Cavitation is a detrimental condition that often occurs in centrifugal pumping units. Cavitation can reduce pump efficiency, cause vibration and noise, and lead to serious damage to the pump's impeller, pump housing, shaft, and other internal parts. Cavitation occurs when the pressure of the fluid in the pump drops below the vaporization pressure, causing vapor bubbles to form in the low-pressure area. These vapor bubbles collapse or "implode" violently when they enter the high-pressure area. This can cause mechanical damage inside the pump, create weak points that are susceptible to erosion and corrosion, and impair pump performance.

Understanding and implementing strategies to mitigate cavitation is critical to maintaining the operational integrity and service life of the nā pumps casing split .

Types of Cavitation in Pumps

To reduce or prevent cavitation in a pump, it is important to understand the different types of cavitation that can occur. These types include:

1.Vaporization cavitation. Also known as "classic cavitation" or "net positive suction head available (NPSHa) cavitation," this is the most common type of cavitation. Split casing pumps increase the velocity of the fluid as it passes through the impeller suction hole. The increase in velocity is equivalent to a decrease in fluid pressure. The pressure reduction may cause some of the fluid to boil (vaporize) and form vapor bubbles, which will collapse violently and produce tiny shock waves when they reach the high-pressure area.

2. Turbulent cavitation. Components such as elbows, valves, filters, etc. in the piping system may not be suitable for the amount or nature of the pumped liquid, which can cause eddies, turbulence and pressure differences throughout the liquid. When these phenomena occur at the inlet of the pump, they can directly erode the inside of the pump or cause the liquid to vaporize.

3. Blade syndrome cavitation. Also known as "blade pass syndrome", this type of cavitation occurs when the impeller diameter is too large or the internal coating of the pump housing is too thick/the pump housing inner diameter is too small. Either or both of these conditions will reduce the space (clearance) within the pump housing to below acceptable levels. The reduction in clearance within the pump housing causes the fluid flow rate to increase, resulting in a decrease in pressure. The pressure reduction may cause the fluid to vaporize, creating cavitation bubbles.

4.Internal recirculation cavitation. When a center-split pump is unable to discharge fluid at the required flow rate, it causes some or all of the fluid to recirculate around the impeller. Recirculating fluid passes through low and high pressure areas, which generates heat, high velocity, and forms vaporization bubbles. A common cause of internal recirculation is running the pump with the pump outlet valve closed (or at a low flow rate).

5. Air entrainment cavitation. Air can be drawn into the pump through a failed valve or loose fitting. Once inside the pump, the air moves with the fluid. The movement of the fluid and air can form bubbles that "explode" when exposed to the increased pressure of the pump impeller.

Factors that contribute to cavitation - NPSH, NPSHa, and NPSHr

NPSH is a key factor in preventing cavitation in split casing pumps. NPSH is the difference between the actual suction pressure and the vapor pressure of the fluid, measured at the pump inlet. NPSH values must be high to prevent the fluid from vaporizing within the pump.

NPSHa is the actual NPSH under the pump's operating conditions. Net positive suction head required (NPSHr) is the minimum NPSH specified by the pump manufacturer to avoid cavitation. NPSHa is a function of the suction piping, installation, and operating details of the pump. NPSHr is a function of pump design and its value is determined by pump testing. NPSHr represents the available head under test conditions and is typically measured as a 3% drop in pump head (or first stage impeller head for multistage pumps) to detect cavitation. NPSHa should always be greater than NPSHr to avoid cavitation.

Strategies to Reduce Cavitation - Increase NPSHa to Prevent Cavitation

Ensuring that NPSHa is greater than NPSHr is critical to avoiding cavitation. This can be achieved by:

1. Lowering the height of the split casing pump relative to the suction reservoir/sump. The level of fluid in the suction reservoir/sump can be increased or the pump can be mounted lower. This will increase NPSHa at the pump inlet.

2. Increase the diameter of the suction piping. This will reduce the velocity of the fluid at a constant flow rate, thereby reducing suction head losses in piping and fittings.

2.Reduce head losses in fittings. Reduce the number of joints in the pump suction line. Use fittings such as long radius elbows, full bore valves, and tapered reducers to help reduce suction head losses due to fittings.

3.Avoid installing screens and filters on the pump suction line whenever possible, as they often cause cavitation in centrifugal pumps. If this cannot be avoided, ensure that screens and filters on the pump suction line are regularly inspected and cleaned.

5. Cool the pumped fluid to reduce its vapor pressure.

Understand NPSH Margin to Prevent Cavitation

NPSH margin is the difference between NPSHa and NPSHr. A larger NPSH margin reduces the risk of cavitation because it provides a safety factor to prevent NPSHa from falling below normal operating levels due to fluctuating operating conditions. Factors that affect NPSH margin include fluid characteristics, pump speed, and suction conditions.

Maintaining the Minimum Pump Flow

Ensuring that a centrifugal pump is operating above the specified minimum flow is critical to reducing cavitation. Operating a split case pump below its optimal flow range (allowable operating area) increases the likelihood of creating a low pressure area that can induce cavitation.

Impeller Design Considerations to Reduce Cavitation

The design of the impeller plays an important role in whether a centrifugal pump is prone to cavitation. Larger impellers with fewer blades tend to provide less fluid acceleration, which reduces the risk of cavitation. Additionally, impellers with larger inlet diameters or tapered blades help manage the flow of fluid more smoothly, minimizing turbulence and bubble formation. Using materials that resist cavitation damage can extend the life of the impeller and pump.

Using Anti-Cavitation Devices

Anti-cavitation devices, such as flow conditioning accessories or cavitation suppression liners, are effective in mitigating cavitation. These devices work by controlling the fluid dynamics around the impeller, providing a steadier flow and reducing the turbulence and low-pressure areas that cause cavitation.

The Importance of Proper Pump Sizing in Preventing Cavitation

Selecting the right pump type and specifying the correct size for a specific application is critical to preventing cavitation. An oversized pump may not operate as efficiently at lower flows, resulting in an increased risk of cavitation, while an undersized pump may have to work harder to meet flow requirements, which also increases the likelihood of cavitation. Proper pump selection involves a detailed analysis of maximum, normal and minimum flow requirements, fluid characteristics and system layout to ensure the pump operates within the specified operating range. Accurate sizing prevents cavitation and increases the efficiency and reliability of the pump throughout its life cycle.