Mechanical seal problems reference
A quick reference to prevent potential seal and pump problems. 8-6
The biggest advantage of experience is you have hopefully learned what can get you into trouble. The following information has been explained in detail in previous technical papers, but I still see the same problems re-occurring on a daily basis.
Take a few minutes and look at the following. It might save you a seal or pump failure.
- Carbon seal face. Any form of carbon is usually not acceptable in the following applications:
- Oxidizers, they combine with carbon to form CO & CO2
- Halogens (most of them end in the letters “ine”) chlorine, bromine, fluorine, astintine & iodine
- Where color contamination can be a problem.
- Some de-ionized water applications.
- Hot petroleum products if you are concerned about fugitive emissions.
- A special carbon is used for cryogenic and hot dry air applications. Moisture is needed to make the graphite release from the carbon-graphite mixture, and in these applications the needed moisture is not present. A special carbon with an imbedded organic is made to satisfy these applications.
- Ceramic grade 99.5 is not a satisfactory hard face in hot applications because of its poor thermal conductivity. Alpha grade silicone carbide or tungsten carbide are much better choices.
- Ethylene Propylene Rubber O-Rings will be attacked by petroleum products and this includes any petroleum grease that might be put on the O-ring during the installation process.
- Kalrez® grade 3018 is not satisfactory if the temperature is below 600°F. (315°C.) The material is too hard at these lower temperatures.
- Nickel base tungsten carbide can cause galvanic corrosion problems with stainless steel shafts.
- Reaction bonded silicone carbide is not satisfactory for caustic or most high PH materials.
- Viton® O-rings are not generally satisfactory in water based fluids. This also includes steam cleaning or flushing the lines with water based caustic solutions. Grade 747-75 fluorocarbon is O.K. if the water is cold, but ethylene propylene rubber is still your best choice as long as the temperature does not exceed 300°F (150°C.).
- White Chemraz is not recommended for most high PH fluids. Do not use it with:
- Acetaldehyde, Ammonia + Lithium metal solution, Aqua Regia, Black liquor, DI water, Ethyl Formate, Ethylene Oxide, FC 75, Freon 113 -114 – 114B2 – 115 – 142B- C318 – PCA – TF, Fuming Sulfuric Acid, Green Sulfate Liquor, KEL-F- Liquids, Lye, Magnesium Hydroxide, Red Fuming Nitric Acid, Potassium Hydroxide, Sodium Hydroxide (Caustic), Fuming Sulfuric Acid, and White Liquor.
- If you choose the wrong elastomer it will be attacked by the fluid and break down. For the first few days the seal will work very well because the elastomer has become “slimy” and moves easily. The elastomer will then “swell-up” and lock-up the moveable seal components.
Remember that chemical attack can be accelerated by temperature, fluid concentration, and stress. Past plant experience is your best indicator of what seal and pump materials to use.
- Ammonia compressor; use Neoprene for the O-ring because the fluid is a combination of ammonia and petroleum oil.
- Black Liquor, as found in paper mill applications can be either sulfite or sulfate. Sulfate (high PH) is the most common and ethylene propylene can be used for the O-ring material if the temperature is below 300°F (150°C). If the temperature is too high, Kalrez is a good choice. White Chemraz is not recommended in these higher temperature caustic applications.
- Boiler feed pump applications vary a great deal. In some cases they are nothing more than a simple hot water application, but in other instances a very high pressure is involved. In any case, cooling is needed in the stuffing box to insure long seal life. High pressure applications also require a heavy duty seal design.
- Caustic. If the concentration is over 50% Monel metal will probably be needed. The metal selection depends upon the temperature and stress.
- Ethylene Oxide will penetrate into most elastomers and explode out the other side of the O-ring. Use two seals and pressurize between them. Ethylene Oxide is a dangerous product, so two seals should be used in any case.
- Halogens attack most carbon faces and will penetrate the Teflon® encapsulated O-rings like Vanway, Creavey and & 76 style.
- Hot oils. Coking is always the problem. The seal area must be cooled. Coking is a function of temperature and time and is independent of the presence of oxygen. If you want to seal fugitive emissions you will have to go to two hard faces. Even the best of carbons show some blistering in these applications. In other words, a metal bellows seal will not eliminate the need for stuffing box cooling.
- High temperature applications. Most metal bellows seal designs incorporate a low expansion holder (Invar 36 or Carpenter 42) to retain the carbon face. This holder is also frequently used as a vibration damper to prevent seal face separation problems caused by “slip stick.” If you lose cooling in these applications the pump shaft expands at a rate three times that of the low expansion steel vibration damper and can cause the seal faces to be pulled open.
- Kaoline (china clay) will penetrate lapped seal faces because the solids are less than one micron in size. You will need two seals with a pressurized barrier fluid between the seals. Water is a good choice for this barrier fluid.
