CO2 Seal Leakage.

[shyamshins]

Dear all forum members,
I am facing problem regarding seal which is used for sealing CO2 at -20deg. celcius.This is Single inside with cooled insert.
Product is CO2
Seal size is 1.875"
Suction Press.20.28kg/cm2
Disch.Press.29kg/cm2
Box press.20kg/cm2
Vap.press.20kg/cm2
Temp. -20 Deg.celcius
Sp.grv.1.07
Pump Rpm.2900. Make of pump is microfinish
When I install seal on pump it runs for 10 to 12 days and then it fails.This is used for unloading Liquid carbon dioxide CO2 from truck container.Flushing plan provided for this seal is API Plan 11 and it has lubricated insert which is lubricated by Ethylene glycol.
Problem was that carbon dioxide is entering pot which stores Ethylene glycol & pot press. rises. Lubricated insert has 2nos of orings which are of BUNA-N this happened 2 to 3 times I also checked for damage of O ring of insert but O ring is in good condition.
Request you to guide how I can solve this problem.


Red Flag

 

[JJPellin]

We had a lot of trouble with our C02 loading pumps for many years. We tried a Glycol drip to lubricate the seal. This did not work, so we eliminated it. The key for us seemed to be good insulation to keep the pump as cold as possible, training for the operators that they did not need to use a steam hose to melt the frost off of the pump, controls to keep the pump from running dry or dead-headed and a simple plan 11 flush. We also used Buna-N o-rings and saw problems. The Buna seemed to be absorbing C02 resulting in explosive decompression. I don't recall what other materials we may have tried. Our pumps were vertical in-line configurations and were used to load C02 from our tanks into trucks. The pumps were started and stopped by the truck drivers.

Johnny Pellin

 

[btrueblood]

Might try Butyl rubber o-rings, they tend are less permeable to CO2, thus (should be) less susceptible to decompression damage.

 

[shyamshins]

JJPellin sir,

These pumps are horizontal pumps can we use kalrez elastomers?

 

[flexibox]

Can you post a drawing of the seal?

 

[JJPellin]

I have never attempted to use Kalrez in cryogenic service. Our material application table lists Kalrez as rated to -40 °C. In general, I would expect a horizontal pump to be more reliable than a vertical in-line. It could be that vaporized C02 is building up in the pump case or seal chamber. You could add a close clearance throat bushing to increase seal chamber pressure to suppress vaporization. Good insulation is still vitally important.

Johnny Pellin

 

[shyamshins]

Flexibox just give me somwtime I will upload this CO2 seal drwg. as this seal has agin failed.

 

[shyamshins]

Flexibox,
I have attached file. Pls. help all forum members as this seal has failed again.

 

[shyamshins]

Flexibox

I have attached drwg. of seal pls provide me solution.

 

[JJPellin]

This is depicted as a non-cartridge seal, which increases the number of potential problems.

First and foremost, this is a mechanical seal. It needs to be installed using proper installation practices including cleanliness. It needs to be set to the correct running length. Because this is a cryogenic service, it is important that the mechanics do not use “white grease” to lubricate the o-rings. This silicone based grease is commonly used by our mechanics. At cryogenic temperatures, it crystallizes and causes failure. The shaft must be running down the center of the seal chamber. The allowable non-concentricity should be less than 0.004”. The shaft must be perpendicular to the gland mounting face. The maximum face run-out should be 0.003”. The shaft must be straight. The maximum allowable run-out on the shaft at the seal must be 0.002”.

I don’t particularly care for this design. I don’t see a need for the complex lubricated insert with the glycol drip. But, in your first post, you indicated that product is flowing up into the pot that supplies the glycol. This suggests that the seal faces are running hard on the ID. This would allow process pressure down to the location of the glycol port. Faces running ID high would suggest high temperature at the face surface. Review the face materials to see if a better combination could reduce the heat generated.

I would convert to a cartridge seal design. I would replace this seal type with a standard seal without the glycol lubrication. I think that the glycol is causing the seal faces to run hotter and the thermal rotation is allowing the C02 pressure to travel up to the pot. I would verify all of the mechanical parameters I have listed above. I would examine the vibration closely for indications of imbalance, misalignment or cavitation. I would correct any imbalance or misalignment identified. I would examine the operation conditions of the pump, including NPSH margin, operation point relative to BEP.

In our application, we ended using a multi-spring pusher seal with a Teflon wedge for a secondary seal. This seems like a very strange choice for cryogenic. But, it worked well. It may be that it allowed enough leakage to keep the seal cold and the ice ball that formed limited the external leakage.


Johnny Pellin

 

[shyamshins]

JJPellin

Can you pls. explain me meaning of "
1) This suggests that the seal faces are running hard on the ID
2) Faces running ID high would suggest high temperature at the face surface.
3) The thermal rotation is allowing the C02 pressure to travel up to the pot.

JJpellin did you replaced your seal with double cartridge seal?

 

[JJPellin]

Seal faces are contacting at the inside diameter. There is a gap between the faces at the ouside diameter. This allows the product to reach the glycol port under enough pressure to travel up to the supply pot. The faces could be open at the outer diameter because of excess heat generated at the face.

Johnny Pellin

 

[JJPellin]

I used " to designate inches. We used a single seal.

Johnny Pellin

 

[shyamshins]

JJPELLIN,

In your previous reply to my query you have stated that glycol is causing seal faces to run hotter but, seal pot is provided with cooling water inlet and outlet to cool glycol still is it possible that glycol can heat seal faces?

Sir, with ref. to previous thread, how can gap be created between stationary and rotary face as rotary face is set at proper length according to seal design and seal face in under spring tension because of comp. unit?

Will glycol freeze at -20 degree Celsius(As glycol is not coming in contact with CO2) when seal is in operation as heat will be generated between seal faces and glycol is used to cool and lubricate seal faces?

With ref.to drwg. part no.6 is insert mounting i.e. O RING of BUNA N can this get decompress(i.e.Hardened) as this will come is contact with liquid CO2 during operation and then carbon dioxide can travel to pot is this possible?

 

[JJPellin]

Heat is relative. It is not necessary for the surface of the seal face to be "hot." It is only necessary for the surface to be much hotter than the rest of the seal face. Cold glycol has much more viscosity than liquid CO2. Shear in that thin film of glycol generates heat. Differential thermal expansion results in distortion that causes a gap to open at the outside diameter .

Why do you think you need glycol?. Seal faces can run on liquid CO2.


Johnny Pellin

 

[shyamshins]

JOHNNY PELLIN,


THANK YOU VERY MUCH FOR YOUR VALUED REPLY.