The sealing of high pressure and hard vacuum 5-7
High pressure does three things that will damage any mechanical seal :
- 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 faces.
- 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.
- 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.
The 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 are:
- 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 holder.
- 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 choice.
- 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.