Selection criteria for suction impellers of centrifugal pumps

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    Selection criteria for suction impellers of centrifugal pumps In the third and final part of this article, J.F. GUlich of Sulzer Pumps, discusses the prevention of cavitation damage and concludes by presenting hydraulic criteria that can be used for the selection of suction impellers.

    6. P revent ion o f cav i ta t ion damage

    The risk of cavitation damage must be minimised by the following means:

    (1)An appropriate hydraulic design of the structures and components upstream of the impeller must provide a flow distribution as uniform as practical. This applies for the suction piping layout, the sump design for a wet pit installation, or the radial inlet bends of multi-stage and double-entry pumps.

    (2)The hydraulic design of the impeller must optimise the pressure distribution on the blades in order to create as low cavity volumes as possible for any given NPSH and to minimise the driving pressure differential in the zone of bubble implosion (as discussed in section 3.1 "concept L" and illustrated by case histories 5.1 to 5.3).

    (3)Selection of the right pump for the specified service: most pumps will operate at partload with a higher risk of cavitation damage than near BEP - be it due to higher incidence or to vortices created by suction recirculation. Over-sizing of the pump usually entails economic losses due to additional power consumption and frequently in terms of increased maintenance cOStS.

    (4)Determination of the NPSH avail- able necessary to ensure operation without erosion and/or noise and vibrations caused by cavitation by appropriate margins or analysis of risk of cavitation damage.

    (5)Selection of appropriate materials.

    Whereas items (1) and (2) focus on the minimisation of the cavity volume at any given flow and inlet pressure, items

    (4) and (5) attempt to exclude the risk of damage when operating with a limited amount of cavitation. This is precisely what most pumps do, since it is neither economical nor necessary in the majority of applications to suppress cavity formation at the pump inlet completely by providing plenty of NPSHA or choosing a sufficiently low pump speed.

    The risk of cavitation damage can be assessed quantitatively - albeit with considerable scatter [12, 7]. The damage prediction correlations given in ref. 12 have been validated since by additional testing [25] and extended to procedures for cavitation diagnosis [26]. The damage prediction method has been used to calculate the cavity length allowable to obtain an impeller life of 40000 hours with stainless steel impellers, Fig. 10. The allowable cavity length has been plotted against the available NPSH; the reason is that the hydraulic cavitation intensity increases with the cavity volume (length) and the driving pressure differential in the implosion zone as quantified by the suction pressure (i.e. NPSHa). The calculation is based on a metal loss due to cavitation of 6 mm in 40000 hours with stainless steel tensile strength of R m = 800 N/mm 2. To estimate the cavity length allowable for other conditions read Lca v from the graph and transform to actual conditions by use of Eq. (8):

    f R ~ 0.7/, I Ae ~ o.35

    (8) [ m,Flef J [ k Ref )

    Rm,Re f = 800 N/mm2; Ii.,Ref = 40000 hrs; AeRe f = 6 ram. As has been discussed extensively in [12] and [23] cavitation damage prediction is sub- ject to a very large scatter due to the

    various parameters influencing cavi- tation erosion; Fig. 10 can therefore just show trends and give an order of magnitude. Cavity thickness and volume increase with cavity length. For LcaffTia > 0.3 to 0.5 this effect represents a considerable uncertainty (refer also to case 5.1).

    'When selecting the speed and size of a pump for a given NPSHA or when determining the NPSH A necessary for the safe operation of a given pump typical applications may be distinguished:

    A: Service when pumping water below 200 *C

    Pumps with an impeller eye tip speed u 1 > 75 m/s must operate virtually above cavitation inception, since very small cavities can lead to erosion. For these pumps the impeller eye diameters are minimised in order to keep the tip speed u 1 and NPSH i as low as possible. This group of pumps include high-speed injection pumps and boiler feedpumps for very large nuclear or fossil power plants.

    When selecting pumps with impeller eye tip speeds between 50 < u 1 < 75 m/s it must be ensured that the cavity length is known and that it is limited indeed in order to avoid excessive cavitation damage (usually a booster pump is required to create a sufficient NPSH A in such applications). Typical examples are injection pumps for oil fields and boiler feedpumps for medium size power stations. A rational choice can be made using the methods mentioned above, which allow a quantification of the expected impeller life when the cavity length is known from tests or can be estimated with a reasonable degree of confidence [12, 7].

