Mechanical Seal Dry Running Test: Methodology, Evaluation Criteria, and Industrial Applications

The mechanical seal dry running test evaluates how a rotating seal performs when its liquid lubrication film is completely lost, simulating real-world upsets such as loss of prime, tank run-down, blocked flush lines, or vapor-space operation. By measuring leakage, face temperature, friction torque, and wear under unlubricated conditions, seal manufacturers and end users can verify upset tolerance, validate face material pairings, and reproduce field failures. This article explains the test principles, sample types, measured parameters, procedures, applicable standards, and typical industrial applications.

Technical Overview

A mechanical seal is a precision device that controls leakage along a rotating shaft by maintaining controlled contact between two lapped sealing faces. Under normal operation, these faces are separated and lubricated by a thin fluid film only a few micrometers thick, which carries away frictional heat and prevents face-to-face contact. When this film is lost, the seal enters a dry running condition in which direct asperity contact generates intense localized heat at the interface.

The mechanical seal dry running test, sometimes called a dry friction or dry burn test, is a controlled laboratory procedure that deliberately operates the seal without liquid lubrication at a defined speed and for a defined duration. Its purpose is to quantify short-term survivability, thermal robustness, leakage behavior, and face material performance under the very condition that causes a significant share of premature seal failures in the field.

During dry operation, the friction coefficient at the interface typically rises sharply, local face temperatures can reach several hundred degrees Celsius within seconds or minutes, and a cascade of degradation mechanisms may begin: carbon graphite faces may blister, glaze, or craze; hard faces such as silicon carbide or tungsten carbide may develop thermal checking cracks; and elastomeric secondary seals may harden, coke, or decompose. A structured dry running test reveals how quickly each of these mechanisms develops and whether the seal can survive brief process upsets without catastrophic loss of containment.

Test programs are generally structured in alignment with recognized frameworks such as ISO 21049 for shaft sealing system qualification, EN 12756 for dimensional and material designation practice, and GB/T 6556 for principal dimensions of mechanical seals, while supplementary face material screening may draw on tribological methods such as ASTM G99.

Testing Samples

Component Mechanical Seals

Pusher-type component seals, including single-spring, multi-spring, and wave-spring designs as well as elastomer bellows seals, are commonly submitted for dry running evaluation. These seals are assembled piece by piece onto the test spindle, and their face load is set by verifying the installed spring compression against the manufacturer's drawing.

Cartridge Mechanical Seals

Single and dual cartridge seals arrive as pre-assembled units with a built-in setting mechanism, making them well suited to standardized testing. Cartridge designs intended for pumping services conforming to ISO 21049 are frequently qualified with dry running endurance as an upset condition, particularly for applications with known risk of loss of prime or gas entrainment.

Gas-Lubricated and Mixer Seals

Dry gas seals and large-diameter mixer or agitator seals are engineered to operate with gas at the interface or in a vapor space above the liquid level. Dry running tests for these samples verify tolerance to barrier gas loss, level drop, and extended vapor-phase operation at the low speeds typical of top-entry and bottom-entry agitators.

Face Material Couples

Because dry running performance is dominated by the face material pairing, test programs often compare multiple couples side by side, including resin-impregnated carbon graphite against reaction-bonded or sintered silicon carbide, antimony-filled or bronze-filled carbon against tungsten carbide, and hard-hard combinations of silicon carbide against silicon carbide.

Testing Items

A comprehensive dry running test program captures both real-time operating data and post-test forensic evidence. The principal measured items include:

• Leakage rate, quantified gravimetrically or volumetrically in grams per hour or milliliters per hour during and immediately after dry operation
• Seal face temperature, measured with thermocouples embedded in or near the stationary face and supplemented by infrared thermography
• Friction torque and power consumption, acquired through a calibrated torque transducer on the test spindle
• Survival duration, defined as the elapsed time from loss of liquid to test termination or functional failure
• Face wear, characterized by mass loss, wear scar depth, and wear track width using profilometry and optical microscopy
• Face flatness, verified before and after testing by monochromatic light-band inspection against the specified helium light band tolerance
• Surface integrity of the faces, including blistering, glazing, heat checking, microcracking, and coking deposits
• Condition of secondary seals, including hardness change, compression set, embrittlement, and chemical degradation of O-rings and gaskets
• Operational anomalies such as audible squeal, vibration signatures, and transient torque spikes that signal the onset of face damage

Testing Methods

Test Apparatus

A typical dry running test rig consists of a variable-speed spindle driven by a frequency-controlled motor, a seal chamber or housing adapted to hold liquid and then be drained through a bottom port, and a full instrumentation chain. Key instrumentation includes a tachometer, a torque transducer, thermocouples positioned close to the sealing interface, an infrared camera for surface temperature mapping, and a drip tray with a precision balance for leakage collection. The rig must accommodate the seal size range, medium pressure where applicable, and rotational speeds representative of the intended service, commonly up to several thousand revolutions per minute.

