Pump and Seal for Liquid CO2 1

[JJPellin]

We are having a lot of trouble with a pair of pumps used to transfer liquid CO2. The pumps are vertical in-line pumps that use a special type of gas seal designed for cryogenic service. The wear parts (bushing and wear rings) have been converted to a non-metalic material that is more tolerant of dry run. I am wondering what other pump or seal technology is being used successfully in similary services. Specifically, what other pump configurations are working well for others? Is anyone using seal-less pumps in services such as this? Any suggestions will be greatly appreciated.

 

[25362]

thread798-63268.

 

[insult2injury]

A problem that I've seen overlooked in the past with such systems is the flow being thermally choked in pipeline. This can cause all sorts of problems for the pump not to mention the risk of a pipe bursting. I'm not saying that's your problem, just something to be aware of.

 

[Montemayor]

JJ:

If you haven't pumped LCO₂ before, I can relate to your concern and questions. But I can put your mind at ease by relating that it is an easy chore and it doesn't take a pump as sophisticated as what you describe.

Firstly, LCO₂ is certainly not a "cryogen" and shouldn't be classed as one. Therefore, it doesn't need an expensive cryogen pump to be pumped successfully and with consistent results. Additionally, sealless technology is an over-kill as well. I've used gear pumps (Smith MC2) and common centrifugals with the expected success. No "fancy" or expensive seal arrangements or seal "systems" were employed. You don't mention your pump make or seal manufacturer or type so I can't comment on what I can't identify or relate to.

As Insult states, sometimes it's not the pump or the bells and whistles; sometimes it's just making sure that pure liquid is simply getting to the impellar.

 

[JJPellin]

Actually we have been pumping liquid C02 for about 25 years. These loading pumps have been inservice that entire time and have been unreliable for that entire time. The pumps are United vertical inline API pumps. The seals have been redesigned many times over the years. The lastest design is a John Crane Type 285 designed for light vaporizing products. I am told that it works like a gas seals and depends on some vaporizing product. The bushings and wear rings in the pump were converted to Dupont Vespel non-metalic material for dry run. The pump are below the tanks and piped to be self venting. In other words, the pumps should be flooded with liquid product at all times. Failures seem to come in clusters and may have something to do with the operators or the truck drivers. Truck drivers actually start and stop the pumps to load their trucks. They may be running the pumps dead-headed after they close their valves. The operators don't like to see a big ice ball around the pumps and have been found to throw a steam hose on them to thaw them out on occasion. Obviously either of these abuses could result in pump failure. But I am still not convinced that I have the best pump selection for the service. I could use a gear pump but would need a spill-back line to the tank and PSV protection for dead-head condition. Is CO2 a good enough lubricant to run a cast iron gear pump? Thank you for your replies.

 

[Montemayor]

JJ:

I've run cast iron pumps on LCO₂.

However, I believe you are using your vertical centrifugals on truck loadings from main storage tanks. If that's the case, I don't see how you can dead head the pump since both the storage tank and the truck tank are equalized -- at least, they should be. That makes it a case where you really must have dense (I mean really dense) truck drivers.

Even with a centrifugal I would install a "spill-back" valve and line in order to avoid dead-heading the pump (regardless of whether it is positive displacement or centrifugal). From what you describe, I wouldn't chase the pumps; I'd chase after the operating procedures and the drivers who are supposed to be applying them.

Basically, you've got a rather benign operation and from what I gather of your description, a very favorable hydraulic installation - positive, flooded suction head, good seals, API construction, centrifugal design, etc. You should be out playing 18 holes of golf instead of chasing around after this operation.

 

[Artisi]

As you haven't detailed the actual problems - only intimated what it might be seals, bushes and wear rings it would be interesting to know what the failures are or have been over the 25 years and have they been the same or a related problem. Or do you solve one problem and then uncover another.

I have never been involved in pumping LCO2 and if it's a product / seal material incompatibility problem then I will leave this to others - but would be interested if it is a problem coming from a mechanical / operation failure mode.

