Herramientas BPM

Net positive suction head, or NPSH, actually consists of two concepts: the net positive suction available (NPSH_A), and the net positive suction head required (NPSH_R). The definition of NPSH_A and NPSH_R, as given by the Hydraulics Institute (1994), were presented in Sec. 5.1.

5.4.1 Net Positive Suction Head Available

Figure 5.1 visually depicts the concept of NPSH_A. Since the NPSH_A is the head available at the impeller, friction losses in any suction piping must be subtracted when making the calculation. Thus, the equation for determining NPSH_A becomes

(5.32) where

h_atm = atmospheric pressure (ft or m).

h_s = static head of water on the suction side of the pump (ft or m) (h_s is negative if the water surface elevation is below the eye of the impeller).

h_vp = vapor pressure of water, which varies with both altitude and temperature (ft or m), and

h_L = friction losses in suction piping (ft or m), typically expressed as the summation of velocity heads (KV²/2g) for the various fittings and pipe lengths in thesuction piping.

Key points in determining NPSH_A are as follows (Sanks et al., 1998): (1) the barometric pressure must be corrected for altitude, (2) storms can reduce barometric pressure by about 2 percent, and (3) the water temperature profoundly affects the vapor pressure. Because of uncertainties involved in computing NPSH_A, it is recommended that the NPSH_A be at least 5 ft (1.5 m) greater than the NPSH_R or 1.35 times the NPSH_R as a factor of safety (Sanks et al., 1998). An example of calculating NPSH_A, is presented in Section 5.5.

5.4.2 Net Positive Suction Head Required by a Pump

The Hydraulics Institute (1994) and Sanks et al. (1998) have discussed the concept and implications of NPSH_R in detail. Their discussions are presented or summarized as follows.

The NPSH_R is determined by tests of geometrically similar pumps operated at constant speed and discharge but with varying suction heads. The development of cavitation is assumed to be indicated by a 3 percent drop in the head developed as the suction inlet is throttled, as shown in Fig. 5.10. It is known that the onset of cavitation occurs well before the 3 percent drop in head (Cavi, 1985). Cavitation can develop substantially before any drop in the head can be detected, and erosion indeed occurs more rapidly at a 1 percent change in head (with few bubbles) than it does at a 3 percent change in head (with many bubbles). In fact, erosion can be inhibited in a cavitating pump by introducing air into the suction pipe to make many

FIGURE 5.10 Net positive suction head criteria as determined from pump test results.

bubbles. So, because the 3 percent change is the current standard used by most pump manufacturers to define the NPSH_R, serious erosion can occur as a result of blindly accepting data from catalogs. In critical installations where continuous duty is important, the manufacturer should be required to furnish the NPSH_R test results. Typically, NPSHR is plotted as a continuous curve for a pump (Fig. 5.11). When impeller trim has a significant effect on the NPSH_R, several curves are plotted.

The NPSH required to suppress all cavitation is always higher than the NPSH_R shown in a pump manufacturer’s curve. The NPSH required to suppress all cavitation at 40 to 60 percent of a pump’s flow rate at BEP can be two to five times as is necessary to meet guaranteed head and flow capacities at rated flow (Fig. 5.10; Taylor, 1987). The HI standard (Hydraulics Institute, 1994) states that even higher ratios of NPSH_A to NPSH_R may be required to suppress cavitation: It can take from 2 to 20 times the NPSH_R to suppress incipient cavitation completely, depending on the impeller’s design and operating capacity.

If the pump operates at low head at a flow rate considerably greater than the capacity at the BEP, Eq. (5.33) is approximately correct:

(5.33)

where the exponent n varies from 1.25 to 3.0, depending on the design of the impeller. In most water and wastewater pumps, n lies between 1.8 and 2.8. The NPSH_R at the BEP increases with the specific speed of the pumps. For high-head pumps, it may be necessary either to limit the speed to obtain the adequate NPSH at the operating point or to lower the elevation of the pump with respect to the free water surface on the suction side i to increase the NPSH_A.

FIGURE 5.11 NPSH required to suppress visible cavitation.

5.4.3 NPSH Margin or Safety Factor Considerations

Any pump and piping system must be designed so that the net positive suction head available (NPSH_A) is equal to, or exceeds, the net positive suction head required (NPSH_R) by the pump throughout the range of operation. The margin is the amount by which NPSH_A exceeds NPSH_R (Hydraulics Institute, 1994). The amount of margin required varies, depending on the pump design, the application, and the materials of construction.

Practical experience over many years has shown that, for the majority of pump applications and designs, NPSH_R can be used as the lower limit for the NPSH available.

However, for high-energy pumps, the NPSHR may not be sufficient. Therefore, the designer should consider an appropriate NPSH margin over NPSH_R for high-energy pumps that is sufficient at all flows to protect the pump from damage caused by cavitation.

5.4.4 Cavitation

Cavitation begins to develop in a pump as small harmless vapor bubbles, substantially before any degradation in the developed head can be detected (Hydraulics Institute, 1994). This is called the point of incipient cavitation (Cavi, 1985; Hydraulics Institute, 1994).

Studies on high-energy applications show that cavitation damage with the NPSH_A greater than the NPSH_R can be substantial. In fact, there are studies on pumps that show the maximum damage to occur at NPSH_A values somewhere between 0 and 1 percent head drop (or two to three times the NPSH_R), especially for high suction pressures, as required by pumps with high impeller-eye peripheral speeds. There is no universally accepted relationship between the percentage of head drop and the damage caused by cavitation.

There are too many variables in the specific pump design and materials, properties of the liquid and system. The pump manufacturer should be consulted about NPSH margins for the specific pump type and its intended service on high-energy, low-NPSH_A applications.

According to a study of data contributed by pump manufacturers, no correlation exists, between the specific speed, the suction specific speed, or any other simple variable and the shape of the NPSH curve break-off. The design variables and manufacturing variables are too great. This means that no standard relationship exists between a 3, 2, 1, or 0 percent head drop. The ratio between the NPSH required for a 0 percent head drop and the NPSH_R is not a constant, but it generally varies over a range from 1.05 to 2.5. NPSH for a 0, 1, or 2 percent head drop cannot be predicted by calculation, given NPSH_R.

A pump cannot be constructed to resist cavitation. Although a wealth of literature is available on the resistance of materials to cavitation erosion, no unique material property or combination of properties has been found that yields a consistent correlation with cavitation damage rate (Sanks et al., 1998).