2.3 Performance modifications for varying pipelene applications

The performance of pipeline pumps often needs to be altered to accommodate varying liquid transmission conditions. These generally require that the pump be physically modified to meet the new conditions. The following are considered for such a modifications by the pipeline industry:

• Impeller change. In order to change the specific speed, impeller size may be changed to meet the new demand on performance. Pump manufacturers can usually offer several different impeller diameters and vanes that will fit the pump casing without any further internal modifications. The effect of changing the impeller characteristics are illustrated in Figs. 2-20 and 2-21.

  Figure 2-20. Changing performance by impeller vane number

• Restaging. Pumps with multistaging capabilities can be restaged (up or down) to meet the change in pressure, head, or flow requirements. For example, if an entire pressure range is not needed for a particular period of time, a number of impellers can be removed to meet the required conditions. Manufacturers provide destaging kits to block off the unused pump impeller areas to maintain efficiency (Fig. 2-22).

  Figure 2-21. Changing pump performance by impeller vane angle β2

• Impeller under-filing and over-filing. This is undertaken to alter the performance of a pump. It involves modifying the flow area of the impeller by grinding metal off the impeller outlet vanes (Fig. 2-23).

Figure 2-22. Changing performance by re-staging

• Impeller volute chipping. This is a technique that is used to alter the outlet flow area of the pump casing in order to modify performance (Fig. 2-24). Again, caution is needed to avoid making costly mistakes. Impeller volute inserts. This technique (Fig. 2-25) involves inserting special removable volutes into the pump to allow for a wider performance range. It also allows for more accurate and close control over the performance.

2.3.1 Cavitation

Cavitation is the rapid formation and collapse of vapor bubbles formed in a pump the whenever the local absolute pressure of the liquid falls below its vapor pressure. These bubbles collapse rapidly and violently when the local absolute pressure increases due to kinetic forces being imparted by the impeller (Fig. 2-26) [9]. Implosion near a solid wall may be described as

• deviation of the bubble shape from spherical symmetry;

• development of a fluid micro-jet moving from the opposite of the solid wall through the bubble;

• microjet strike at the wall (directly if the distance between bubble and wall is smaller-than-bubble radius) with velocity up to 1000 m/s;

• bubble degeneration to a torus, microbubbles remain;

• implosion of microbubbles, emitting a high-frequency, high-amplitude pressure wave

Therefore, collapsing cavitation bubbles cause noise (often sounds like gravel going through the pump), vibration, and erosion of material from the impeller (Fig. 2-27). Damage due to cavitation may take the following forms:

• material fatigue

• microscopic plastic surface deformations ("pits"), statistically distributed.

• pits overlapping.

• material removal, cavitation erosion.

• components functionality loss.

Figure 2-23. Changing performance with under-filling

The following, singularly or in combination can influence or can reduce cavitation aggressiveness [10 & 11]:

• NPSHA

• pump blade geometry

• fluid momentum on pump suction side

• contents of solids and dissolved gases

• air injection into the cavitation zone

• flow velocity, rotational speed

Figure 2-24. Changing performance with volute chipping

Cavitation control is a very important consideration in any liquid system, and thus, my cavitation-induced conditions must be avoided when operating centrifugal pumps. The forrmation of cavitation can be further understood by considering the following:

• A local pressure drop causes an increase in flow velocity and, hence, an acceleration.

• A liquid will boil when the local pressure falls below its vapor pressure.

• A vapor occupies many more times the volume of a liquid.

If a liquid is accelerated in such a manner that the local pressure falls below the liquid I vapor pressure, the liquid will transform into the vapor phase, which results in the formation of bubbles. If the local pressure recovers, the vapor bubbles will transform themselves I back into a liquid. There is a tremendous volume change during transformation, because I collapsing bubbles release a large amount of energy. Because the bubbles are very small, the resulting impact loads on the surrounding metal can be significant. This can result in physical damage to the metal and thus the creation of high noise levels.

Figure 2-25. Changing performance with volute inserts

Some liquids (such as water) are more difficult to handle from a cavitation point of view. When the vapor pressure of a homogenous fluid such as water is reached, the entire fluid begins to change phase, resulting in the formation of a large number of damage-causing bubbles. For a nonhomogeneous fluid such as a hydrocarbon, only the light ends (such as condensates) with low specific volume are affected.

Figure 2-26. Cavitation bubble implosion onto a solid surface, arrows indicate fluid pressure (after [9])

Vapor pressures of some liquids commonly transported by pipelines are shown in Fig. 2-28. Figure 2-29 also provides similar information for our fluids.

Figure 2-27. Typical pump impeller destroyed by cavitation [9]

In a centrifugal pump, the fluid is accelerated by the impeller. The area of lowest pressure in the pump suction system, as shown in Fig. 2-30, is the eye of the impeller at cross section A-A. If the pressure falls below the vapor pressure of the liquid, vapor bub­bles form. As the mixture of liquid and bubbles continue through the pump, the pressure increases and the bubbles return to the liquid state.

Figure 2-28. Vapor pressure of liquids commonly transported by pipelines

Damage to the impeller occurs where the bubbles collapse as shown at cross section B-B. This location varies for different impellers and different suction conditions:

As indicated previously, the effects of cavitation include:

• Noise and vibration

• Pump damage (e.g., pitting of the impeller)

• Falloff of pump performance and efficiency

Cavitation in centrifugal pumps can be recognized by a characteristic noise, which sounds just like it is trying to pump gravel. A typical break-off in the performance curve of a pump due to cavitation is shown in Fig. 2-31.

Figure 2-29. Vapor pressure of some fluids