Mechanical Seal Failures: Causes, Diagnosis and Maintenance

The main causes of mechanical seal failure are dry running, cavitation and vibration, installation errors, overheating and heat checking, abrasive particles, chemical incompatibility of elastomers, pressure fluctuations, crystallization, and inadequate flush or cooling. A seal usually leaks because of the conditions it runs in rather than a defect of its own, so a lasting fix comes from finding the root cause by reading where the leak starts and what marks it leaves on the faces.

A mechanical seal works with two precision faces, one rotating with the shaft and one stationary in the housing, separated by a very thin liquid film. The faces are typically lapped flat to within a few helium light bands (one band is about 0.3 µm), that is, below about one micron, and the film itself is typically on the order of a micron. Anything that disrupts this film, distorts the faces or damages the secondary seals eventually shows up as leakage. For the role of each component, see what is a mechanical seal.

In the field, one symptom can have several causes: rising leakage may result from dry running, an installation error or chemical attack. This guide first separates leaks by location, then explains each failure cause, makes diagnosis concrete with a symptom-to-cause table and post-removal inspection steps, and closes with a preventive maintenance checklist.

The first diagnostic question is where, not why. A typical seal has several possible leak paths: between the two faces that form the primary seal; past the secondary seal (O-ring, bellows or wedge ring) that seals the rotating unit to the shaft or sleeve; past the static gasket that seals the stationary face to the gland; at the gasket where the gland meets the seal chamber; and, in cartridge designs, past the O-ring between sleeve and shaft. Each path points to a different family of causes.

Leakage across the faces usually emerges from the gap between the gland and the shaft or sleeve and often varies with operating conditions. Because a leaking rotating secondary seal can appear at the same spot, the two can often be told apart only after removal. Leakage at the gland-to-housing joint comes not from the seal itself but from the gland gasket or unevenly tightened bolts. Mistaking liquid from the pump casing, a drain plug or a pipe connection for seal leakage is also a common error.

The second distinction is whether the seal leaks at standstill or only while running. If it also leaks when the pump is pressurized but not rotating, the static gaskets, a cut or damaged secondary seal (O-ring, bellows), a damaged face or faces that cannot close are the first suspects. Leakage that starts only during operation, or changes with speed and temperature, points more often to dynamic causes such as misalignment, vibration, thermal distortion or vaporization at the faces. This is not a strict rule, but it narrows the list of suspects considerably before disassembly.

• Clean and dry the leak area, then watch where the first new drop comes from.
• Observe the leak separately with the pump pressurized at standstill and while running.
• Use color, odor and temperature to tell whether the leaking liquid is process fluid or flush, barrier or quench fluid.
• Record whether the leak is a steady drip, an irregular spray or vapor; hissing and vapor suggest vaporization at the faces.
• Confirm that the gland bolts are evenly tightened and the gland gasket is in place.
• Check the casing, drains and pipe connections separately to rule out leaks that do not come from the seal.

Dry running is the partial or complete loss of the liquid film between the faces and is one of the best-known causes of mechanical seal failure. Once the film is gone, the faces touch directly and frictional heat rises within a very short time. Typical triggers are starting the pump without venting, loss of fluid on the suction side, an empty tank, a valve left closed, and air or gas trapped in the seal chamber. In vertical pumps the seal chamber is often at the highest point, so air pockets are more likely to form.

The marks of dry running are usually distinct: rapid wear and an overly glossy wear track on the carbon graphite face, with carbon dust around the seal; heat cracks and discoloration on the hard face; hardening, cracking and burn marks on elastomers near the faces. Silicon carbide pairs, in which two hard faces run together, have no self-lubricating carbon layer and are therefore generally more sensitive to dry running than pairs with a carbon face.

A similar mechanism is vaporization of the fluid between the faces, known as flashing. With hot water, light hydrocarbons or liquefied gases operating close to their boiling point, frictional heat can turn the film into vapor; the faces open and close intermittently, the seal hisses, emits vapor and leaks erratically, and the face edges may show chipping. In this case, keeping seal chamber pressure and temperature away from boiling conditions, through cooling, pressurization or a suitable flush plan, is generally a more lasting fix than replacing the seal.

