Oversizing Pumps

One of the following steps should be taken when pump oversizing is experienced  

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Design equipment to prevent excessively fast feed. Do not oversize pumps or control valves... 

A major consideration with these reboilers is pump-system compatibility. Since the liquid is near its boiling point, and liquid head is costly, NPSH (net positive suction head) is critical. Oversized pumps can be detrimental to NPSH and should be avoided (134, 253, 358). One case has been reported (134) of poor reboiler performance due to pump oversizing reboiler performance was the same whether power to the pump was on or off. 

FIgur 15.8 Pump and forced-circulation reboiler compatibility. (a) Oversized pump, expect NPSH problems (6) well-sized pump. 

Pump manufacturers have established guidelines to ensure each pump they supply is not exposed to conditions that result in cavitation. The design standard is called NPSHR or net positive suction head required. The NPSHR takes into account any potential head losses that might occur between the pump s suction nozzle and impeller thereby ensuring the liquid does not drop below its vapour pressure (bubble point). The NPSH is a measure of the proximity of a liquid to its vapour pressure, and must exceed the pump manufacturer s pump NPSHR. There are two process variables that can be adjusted, in case the available NPSH is less than the NPSHR raise the static head and lower friction losses. Conversely, the NPSHR can be reduced by using a larger, slower speed pump, a double suction impeller, a larger impeller inlet area, an oversized pump and a secondary impeller placed ahead of the primary impeller. 

Some effort should be made to avoid low available NPSH values (2 to 5 feet) because this may result in the pump manufacturer offering an oversized pump that will operate at part capacity and low efficiency. Whenever possible, excess NPSH over the requirements of the pump should be available. When a pump is handling a boiling liquid, the available NPSH is almost entirely dependent upon the elevation of the liquid level in the suction vessel. 

FIGURE 8-23 Restraining the flow by extending the cutwater and modifying the throat of the volute or liner, or decreasing the suction diameter of the impeller are methods for correcting oversized pumps. 

Shafts of slurry pumps have broken at the shaft thread, simply because the radial load was too high and caused rapid fatigue failure. It is therefore strongly recommended to limit the minimum flow rate to half the best efficiency flow rate at the given speed. Throttling an oversize pump is not recommended at all. Downsizing or reducing the speed of the pump is essential to avoid excessive radial load on the pump shaft. 

Wide voltage fluctuations may be prevalent in a rural distribution system, particularly in developing countries. In such cases it is common practice for users to select an oversized machine for their needs. Accordingly, the motors employed for loads such as pumps, thrashers and winnowers are normally over-rated and underutilized. Also the same motor may have to cater for different types of loads, at different times, and these loads may be much less than the motor rating. 

Rarely do design engineers request the L/D faetor in their quotes. Some engineers don t know they have the option. Most pumps are bought based on priee, and beeause a high defleetion resistanee (low L/D Faetor) indieates a larger diameter shaft with oversized bearings these U pe pumps don t normally win a competitive hid process. 

This could also be induced by design, if the pumps are oversized, or by high velocity and friction head in discharge piping of inferior diameter. 

Lube oil pumps are typieally heavy-duty eentrifugal or positive displaeement type with a design eapaeity that eovers the full range of antieipated operation. They ean be either direet-driven by eleetrie motors or eoupled with disk or diaphragm-type eouplings. Experieneed turboexpander manufaeturers will supply motors and eouplings that are at least 25% oversized. 

For low available NPSH (less than 10 feet) the pump suction connection and impeller eye may be considerably oversized when compared to a pump not required to handle fluid under these conditions. Poor suction condition due to inadequate available NPSH is one major contribution to cavitation in pump impellers, and this is a condition at w hich the pump cannot operate for very long without physical erosion damage to the impeller. See References [11] and [26]. 

Impeller Since the Sundyne straight, radlal-bladed Impeller does not require close running clearances, It eliminates the necessity for oversizing to compensate for performance deterioration common to conventional pumps. Clearance of the Sundyne Impeller Is 0.030 to 0.070 inch. 

Motors shall have power ratings, including the service factor (if any), at least equal to the percentages of power at pump rated conditions given in Table 3-1. However, the power at rated conditions shall not exceed the motor nameplate rating. Where it appears that this procedure will lead to unnecessary oversizing of the motor, an alternate proposal shall be submitted for the purchaser s approval. 

Desalter units generally will produce a dehydrated stream containing like amounts of BS W from each stage. Therefore, BS W can be considered pan through volume and the dilution water added is the amount of water to be recycled. The recycle pump, however, generally is oversized to com- >cnsate for difficult emulsion conditions and upsets in the system. 

There will be a small loss in motor efficiency, by using an oversized motor. As process operators and engineers, we can ignore this effect. It is good engineering practice to purchase new motors for the maximum-size impeller that can be installed in a pump. 

With regard to specific types of equipment, the safety factor practices of some 250 engineers were ascertained by a questionnaire and summarized in Table 1.4 additional figures are given by Peters and Timmerhaus (References, Section 1.1, Part B, pp. 35-37). Relatively inexpensive equipment that can conceivably serve as a bottleneck, such as pumps, always is liberally sized perhaps as much as 50% extra for a reflux pump. In an expanding industry it is a matter of policy to deliberately oversize certain major equipment that cannot be supplemented readily or modified suitably for increased capacity these are safety factors to account for future trends. 

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