Dynamic pressures

Let's consider now a system with dynamic pressures and a constant elevation. A classic example of this would be where a pump feeds a sealed reactor vessel, or boiler. The fluid level in the reactor would be more or less static in relation to the pump. The resistances in the piping, the Hf and Hv, would be mostly static although they would go up with flow. The Hp, pressure head would change with temperature. Consider Figure 8-14.

The system curve, once again, is the visual graph of the four elements of the TDH. The Hp is stacked on top of the Hs. The Hp changes with a change in temperature in the reactor. If the reactor were cold, the Hp would be minimum or zero. We'll call this HpP When the tank and fluid are heated, the Hp rises to its maximum. This is represented as Hp2 on the graph (Figure 8-15).

Figure 8-14

Figure 8-15

Let's say that the needs of the system require X flow. Now we search for a pump with a BEP at X gpm, at a head tailing right between 11pj and Hp2 on the system curve. See the next graph (Figure 8-16).

The system's AHp should tall within the pump's primary or secondary sweet zone. At the beginning of the operation, with the cold reactor vessel, the pump operates to the right of the BEP but within the sweet zone, and as the reactor vessel is heated, the pump migrates on its curve toward the left, crossing the BEP, to the other extreme of its sweet zone. When the reaction is completed and the tank cools, the pump





migrates again on its curve, this time toward the right, crossing the BEP and comes to rest on the right end of its sweet zone. See the next graph (Figure 8-17).

Again, we see the importance of the pump family curve, with its concentric ellipses of efficiency. It shows that in the beginning of the operation, the pump is operating to the right of its BEP. As the pressures rise in the system, the pump moves toward the left of the BEP. When the temperatures and pressures are reduced at the end of the process, the pump migrates again on its curve to the right of the BEP. During the entire operation, the pump is inside its primary or secondary sweet zone.

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