Pump cavitation in feed water systems - remedies

. The Net Positive Suction Head incipient is a multiple of the Net Positive Suction Head Required in centrifugal pumps. With limited static heads of the feedwater systems, the suction impellers of the feed pumps work in cavitation. The article discusses the ways to deal with the problem of cavitation in such systems.


Introduction
Cavitation in industrial pumps reduces the head, efficiency, leads to excessive vibration and finally cuts the impeller© s operating time off due to damage caused by cavitation erosion. This phenomenon is caused by excessive static pressure drop in the pump suction nozzle. Fig. 1 shows the stages of cavitation and their effect on the pump head while maintaining a constant flow. . The static pressure before the pump is determined by the characteristics of the suction side of the pumping system determined by the Net Positive Suction Head available.

Net Positive Suction Head Available of a feedwater system
A diagram of a typical steam generation system in power plants and hot water generation system in an industrial plant is shown in Figure 2.

Fig. 2. Feedwater system
The low-pressure side of the system consists of a feedwater tank with a degasser, a suction pipeline with fittings (filter, elbows, etc.), feed water pump with or without a booster pump. The water in the feed pump is of the temperature above 120C. This affects the vapor head, which for such temperatures is Hv > 22m. The disposable excess of the system is determined by the difference between the total amount of energy at the end of the suction system (inlet to the feed pump) and the evaporation height. It can be converted to the form: wherein: Taking into account that the pressure in the feed water tank nearly equals vapor pressure Typical static heads in industrial plants are HS = 2530m. The losses for optimal flow usually do not

Net Positive Suction Head Required margin
A commonly used cavitation characteristics in industry is a 3% drop in head, i.e. NPSHR = NPSH3. Manufacturers provide the NPSH3 characteristic for rated speed. For a suction impeller of a suction specific speed nss = 230 (single-suction i = 1 commercially available pump with a specific speed nq = 20), rated rotational speed n = 5000 1 / min, and flow rate QBEP = 520m 3 /h, NPSH3 can be expected: This is a relatively large value. Such NPSH can be reduced in three ways:

Incipient cavitation and NPSHi/NPSH3 ratio
To determine the safe pump operation area without cavitation, the ratio of NPSH for incipient cavitation and 3% cavitation is needed. Figure 3 shows such ratio for pumps with a specific speed from nq = 16 to nq = 90. The ratios range between about 4 and 6. For very good suction impellers, they are lower and range from 2 to 3 [1,4]. Figure 4 shows NPSH ratios for a typical feedwater system with the static head HS = 25m, a feedwater pump with a double-suction impeller and 25% margin, and NPSHi for incipient cavitation. All is related to NPSH3BEP = 10m. NPSHi values as a function of flow rate were estimated based on the studies in [4,5,6,7]. The field below the NPSHi curve is the cavitation area so feedwater pump operates in cavitation because of NPSHA  NPSHi.

Feedwater system with booster pump
The use of a low-rotational speed booster pump significantly improves the suction conditions of the feed water pump. The booster pump power is often around 5% of the feed water pump power. With efficiency of over 80%, additional losses associated with booster pump operation reduce the efficiency of pumping into the boiler by less than a percent. With a booster pump's head of 6 to 8 times the NPSH3 of feed water pump the latter can achieve a wide range of operation without cavitation. The simple structure of the booster pump allows for its long trouble-free operation.  5. Impact of the booster pump on a feed water pump operation Figure 5 shows NPSH of the system with a booster pump where non-cavitation operation of the feed pump is guaranteed for flow rates Q <1.2QBEP . The presence of the booster pump slightly increases system complication and decreases efficiency, but extends trouble-free operation time by over 100,000 hours. Despite the tendency to eliminate booster pumps, the final decision should be preceded by LCC analysis.

New materials for suction impeller
At existing static heads of the system, without the booster pump the suction impeller of the feed pump operates in cavitation. The conditions for suction impellers material is operation for 40,000 hours. Typical chromium cast steels traditionally used, e.g. GX20Cr14, does not meet this condition but alloys are offered whose cavitation erosion resistance is at least ten times larger. Table 3 contains materials ordered by increasing cavitation resistance according to KSB Lexicon

Conclusion
The feed pumps operation in power plant systems takes place at static heads of 2530m resulting from typical elevation heights of the feed water tank. Even for suction impellers with large suction specific speed and relatively large NPSH margin it leads to cavitation operation. One way is to avoid cavitation by using a booster pump. The reliability of single-stage, double-suction pumps working usually as booster pumps is very high due to the lack of balancing system, low rotational speed, and simple structure. The disadvantage of the system with a booster pump is slightly lower efficiency reaching a fraction of a percent. Therefore, the assessment of the system operation with or without a booster pump should be preceded by LCC analysis. Without a booster pump, an acceptable impeller lifetime of 40,000 hours requires materials with high resistance to cavitation erosion. New cast steels with erosion resistance more than ten times higher than typical chromium cast steels are the answer to these needs. However, further material advancement extending impeller lifetime is required.