Back to Back vs Face to Face Seal Arrangement

[h4m]

Could someone please provide any insights on below questions as per his/her experience regarding selection of pump double seal arrangement.

Between Face-to-Face and Back-to-Back arrangement which one is better for handling solid particles of around 2mm size.

Is it necessary that flexible elements be the rotating elements as well in Back-to-Back arrangement.

Are there any published guidelines regarding seal arrangement selection.

If someone could provide some useful references regarding this that would be really appreciated.

 

[SB1976]

Hi,

Regarding your two questions :

1. Solid containing pumped liquid is an abrasive service, for which a shorter length of shaft seal is required in view of reliability considerations. F-B seals requires the longest, followed by F-F seals and then B-B (which requires the least of the shaft seals lengths). But, shorter seal length also implies certain disadvantages : limited axial movement, inside mounted cartridge and potential loss of cooling functionalities by the barrier fluid. All these need to be evaluated before a final selection is made. But the primary choice shall always be B-B type.

2. Whether flexible elements be rotating or stationary, API STD 682 serves as a useful guide for this:
(a) Recommends stationary flexible elements in case seal-face surface speed at mean seal face diameter > 23 m/s.
(b) Design considerations are highlighted (seal balance diameter, misalignment issues, runout issues)

Trust above helps.

 

[hydtools]

Face to face tend to trap carry-over oil and increase seal pressure. Hydro-lock?

Ted

 

[Pandeyji]

For solid particles , both F/F and B/ B seal should not be used.
F/B ( Tandem seal is best.
Face combination of inboard seal should be hard face combination.
Spring should be out of product.
Stationary spring seal is always better than rotating seal.
Technically stationary and rotating Seal as per API 682 are equivalent. Their recommendation is based on population of the seal in field.
Choice is left to user.
Indian Oil Corporation has standardised on above recommendation.