- Latex balls up between the seal faces. Dual seals with a pressurized water barrier fluid have been used in this application, and non contacting gas seal seem to be the current choice, but flushing with a small amount of cold water seems to be the only satisfactory solution to this application.
- Paper stock always requires a small amount of flushing water. You cannot use suction recirculation and centrifugal force to separate the stock from the water because of the stock’s low specific gravity. If the pump is trying to “lift” paper stock it will almost always cavitate.
- Pipe line applications almost always involve high pressure. Heavy duty seals should be used in these applications.
- Products that freeze (cryogenic). Watch out for moisture outboard of the seal. Dual seals with anti-freeze circulating in a convection tank is your best bet. Do not put any grease on the seal faces. It will freeze also.
- Salt water. Coat the O-rings and all clamped surfaces with Zinc Oxide paste to prevent corrosion at these locations.
- Sulfuric acid. Alloy 20 metal is usually needed for these applications. Any leakage will cause severe corrosion as the product is diluted.
Horizontally split pumps:
- Suction recirculation will not work if the stuffing box is at suction pressure. Most single stage designs fit into this category
- The face of the stuffing box must be resurfaced to get a good gasket seal.
- If you are making a new gasket between the casing halves, be sure to have it extend outside the stuffing box face and then trim it flush after the halves are tightened together.
- Be sure to seal between the sleeve and the impeller. This is a potential leak path after a mechanical seal is installed.
- Some sleeves terminate under the seal. Check that you will not have a corrosion problem if the sleeve and shaft are different materials,
- Sometimes a new gasket will extrude into the sides of the stuffing box when the two halves of the pump are bolted together. The gasket can then rub against the side of the seal interfering with its movement.
- You will need either a stationary mechanical seal or some type of self aligning feature to seal these pumps successfully.
Flyte sewage pumps can be converted to a single mechanical seal if a special adapter is made. It’s worth the problem. You only have to seal the bearing cavity in this application
- Discharge recirculation can act as a sand blaster against the seal body. This can be a big problem with the thin metal plates found in metal bellows seals.
- Dual seal barrier or buffer fluid. Oils should be your last choice as a barrier or buffer fluid because of oils’ low specific heat and poor thermal conductivity. You will definitely need a pumping ring if you are going to use a convection tank.
- Quenching. An excess of water or steam can easily get into, and ruin the bearings.
- Suction recirculation is not effective in the following:
- Flowserve (Duriron) pumps, because of their semi- open impeller design.
- If the fluid is close to its vapor point. flashing will occur when the stuffing box pressure drops.
- If the specific gravity of the solids is lower than the fluid. If the solids float, centrifugal force will throw the liquid to the outside leaving the solids against the seal components. Paper stock is a good example of this.
- Single stage, double ended pumps where the stuffing boxes are at suction pressure.
- Seal set screws are normally manufactured from corrosion resistant materials and are therefore softer than normal set screws. This means they can slip if reused. You can substitute hardened set screws in most cartridge seal applications.
- Do not use any type of set screw on non-metallic shafts. Seals must be clamped to the non metallic shaft or sleeve.
- Split seal designs. Many designs can seal either a pressure or a vacuum, but not if the application alternates between them. You can run into this problem in some mixer applications.
- Troubleshooting hints
- Are other seals working in this application? If they are, you know the materials are alright. Now you must decide what is different about this application.
- Has the seal been repaired? You may be looking at a rub mark, discoloration, or corrosion that is not relevant to this application.
- Do not let the welder use the pump as an electrical ground. You can ruin the seal or bearings in the process.
- Pumping off of the best efficiency point will not excessively deflect the shaft with the following centrifugal pump designs:
- Double volute casings.
- Multi stage designs.
- Diffuser or turbine pump designs.
- Be sure to level the pump when you do an alignment.
- If you trim the impeller, file the tips and re balance the assembly.
- The next time that you look at the pump discharge gauge, remember that the pump pumps the difference between the suction and discharge heads. You must subtract a positive suction head to determine what head the pump is really creating.
- Bearing lip or grease seals have a useful life of less than 90 days and will cut and score the shaft because of fretting corrosion.
- Never cool a bearing housing because it will shrink and over compress the bearing. Cool only the bearing oil.
- Flushing the system with steam or a cleaner seldom flushes out the stuffing box of the pump.
- Do not circulate shop water through the cooling jacket on a high temperature pump. Condensate or low pressure steam is a better choice. Be sure to install a thermal bushing in the end of the stuffing box to get effective temperature control in the seal area. Make sure you come into the bottom of the jacket and out the top to vent any air that might be trapped in the jacket.
® DuPont Dow elastomer
- On February 18, 2018