Question

Question	consider not only the mechanical seal needs, but the hydraulic requirements of the pump as well, before answering the questions. Problem description: Please refer the image below. Two pumps, pump #1 and pump #2 run continuously and both draw liquid from the distillation tower. If the fluid level drops below a specified value, pump #2 draw-off control valve will begin to close in order to allow the fluid level in the Distillation Tower to rise. The draw off valve only modulates for a very short time and never fully closes as the tower fills rapidly when this valve begins to close. During this time the pump noise levels rise. The seal life in pump # 2 is far shorter than that of pump #1. The seal in pump #2 is of the unbalanced type. Analysis of the failed seal shows the tiny cracks on the seal and damage of the O-rings. Production do not want to change to dual seals because of interchangeability between designs Q1a) Describe the initial mechanism that started the process of failure of the seal in Pump #2? (Hints: Consider the suction pressure of the pump and phenomenon of cavitation) Q1b) Describe two possible solutions that could prevent seal failure

Answer 1

Two pumps, pump #1 and pump #2 run continuously and both draw liquid from the distillation tower. If the fluid level drops below a specified value, pump #2 draw-off control valve will begin to close in order to allow the fluid level in the Distillation Tower to rise. The draw off valve only modulates for a very short time and never fully closes as the tower fills rapidly when this valve begins to close. As, the draw off valve never fully closes as tower fills rapidly, the suction pressure of the pump #2 which responsible for the working of pump comes into picture, it creates low pressure area between the valve and the motor in the pump.

This low pressure creates bubbles inside the water in the pump and begin to escape inside pump in the form of high speed turbulence of water. This high force water strikes everywhere in the pump and also to the seal of the pump. During this time, the pump noise levels rise. This phenomenon is called cavitation. This results in starting of the damage of seal of the pump #2 because it also uses unbalanced seal. Repetitions of this phenomenon leads to permanently failure of the seal.

(b) There are various solutions to this problem. But the most economical solutions is the establishment of new input of water to the Distillation tower which fills the tank and it ensures that the level of the water must not be lowered from a specified value. This completely resolves the problem.

But in any case, we can't add another input of water in Distillation tower.

The draw off valve should be completely closed when the water filled into the Distillation tower.

When draw off valve is closed completely. The suction pressure would not create a low pressure region and the cavitation phenomenon would not occur.

This also eliminates our failure of the seal of Pump #2.

Another temporary solution is :

We should replace the seal of Pump #2 with a better seal which has more life than the previous seal. But this only reduces our problem instead of eliminating the problem.