    42 WORLD PUMPS April 2001 0262 1762/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved

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    For pumps with an impeller eye tip speed U 1 < 50 m/s the cavity length may not be known. An adequate mar- gin or "safety factor" is to be applied on the NPSH3 to determine the NPSH A necessary to ensure trouble-free oper- ation (table 1 or other rationale).

    In rare applications pumps are operated with cavitation control: the flow rate pumped is then determined by the extent of cavitation which causes that amount of head drop that the remain- ing head is able to overcome the head required by the system characteristic. With stainless steel impellers and u I < 20 m/s cavitation erosion may stay within acceptable limits.

    B: Service when pumping hydrocarbons

    Hydrocarbon pumping applications present a much lower risk of cavitation erosion than water pumping systems. The NPSH available is determined by safety factors (table 1 or other rationale). For pumps with high tip speed ul > 50 to 60 m/s the cavity volume must be limited too in order to avoid excessive noise and vibration (case history 5.2) or even erosion.

    7. Conc lus ions

    The hesitation of some pump users to accept pumps with suction specific speeds above a certain level (e.g. ns~ = 213) originates from investiga- tions on pumps designed 30 to 40 years ago [1, 3]. Extensive research and development of high energy pumps in the past 30 years have produced a better understanding of the flow in pumps and of the reasons for earlier failures. As a result a new design concept has emerged for suction impellers with low incidence and flat pressure distributions. Former high nss impellers combined a large eye with large incidence leading to extensive flow separation at partload (often even at BEP). To compensate the counter-productive effect of the large incidence the eye diameter had to be increased even more than with an optimum design to get the desired suction specific speed, refer to Eq. (5). Thus unnecessarily high impeller tip speeds u 1 have been combined

    200

    E E .c 150 C~ ,.

    ~100

    .O

    ,~ 50

    Cold water gas cont snt 24 ppm or boiler feed water above 140 "C

    Cold water gas content 0.05

    0 50 100 150

    NPSH A [m]

    with inlet flow separation and recirculation to create noise, vib- rations and cavitation damage (e.g. test data in section 5.1).

    By definition the suction specific speed is a relation between speed n, flow QBEP and NPSH3,BEp which is related to the stream-wise pressure distribution Ap = f(L) along the impeller blades. Ap = f(L) depends on the uniformity of the approach flow, leading edge profile and - most important - on the stagger angle of the blades or 131B. This experimental fact - expressed by Eq. (4) - has been ob- served on radial impellers as well as inducers, but is not documented in "classical pump theory" as given by Eq. (2). It demonstrates that the low pressure spike, in other words: "the quality of the flow", improves as the angle of approach flow decreases or the impeller eye increases. For this reason the NPSH decreases (in spite higher Ul) - and the suction specific speed increases - as approach and blade angles decrease. The lower the blade inlet angle 13 m, the lower the flow incidence at part load: roughly speak- ing an impeller with 1318 = 24 experi- ences at q* = 0.5 an incidence of i 1 = 12 , while an impeller with ~IB -- 100 operates at that flow with i I = 5 only.

    While the stream-wise quality of the flow increases with lower angles 131a, this is not necessarily true for the span-wise pressure distribution, which has a dominating impact on impeller inlet recirculation. There is further an interest to keep the tip speed ul low,

    since the cavitation effects and the always present hydraulic excitation forces increase exponentially with ul. Consequently an optimum impeller eye diameter must be found. For a given application with given NPSH^ the procedure described in section 3.2 and Fig. 4 can help to find that optimum. The procedure is based on a margin on NPSH 3 which increases with the tip speed ul, Eq. (TI.1) in table 1. Stream-wise and span-wise pressure distributions are not suff- iciently related to use nss as a criterion for intensity (and less important: onset) of inlet recirculation: neither does a low or moderate n,s guarantee low recirculation (refer to case 5.4) nor does a high nss imply excessive vibrations or recirculation, see also [8]. As has been shown by the experiments in section 5 and table 3 cavitation erosion, recirculation and vibrations are not correlated with the suction specific speed.

    During part load flow recirculation the flow in the impeller is fully 3- dimensional and unsteady and depends on many parameters inclu- ding the shapes of the impeller section