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What's on your seal faces? Heat checking? Coking? Blistering?
The appearance of these blemishes on your seals is a sure sign the seal is on its way to failure if it hasn't already failed. Read on to understand what causes these problems and how to troubleshoot mechanical seal issues like these.
The appearance of radial cracks originating from the center of the metal or ceramic ring is referred to as heat checking. According to the Fluid Sealing Association (FSA), these cracks in the seal face act as cutting edges, which eventually wear out the seal because of the consequent scraping action.
Heat checking most often occurs with seals applied to applications with low specific gravity liquids at high speeds and pressures. When the seal is in operation, one might notice the seal drips steadily, no matter if the shaft is rotating or stationary. Sound may present due to flashing or face popping.
Possible causes of heat checking include an inadequate amount of seal face lubrication, inadequate cooling, or vaporization at the seal face.
To protect against heat checking make sure an adequate amount of lubricant is applied to the seal. You can also go a step further and confirm adequate coolant flow at the face to ensure self-generated heat is carried away from the seal face.
Other corrective actions include:
Grooving and wear typically appear on the mating ring. In this case, the primary ring has grooved the mating ring evenly in a 360-degree wear pattern. One might notice the issue when the seal drips steadily no matter if the shaft is rotating or stationary.
Grooving can happen for a couple of reasons. Poor lubrication from the sealed liquid could be a culprit. This is common especially when both seal faces are made from hard materials.
Abrasives embedded in the softer primary ring material might also cause this recognizable wear pattern.
To prevent leakage, try some of these corrective solutions:
Coking leaves a black build-up, or abrasive sludge on the atmospheric side of the seal. This build-up causes rapid wear of the seal faces, usually caused by oxidation. Signs of coking tend to develop when the seal is operating under excessive temperatures or by using a dirty or contaminated flush, among other things.
Flushing the seal from a cool, clean external source, or even switching to a hard seal face material that can withstand oxidation helps to prevent coking. If signs of coking appear, use steam to remove sludge and debris from the atmospheric side of the seal.
Chemical attack is a culprit for pitting and corrosion of mechanical seals. Seal failure caused by chemical attack typically results from using the wrong seal materials for the operating environment, as evidenced in the picture to the right. Pitting can also occur when the seal runs dry, causing gases to escape and implode on the seal face.
To combat corrosion, understand the fluid's chemical breakdown, then switch out the seal materials. When selecting seal materials, always consider normal operating conditions and non-process activities, such as cleaning, steaming, acid and caustic flushes.
Blistering is a symptom of thermal attack and is characterized by small circular sections that appear raised on the carbon seal faces. Blistering is a common cause for seal leakage because the blisters cause separation between the seal faces during operation. As we discussed in our most popular eBook, 36 Ways to Kill Your Pump, seal faces are incredibly flat and any amount of contamination (such as blistering) will cause the faces to not align and leak. Blistering is also a common occurrence in seals that start and stop frequently.
The causes of blistering typically generate from highly viscous fluids. When the seal heats up, the oil from the viscous fluid is rapidly driven out from the pores in the seal face. Improper cooling and circulation on the seal face will also increase your chance of blistering.
Eliminate frequent starts and stops of rotating equipment that contain mechanical seals to help prevent blistering. If possible, substitute a non-porous seal face material to prevent the oil from penetrating the seal face in the first place.
Got an issue with a seal that doesn't fit any of these common problems? Ask us about it! We gladly provide technical assistance to businesses and municipalities in Wisconsin and upper Michigan.
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