suggestion for low pressure water, rotating shaft seal

[abeschneider]

Hi,
I'm trying to specify seals for an application as follows:
- submerged, freshwater (river water; not many particulates; some chance of suspended sand)
- max pressure: 3 psi
- max vacuum: 8 psi
- shaft speed: 520 RPM, continuous, single-direction (non reversing)
- surface speed: 1.4 m/s (4.5 ft/s)
- shaft hardness: 25-27 Rc
- desired life: at least 2 years continuous operation

I'm having trouble figuring out what seal to choose; it seems a mechanical seal is overkill given the low pressure and speed. Packing is leaky and maintenance-heavy. I've heard O-rings "are not good in dynamic situations". And lip seals damage the shaft and wear very quickly.

What is a good suggestion for the main shaft seal?

By the way, the shaft bearings (ball) are on the "dry" sides of the shaft (think of the application as a shaft going through a box filled with water under the above pressure). Should I specify both shaft seals at the "box walls" as well as bearing isolators?

Thanks for your help.

 

[scalleke]

Seems you have made the choice already.

Mechanical seals may seem like overkill but they are the only thing that will give you the service life that you need and if you select them double with a barrier fluid they will work as well on the vacuum as with pressure in the pump.

scalleke

 

[longeron]

Just to make sure I understand the application:

Shaft goes through the wall of this vessel or container into a river. Exposed bearings help to support this shaft inside the vessel or container? Or are the bearings in some bearing housing? Are the bearings packed with grease or do the use a flinger and oil sump?

If these are like a set of pillow block bearings, I'd use a labby type seal on the housing. Otherwise I'd suggest a magnetic type bearing housing seal instead of a lip seal. They provide much better isolation for oil than most anything else out there.

For the wall of this vessel and the shaft I'd suggest the same kinds of systems used in Marine applications and large mixers in tanks:

Install an inflatable diaphragm seal around the shaft on the vessel wall. This will allow you to change a mechanical seal without having to worry about the river water entering the vessel during maintenance. Before working on the mechanical seal you inflate the bladder which then seals against the shaft and the vessel. Deflate it when you're done performing maintenance and operate the equipment.

I'd suggest a split seal for this application as it does not sound like removing the bearings and associated equipment will be easy. Choose a split seal with two hard faces if you are concerned about silt, sand, and other abrasives.

Most of the major seal manufacturers offer these kinds of seals.

John Crane Type 74 bladder and Type 37 or 37FS split seal
Flowserve MSS split seal.
Chesterton 442 split seal. ISS bladder type seal