Mechanical Seal

Bultum · Mar 3, 2020

Hi,

We have a situation where we operate three horizontally split case centrifugal pumps that are connected to the same suction line and discharge line. When operating under suction mode, the gland packing are known to leak air into the system. Usually it's one pump duty and two of the pumps on standby.

I have been told that to prevent air leak into the system, a flushing line that bleeds water from the pump volute and provides pressure on the packing gland can be installed. This should solve the air leak through the duty pump. However air would still leak into the pumps through the packing gland of the standby pumps.

Hence, i am thinking of mechanical seals on all three of the pumps. My question is how good are mechanical seals under very low or vacuum condition? Would low pressure in the pump push the two faces apart to leak air into the system?

Thanks for any information

LittleInch · Mar 3, 2020

Can't you just isolate the inlet lines of the standby pumps?

Or actuate the valves which are there.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

JJPellin · Mar 3, 2020

You can add the discharge flush as you indicate. If you take the flush stream off of the main header, downstream of the check valve, it will flush the packing on all pumps, even the ones that are not running. If you choose to go with mechanical seals, that will work as well. Most seals are designed to take slight reverse pressure with no impact. However, even with mechanical seals, the reliability would likely be better if you provide a flush line to cool and lubricate the seal. We use this flush arrangement on pumps in vacuum service on most of our surface condenser systems.

Johnny Pellin

TugboatEng · Mar 3, 2020

Does your system have sufficient NPSH if it's drawing other pump casings into vacuum?

JJPellin · Mar 3, 2020

Yes. The stream is water at perhaps 90 F. There is sufficient elevation head for the pumps to run under partial vacuum with no cavitation problems.

Johnny Pellin

Bultum · Mar 3, 2020

Hi All,

Thank you for the response.

#littleinch - That is definitely one option. I will look further into that.

#JJpellin are you saying if we go with mechanical seals, flushing would need to be provided for all the pumps simultaneously? is that the case for pumps on your surface condenser system? There needs to be some sort of flow control to prevent high velocity water from damaging the mechanical seals?

JJPellin · Mar 3, 2020

Yes. You know the discharge pressure. You know the suction pressure. Size an orifice to provide 1 gpm per inch of seal diameter.

Johnny Pellin

Artisi · Mar 3, 2020

Of course, the best solution from this point is to get a mechanical company involved.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

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