A 70/30 balance ratio means that 70% of the seal area is seeing the stuffing box pressure and 30% is not seeing the hydraulic pressure.
The ratio is obtained by placing the seal face area some distance above or below the balance line formed where the dynamic elastomer slides or rolls on the shaft or sleeve.
Please note that the dotted line in the middle of the seal face does not place 50% of the seal face area above and 50% of the area below this line.
To get a 50/50 balance the line would have to be about 75% above the face inside diameter.
Metal bellows seals do not have this sliding elastomer. They use an “effective diameter” to determine the balance ratio.
The effective pressure area in a bellows seal is just about half way through the cross section of the bellows convolutions. The seal manufacture places 70% of the seal face area above this line.
Unfortunately this line shifts up and down with changing stuffing box pressure, so bellows seal hydraulic balance is not as accurate as the balance we get with elastomer designs.
- On February 14, 2018