DAMAGE OF THE METAL CASE OR BODY OF THE SEAL ST011
Corrosion is the main problem. Here are some types of corrosion we find with stainless steel components. You will find details about these different corrosions in the alphabetical section of this book
- General or overall. This is the easiest to see and predict. The metal has a “sponge like” appearance. Overall corrosion always increases with an increase in temperature.
- Concentrated cell or crevice corrosion. Caused by a difference in concentration of ions, or oxygen in stagnant areas causing an electric current to flow. Common around gaskets, set screws, threads, and small crevices.
- Pitting corrosion. Found in other than stagnant areas. Extremely localized. Chlorides are a common cause. Can be recognized by pits and holes in the metal.
- Stress corrosion cracking. Threshold values are not known. A combination of chloride, tensile stress, and heat are necessary. Chloride stress corrosion is a serious problem with the 300 series of stainless steels used in industry. This is the reason you should never use stainless steel springs or stainless metal bellows in mechanical seals.
- Inter granular corrosion. Forms at the grain boundaries. Occurs in stainless steel at 800-1600 F. (412-825 C.), unless the part has been stress relieved. A common problem with welded pieces. Stabilizers such as columbium are added to the stainless steel to prevent this. Rapid cooling of the welds, the use of 316L and stress relieving after the welding are the common solutions.
- Galvanic corrosion. Occurs with dissimilar materials in physical contact, in a liquid and connected by an electrical current. Common in brine, caustic, and salt water applications.
- Erosion / Corrosion. An accelerated attack caused by a combination of corrosion and mechanical wear. Vaporization, liquid turbulence, vane passing syndrome, and suction recirculation are special cases often called cavitation. Solids in the liquid and high velocity increase the problem.
- Selective leaching. Involves the removal of one or more elements from an alloy. Common with demineralized or deionized water applications.
- Microorganisms, that will attack the carbon in active stainless steel.
Rubbing all around the metal body.
- A gasket or fitting is protruding into the stuffing box and rubbing against the seal.
- The pump discharge recirculation line is aimed at the seal body.
- The shaft is bending due to the pump operating off of its best efficiency point.
- Pipe strain.
- Misalignment between the pump and its driver.
- A bolted on stuffing box has slipped and the stuffing box inside diameter is contacting the rotating seal outside diameter.
- Thermal growth of the pump wet end.
Partial rubbing on the metal body.
- Bent shaft.
- An unbalanced impeller or rotating assembly.
- Excessively worn or damaged by corrosion or solids in the product.
- The product has attached its self to the impeller.
- The impeller never was balanced.
- The impeller was trimmed, and not re balanced.
- The seal is not concentric with the shaft, and is hitting the stuffing box I.D.
Discoloration. Caused by high heat. Stainless steel changes color at various temperatures. The following graph will give you some guidelines:
NOTE: To tell the difference between discoloration of the metal caused by high heat, and product attaching to the metal part, try to erase the color with a common pencil eraser. Discoloration caused by high heat will not erase off.
Some of the product is sticking to the metal surfaces restricting their movement.
- Heat is the main cause. Heat will cause products to :
- Build a film.
- Become very viscous
- The product pressure has dropped. Many fluids solidify with a drop in pressure. Paint is a good example of this. The paint solidifies when the solvents evaporate.
- Air or oxygen is getting into the system. It can enter from:
- Valves above the water line.
- Through the stuffing box.
- The product was not deaerated.
- The pump suction is not completely submerged.
- The bypass return is too close to the pump suction.
- The liquid is vortexing in the suction line.
- A non O-ring elastomer is being used in the seal allowing air to enter the stuffing box when you are sealing a vacuum application.
- The system protective oxide coating is depositing on the sliding metal components and restricting their movement. In almost all cases this passivated material is a ceramic. A good example of this ceramic coating is the Magnetite that forms on carbon steel.
The Teflon® coating is coming off some of the metal parts.
- Be sure to remember that these coatings are very porous. They do not provide corrosion resistance. The product you are sealing will penetrate the Teflon® coating and then proceed to attack the base metal. If you want corrosion resistance you are going to have to sleeve the material with a layer of Teflon® at least 0.060 inches (1.5 mm) thick
- A Teflon® coating is put on mechanical seal components for a couple of reasons
- To stop solids or films from sticking to the seal component and interfering with the movement of the seal.
- To provide a smoother surface for the dynamic elastomer (O-ring) to flex and roll.
- To prevent an elastomer from attaching its self to the shaft during shut down periods. Without the Teflon® the elastomer can settle into the metal surface irregularities causing a high “break away torque”.