Damage of the seal hard face
DAMAGE OF THE SEAL HARD FACE ST009
- Caustic and other high pH fluids attack some ceramics and silicon carbides. Check to see if your seal face material contains silica. As an example: both reaction bonded silicon carbide and 85% ceramic have this high silica content.
Cracked or broken.
- The product is solidifying between the faces. Most hard faces have poor tensile or shear strength.
- Excessive vibration will cause cracking at the drive lug location.
- A cryogenic fluid is freezing a lubricant that was put on the face.
- The elastomer is swelling up under an outside seal face. This problem can also occur if the seal design allows a spring to contact the inside diameter of the hard face. You see this design problem in most dual, “back to back”, rotating seal designs
- The shaft is hitting the stationary face or the rotating seal face is hitting a stationary object. This is a good reason for converting your pump to an oversized stuffing box.
- Poor packaging.
Heat check (common with coated or plated faces)
- Heat check or cracking of the hard face is caused by a high heat differential across the face. Most hard coatings have only one-third the expansion rate of the stainless steel base material.
Hard coating coming off the face.
- The base material not compatible with the sealed product. These coating are very porous so if the product attacks the base material the coating will come off in sheets.
- The plating process was not applied correctly.
- High heat can cause a problem with the differential expansion rate between the coating and the base material.
Let’s analyze the wear track on the hard face. We will be looking for:
Deep grooves or excessive wear caused by.
- Solids imbedded in the carbon are causing the problem.
- The seal faces opened letting the solids penetrate between the lapped faces. The carbon is softer than the hard face so the solids penetrate into the carbon.
- Someone used lapping powder to lap the carbon face and the lapping powder is imbedded into the carbon. The carbon should have been lapped dry, on ceramic stones.
The wear track is wider than the carbon. The shaft is having run-out problems
- Worn bearings.
- A bent shaft.
- An unbalanced impeller.
- The sleeve not concentric with the shaft.
- The seal not concentric with the sleeve.
- You are using a pump seal in a motion seal application.
- In a stationary seal design, the stationary carbon is often not centered to the shaft causing a wiping action.
The graphite wear track is narrower than the carbon.
- The soft face (carbon) was distorted by pressure.
- The hard face was over tightened against an uneven surface. It is now either concave or convex.
- The hard face clamping forces are not “equal and opposite”. You probably have two different width gaskets on either side of a clamped hard face.
- The face never was flat or it was damaged during shipment.
Non-concentric pattern. The wear track is not in the center of the hard face.
- The shaft is bending because the pump is running off of its best efficiency point.
- Poor bearing fit.
- Pipe strain.
- Temperature growth is distorting the stuffing box.
- The stationary face is not centered to the shaft.
- Misalignment between the pump and its driver.
Uneven face wear. The hard face is distorted:
- High pressure.
- Excessive temperature.
- Over tightening of the stationary face against the stuffing box.
- The clamping forces are not equal and opposite.
- The hard face is not wide enough. It needs more mass to resist the clamping forces.
- You are using a two-bolt gland, and the gland is too thin causing it to distort the stationary face.
The product is sticking to the seal face. The product is changing state and becoming a solid. Most products solidify for the following reasons:
- A change in temperature.
- A change in pressure.
- Dilatants will solidify with agitation. As an example: cream becomes butter.
- Some products solidify when two or more chemicals are mixed together. Like epoxy glue.
The hard face is not flat.
- Mishandling. Parts get dropped and the worker is afraid to tell the boss. He put the part back in inventory without telling any one what happened.
- Poor packaging.
- The hard face has been installed backwards and you are running on a non-lapped surface.
- It was shipped “out of flat.”
- On February 18, 2018