Starting NPSH requirement

The need to accelerate the fluid in the suction of a pump is called the starting net positive suction head (NPSH) requirement To calculate this starting NPSH requirement, let s assume  

If the operator increases the flow rate uniformly over a period of 100 seconds, the change in momentum is 

One lb mass equals 1 lb force times s /ft. Therefore, the force needed to accelerate the water in the pump s suction is 

The area of the 8-inch suction line is 50 in. Therefore, the starting pressure loss is 

The need to accelerate liquid in the suction line of a pump leads to a difficult operating problem, which occurs on start-up. Just before the pump shown in Fig. 34.5 is put on line, the velocity in the suction Hne is zero. The energy to increase the velocity (i.e., accelerate) the liquid in the suction line must come from the pressure of the liquid at the pump s suction. As the pump s discharge valve is opened, the velocity in the suction line increases, reducing the pressure at the suction of the pump. The faster the discharge valve is opened, the greater the acceleration in the suction line, and the greater the loss in the pump s suction pressure. 

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This results in a temporary loss of pressure at the suction of the pump. This temporary loss of pressure is called the starting NPSH requirement. The more quickly the operator opens the discharge valve of a pump, the more rapidly the liquid accelerates in the suction line. This increases the starting NPSH required. 

The longer the suction line and the larger the diameter of the line, the more mass has to be accelerated. This also increases the starting NPSH required. If the sum of the frictional loss in the suction line, plus the running NPSH, plus the starting NPSH, equals the available NPSH, then the pump will cavitate on start-up. 

The experienced design engineer always allows for the starting NPSH requirement when determining the elevation of a vessel. It is hard to say whether to allow an extra 2 ft, or an extra 10 ft. It depends on the size of the suction line, and how carefully the operators are likely to be when starting the pump. But if the designer forgets this factor, then the plant operators are sure to notice the omission. 

This pump is presumed to run fine once it is running. The available NPSH is such that it exceeds the running NPSH. So how can I provide a temporary increase in the available NPSH, to satisfy the temporary starting NPSH requirement ... 

A pump seal ought to last about six years. Running a seal dry for a few seconds during start-up can reduce the seal life by six months. For a self-flushed pump that loses suction pressure on start-up, there will be little or no seal flush pressure until a normal pump suction pressure is established. (See section on starting NPSH requirements in Chap. 36.) Also, a pump that cavitates on start-up, due to the loss of suction pressure, will vibrate, which also contributes to the damage to the pump s mechanical seal faces. 

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