In a centrifugal pump, the flow area at the eye of the pump impeller is smaller than either the flow area of the pump suction piping or the flow area through the impeller vanes. When the liquid being pumped enters the eye of the centrifugal pump, there is a significant decrease in the flow area. This results in an increase in flow velocity, which is accompanied by a decrease in pressure.
The greater the pump flowrate, the greater the pressure drop between the pump’s suction and the eye of the impeller. If the decrease in pressure is high enough, or the temperature is high enough, the liquid will flash vapor when the local pressure falls below the saturation pressure of the liquid being pumped. Any vapor bubbles formed as a result of pressure drop at the eye of the impeller are swept towards the oncoming impeller vanes by the flow of the liquid. When bubbles enter a region where local pressure is greater than the saturation pressure farther out on the impeller vane, the vapor bubbles will abruptly collapse, causing a physical shock to the leading edge of the impeller vane. The formation and subsequent collapse of vapor bubbles in a pump is called cavitation.
|Here a pump has been removed from service due to damage from cavitating process conditions.|
Cavitation degrades the performance of a centrifugal pump, resulting in a fluctuating flowrate and discharge pressure. The physical shock caused by the vapor bubbles creates small pits on the leading edge of the impeller vane. Each individual pit is microscopic in size, but the cumulative effect of millions of these pits formed over a period of hours or days can literally destroy a pump impeller. In addition, cavitation may cause excessive pump vibration, which could also damage pump bearings, wearing rings and associated seals.
Common Causes of Cavitation
Cavitation occurs when bubbles in the flow stream collapse after passing into higher regions of pressure, causing noise, vibration, and potentially damaging pump components, thus reducing the pump’s efficiency and limiting its ability to generate head. There are several causes for bubbles forming in the flow stream, including vaporization, recirculation, vane passing syndrome, flow turbulence and air ingestion.
Vaporization: Hydraulic fluids may contain up to 12 percent dissolved air by volume. Certain operating conditions can make this dissolved air vaporize. A fluid vaporizes when its pressure becomes too low or its temperature is too high. All centrifugal pumps have a required head (pressure) at the suction side of the pump to prevent this vaporization. This head requirement is supplied by the pump manufacturer, and it is provided with the assumption that fresh water at 20 C (68 F) is the liquid being pumped.
Since there are pipe losses from the source to the suction of the pump, the head must be determined after the losses are calculated. Alternatively, vaporization can also occur when the pump is not cooled properly by the appropriate amount of water flow. This will cause the liquid within the pump to vaporize as a result of heat buildup, and therefore cause bubbles to form. The vaporization of dissolved air can occur at the pump inlet as a result of excessive lift, a clogged or undersized reservoir breather, a poorly designed pump inlet, inadequate cooling of the pump, or clogged suction filters. In order to cure vaporization problems, the suction head must be increased, the fluid temperature lowered, the net positive suction head required (NPSHr) decreased, and/or heat buildup in the pump must be avoided.
Flow Turbulence: Fluids are always preferred to flow through the piping at a steady velocity. Agitation in the supply reservoir, clogged inlet filters, and corrosion can alter the velocity of the liquid — and any time the velocity of a fluid changes, its pressure changes. In multiple-pump arrangements, flow turbulence may occur whenever fluid streams of different velocities merge, giving rise to shear due to friction between the fluid streamlines. This scenario can be avoided by following piping layout best practices, such as:
• Providing 10 diameters of pipe between the pump suction and the first elbow.
• Locating the suction bells in separate bays so that one pump suction does not interfere with another.
Recirculation: Recirculation occurs as a result of low flowrates through the pump.
This condition is visible on the loading edge of the vane. There are two types of
recirculation, which can occur together or separately at suction side and discharge side. Both types work by the same phenomenon of reverse flow of fluid entering the pump suction nozzle. This results in high velocity vortexes either near or in the impeller eye, in the pipe close to the suction nozzle, or in the suction nozzle itself. High velocities result in low localized pressures, which may drop below the vapor pressure of the fluid, giving rise to cavitation. Cavitation damage observed on the pressure side of the inlet vanes near the eye of the impeller is a sign of suction recirculation.
