Double lip seal effectiveness on fan motor shaft for biogas 2

[USAeng]

We have fans that send biogas to a flare. We had a neoprene seal that is basically a hole cut in neoprene with the motor shaft stuck through then the impeller was direct drive mounted to the motor. That leaked too much gas so the OEM switched to a Teflon plate with a hole machined that the shaft fit through with a tight tolerance- still direct drive - still leaked. Now we are looking at a coupled design which gives them more room to put in a double lip seal around the shaft.

I believe this will work but wonder if anyone has an experience to share? We also looked at a John Crane seal but they were very expensive.

We are running the blowers at 3450RPM but only for about 5-10 minute intervals normally about 8 times daily. If the plant is down then they will flare for longer periods but that will be abnormal.

Thanks for any advice

 

[tbuelna]

What is the diameter of the shaft? What is the temperature range and pressure delta the lip seal will see? Is there any type of lubricant present at the sliding interface between the shaft and lip seal?

The most common rotating shaft seals are lip seals that employ a garter spring to maintain a consistent contact force between the elastomer seal lip and shaft surface. The typical surface rubbing speed limit with elastomer lip seals is around 4000fpm.

Teflon is self-lubricating, but it makes a crappy lip seal material due to its tendency to cold-flow.

Take a look at the SKF website for detailed information on elastomeric lip seals.

Good luck to you.
Terry

 

[USAeng]

Pressure diff is 15"WC. Shaft dia is 1.5". No lube present. That are looking at Teflon and Viton.

 

[USAeng]

Would be interested in more info on problem of cold flow with Teflon [pre][/pre]

 

[USAeng]

Googled the cold flow- good to know and makes sense. I see there are products they make that are a combination of materials too

 

[Tmoose]

>>Could<< lube be present?

Do you have a supply of compressed air available?

 

[USAeng]

Unfortunately nowhere closeby. We did think of doing that

 

[Tmoose]

Mounting a lip seal will likely require jumping up a level or two in construction and assembly sophistication compared to holes in neoprene and teflon plate. Also Lips seals have pretty limited tolerance for radial runout and eccentricity.

A nice Forsheda V-ring slid onto the shaft "behind" the impeller and bearing on a nicely finished portion of the fan housing would seal against the face of the housing and has about 10X greater tolerance for radial runout and eccentricity.

http://www.skf.com/binary/21-6109/vr1ad_tcm_12-6109.gif

Vanes on the back side of the impeller can lower the pressure in the seal area, reducing the sealing requirements.

 

[USAeng]

Those are some pretty good ideas. I'll look into those more tomorrow. Thanks

 

[tbuelna]

With a 1.5" dia shaft and a deltaP of just 15"H2O, you might get acceptable performance from a PTFE lip seal.

http://www.seals.saint-gobain.com/u...Lip-DynaLip_Docs/OmniLip-Brochure-BST4184.pdf