Pump Wisdom. Robert X. Perez

Pump Wisdom - Robert X. Perez


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impeller's periphery. After the fluid exits the impeller, it gradually decelerates to a much lower velocity in the stationary casing, called a volute casing, where the fluid stream's kinetic energy is converted into pressure energy (also called pressure head). The combination of the pump suction (inlet) pressure and the additional pressure head generated by the impeller creates a final pump discharge pressure that is higher than the suction pressure [3].

      Source: SKF USA, Inc. [1].

Schematic illustration of typical process pump with suction flow entering horizontally and vertically oriented discharge pipe leaving the casing tangentially.

      Source: Emile Egger & Cie. [2].

      Head is related to the difference between discharge pressure and suction pressure at the respective pump nozzles. Head is a simple concept, but this is where consideration of the impeller tip speed is important. The higher the shaft rpm and the larger the impeller diameter, the higher will be the impeller tip speed – actually its peripheral velocity.

Schematic illustration of a semiopen impeller with five vanes. Schematic illustration of typical H–Q performance curves are sloped as shown here.

      For reasons of material strength and reasonably priced metallurgy, one usually limits the head per stage to about 700 ft. This is a fairly important rule‐of‐thumb limit to remember. When too many similar rule‐of‐thumb limits combine, one cannot expect pump reliability to be at its highest. As an example, say a particular impeller‐to‐shaft fit is to have 0.0002–0.0015 in. clearance on average size impeller hubs. With a clearance fit of 0.0015 in., one might anticipate a somewhat greater failure risk if this upper limit were found on an impeller operating with maximum allowable diameter.

      The generalized depictions in Figure 1.4 also contain a curve labeled NPSHr, which stands for Net Positive Suction Head required. This is the head of liquid that must exist at the edge of the inlet vanes of an impeller to allow liquid transport without causing undue vaporization. It is a function of impeller geometry and size and is determined by factory testing. NPSHr can range from a few feet to a three‐digit number. At all times, the head of liquid available at the impeller inlet (NPSHa) must exceed the required NPSHr.

      The rate of flow through a pump is labeled Q (gpm) and is plotted on the horizontal axis (the x‐scale). Note that for a given speed and for every value of head H we read off on the y‐axis, there is a corresponding value of Q on the x‐axis. This plotted relationship is expressed as “the pump is running on its curve.” Pump HQ curves are plotted to commence at zero flow and highest head. Process pumps need a continually rising curve inclination and a curve with a hump somewhere along its inclined line will not serve the reliability‐focused user. Operation at zero flow is not allowed and, if over perhaps a minute's duration, could cause temperature rise and internal recirculation effects that might destroy most pumps.

      But remember that this curve is valid only for this particular impeller pattern, geometry, size, and operation at the speed indicated by the manufacturer or entity that produced the curve. Curve steepness or inclination has to do with the number of vanes in that impeller; curve steepness is also affected by the angle each vane makes relative to the impeller hub. In general, curve shape is verified by physical testing at the manufacturer's facility. Once the entire pump is installed in the field, it can be re‐tested periodically by the owner–purchaser for degradation and wear progression. Power draw may have been affected by seals and couplings that differ from the ones used on the manufacturer's test stand. Occasionally, high efficiencies are alluded to in a manufacturer's literature when bearing, seal, and coupling losses are not included in the vendor's test reports.

      Regardless of flow classification centrifugal pumps range in size from tiny pumps to very big pumps. The tiny ones might be used in medical or laboratory applications; the extremely large pumps may move many thousands of liters or even gallons per second from flooded lowlands to the open sea.


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