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:
- The product is dangerous.
- A seal leak could cause a pollution problem.
- The product is very costly.
- Unscheduled down time is too expensive.
- You need fugitive emission protection.
- It is a sensible way to institute a predictive maintenance program for mechanical seals.
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:
- Since systm pressure can vary, the barrier fluid pressure must be kept at one atmosphere higher than the maximum stuffing box pressure and that pressure is very hard to predict because of pressure surges, cavitation, and water hammer in the system.
- Barrier fluid pressure can vary depending upon its source. Shop water is notorious for pressure variations.
- If the system pressure exceeds the barrier pressure, the inner seal can blow open.
- If the barrier fluid piping or fittings are damaged or leaking, the inner seal will blow open allowing the product to escape to atmosphere. Remember that you purchased the second seal to prevent that possibility.
- Some mixer applications alternate between pressure and vacuum.
- If the outside seal wears out, or fails prematurely the barrier fluid pressure will drop, and the inner seal will blow open. In other words, if the seal works properly, both seals will fail at the same time.
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.
- The dynamic 0-ring is placed in an odd shaped groove.
- The stationary seal face is wider than the previous version. The O-ring cross section represents 40% of the area of the seal face with 30% of the face area outside and inside of the O-ring area.
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.
- You can elect either a higher or lower barrier fluid pressure. It’s your choice. With a lower barrier fluid pressure you can just about eliminate the possibility of product dilution.
- A rupture in the barrier fluid pressurizing system will not open the inner seal face, allowing the product to escape to atmosphere.
- Water hammer, cavitation, or pressure surges will not blow the seal open. The faces always shut with the higher pressure, the way they’re supposed to.
Are there any disadvantages to this design? Yes, a couple:
- A wide seal face is required restricting the use of the seal to mostly mixer applications because of the additional radial room needed. There seldom is enough room in the typical centrifugal pump stuffing box to accommodate the cartridge version of this design.
- Because one half of the dynamic O-ring groove is mounted in the gland and the other in the seal face, The O-ring must slide when the pressure reverses, and in some slurry applications it will “hang up” unless flushing water is available.
- Extra cost is usually involved if it is not a standard seal with your supplier.
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:
- A narrow seal face can be used.
- The seal will fit conventional stuffing box bore dimensions.
- The dynamic O-ring does not have to slide when the pressure reverses. This is a very important feature when sealing slurries, viscous fluids and liquids that can crystallize
- Manufacturing cost is reduced.
- Conventional O-ring cross sections can be used.
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.