid as a result of pressure quickly dropping below vapor pressure. More technically speaking, pump cavitation results from inadequate net positive suction head (NPSH). The available NPSH (NPSHa) is a quantifiable value for the absolute pressure* at the pump’s inlet port, minus the liquid vapor pressure. NPSH required (NPSHr) is a measured value that dictates the absolute pressure* that is needed at the pump’s inlet port to prevent the pump from cavitating. This value comes from testing by the pump manufacturer and is included with the performance curves.
* For kinetic pumps, like centrifugal pumps, head is used (commonly in units of feet or meters). For positive displacement pumps, like gear pumps, pressure is used (commonly in units of PSI, bar, or feet of water).
NPSHa must be greater than NPSHr to prevent cavitation. If not, vapor-filled cavities form in the pump’s inlet and get carried by the rotation of the pump to the outlet side. Here, exposed to high discharge pressure, these cavities collapse. See Figures 2 and 3 from a video of an internal gear pump with a clear head to facilitate the view of the gears. In Figure 2, the pump is running under normal conditions, the pump is running quietly and at full capacity. In Figure 3, the suction valve has been almost entirely closed to create a low NPSHa condition. As a result, vapor bubbles are made at the left, carried by the pump rotation counterclockwise to the outlet side; vapor then collapses when exposed to high pressure. This collapsing of vapor cavities results in noise and loss of capacity.
Symptoms of a Cavitating Pump
End users and operators do not typically request for a pump to be assessed if it is not making an unusual sound; when the pump is loud this can be the first indicator that a pump is cavitating. Descriptors like ‘growly’, ‘rumbling’, or ‘gravelly’ are common ways to describe an atypically loud sound coming from a cavitating pump. Intermittent noise is the second indicator of a cavitating pump. Although users may not experience these noises at the onset of a season, as the weather begins to change, they may become more prevalent.
It is often loudest when the liquid is more viscous, the supply tank is near empty, if the pump runs faster, when the strainer has not been cleaned, or when the inlet conditions are performing poorly. The third potential indicator is the level of the flow rate, specifically if the flow rate is lower than expected. In this scenario, end users typically report the lack of speed during the emptying of tanks. This is best confirmed with a meter, but this information is commonly more anecdotal; when the pump is slow, it takes longer to move product. There is cause for concern beyond the annoyance of noise and the inconvenience of reduced flow. Over time, cavitation can result in pitting and wear to critical pump internals, resulting in unplanned downtime and costly repairs. These symptoms are a call to action to address cavitation before more serious problems arise.
Individually, the indicators – or symptoms – could be due to other root causes, including wear, relief valve bypass, or aeration. An inexperienced troubleshooter (someone who “knows enough to be dangerous”) may guess that cavitation is the root cause, and this is often correct. But before making sweeping changes to the piping or pump, it is best to verify this hypothesis with calculation and testing.
Beginning with how NPSHa is calculated. The equation is as follows:
Where NPSHa is net positive suction head available.
Ha is applied pressure at the surface of the liquid. This value equals atmospheric pressure in absolute units for liquids pulled from vented tanks, drums, totes, trucks, etc. Atmospheric pressure varies with altitude, so pumps at higher altitudes are often more prone to experiencing cavitation issues than those near sea level.
Hz is the pressure exerted due to the vertical distance between the surface of a liquid in the supply tank and the centerline of the pump’s inlet port. If the liquid level is above the pump (static suction head), the added value increases NPSHa. This value is subtracted if the liquid level is below the pump (suction lift), decreasing NPSHa.
Hf is total friction losses in the pipes, valves, fittings, strainers, etc., from the tank to the pump inlet. It is always subtracted, further decreasing NPSHa.
Hvp is the vapor pressure of the liquid at the pumping temperature. It is always subtracted, further decreasing NPSHa. The higher the vapor pressure, the easier it is to transition to a gas, causing cavitation.
It is important to note that while any units can be used (PSI, bar, ft. of water, etc.), it must be held consistent for each of these coefficients.
Calculations are recommended before pump selection, but this exercise may be moot after pump installation. First, variables may be different than what the original datasheet shows. Viscosity may vary, the roughness/inner diameters of the piping system may be worse than ‘like new’ conditions, a valve may be partly closed, or some other obstruction may be hidden inside the pipes. Running these calculations may help to give an operator an idea of what the NPSHa should be. Once a system is up and running, however it makes more sense to take a direct measurement. By installing a suction gauge at or near the pump’s inlet port, the pressure at the pump can be measured. Converting this value to an absolute pressure (adding atmospheric pressure) and subtracting the liquid’s vapor pressure solves for NPSHa. This NPSHa value will be compared to the NPSHr of the pump provided by the manufacturer. Suppose NPSHa (calculated or measured) is less than NPSHr ; in that case, it is confirmed that the original hypothesis is correct, and the symptoms of noise and loss of flow are a result of cavitation.
Cures for Cavitation
There are two paths to follow when addressing cavitation:
The first is to improve NPSHa.
Little can be done to address Ha (atmospheric pressure) or Hvp (vapor pressure) in reference to the equation above, which leads to only opportunities to improve Hz (pressure due to head) and Hf (friction losses). To improve Hz action is required to raise the minimum liquid level in the supply tank and if the possibility of lowering the pump is an option, this will be helpful. To improve Hf the reduction of the length of inlet pipes, the increase of the diameter of the inlet pipe and the reduction of the viscosity of the liquid through heating and insulating the tank and pipes will become crucial.
Ensuring the lines and strainers are clean and free from buildup is another important method of troubleshooting, as this may restrict the quality of the flow in the pump. Of course, the second path could be to reduce the pump NPSHr which involves slowing down the pump speed; this has the effect of reducing flow. If cavitation occurs intermittently (i.e., when the tank is nearly empty or the viscosity is high), then speed can be dialed down either manually or using logic controls. Increasing the size of the pump may also help, as it will aid in holding the flow at a lower pump speed.
The Cost of Inaction
Some users will accept the symptoms, learning to live with excess noise and decreased flow. But these symptoms may be early indicators of problems down the road. It is essential to understand the signs of cavitation and how to diagnose so that it can be addressed to avoid downtime and maintenance costs.
ABOUT THE AUTHOR
Chad Wunderlich is a Distributor Development Manager with Viking Pump® with more than 20 years of industry knowledge regarding pump technology. His accomplishments include training more than 2,000 Viking Pump School graduates, trainings in 15 countries, and a patent to his name.