Pumps larger impellers

When considering a larger impeller to expand a pump s capacity, try to stay within the capacity of the motor. Changing impellers is relatively cheap, but replacing the motor and its associated electrical equipment is expensive. The effects of a larger diameter (d) are approximately as follows ... 

The point where the flow rate of 275 gpm intersects the system curve in Fig. 8-2 (at 219 ft of head) falls between impeller diameters of l and 7 in., as indicated by the O on the line. Thus, the P in. diameter would be too small, so we would need the 7 in. diameter impeller. However, if the pump with this impeller is installed in the system, the operating point would move to the point indicated by the X in Fig. 8-2. This corresponds to a head of almost 250 ft and a flow rate of about 290 gpm (i.e., the excess head provided by the larger impeller results in a higher flow rate than desired, all other things being equal). 

Increasing the size of the impeller by 10 percent will increase the amperage load on the motor driver by 25 percent. For many process pumps, this would require a new motor and breaker, to support the larger impeller. 

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. 

Furthermore, the diameter of the circulation pump plays a role. As the pressure drop in the system is now lower, a pump with a larger impeller that rotates more slowly can be used. According to the equation for the power consumption N, there is a connection between the impeller diameter D and the impeller speed n ... 

Pumps with larger impeller diameters (i.e., lower tip speeds and therefore lower nucleation rate) are used in DTB crystallizers in order to produce coarser crystallized masses. 

Fias the size of the original pump impeller been increased A larger impeller can cause the pump s motor to draw more amperage. 

Uquid viscosity is higher than spedfted for pump. Options Reduce fluid viscosity by heating or diluting. Install a larger impeller if possible. Replace centrifugal with rotary pump. 

Experimental closed tank results for sands with average particle sizes of 0.18 and 0.5 mm in pumps with impeller diameters of 0.35 and 0.6 m, respectively, were reported by Herbich [5]. Slurry densities were up to about 1400 kg/m. It was found that the NPSHR-values (expressed in m of slurry) were similar to the water values, independent of the slurry density. Similar results were also reported by Herbich [51 and Ladouani et al. [6] for non-settling clay-silt slurries with densities of up to 1300 kg/ m in pumps with impeller diametres less than 0.275 m. Ladouani et al. [6] used an open-tank loop arrangement. Detailed inspection of their data indicates that the independence of the slurry density on NPSHR was limited to flow rates smaller than about 70% of the best efficiency point (BEP). With larger flow rates, NPSHR increased with increasing slurry densities, giving values from 1 to about 2 times the water values in the BEP-region. 

Installation only of a larger control valve, or larger valve trim, will save zero energy regardless of the result of the above calculation. In addition to modification of the control valve, the upstream centrifugal pump must also be modified. Assuming that the pump is driven by a fixed-speed AC motor, the size of the pump s impeller has to be reduced. The smaller impeller plus the larger control valve will result in ... 

The larger the ratio of impeller diameter to tank diameter, the less mixer power required. Large, slow speed impellers require a low er horsepow er for a given pumping capacity, and solid suspension is governed by the circulation rate in the tank. 

Coarse abrasives Single suction. Not available for full range of ratings, that is, small capacities not too easily obtained. Often have very large impellers operated at slow speeds for use when solids larger than 1" diameter are the standard diet. This would be of the type called dredge pumps handling sizeable rocks. Closed. 

Recognizing the flexibility of the affinity laws, it is better to select an original pump impeller diameter that is somewhat larger than required for the range of anticipated performance, and then cut this diameter dowm after in-service tests to a slightly smaller diameter. This new performance can be predicted in advance. Once the impeller diameter is too small, it cannot be enlarged. The only solution is to order the required large impeller from the manufacturer. 

A well-established and effective method of ensuring a laminar flow to the eye of the impeller is to provide the suction of the pump with a straight mn of pipe in a length equivalent to 5-10 times the diameter of that pipe. The smaller multiplier would be used on the larger pipe diameters and vice versa. 

You want to know what pressure a pump will develop with a liquid having an SG of 1.4 at a flow rate of 300 gpm using an impeller with a diameter of 12 in. driven by a motor running at 1100 rpm. You have a similar test pump in the lab with a 6 in. impeller driven by an 1800 rpm motor. You want to run a test with the lab pump under conditions that will allow you to determine the pressure developed by the larger pump. 

Pumps shall be capable of at least a 5 percent head increase at rated conditions by replacement of the impeller(s) with one(s) of larger diameter or different hydraulic design. 

Over and above this, and particularly for the larger types with long rotor blades, airing the pump to atmospheric pressure while the impellers are rotating may be done only when observing exactly the rules given in the operating instructions. Under certain circumstances it is possible to operate turbomolecular pumps under exceptional conditions. 

AP remains constant. But the lower required feet of head permits the pump to operate further on its performance curve. This produces more volumetric (GPM) flow. It also requires more work to pump the greater flow. So we have to have sufficient excess amperage on the motor driver, to accommodate this maneuver. But that is the price we pay for expanding the pump s capacity, without increasing the impeller diameter. Of course, the larger-diameter impeller might still require more motor amperage. 

We noted in Chap. 23 that increasing the impeller diameter might require a larger motor for an existing pump. Suppose, then, when a new pump is purchased, we install a 100-hp motor sized for the maximum 10-in impeller, which the pump could accommodate. Actually, only an 8l/2-in impeller would be used in the pump initially. This 8V2-in... 

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