How to kill a mechanical seal part II
In my previous article I have mentioned the healthy working conditions of a mechanical seal and dry running as one of the causes of seal failure. For the topic of this article I had to borrow Daniel the Mantis Shrimp. Daniel has mastered the principles of Bernoulli and can be a nasty little fellow when he is hungry! He uses his interesting make- up and bright colours by probably making his prey curious. Once his prey gets too close, he uses his fistlike clubs to smash its victims’ shell with a speed up to 23 meters per second, by generating a vacuum bubble that implodes. For the sake of comparison, that’s about the same force generated by a .22 caliber bullet!
Cavitation
When we look at cavitation in a pump, something similar happens, except that here are thousands or millions of these vacuum bubbles in a pump. Is Daniel to blame for these? No, he probably wouldn’t survive in a pump anyway. But what causes it then? Here is a short list of most common causes:
1. A poor suction pipe work design
2. A low suction tank level
3. A product close to its boiling point
4. Build-up in suction pipe work
5. Clogged suction strainer
6. Blockage such as partially closed suction valve
7. A globe or butterfly valve installed in place of gate valve
8. Flange gasket that is incorrectly installed
9. A pump with a higher speed (RPM) has been installed
10. A pump with a higher capacity or flow has been installed
11. The system temperature has changed
12. The system discharge has changed
After opening a pump and taking a look at the impeller, it looks something like this:
So what are its symptoms, what to look for?
1. Vibration of the pump
2. Noise (as if pumping gravel or marbles instead of a fluid)
3. Loss in capacity
4. Reduced electrical consumption
5. Seal damage
6. Internal damage to the impeller and casing as shown above
Just a quick reminder of what the 2 main reasons are for a seal to break down: the seal faces open up too much or it becomes damaged.
When seal faces operate with an opening of 1 micron ( 0,001 mm) it is not hard to imagine what will happen to it. Chipped of metals can have abrasive effects on the seal faces, not to mention the seal faces hitting each other due to vibrations, just to name a few.
So cavitation forms bubbles in a fluid, where the fluid vaporizes when the pressure is too low. The bubbles implode at the speed of sound, where it damages more than just the inside of the casing and the impeller, but also the seal. Once established, it is vital to act as fast as possible to remedy this.
Overheating
Another cause of seal failure is overheating. This usually happens when the fluid film is approximately + 30 ℃ or 85 °F over the product temperature. Keep in mind that extra caution is required when the product is close to its vapor point as the fluid film can vaporize, which can result in chattering seal faces. Always check the dual seal vessel levels as well as the pressures.
In my next article I will continue on more causes of seal failures such as shaft deflection…
Until then! Take care!