Test Sequence

The standard test sequence is designed to separate normal operation from the dry running upset so that any degradation can be attributed unambiguously to the unlubricated phase:

• Pre-test metrology: record face flatness, surface roughness, and component masses; photograph and document all parts
• Installation: assemble the seal on the spindle with verified spring compression and correct drive engagement
• Baseline wet run: operate with liquid at rated speed and pressure to confirm stable temperature and acceptable leakage before the upset
• Drain phase: remove the liquid from the chamber while the shaft continues to rotate at the specified test speed
• Dry running phase: continue unlubricated operation for the specified duration, logging temperature, torque, and leakage continuously; common durations range from a few minutes for screening tests to one hour or more for qualification programs
• Cooldown and teardown: allow controlled cooling, then disassemble for post-test metrology and destructive inspection of faces and secondary seals

Measurement and Evaluation

All channels are logged at a fixed sampling rate so that the time of onset of thermal runaway, torque instability, or leakage excursions can be identified precisely. Acceptance criteria are agreed before testing and typically include a maximum permissible leakage rate, absence of catastrophic face failure such as cracking or spalling, wear depth within the drawing tolerance, retention of face flatness within the specified light band limit, and secondary seals that remain dimensionally and mechanically functional. A full report presents the time histories, before-and-after metrology, photographic evidence, and a conclusion on whether the sample satisfies the specification.

Reference Standards and Practices

• ISO 21049, which defines the qualification framework for shaft sealing systems on centrifugal and rotary pumps, within which dry running endurance is commonly assessed as an upset condition
• EN 12756, which governs principal dimensions, designations, and material codes for mechanical seals used in test planning and reporting
• GB/T 6556, which specifies the principal dimensions and designation conventions for mechanical seals and supports sample preparation and interchangeability checks
• ASTM G99, a pin-on-disk wear test method used to screen candidate face material couples under unlubricated sliding before full-scale seal testing

Applications

Dry running test data directly supports seal selection, product qualification, and root cause analysis across process industries wherever the liquid film at the seal faces can be interrupted. Typical applications include:

• Chemical and petrochemical plants, where flashing, cavitation, or loss of prime can momentarily starve the seal of liquid
• Oil and gas facilities, including pipeline, refining, and offshore pumping services that demand documented upset tolerance
• Water and wastewater treatment, where dry-pit and intermittent-duty pumps are exposed to repeated rundown events
• Mixers, reactors, and agitators that operate partially or entirely in the vapor space above the liquid level
• Food, beverage, and pharmaceutical services, where clean-in-place cycles may temporarily interrupt flush supply to the seal
• Seal manufacturer research and development, used to compare face materials, optimize balance ratios, and validate thermal design
• Production quality control and benchmarking of incoming seal lots against established performance baselines
• Failure analysis laboratories that reproduce suspected dry running damage to confirm or exclude root cause hypotheses

FAQ

What is a mechanical seal dry running test?

It is a laboratory test in which a mechanical seal is operated at a defined speed without any liquid at the sealing interface, in order to measure leakage, temperature rise, torque, wear, and survivability under unlubricated conditions. It simulates process upsets such as loss of prime, tank run-down, or blocked flush lines.

How long can a mechanical seal survive dry running?

Survival time depends on speed, face load, materials, and design. Conventional carbon versus silicon carbide pairs commonly tolerate short upsets measured in seconds to a few minutes, while special dry-running grades and gas-lubricated designs can extend this substantially. The dry running test establishes the documented endurance limit for a specific seal configuration.

Which face material pairing performs best in dry conditions?

Self-lubricating resin-impregnated carbon graphite running against solid silicon carbide is the classic pairing for limited dry running capability. Hard-hard pairs such as silicon carbide against silicon carbide or tungsten carbide against tungsten carbide generate more frictional heat and generally require engineered dry running features to survive unlubricated operation.

Is a dry running test the same as testing a dry gas seal?

No. Dry gas seals are engineered with hydrodynamic lift features that maintain a controlled gas film, so gas operation is their normal duty. A dry running test on a liquid-lubricated seal instead recreates the accidental loss of the fluid film, which is a fault condition for that design.

What are typical acceptance criteria after the test?

Common criteria include leakage within the agreed limit, no cracking, spalling, or blistering of the faces, wear depth within drawing tolerance, retention of face flatness within the specified light band limit, and secondary seals that remain functional without excessive hardening or compression set.

Can a seal be reused after a dry running test?

In qualification and benchmark testing the sample is treated as sacrificial and is not returned to service. In some routine quality control tests, reuse may be permitted only if every metrological and inspection criterion is met and documented by the responsible engineer.