What are your mechanical seal faces showing you? Heat checking? Coking? Blistering?
The appearance of this damage 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.
Heat Checking
The appearance of radial cracks originating from the centre of the metal or ceramic ring is referred to as heat checking. These cracks in the mechanical seal face act as cutting edges, which eventually wear out the seal because of the consequent scraping action.
Heat checking most often occurs with mechanical seals applied to applications with low specific gravity liquids at high speeds and pressures. When the mechanical 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 vaporisation at the seal face.
To protect against heat checking make sure an adequate amount of lubricant is applied to the mechanical 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 mechanical seal face.
Other corrective actions include:
Checking the mechanical seal chamber pressure for adequate vapor pressure margin
Checking the mechanical seal setting for proper working height
Checking for proper running clearances between the shaft and primary ring at operating temperature
Reviewing flush system design, increasing flush flow rate
Reviewing details of mechanical seal selection
Using distributed flush design instead of single point flush
Checking for mating ring distortion
High Wear and Grooving
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 mechanical 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 mechanical seal faces are made from hard materials.
Abrasives embedded in the softer primary ring material might also cause this recognisable wear pattern.
To prevent leakage, try some of these corrective solutions:
Increase cooling of mechanical seal faces
Check procedures for lapping carbon primary ring. Do not use abrasives for lapping hard mating ring
Check for abrasive particles in the pumped liquid
Check for a dead-ended seal chamber
Coking
Coking leaves a black build-up, or abrasive sludge on the atmospheric side of the mechanical seal. This build-up causes rapid wear of the seal faces, usually caused by oxidation. Signs of coking tend to develop when the mechanical seal is operating under excessive temperatures or by using a dirty or contaminated flush, among other things.
Flushing the mechanical 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.
Pitting and Corrosion of Mechanical 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. Pitting can also occur when the mechanical 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 on Mechanical Seal Face
Blistering is a symptom of thermal attack and is characterised by craters, voids or pits of removed material or small circular sections that appear raised on the carbon seal faces. Blistering is a common cause for mechanical seal leakage because the blisters cause separation between the seal faces during operation.
Mechanical 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 mechanical seals that start and stop frequently.
The causes of blistering typically generate from highly viscous fluids. When the mechanical 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 mechanical seal that doesn't fit any of these common problems? Ask us about it! Specialised Flow Solutions gladly provide technical assistance to pump repairers and end-users Australia wide.