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Centrifugal pumps lexicon

The designation NPSH is the English. Abbreviation for "N et P ositive S uction H ead" and an important parameter forevaluating the absorbency of a centrifugal pump. It enables a statement about the safety against effects of cavitationduring operation. In standard EN 12723, the concept of "is positive suction head" used as a synonym for the NPSHvalue. As to both calculate different reference planes are defined, they can be in the numerical value by z s differ(geodetic height difference between the two reference planes S and S '). In practice, only the NPSH value is used.

In the course of flow through the impeller of a centrifugal pump there is in particular at the blade channel entranceinitially a lowering of the static pressure with respect to the value before the impeller. The amount of pressurereduction is from the speed, density, geometry of the impeller occurrence and viscosity of the pumped liquid and theoperating point dependent and velocity profile of the incoming flow.

To avoid cavitation or to limit to an acceptable level which has to pressure upstream of the impeller by a certainamount above the vapor pressure of the conveying medium are. In order to assess the occurrence of the extent or theeffects of cavitation in a centrifugal pump, two NPSH values are to be compared. These are the required for the pump(NPSHR with "R" for required) and that of the existing plant NPSHA (NPSHA with "A" for available).

NPSH of system

The NPSH value of the pump system (NPSHA) is defined as

The site s refers to the middle of the suction connection. In the absence of pump suction port as in a pipeline pumpwith welded connection (Einschweißpumpe) or submersible pump with inlet nozzle has an appropriate place to be thiss defined and identified when specifying the NPSHA value. See Fig. 1 NPSH

Closeup fig. 1 NPSH: The State of the reference points s' for NPSHWert and s for the net positive suction head (in thisexample, the impeller incident flow from below)

The total pressure at the point S can be expressed as:

The reference point s' for the NPSH is the intersection of the pump shaft axis with the direction perpendicular to theirplane through the external points of the blade inlet edge at the center of the impeller., See Fig. 2 NPSH

Closeup fig. 2 NPSH: The Situation of the reference point s' at various impellers

Thus, the NPSH value of the system is calculated as follows:

The values in the inlet cross section of the plant is true:

The loss amount also includes any losses through inlets and accessories and fittings etc.

NPSH of pump

The NPSH value of the pump (NPSHR) is similarly defined as the NPSH value of the plant with the same meanings ofthe standing inside the brackets symbols:

A major difference is that the sum of the indicated by the parenthesessizes must not fall below a certain pump and application-specificminimum value (min).

Is it not so that Kavitationkriterien for centrifugal pumps are violated.

When specifying the NPSHR value it is imperative to specify the associated cavitationcriterion with. These criteria may include:

Cavitation inception (incipient cavitation) NPSH

a certain expansion of the cavitation on the blades

The beginning of the delivery head drop due to cavitation (NPSH 0)

cavitation head drop by 3% (NPSH 3)

The three criteria are first mentioned less common and the proof of the criterion NPSH i requires a very largeexperimental outlay. Frequently, therefore, is NPSHR = NPSH 3 agreed. All quoted Kavitationskriterien and theirderived NPSH values change according to the operating point. See Fig. 3 NPSH

Closeup fig. 3 NPSH: NPSHR according to various criteria as a function of the relative conveying flow Q / Q stoßfre

The curves for by a particular impeller NPSHR required are on the relative flow rate applied. As a parameter, themanifestations of cavitation are known, for example. The length (L Bl) of the resulting bubble towing in relation to the blade

pitch (t) (see blade cascade). If in the following graph, the curve of the system-side NPSHA-value transfer, the expectedappearance of the cavitation can be read depends on the flow rate. The uppermost curve (NPSH i) indicates the state of

incipient cavitation. Is NPSHA so high that NPSH i is exceeded, no cavitation occurs and the impeller runskavitationsblasenfrei. The farther NPSHA drops, the more increases the Kavitationsschleppenlänge.

From a minimum of which lies at the intersection of the lines for pressure and suction cavitation, increase the bubblehauling lengths by partial and overload out at at constant NPSHA. The flow rate at which present this minimumcorresponds to the flow direction of the shock-free entry, which leads to the lowest over-speeding on theSchaufeldruck- and -saugseite. It is therefore also known as shock-free flow (Q bumpless) referred.

If the flow rate (Q) is smaller than the shock-free flow (Q bumpless), cavitation occurs on the Schaufelsaug- and reverse flow

rate ratio on the blade pressure side. To determine NPSHR test experiences are most crucial, in particular:

the conversion of the NPSH value of the pump from one to another speed

in similar pumps from one to another pump size

from one to another delivery medium (particularly in the presence of air in dissolved or undissolved state) (seegas content in the fluid)

The operating point of a centrifugal pump can only be a continuous operation point when this point is considered in:

The following simple relationship exists between the NPSH value and no longer in usePositive Suction Head:

For horizontal pumps is due to the lack of difference in height (z s = 0) nodifference between the NPSH and positive suction head. In connectionwith the NPSH value and the following characteristics are used:

In the extraction of hydrocarbons or water with high temperatures the measured NPSH is 3 value smaller than in coldwater. Thus, the actual required NPSH value for hydrocarbons or hot water can be reduced in acceptance tests withcold water:

Hydrocarbons after HI (Standards of the H ydraulic I NSTITUTE, New York)

Hot water See Fig. 4 NPSH

Closeup fig. 4 NPSH: correction factor f for NPSH3 at promoting Heißwassser (based on KSB measurements)