CARTRIDGE SEALS OVERHEATING PROBLEMS ST020
Excessive heat can cause multiple problems with mechanical seals:
- The elastomer (rubber part) can be damaged.
- Some seal faces can be damaged.
- Carbon-graphite faces can pit as trapped air expands within the carbon, or the product carbonizes and pulls out pieces of the seal face.
- Plated faces can heat check and crack, causing rapid carbon face wear.
- The filler in some carbon /graphite compounds can melt or oxidize at elevated temperatures.
- Critical dimensions can change, causing the lapped seal faces to go out of flat and leak prematurely (especially fugitive emissions).
- The sealed product can change state and :
- Vaporize between the faces opening them.
- Crystallize on the moving components, restricting their movement.
- Change fluid viscosity restricting the ability of the seal to follow run out.
- Solidify, making the seal inoperable.
- Build a film on sliding components and the lapped seal faces.
- Carbonize or coke restricting the seal movement and opening the lapped faces.
- Corrosion always increases with increasing product temperature.
Some heat problems are not seal design or seal installation related:
- An inefficient heating-cooling jacket.
- A layer of calcium or some other similar product has built up on the jacket walls interfering with the heat transfer.
- The coolant is flowing too rapidly through the cooling jacket.
- A thermal bushing was not located in the end of the stuffing box.
- If steam is being used as the coolant, the pressure is too high. The temperature of steam is directly related to its temperature.
- The fluid is not dead-ended in the stuffing box. There is either suction or discharge recirculation of the pumping fluid.
- Clearance between the seal outside diameter and the stuffing box bore is not sufficient.
- The shaft material is conducting the product heat to the cartridge static elastomer and other components. As an example: carbon steel conducts heat much better than a stainless steel shaft.
- The convection tank is not convecting properly.
- The convection tank is running backwards.
- The dual seal barrier or buffer fluid has been shut off.
- The product has a low specific heat and poor conductivity. Oil is a good example of such a product.
- The outside quench has failed, or an operator has shut off the quenching fluid thinking he has solved a seal leak.
- The seal faces were over-compressed during the installation process.
- A wrong measurement was used.
- The mechanic did not read or understand the print dimension.
- The pump sleeve moved as the impeller was tightened on the shaft.
- The spring compression measurement was taken at the wrong place. The stuffing box face is the only safe reference point.
The cartridge seal design has a major affect on heat generation and heat sensitivity:
- Unbalanced mechanical seals generate more heat than hydraulically balanced mechanical seals.
- Two hard faces generate more heat than carbon/graphite vs. a hard face.
- Silicon carbide and tungsten carbide dissipate heat faster than 99.5 ceramic or carbon&endash;graphite.
- The location as well as the grade of the elastomer can be critical in temperature sensitive applications.
- In dual seal applications, convection systems are not as efficient as pumping rings or forced circulation of the barrier fluid system. When oil is used as a barrier fluid forced circulation, or the use of a pumping ring is mandatory.
The above problems are not unique to cartridge seals. There are however, some problems that are unique:
This is an illustration of a stationary cartridge seal design. It is called a stationary design because the spring loaded seal face does not rotate with the shaft. It is called a cartridge because the rotating unit is mounted on a sleeve that is attached to the shaft, outside the stuffing box.
- Pushing the seal gland along the shaft and against the stuffing box face can over compress the seal because of the friction between the shaft and the cartridge sleeve static elastomer. In dual seal applications, the inner seal can over compress as the outside seal looses some of its compression. Be sure to reset the spacing device (usually “clips” of some type) prior to locking the seal to the shaft.
- Some open impeller pump designs (Duriron as an example) Adjust to the back plate rather than the volute. Be sure to reset the cartridge after the impeller adjustment.
- Cartridge set screws can slip on a hardened sleeve. The system pressure can then over compress the seal.
- Higher pressure applications or water hammer can move the set-screws and over compress the seal faces.
- Be sure to re-tighten the adjusting nuts after making the impeller micrometer adjustment on those pumps that uses that adjustment method. The Chesterton System #1 pump is a good example of this design.
- Make sure the centering-positioning clips are in place when installing or resetting the seal for proper face loading.