Long term resistance changes

In the long term, filters and strainers become clogged: this is their purpose. Minerals and scale start forming on the internal pipe walls and this reduces the interior diameters on the pipe. A 4 inch pipe will eventually become a 3.5 inch pipe. This moves the pump on its curve because as the pipe diameter reduces, the velocity must increase to maintain flow through a smaller orifice. The Hf and Hv increase by the square of the velocity increase.

Also in the long term, the equipment loses its efficiency, and replacement parts are substituted in a maintenance function. Also, the plant goes through production expansions and contractions: new equipment is added into the pipes. In short, the system and its elevations and pressures, its resistances and velocities, are very dynamic. The BEP of the pump is static.

What must be done is establish the maximum flow, and the minimum flow, and implement controls. Regarding filters, you've got to establish the flow and pressure (resistance) that corresponds to the new, clean filter, and determine the flow and resistance that represents the dirty filter and its moment for replacement. These points must be predetermined. The visual graph of the system curve with its dynamic resistances are seen in this example filtering and recirculating a liquid in a tank. Consider the following graphs (Figures 8-18 and 8-19).

Figure 8-18

Figure 8-18

Figure 8-19



Figure 8-19

As mentioned earlier, the system curve with the clean and dirty filters should coincide within the sweet zone of the pump on its curve. (Figure 8-20 and Figure 8-21).


Figure 8-20

Figure 8-21

The pump will run to the right of its BEP within its sweet zone with the new filter, and slowly over time, move toward the left crossing the BEP as the filter screen clogs (Figure 8-21 and Figure 8-22).

Figure 8-22___

On superimposing the curve of a single pump over this system curve, we see that the system extremes are too wide for the pump to cover on its curve (Figure 8-22).

You should install pressure sensors that transmit a message to shut-off the pump, sound an alarm, or indicate to the operator that the moment to change the filter has arrived. With a new filter installed, the pump begins operating again to the right of the BEP within the sweet zone and slowly over time proceeds moving toward the other end of the sweet zone.

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