Detecting the signs of cavitation, and correctly identifying and understanding the type of cavitation present, can help an operator prevent serious damage.
HOW CAVITATION OCCURS
The pressure of the liquid in a centrifugal pump drops as it flows from the suction flange through the suction nozzle and into the impeller (Fig. 1). The amount of pressure drop is a function of many factors, including pump geometry, rotational speed, frictional and hydraulic shock losses, and flowrate.
If the pressure at any point within the pump falls below the vapor pressure of the liquid being pumped, vaporization or cavitation will occur. For example, water at 100°F (38°C) will boil or vaporize if exposed to a vacuum of 28-in Hg.
DETERMINING NPSHr AND NPSHa
The minimum head required to prevent cavitation with a given liquid at a given flowrate is called the net positive suction head required (NPSHr). In a pumping system, the difference between the actual pressure or head of the liquid available (measured at the suction flange) and the vapor pressure of that liquid is called the net positive suction head available (NPSHa).
The pump manufacturer determines the NPSHr of an impeller pattern by conducting a suppression test using water as the pumped fluid. These tests are usually only made on the first casting for an impeller pattern, not on individual pumps.
Normally, the NPSHr plotted on the traditional pump curve is based on a 3% head loss due to cavitation, a convention established many years ago in the Hydraulic Institute Standards. Permitting this large a head loss means that cavitation would already have been occurring, at some higher flow condition, before performance loss was noticed.
Variations in excess of 5 ft. in the NPSHr will occur between pumps of the same size and model equipped with the same impeller pattern. This variation is due to impeller eye area differences, including vane angles, surface finish and fluid channel areas; wear-ring clearance differences; and suction nozzle passage area variations.
Further, the NPSHr will actually tend to increase with a reduction in flow. Thus, it is imperative that a margin is provided between the NPSHr as quoted by the manufacturer and the NPSHa at the operating conditions.
A reasonable margin of 5 ft at rated flow is commonly accepted by end users for most services. For known problem applications, such as vacuum tower bottoms and some solvents, this margin is often increased to 10 ft.
Cavitation can obstruct the pump, impair performance and flow capacity, and damage the impeller and other sensitive components. In short, the pump has a heart attack called cavitation.
Classical cavitation occurs when the absolute pressure of a moving liquid is reduced to, or even below, the vapor pressure of the liquid in the impeller eye (Point 4 in Fig. 1). Bubbles are formed as a result of this pressure drop. Lower pressures in the impeller eye are caused by variations in velocity of the fluid and friction losses as the fluid enters the impeller.
The bubbles are caught up and swept outward along the impeller vane. Somewhere along the non-visible side of the impeller vane, the pressure may once again exceed the vapor pressure and cause the bubbles to collapse.
Implosions of these vapor pockets can be so rapid that a rumbling/cracking noise is produced (it sounds like rocks passing through the pump). The hydraulic impacts caused by the collapsing bubbles are strong enough to cause minute areas of fatigue on the metal impeller surfaces. Depending on the severity of the cavitation, a decrease in pump performance may also be noted.
The first reaction to a cavitation problem is usually to check the NPSHa at the eye of the impeller and compare this to the NPSHr by the impeller design. The ratio of NPSHa/NPSHr must be sufficiently large to prevent the formation of cavitation bubbles.
Keep in mind that very few process applications call for a pump to handle a pure liquid such as water. Most services handle a mixture of various components (e.g., crude oil, blended gasoline or even paint). As such, they will have a range of vapor pressures or boiling points, which depend on the volatility of each component.
Cavitation damage to a centrifugal pump may range from minor pitting to catastrophic failure and depends on the pumped fluid characteristics, energy levels and duration of cavitation. However, most of the damage usually occurs within the impeller; specifically, to the leading face of the nonpressure side of the vanes. This is the area where the bubbles will normally begin to collapse and release energy onto the vane. The result will be a pockmarked, rough surface.
Recirculation cavitation (also called rotating stall or separation) is a term used to describe the formation of vapor-filled pockets. This type of cavitation is less well known and understood than classical cavitation, though it is probably more common. It is most often caused by suction recirculation (as shown at Point 'Z' in Fig. 2).