In discharge recirculation, fluid exiting the pump discharge nozzle or the impeller discharge side at low rates may be reversed, resulting in high velocity vortexes in between the two directions of flow. This results in low localized pressures and these pressures may drop below the vapor pressure of the liquid, causing cavitation. This type of recirculation may result in cavitation damage at the discharge side of the impeller periphery, inside the discharge nozzle, at the cut water(s), or in the pipe close to the discharge nozzle.
The occurrence of recirculation cavitation within a pump at low flowrates is an inherent function of pump type and design. This has always been a problem with low NPSH pumps. The term suction-specific speed serves as guidance in determining how close the pump best efficiency point (BEP) is being operated to prevent the problem. However, pumps with lower suction-specific speed and lower pump-specific speed are more resistant to recirculation cavitation effects.
The Vane Passing Syndrome: This cavitation damage results when the outer diameter of the impeller passes too close to the pump cut water. This causes the velocity of the liquid to increase as it flows through the small passage, thus lowering the fluid pressure and causing local vaporization and pulsation. The bubbles that form then collapse at the higher pressure just beyond the cut water. This damage is limited to the discharge edge of the impeller shroud, the center of the impeller vane, and possibly to the pump casing downstream and directly behind the cut water. If the pump has a closed impeller, then the damage will not extend into the shrouds. This can be prevented if the minimum impeller tip to cut water clearance is 4 percent of the impeller diameter in the smaller impeller sizes (14 feet); a 6 percent clearance in the larger impeller sizes (greater than 14 feet) is recommended.
Air Ingestion: A centrifugal pump can handle 0.5 percent air by volume. If there is an increase in the percentage of air by volume, its outcome can become disastrous. In air ingestion, vapor bubbles are not formed in the eye of the impeller, but occur as a result of seeping in of external air. Air entrainment commonly occurs at the pump as a result of:
• Valves located above the fluid line
• Faulty pump shaft seal
• Incorrectly adjusted load control valves
• Leaking flanges
• Porous intake lines
• Low reservoir fluid level
• Bypass line installed too close to the suction line
• Passage through the packing stuffing box.
Air ingestion seldom causes damage to the impeller or casing. Its main effect is loss of capacity.
Cavitation Prevention & Control
If a centrifugal pump is cavitating, several changes in the system design or operation may be necessary to increase the net positive suction head available (NPSHa) above the net positive suction head required (NPSHr) to stop cavitation:
• Increasing the NPSHa: This is achieved by increasing the pressure at the suction of the pump. For example, if a pump is taking suction from an enclosed tank, either increasing the pressure in the space above the liquid or raising the level of the liquid in the tank will increase the suction pressure. It can also be achieved by decreasing the temperature of the pumped liquid, thereby decreasing suction pressure. This is accomplished by properly cooling the liquid being pumped.
• Reducing the NPSHr: The NPSHr is not constant for a given pump under all conditions. Initially, the NPSHr of a pump increases as fluid flowrate through a pump increases. Therefore, a reduction of flowrate through a pump by throttling a discharge valve decreases the pump’s NPSHr. Another factor is pump speed. The faster the impeller of a pump rotates, the greater the NPSHr. Therefore, if the speed of the pump is reduced, the NPSHr of the pump decreases. To check this, only limited adjustments are required without commissioning additional parallel pumps, if available.
Faulty sounds produced by pumps are referred to as a noise, and this is an indication that a centrifugal pump is cavitating. Other indications of cavitation are a pump that is consuming more power than expected, fluctuating discharge pressure, etc. In good process operations, these indications should be detected as early as possible and addressed immediately. Proper equipment maintenance will prevent the occurrence of most air contamination problems. As in all troubleshooting situations, when air contamination, air ingestion, vane passing syndrome and/or recirculation occur, an understanding of the problem and a logical process of elimination are required to reach an appropriate resolution.
Nwaoha Chikezie recently completed a term as an operator (student trainee) with Port Harcourt Refining Company (PHRC, www.nnpcgroup.com) in Nigeria, and is currently working on several research projects involving flow systems design, including an initiative with the Caribbean African Student Exchange Initiative (CASEI). As part of his research, Mr. Chikezie has authored a number of engineering articles in leading international journals. Mr. Chikezie is a member of SPE, ASME, AIChe, IMechE, ICE, IGEM and Nigerian Gas Association (NGA). He can be reached at email@example.com or +234-703-135-3749.