Abstract. 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.
1 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.
Fig. 1. Stages of cavitation at constant flow rate. 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.
2 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:
Typical static heads in industrial plants a re HS =2 5÷30m. The losses for optimal flow usually do not exceed a few meters, hence the typical values of hydraulic resistance in equation (2) = 2 ÷ 3m. The NPSHA characteristic limits the pump operation. To avoid cavitation, the Net Positive Suction Head Required must be smaller.
3 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 = 520m3/h, NPSH3 can be expected:
This i s a relatively large value. Such NPSH can be reduced in three ways:
1. Use of impeller of larger suction specific speed. For the suction specific speed nss = 280 and the same rated pump parameters, NPSH3 13 be obtained.
2. Use double suction impeller with the same parameters n, nss, QBEP . It allows to obtain NPSH3 20% smaller.
3. Reducing the rotational speed of the pump while increasing the number of stages and impellers diameter to maintain the required head. For example, reducing the speed down to n = 3500 1 / min reduces the Net Positive Suction Head to NPSH3 10m. To avoid cavitation some margin is recommended. The NPSH margin is different depending on source literature, for example [1] gives:
However, they are larger for high energy pumps. McGuire [3] reports:
The correct pump operation is defined by NPSHA ≥ NPSH3 + NPSH margin but avoiding cavitation imposes a stricter condition NPSHA NPSHi .
Table 2. Examples of NPSH margin according to [3].
4 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.
Fig. 3. NPSHi/NPSH3 ratio for the data from [4,5,6,7] as a function of specific speed.
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 a s 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 feed water pump operates in cavitation because of NPSHA ≥ NPSHi.
5 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.
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<1.2 QBEP. 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.
6 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. GX 20Cr14, 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
Table 3. Cavitation erosion: Materials graded by increasing cavitation resistance; weight loss index for typical cast metals (based on grey cast iron JL 1040 with an index of 1.0)
Name | Type | index |
Cast steel | GP240GH+QT | 0.8 |
Tin bronze | CC480K-GS | 0.1 |
Cast chrome steel | GX20Cr14 | 0.2 |
Aluminium multi-alloy bronze | CC333G-GC | 0.1 |
Cast chrome nickel steel | GX5CrNi19-10 | 0.05 |
Noridur | GX3CrNiMoCuN24-6-2-3 | 0.02 |
Leading producers offer cast steels with increased resistance to cavitation erosion e.g. Noridur and Noriclor (KSB) or X-Cavalloy (Flowserve). They are compared in table 4.
Table 4. Commercial cast steels resistant to cavitation erosion. Noridur and Noriclor are based on chrome, nickle and molybdenum whereas X-Cavalloy distinguishes by a large share of 15.5% Manganese.
Noridur | Noriclor | X-Cavallo | |
C | .040 | .40 | 0,1 |
Si | 1.5 | 1.0 | 0, 5 |
Mn | 1.5 | 1.0 | 15, 5 |
Cr | 23.0-26.0 | 22.0 - 25.0 | 18 |
Ni | 5.0 - 8.0 | 4.5 - 6.5 | 0, 5 |
Mo | 2.75 - 3.5 | 4.5 - 6.0 | - |
Cu | 2.75 - 3.5 | 1.5 - 2.5 | - |
N | 0.10 - 0.2 | 0.15 - 0.25 | 0, 25 |
Comments