SUBJECT: The sealing of high pressure and
hard vacuum 5-7
pressure sealing problems
High pressure does three things that will damage any mechanical
- It will create a high axial hydraulic load on the seal faces.
This load will be in addition to the axial force created by the
single spring, multiple springs, or metal bellows that are used to
create the initial face loading in popular seal designs. This
excessive axial loading can:
- Generate heat that could be detrimental to one or more of
the seal components such as the elastomer or in some cases, the
product that you are sealing.
- It can cause excessive wear in a short period of time. This
will be a very important consideration when you are sealing
non, or poor lubricating fluids. Thixotrophic fluids (they lose
their viscosity when agitated) will also be affected.
- If the product is a poor lubricant or a non lubricant, a
high axial load can cause "slip stick" problems that can chip
the carbon outside diameter and possibly open the lapped seal
- It can change critical dimensions, such as the lapped seal
faces going out of flat.
- It can distort one or more of the seal components causing the
lapped seal faces to go "out of flat."
- Seal faces are subjected to "hoop stresses" that attempt to
shrink the material. Since these faces are seldom designed as a
"solid block" the affect is to alter the lapped face flatness.
Finite element analysis design techniques help, but are still
limited in practice.
- High pressure can extrude the elastomer (rubber part) in many
seal designs, either "locking up" the seal or causing leakage
where the elastomer was extruded. In almost every case the
elastomer suffers permanent damage.
excessive hydraulic pressure can come from several sources that
- The normal system pressure. In most single stage pump
applications the stuffing box pressure is slightly higher than
suction pressure, but multi stage pump applications, boiler
circulating pumps, and some pipe line service pumps can experience
very high stuffing box pressures.
- Water hammer and pressure surges can cause a very high
temporary pressure in the system.
- Unusual system operation is another cause. The rapid opening
and closing of valves can cause these surges of pressure.
- A loss of power to a running pump can cause vacuum pockets in
the lines. As the liquid rushes to fill up these vacuum voids,
very high pressures can be experienced.
solution to high pressure sealing falls into three separate
categories. You must decide which of the approaches makes the best
sense in any given application. The three approaches you can use
- Build a seal that can handle the excessive pressure.
- Select hydraulic pressure balanced seal designs to lower
the axial load.
- Higher modulus materials are seldom available. You will
have to go to a finite element stress analyzed design. Look for
seal components that have uniform thickness cross sections, or
go to larger cross section seals that will require more
stuffing box radial room.
- Laminated bellows are available for many higher pressure
metal bellows applications (same principal as plywood).
- Higher durometer o-rings with non-metallic back up rings
are available to prevent elastomer extrusion.
- Stage the seals in an application so that several seals will
be sharing the pressure.
- Tandem sealing with an intermediate lower barrier fluid
pressure is the most common. In some nuclear applications three
seals have been connected in tandem to handle the high
pressure. Tandem and other types of multiple seal arrangements
take a great deal of axial room. In every case you are moving
the first seal further away from the bearings so shaft
stabilization becomes very important. You should also remember
that the multiple units are acting as a single seal. In other
words if you fail one of the seals, you fail them all.
- Lower the pressure in the stuffing box.
- Locking a restriction bushing into the bottom of the
stuffing box and then connecting a suction recirculation line
from the bottom of the stuffing box to a lower pressure
location in the system is the normal way to accomplish this.
Watch out for erosion of this bushing, especially in abrasive
applications. Be aware that if stuffing box pressure is near
the product vapor pressure, flashing could occur in the
stuffing box or between the lapped seal faces.
- You can cross-connect stuffing boxes in a multiple stage,
double ended pump design. Keep in mind that this will not work
with single stage centrifugal pumps.
Vacuum means less than atmospheric pressure and vacuum sealing
falls into two categories:
Normal vacuum. This vacuum is usually measured in inches or
millimeters of mercury.
- This is the vacuum found in condensers, evaporators and at the
suction side of the pump every time you use the centrifugal pump
to lift liquid.
- Hydraulic balanced seal designs can handle this vacuum because
vacuum only means one atmosphere of pressure (15 psi. or one bar)
coming from the other side of the seal.
- O-rings are preferred for the elastomer design. Continuous
o-rings can seal either vacuum or pressure. They also have the
ability to flex and roll to compensate for shaft movement.
- Carbon/ metal composite seal faces are satisfactory as long as
the carbon is sealed at the inside diameter to prevent the
pressure from penetrating behind the carbon, upsetting the
hydraulic face balance and possible blowing the carbon out of its
vacuum sealing. This vacuum is normally measured in microns,
micro-inches, or portions of a millimeter of mercury (Torr).
- Elastomers are not acceptable for hard vacuums. The vacuum
will cause the elastomer to "out gas" increasing the elastomers'
density and reducing the volume to a point where leakage is
possible. All metal seal designs will probably be your first
- Seal face density and self lubrication can be a real problem
in hard vacuum applications because of the lack of moisture to
release the graphite from the carbon/ graphite compound.
Conventional seal designs are seldom satisfactory in these
applications. A great many materials exist that can solve the
problem, so you will want to contact your seal supplier for the
availability of higher density and self lubricating carbons for
these special applications.
- Tandem dual seals with a higher pressure lubricating barrier
fluid is the most common solution to hard vacuum sealing.
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