The seal is sensitive to the mechanical condition of the pump. Cavitation is the formation of vapor bubbles when the local pressure at the impeller inlet falls below the vapor pressure of the fluid, and their collapse in higher-pressure regions; the resulting pressure pulses and vibration make it harder for the faces to follow each other continuously. Insufficient NPSH, a clogged suction strainer, and operating the pump far from its best efficiency point, particularly at very low flow, are common sources of cavitation and hydraulic instability.

The sources of vibration and misalignment usually lie outside the seal: misalignment between motor and pump, worn bearings, a bent shaft or one with runout, an unbalanced impeller, and pipe strain. As the shaft moves radially or axially, the position of the rotating face relative to the stationary face keeps changing, and the spring and secondary seal wear as they try to follow. The result can be an eccentric or widened wear track on the faces, edge chipping on the carbon face, fretting on the secondary seal, and a groove on the shaft or sleeve under the O-ring.

Before replacing the seal, therefore, bearing condition, shaft runout, axial end play and coupling alignment should be measured and compared with the limits in the manufacturer's instructions. With repeat failures, the relationship between the vibration trend and seal life is one of the strongest clues as to whether the root cause lies in the seal or in the pump. A new seal installed without correcting the pump will most likely fail with the same marks.

When a newly installed seal leaks in its first hours or days, the cause is often installation or startup. In component seals the most critical setting is the spring working length: if the rotating unit is fixed in the wrong position on the shaft, the spring is either overcompressed, overloading and heating the faces, or undercompressed, so the faces do not close adequately, especially at low pressure. The setting length should be established from the actual dimensions of the seal chamber and gland and the value in the manufacturer's drawing.

Face damage is the second major group of errors. Lapped faces handled with bare hands pick up oil and dirt; set down on a dirty surface or knocked against an edge while being pushed along the shaft, they can pick up scratches and chips that are hard to see; these become leak paths from the first start. O-rings can be cut when passed over a keyway, a threaded end or an unchamfered shaft step; an unsuitable assembly lubricant can swell the elastomer, for example a mineral-oil-based grease on EPDM. Unevenly tightened gland bolts seat the stationary face at an angle and can distort it. Running a rotation-dependent design in the wrong direction is another known cause of early failure.

Startup is the last step of installation and is often rushed. The pump and seal chamber should be filled with fluid and vented before starting; any flush, barrier or quench system should be commissioned before the pump. Motor rotation should be checked with the coupling disconnected where possible, because turning the pump briefly with the seal dry can already damage the faces. In cartridge seals the setting clips should be removed after the gland and sleeve are secured and before the first start. Cartridge mechanical seals, whose setting length is fixed at the factory, greatly reduce errors from setting the spring length and handling the faces; filling, venting and removing the setting clips still depend on the work done on site. For choosing a type by application, see the mechanical seal types guide.

Friction between the faces always generates heat, which must be carried away by the fluid and by circulation in the seal chamber. Because heat generation rises with face load, sliding speed and friction coefficient, an unbalanced seal running at high pressure and speed heats up markedly more than a balanced design. When the heat cannot be removed, the film thins, friction increases and the process can become self-reinforcing.

Heat checking appears on the hard face as fine cracks, mostly running in the radial direction. The crack edges wear the mating face like a cutting tool and leakage rises quickly. Heat checking usually stems from excessive frictional heat that creates local hot spots on the face and from repeated heating and cooling cycles; a hot face suddenly wetted by cold fluid after dry running can accelerate the damage. Thermal distortion is more insidious: the faces lose flatness without cracking, the wear track narrows or becomes uneven around the circumference, and leakage is usually most noticeable during hot operation; if the faces have worn in while distorted, leakage can persist when cold as well. Blistering of the carbon face is another form of damage, seen especially with viscous oils and in frequent start-stop service; it is usually associated with the high shear stress at the moment of initial rotation, and cold starts, when viscosity is higher, can increase the risk.

