Sealing high pressure
SEALING HIGH PRESSURE SS002
High pressure causes at least three problems with mechanical seals:
- The high pressure 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 some seal faces, the elastomer or in many cases the product that you are sealing.
- It can cause excessive face 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.
- High pressure in the pump’s stuffing box can change critical dimensions such as the roundness of the sleeve or seal barrel
- The high pressure can distort one or more of the seal components:
- Distortion can cause 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.
- Distortion of a seal sleeve can increase the hysteresis problems with the dynamic elastomer (the O-ring or elastomer will drag on the sleeve).
- 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 unless a back-up ring has been installed to prevent the extrusion.
The excessive hydraulic pressure can come from several sources that include:
- 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.
- Stage the pressure between dual seals.
- Reduce the pressure in the stuffing box.
In the next paragraphs we will look at each of these alternatives:
- Let’s start with building a seal that can take the excessive pressure without distorting.
- Select hydraulic pressure balanced seal designs to lower the axial load.
- Higher modulus materials are seldom available so:
- You will have to go to a finite element stress analyzed design.
- Look for seal components that have uniform thickness cross sections
- 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 (just like plywood).
- Higher durometer O-rings with non-metallic back up rings are available to prevent elastomer extrusion.
- The next approach will be to stage the seals in an application so that several seals will be sharing the pressure.
- Tandem sealing with an intermediate lower buffer 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.
- The last choice is to 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.
- On February 18, 2018