As the pump is operated back on its curve, eddy currents begin to form in the eye of the impeller. There is no reduction in mass flow through the pump at a given point on this curve. This means that the velocity through the impeller fluid channels must have increased. That is, the eddy currents at the eye have effectively reduced the flow channel size, thereby increasing liquid velocity for the fixed flowrate.
When the velocity increases, the pressure drop due to friction must also increase. If the drop is large enough to cause the pressure to fall below the liquid's vapor pressure, the pump will develop classical cavitation because of the initiating action of recirculation cavitation.
Another cause of recirculation is that as the fluid flows over an impeller vane, the pressure near the surface is lowered, and the flow tends to separate (Fig. 3).
This separated region occurs when the incidence angle--the difference between flow angle and pump impeller vane inlet angle--increases above a specific critical value. The stalled area eventually washes out but is reformed as rotation continues. The area contains a vapor surrounded by a turbulent flowing liquid at a higher pressure than the vapor pressure. This separated region will then fill with liquid from the downstream end. The vapor pocket collapses, which causes damage to the surface of the impeller vane. This may occur up to 200 to 300 times per sec.
Note that this damage will be on the opposite side of the vane as that which occurred with classical cavitation (Fig. 4). This continuous recycling results in noise, vibration and pressure pulsations. These results imitate classical cavitation, and thus recirculation is often incorrectly diagnosed as such.
Because of a cautionary note in API 610, 'Centrifugal Pumps for Petroleum, Heavy Duty Chemical, and Gas Industry Services,' it is widely believed that only high-energy pumps are affected by recirculation cavitation. However, an impeller constructed of cast iron or bronze can erode badly at much lower energy levels.
As the flow at the eye of the impeller doubles back on itself, severe vortexing will take place. These vortices can even pass through the impeller liquid channels to initiate discharge recirculation, as shown in Fig. 2 at Point 'Y.' The further the pump operates from its best efficiency point (BEP), the greater the amount of vortexing.
Most pumps operate in either continuous or intermittent suction or discharge recirculation, especially those designed with high suction-specific speeds (above about 11,000). (Suction-specific speed is an index number calculated from a pump's rotational speed and its capacity and head at BEP.) Impeller internal circulation usually shows up as cavitation noise and erosion damage.
This damage is influenced by impeller inlet tip speed, the degree to which recirculation is taking place, NPSH margin, characteristics of the pumped fluid, pump energy level and materials of construction--all factors that cannot be readily calculated.
Designing the pump for lower suction-specific speeds and limiting the range of operation to flow capacities above the point of recirculation can help end users avoid many of these problems.
Raising the suction head can result in some improvement with recirculation-induced cavitation and increase the likelihood of confusing it with classical cavitation. But it is important to accurately diagnose it because its end results can be more destructive than those of classical cavitation. Recirculation cavitation is also more common than may be expected.
Excessive wear-ring clearances can impact the tendency for formation of both classical and recirculation cavitation. As the increased leakage at Point 'Z' of Fig. 2 disturbs the flow pattern into the eye, the tendency for pockets and bubbles to form is increased. A doubling of the wear-ring clearances can raise the NPSHr of a given impeller pattern by as much as 40% to 45%, which will help to suppress these tendencies.
EVALUATING CAVITATION DAMAGE
As both types of cavitation generate noise and vibration, about the only way to distinguish between them is by a careful inspection of the damaged impeller. The following two simple statements and Fig. 2 can be used to guide the inspection.
1. Classical cavitation. Damage is located on the non-visible or underside of the vane. It starts near the leading edge and can extend up to approximately two-thirds of the vane length before the pressure implodes the bubbles. Either feeling or looking at the underside of the vane with a mirror is necessary to evaluate the damage.
2. Suction recirculation. Damage is on the visible or the pressure side of the vane's leading edge (Figs. 2, 3 and 4). If tip recirculation has occurred, damage will be on the visible or pressure side of the vane near shroud walls (Point 'Y' of Fig. 2).