Mechanical Seal hydraulic balance 8-01
Seal hydraulic balance is one of the most effective tools we have to counter the detrimental affects of heat being generated in the pump stuffing box. The original patent for hydraulic balance was granted in 1938, but the concept has never been adopted by the “original equipment manufacturer” (O.E.M.), and so to this day it remains only an “after market” product.
Hydraulic balance is very easy to understand; please look at the following diagram:
A = The spring loaded face with an area of 2 in2 (6 cm2)
B = The stationary face held to the front of the stuffing box by gland “G”
P = The hydraulic pressure in the stuffing box is given as 100 psi (10 Kg./cm2)
To understand hydraulic balance you must know that:
- Pressure (lbs./in2) x Area (in2) = Force (lbs.) or
- Pressure (Kg/cm2) x Area (cm2) = Force (Kg. *)
* Multiply this number by gravity (9.8 m/sec2) and you get Newtons of force.
There are at least two forces closing the seal faces:
- The mechanical spring force.
- The hydraulic force caused by the stuffing box pressure acting on the seal face area.
There are at least three forces trying to open the seal faces:
- A hydraulic force is created any time there is fluid between the seal faces.
- A centrifugal force created by the action of the fluid being thrown outward by the rotation of the pump shaft.
- A hydrodynamic force created because trapped liquid is, for all practical purposes, non compressible.
Let’s look at these forces individually and in a little more detail!
First we’ll look at the closing forces:
- A spring load of 30 psi. (2 Kg/cm2) is an industry standard when the seal face is new and a load of 10 psi (0.7 Kg/cm2) should still available when the carbon seal face has worn away. We need this minimum load to prevent normal vibration from opening the lapped faces. You set this load by installing the mechanical seal with the proper amount of compression as shown on the mechanical seal installation print. A tolerance of plus or minus 1/32″ (0,8 mm.) is typical.
- Since the definition of hydraulic force was given as pressure x area :
- 100 psi x 2 in2 = 200 pounds of closing hydraulic force, or
- 10 Kg/cm2 x 6 cm2 = 60 Kg of closing hydraulic force.
Now we’ll look at the opening forces
- First the hydraulic force:
- Testing shows that some times there is a film of liquid between the faces, some times there is only vapor, some times there is nothing at all, and some times there is a combination of all three. This means that if there is liquid or vapor between the faces, it is under pressure trying to force the lapped faces apart. The stationary face (B) cannot move because it’s being held by gland “G”, but the spring loaded face (A) will respond to this force.
Look at the following diagram: If we assume a straight line or linear pressure drop across the seal faces, we would get an average of:
- 50 lbs/in2 x 2 in2 = 100 pounds of force trying to open the seal face,
or 5 Kg/cm2 x 6 cm2 = 30 Kilograms of force trying to open the seal faces.
- Centrifugal force is acting on the spring loaded face (A) trying to spin it perpendicular to the rotating shaft.
- Stationary face (B) is not perpendicular to the shaft because it is referenced against the stuffing box face which is a casting that is not perpendicular or square to any thing. A gasket located between the gland and the stuffing box further compounds the problem. Testing has shown that a surface speed of 5000 fpm. or 25 meters per second centrifugal force is powerful enough to open most mechanical seal faces.
- Seal faces are lapped to within three helium light bands or slightly less than one micron. This slight waviness is enough to generate hydrodynamic lifting forces as we try to compress non-compressible liquid t trapped between the lapped faces.
Two forces closing, and three forces opening the seal faces. If the closing forces are the greater forces, the seal will generate heat that is often destructive, but always a waste of energy and pump efficiency. If the opening forces are the greater forces the seal will leak and that’s never desirable.
A balanced seal, by definition, balances these opening and closing forces so that the seal will not get hot and it will not leak. How is that accomplished? Since the hydraulic closing forces were twice the opening forces (100 psi. vs. 50 psi.) we have installed a sleeve inside the seal to reduce the closing area and reduce the closing force. Look at the following diagram for an explanation:
You can now see that the 100 lbs/in2 (10 Kg/cm2) is now pushing on only 1in2 (3 cm2) because the inner sleeve is attached to the shaft and cannot move. The opening force remains the same. The numbers look like this:
- 100 lbs/in2 x 1 in2 = 100 lbs. Closing
- 50 lbs/in2 x 2 in2 = 100 lbs. Opening or
- 10 Kg/cm2 x 3 cm2 = 30 Kg. Closing
- 5 Kg/cm2 x 6 cm2 = 30 Kg. Opening
As you can see, we have eliminated the hydraulic forces from acting to open or close the seal faces. This leaves only the spring force to close the seal and the hydrodynamic and centrifugal forces to try to open the seal faces. The question then becomes, “can the spring force balance the other two?”
And there is another point to consider; was the pressure drop across the seal faces really linear?
All of these questions were answered way back in 1938 by physical testing. The final design solved the problem by overbalancing the closing hydraulic forces to compensate for:
- The non-linear pressure drop across the seal faces.
- The hydrodynamic opening forces
- Centrifugal opening force.
Look at the following diagram for the final result:
Seventy percent (70%) of the seal face area is exposed to the hydraulic closing force instead of the fifty percent (50%) shown in the previous drawing. This is the standard 70-30 balance used by most mechanical seal companies. The seal designer can increase or decrease the percentage of over balance by changing the stepped sleeve diameter. We would want to do this to:
- Decrease the face loading for low specific gravity fluids and higher speed shafts.
- Increase the face loading for higher viscosity liquids.
All that was required to hydraulically balance the seal was the simple low cost sleeve, but it is this additional cost that is keeping the original equipment manufacturer from adopting the design as his standard. The “bottom line” is that with an un-balanced seal design you either suffer the consequences of adding heat to the stuffing box area, or having to provide cooling to remove the heat that is being generated by the un-balanced seal.
Keep in mind that this heat is never desirable because:
- Heat means a loss of expensive energy.
- Heat will affect the elastomer (rubber part) in the mechanical seal. reducing its’ life.
- Heat can damage some carbon faces by melting the fillers and expanding the air pockets trapped below the surface, causing pits in the carbon that will prevent it from passing a fugitive emission test.
- Some hard faces can be damaged by a rapid temperature change.
- Plated surfaces can “heat check” and crack due to the differential expansion between the coating and the base metal.
- Many products can vaporize at elevated temperature, blowing the faces open and leaving solids between the lapped faces.
- Heat will change the viscosity of many liquids. It many cases it will diminish, but in some cases the viscosity can increase.
- Corrosion always increases with additional heat.
- Petroleum base products can “coke” between the faces.
- Lapped faces can go “out of flat” and critical tolerances change at elevated temperature.
Allways specify hydraulically balanced seals. The unbalanced versions do not make any sense. In addition to low heat generation, balanced seal have other advantages:
- They will allow you to standardize on one seal style for both high and low pressure applications.
- The o-ring version will seal either vacuum or pressure.
- Balanced seals do a good job of compensating for “water hammer” and pressure surges.