Cryo Pumps: Dry Run Seal, Damaged Seal?

Have you considered the idea that you may have a vapour lock condition (Nitrogen gas) occurring at this part of the pump or somewhere in your circuit. This will prevent cooling LIN from entering this area to both cool and lubricate these parts. Similarly another possibility should also be considered is the idea of trapped water vapour solidifying and becoming either a fluid block or scoring debris in the moving parts.

You have a very tricky problem to solve here because operation happens in such a different environment than when one can inspect. Monitor as much telemetry as you can to identify what is happening. Verify that the mechanical drive of this pump is or is not seeing a consistent load. Add a few more thermocouples to the lines to verify that vaporization occurs only where it should.

Good Luck

redfred is probably right on both possibilities. There could be enough heat generated by the impeller, fluid flow through narrow channels, and especially by the seal faces themselves, to cause the LIN to vaporize, and the design of the seal-chamber looks like it could be prone to vapour-locking. I would be concerned about seal-chamber pressure. Do you have any way of measuring this? Is there a recirculation loop (API plan 11) from the discharge (or taken anywhere off the volute) to the seal chamber (this is not shown in the schematic)? I doubt that recirculation past the rear wear-ring would provide enough flow & pressure to the seal chamber. You can see the return channel(s) through the impeller from seal-chamber to pump-suction. If there is no recirc.harness, then seal-chamber pressure will be too low, and prone to vapour-lock.

Your pump supplier/manufacturer ought to be able to provide the right seal for the job (When I was involved with sealed-centrifs I spent much more time on seals than on pumps!), and they should be sorting the problem for you. If not, then you should consult the seal manufacturer. In my day they always knew what they were talking about. If your current pump/seal supplier/manufacturer cannot provide a good solution, there are others who can, as well as those who can point you in the right direction.

I'm completely out of my league here, but it seems like there should be some way to prime the pump and purge the air before putting into service. If such a device exists, have you checked it?

It ain't about air.

A cryopump is a vacuum pump that traps gases and vapours by condensing them on a cold surface.

If I start off by saying I'm out of my league, you're not supposed to come around and make me feel stupid.

Let me do a little homework. I'm amazed that these things work at all. Cool, (pun intended), stuff.

Yes, and?

A cryopump is NOT a vacuum pump. It's just a pump that pumps at low temperature. In terms of centrif principles it is exactly the same as any other. The low temperature affects the material selection (and possibly the design of certain components), but not the hydraulic performance. What enters the pump is a liquid, not 'vapours' and 'gases'. This is what the OP is on about.

To the uninitiated, a vacuum pump describes the function as well as any technical explanation.

A cryogenic pump describes the temperature, not the pressure functions.

You may have stepped it in it ........................again.

Do I care? A pump produces lower pressure on the inlet side than the outlet side.

Call it what you want.

The H man knows his pumps, no doubt.

I step in it all the time, and just scrape it off.

You're still scraping? I just leave it on. Tomorrow's another day.

WoW

You are so far off the mark here that it is not funny. A cryopump pumps cryogenic fluids. The goal of these pumps is to get the fluid from point A to point B without allowing the fluid in A or B to vaporize from too much of a drop in pressure in either container. Now very often a vacuum pump will be nearby because regions will be in a vacuum to reduce heat transfer to the cryogenic fluids. Cryogenic pumps have become one of the more efficient methods of cryogenic cooling used today. In particular they save Helium from being constantly vented from a Dewar because it is instead a closed system where the Helium is reused instead of boiled off. To get below 4K vacuum pumping of liquid Helium is still done for some conditions.

I'm feeling much better now. Lets just assume that his buddy gave him another bottle of expensive tequila..............................he gets smarter by the sip.

Of course, his retort will be that he was just testing the OP.....................cause it smells like homework.

That's what I get for copying Wiki.

Oh well, next time I'll check the source.

Maybe this will help:

http://www.media-awareness.ca/english/resources/special_initiatives/wa_resources/wa_teachers/tipsheets/search_internet_effectively.cfm

That's what got me into trouble in the first place.

Trouble comes with the territory. The option is not saying anything at all...............and that's impossible.

Can you eloborate more on the 'seal arrangement', a schematic would be better.

There is another scenario that may explain the damage your pump seal has experienced. If your liquid Nitrogen has Helium or Hydrogen in suspension you might get localized vapour locks without a high temperature reading. This can happen if the cryogenic lines previously had these liquids in them and an incomplete purging was not accomplished before filling with LN2.

Dear Maria-LNG, What are the discharge pressure & flow rate from the pump?

In such pumps seal material is normally carbon or rulon and mating ring is made of TC or similar hard material. Dry running is a most common cause of seal failure in cryogenic pumps. But as you say this is not the reason, I suspect following possibilities:

1. Mistakes in seal installation: Check with the seal manufacturer about norms for perpendicularity and concentricity while installation. Also the cleanliness. Never touch or wipe the seal mating surfaces. It should be installed immediately after opening the packings from manufacturer instead of keeping it open for long time.

2. How you prime the pump before start & how long you take for priming?

In cryogenic pumps proper priming has utmost importance. For this generally suction valve and vents/drains in pump body and outlet are opened for cooling down until liquid starts coming out from all vents. It takes about 15-20 minutes. If you ignore and start the pump, it is like dry running for some time.

Also during this cooling down LIN vapors will be coming out from seal area which may result frosting of atmospheric moisture near the seal mating surfaces. Some times when you try to rotate the shaft by hand after cooling down, it may not rotate due to sticking with hard Frost. And in this condition when pump is started, it may damage the seal. To avoid it shaft is to be rotated few times while cooling down to ensure it is free. Some manufacturers recommend one or two kick starts in between. Please check with pump/seal manufacturer.

GA. Not because you provided an answer, but you may have uncovered an inherent problem with this pump. Worth a look. We forget that we can do a search within CR4. Good job.

GA from me too. In fact there is one more thread by Peter- Lucas about flexibles for same pump. Removing the flexible pipes at suction and discharge of the pump may be the root cause of seal failure because it can generate excessive stress on pump due to thermal contraction when liquid nitrogen is charged. Direction of stress depend on piping configuration. It was mentioned somewhere by OP or Lucas, that at ambient temp pump rotate smoothly but after cooling make metallic sound when tried to rotate. It proves that.

Hello members

Thank you all for your time and effort and all those useful information,

let me especially thank redfred, holzfeller and pritam for providing such a detailed description.

Indeed, this is the same pump as Peter Lucas mentioned in his thread, he is my colleague, however since he was off for holiday I couldn't reach him, but I knew that he posted some questions on this forum, so, I decided to ask your opinion before making any further decision.

During last two weeks, me and other colleagues were in negotiation with the pump company (in collaboration with a local company) to initially find the cause of the problem and then the most practical and economic solution.

The funny thing is, the pump was used and running properly (or at least without any recognizable problem) during last year on the same set-up (before I join this company!). The story still goes on...

All the Best, Maria