The standard way to control heat and contamination in the seal chamber is a flush and cooling plan; API 682 defines these support systems by standard plan numbers. In brief, API Plan 32 injects clean, compatible liquid into the seal chamber from an external source, keeping hot or dirty process fluid away from the faces, while API Plan 53B is a dual seal arrangement with pressurized barrier fluid whose pressure is maintained by a bladder accumulator. A clogged orifice, a closed valve, an out-of-service cooler or falling barrier pressure are the first items to check before replacing the seal. The choice between Plan 32 and Plan 53B in abrasive service is covered in detail in the guide to mechanical seals for abrasive fluids.

Solid particles that get between the faces act like a lapping paste. The softer face, usually carbon graphite, wears quickly and circumferential scratches and grooves form on the faces; a hard particle embedded in the soft face keeps scoring the mating face. Solids can also build up between spring coils and behind the rotating unit and hinder the spring; in O-ring (pusher) seals, a clogged sliding area prevents the face from following wear.

The location of the wear also carries information. In the common arrangement where process fluid sits at the outer diameter of the faces, scored and grooved wear that starts at the outer edge and progresses inward may suggest particles entering from the process side, whereas a smooth, unscored track concentrated toward the inner or outer diameter points more to distortion than to abrasion. Deep grooves across the whole face may also indicate high solids content or inadequate flushing. Sandy water, mining slurries, paper pulp and crystal-laden solutions are typical settings for this failure; mining applications combine these conditions most severely.

The solution is usually threefold: pairs in which two hard faces run together (silicon carbide against silicon carbide, or silicon carbide against tungsten carbide), designs that protect the spring from the process or use a single large spring, and a flush plan that keeps particles away from the faces. Keep in mind that hard-hard pairs are more sensitive to dry running. Where impact and vibration are also present, materials with higher fracture toughness come to the fore; for face pair selection, see the silicon carbide vs tungsten carbide comparison.

Chemical incompatibility often shows up in the secondary seals before the faces. An elastomer unsuited to the fluid swells, softens and binds in the sliding area or extrudes from its groove; some fluids and high temperatures instead harden, crack and permanently deform the elastomer (compression set). A hardened O-ring can no longer flex in its groove and lets fluid through, especially during temperature changes. Known examples include the limited compatibility of EPDM with mineral oils, of NBR with many aromatic solvents and strong oxidizers, and of standard FKM grades with ketones, amines and hot water or steam.

Face materials can also be attacked chemically. In cobalt-bonded tungsten carbide, acidic media dissolve the binder and grains break out of the face; free silicon in reaction-bonded silicon carbide can dissolve in strong alkalis; the impregnant in carbon graphite can be affected by some aggressive fluids. Dulling, pitting or roughening of the face can be the mark of such an attack. For springs and other metal parts, stress corrosion cracking should also be considered, particularly in chloride-bearing media.

The lasting solution is to choose materials by evaluating the full fluid composition, cleaning chemicals and temperature range together; the selection logic is explained in the guide to material selection for sealing products. In chemical processes where many aggressive chemicals occur together, perfluoroelastomer (FFKM) seals offer broad chemical resistance; for details, see FFKM (Kalrez®) mechanical seals. Kalrez® is a registered trademark of DuPont.

The force holding the faces closed is the sum of the spring preload and the force that process pressure transmits to the faces, while the film pressure between the faces tries to push them apart. This balance can be upset when pressure drops suddenly, when the seal is exposed to reverse pressure (for example loss of barrier pressure in a dual seal, or operation under vacuum), or during sudden pressure spikes such as water hammer. The faces can open briefly and let particles in; a stationary face not designed for reverse pressure can shift in its seat. The result is intermittent leakage, chipped face edges or displaced parts.

Crystallization and drying occur when the very small amount of fluid crossing the faces evaporates on the atmospheric side and leaves salt, sugar, polymer or scale. These deposits can scratch the faces; more importantly, in O-ring (pusher) seals they prevent the axial sliding of the movable face. As the face wears, the spring can no longer push it forward and a gap remains between the faces; this is called hang-up. A seal that ran without trouble for a long time and starts leaking when restarted after a shutdown may suggest hang-up. On removal, deposits are found in the O-ring sliding area and the movable face often does not slide freely by hand in the axial direction; this is the finding that most strongly supports hang-up.

