SUBJECT: Two way hydraulic balance 8-2

There are several reasons why you might want to invest in the higher cost of installing two seals in your pump, or some other pieced of rotating equipment:

Please look at the following diagram and you'll see a major problem with manydual seal applications:

In a typical dual seal application, barrier fluid is circulated between the seals at a pressure at least one atmosphere (14.7 psi, or 1 bar) higher than stuffing box pressure. This presents some operating problems:

Hydraulically balancing the inside seal will not help because with a standard 70-30 balance the seal is hydraulically balanced in only one direction. You can check the paper 8-1 in this series if you are not familiar with standard seal balance.

In that paper we learned how to balance a rotating seal (the spring loaded face rotates). Now we'll see how a stationary seal (the spring loaded face does not rotate) is balanced. Please refer to the following diagram:

If the higher pressure is coming from side "A" you have the classic 70-30 hydraulic balance, but if the higher pressure is at "B" you have a 30-70 balance and the seal faces will open.

In 1939 a solution was found for this very real problem, and it was called "two way balance". Look at the following diagram, it appears to be very similar to the previous diagram with two major exceptions.

If the higher pressure is at side "A" the O-ring "C" is pushed against gland "D" and any sliding action of the seal face will take place at the inside diameter of the O-ring. In that case 70% (40%+ 30%) of the face area would see the pressure and 30% would not. The classic 70-30 balance.

Reverse the pressure to side "B" and the O-ring will bottom against the seal face and all sliding, or flexing will occur on the O-ring outside diameter, allowing 70% (40%+ 30%) of the seal face area to see the pressure and 30% would not.

In other words you would have the same 70-30 balance regardless of the direction of the higher pressure. This presents some very real advantages over non-two way balanced designs.

Are there any disadvantages to this design? Yes, a couple:

In recent years other methods of achieving two way balance have evolved with very obvious advantages. One of them is shown in the following diagram:

The diagram should be self explanatory. You will note that in this version the hydraulic balance reverses from 70-30 to 60-40.

It does not make sense to install two seals into a piece of rotating equipment without this two way balance feature. Be sure to specify it in all future purchases and proposals.

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