Naresuan University
Phitsanulok
Thailand

 

[JJPellin]

Before we converted to the non-metalic wear materials, we had a couple of different failure modes. In some of the failures, we would find the bushing above the impeller wiped out. In these failuires, the wear rings were rubbed hard and destroyed. Other failures during this period were seal related. We did have some materials problems. It seems that liquid CO2 will absorb into some elastomers. When the pressure is removed, you can get explosive decompression failures of the o-rings. We also tended to find seal faces ground off from appearant dry running. Since we put in the non-metalic wear parts and the new seal design, we have mostly seal failures. I have not had a chance to examine any of the more recent seal failures. But I believe that there are indications of dry running and shaft deflection problems.

 

[Artisi]

It seems that most of your failures stem from application / operation problems,including dry running of the seal faces. This is indicated by the rubbing of pump components and seal failure and to me looks like a problem of shaft deflection - the failures of dry running of seals is a seperate application problem in itself.

So really the failures appear to be application / operation type failures, not a pump design fault or inadequacy.

Mis-applications just go round in circles - overcoming one problem just transfers it somewhere else.

You might have overcome the problem of rubbing of componets - you might come up with a seal that can handle the shaft deflection and can run dry.
But next the shaft will fail from fatigue.
What do you do then? - change shaft material / stiffen it to overcome the breakages.
Then the bearings will start to fail from overload and so on and on and on .

This is stating the obvious but stop the pump running at shut-head -
change the mode of operation - make it "Paddy-proof" so the operators can not run the pump at "shut-head". - put in a recirc. line - a pressure switch to shut down the pump from over pressure - a 3 way valve to close the discharge line to the tankers and divert the flow back to the source.









Naresuan University
Phitsanulok
Thailand

 

[Enkidu]

I believe a lot of the older United inline pumps have a poor mechanical design that starts with the coupling to shaft interface. That may be the root problem here. Have you ever tried to contact the manufacturer for upgrades?
Just as a curiosity, can you advise what the original seal and flush type were?

 

[JJPellin]

I am not sure what the original seal design was. I believe it was probably a John Crane type 8 or 9 run with a plan 11 flush. You are correct to be concerned about the rigid coupling. We do blue checks for contact between the coupling and the shafts. We check for concentricity in the seal chamber and run-out on the coupling, motor shaft and pump shaft. If any of these are out of tolerance, we replace the coupling or pump shaft or motor. But even with a perfect installation, the overhung length of this shaft from the nearest true bearing is too great. I am not certain, but I also believe that the pump case is a single volute. With a single volute and long overhung shaft, running away from BEP leads to disasterous shaft deflection and rubs. It's a poor design under the best of conditions. And if the truck drivers decide to close the block valve at the truck with the pumps running, we run dead-headed and don't have a chance. I can train the drivers and post warning signs, but it will happen anyway. I need a pump and system that is more forgiving of abuse. An automatic spill-back to the tank (could be a Yarway-type valve) and a two volute pump with bearings in the pump.

 

[Alanator]

I was discussing the problem of deadheading, BEP, etc., with a colleague. This person was of the opinion that pumps should be started/stopped unloaded.

I think I have seen some good advice from this forum. Any thoughts on the paragraph above? Agree or disagree?

 

[Artisi]

You seem to be answering your own question - if you have had problems for 25 years it's about time someone bites-the-bullet and makes a sound engineering / commercial decision and replaces the pumps installed a sound engineered system.
I know I know - the "bean counters - the figure fakers" just tell you it's a maintenance problem and you need to account for it from the maintenance budget - "Sorry we don't have any money available for capital equipment" - have you heard that before - the ongoing saga of plant engineers throughout the world.

Naresuan University
Phitsanulok
Thailand

 

[Enkidu]

One last thought for you JJPelin.
Inline pumps have come a long way in the last 15 years, but still have not overcome the stigma of their poor past performace. Your comment about the great distance to the first "true" bearing is absolutely correct. That's why pretty much all the vertical inline units now are made with a close clearance "throat bushing" that are in fact hydrodynamic product lubricated bearings - typically made of an upgraded metal impregnated carbon, with an operating clearance of .005"-.008" on diameter. I would suggest considering reworking the cover to fit one - be carful with the thermal expansion difference between the cover, shaft and the bushing. Should you go to a metcarb/graphalloy type bearing in the box, you should also use the old plan 11 to push some fluid through the bearing/throat bushing. This "could" be a relatively inexpensive fix. All this of course is just general speculation without knowing specifics - OK, here's the disclaimer - to be honest, I've no specific experience with liquid CO2. Feel free to abuse me.
Good luck with the pump