As countermeasures, a clean water or steam quench can be applied on the atmospheric side; bellows designs without a sliding O-ring, or single-spring designs more tolerant of solids buildup, can be selected; in severe cases a dual seal with barrier fluid may be more suitable. In metal bellows designs such as MS-350, a flexing bellows rather than a sliding O-ring provides the secondary seal, so O-ring-related hang-up is eliminated; deposits can still build up between the bellows convolutions, however, so the need for a quench in crystallizing service should be assessed separately. Against pressure-induced opening, the first points to address are a seal designed for pressure from either side (double-balanced) with a mechanically retained stationary face where reverse pressure is possible, adequate spring preload, and avoiding sudden valve operations on the process side. A balanced design reduces face load and heat but is not in itself a safeguard against opening.

The table in this section matches common field symptoms with possible causes and first checks. A symptom can have more than one cause; the table is not a priority ranking but a starting point for narrowing the list of suspects before disassembly. If several rows apply at once, look for the common denominator: if there is both a temperature rise and carbon wear, for example, the flush line is often one of the first places to check.

Simply replacing a failed seal with a new one can lead to the same failure again, because the root cause remains. The removed parts are the most reliable record of the failure, so they should be inspected, and photographed where possible, before they are cleaned and before any deposits are wiped off.

The wear track is the most valuable clue. In healthy operation, the wider face shows a smooth track equal to the width of the narrower face and running around the full circumference. A track concentrated on only part of the circumference, or an interrupted track, may suggest a distorted or cocked face; a track wider than the narrow face, shaft runout, misalignment or a miscentered stationary face; a narrow track concentrated toward the inner or outer diameter all the way around, pressure- or heat-induced distortion; and no track or a very faint one, faces that were not adequately loaded: insufficient spring compression, hang-up or opening under reverse pressure.

Elastomers and metal parts should be inspected with the same care. When the findings are evaluated together with temperature, pressure and vibration records and shutdown and startup dates, the root cause usually becomes clear. The step list in this section suggests an inspection sequence that can be applied in the field.

• Before removal, record the leak location, operating conditions and the date of the last startup.
• Mark or measure the position of the rotating unit on the shaft and the setting length before removal.
• Inspect deposits, crystals, discoloration and burn marks before cleaning the parts.
• Compare the width, continuity and position of the wear track on both faces.
• Look for heat cracks, edge chipping and corrosion on the hard face, and for scratches, blistering and edge chipping on the carbon face. If reuse is considered, have flatness checked with an optical flat and monochromatic light.
• Check O-rings and bellows for cuts, swelling, hardening and permanent set, and springs for breakage, corrosion and clogging.
• Look for grooves, fretting and deposits on the sleeve and shaft surface, and for play and wear in the bearings.
• Match the findings with operating and maintenance records and decide on the corrective action before installing the new seal.

Mechanical seal maintenance is less about removing the seal at intervals than about monitoring its operating conditions regularly and catching deviations early. A seal that is running well usually needs no direct intervention; the aim of maintenance is to keep the systems around the seal healthy: the pump, the flush plan, the barrier system and the alignment.

Meccanotecnica Umbra Turkey is the Turkish company of the Italian mechanical seal manufacturer Meccanotecnica Umbra S.p.A. From Ikitelli OSB, Istanbul, it manufactures and supplies mechanical seals, gland packings, gaskets and rotary joints for automotive, home appliance and industrial pump applications. To choose a seal for your application, browse the product catalog, or to have a recurring failure evaluated, contact our engineering team.

Check frequency should be set according to the application, the hazard class of the fluid, the plant's maintenance strategy and the manufacturer's instructions. Taking each measurement at the same point and by the same method makes it possible to read the trend rather than individual values. The checklist table in this section summarizes the check items, how they are carried out and what each one indicates.