How to kill a mechanical seal part III

Did you know that pole dancing nowadays is a mainstream form of fitness practiced in gyms and dedicated dance studios? One of the earliest forms has been tracked eight hundred years back to the Indian sport of Mallakhamba and was considered a national sport. Honestly, ask Mr. Wikipedia, he will tell you! Also it is a great metaphor to get you guys to click on the article and read a bit more about shaft deflection as one of the causes for seal failure to upgrade your knowledge. It beats Daniel the Mantis “Cavitation“ Shrimp anytime if you ask me…Besides I wasn’t allowed to use a picture of an exotic female dancer, so there is that…

Shaft deflection

So what is shaft deflection and what does it do to the seal? Every pump has a so called BEP point or Best Efficiency Point. This basically means that the velocity and pressure are equally distributed around the impeller based on an ideal situation. So when the velocity of the liquid is not distributed equally at all points, the pump shaft can bend. If it were to be demonstrated by a pole dancer, it wouldn’t look smooth or elegant, but rather like someone losing control and possibly taking a fall. Does this mean that even with an ideal BEP point there is no shaft deflection? Unfortunately not! There will always be some shaft deflection because of the pressure and velocity applied. Just like with metal fatigue, eventually the shaft will have to be inspected from time to time and if necessary repaired or replaced. So this has to be checked even if you are operating close to the BEP all the time! The API 610 norms and ISO 5109 norms specify what the allowed tolerances are.

Now imagine the seal face opening to be very narrow (remember the 0,001mm?). So what kind of damage can it do?

1.     It opens seal faces

2.     The seal internals can make contact

3.     Damages the seal faces

What about pumps?

4.     Pump internals can make contact

5.     Throttled discharge valves

It is also prone to happen with older pump designs and it can be common with start-ups.

Misalignment

After the pump comes back from the workshop, always make sure the pump is installed levelled and tension-free. A fuller shaft for example, without a sleeve will usually show less deflection, which is better for the mechanical seal. Tensions can cause distortions with pump flanges, break them off or even tear the pumping house. Bending of the shaft as a consequence of misalignment can happen, which causes vibrations that in turn damages the seal faces.

Solids and abrasion

Because seal faces can be sensitive, such as soft carbon faces that are usually used in single seals, abrasive particles scratch the seal faces, causing it to leak uncontrolled or it can act as a strainer. Extra caution is advised with solids in the bottom of the tanks, debris from cleaning or boil-outs or poorly mixed solids.

Water hammering

Rapidly closing a valve creates a hydraulic pulse, that can be accompanied by a loud bang or pipe vibration, where the pulse can exceed over 100 bar/ 1500 PSI. This will make the seal faces distort or lose its fluid film. Always close valves slowly and be extra careful with auto valves and quick acting solenoid valves.

So we can basically conclude by now that a mechanical seal is a very precise and delicate piece of equipment, that needs to be installed carefully. In my next article I will pay attention to the general seal installation steps, in order to prevent seal leakage due to a faulty installation…

Until then!