Selecting Mechanical Seals for Abrasive and Slurry Service

What makes a fluid abrasive for a seal is the hardness, size, angularity and concentration of its solids; as velocity rises the same particles erode metal parts. A seal for such duty must keep solids out of the face gap and every moving clearance, and resist wear where contact is unavoidable.

In solids-laden fluid a seal must hold the film in the face gap, keep solids out of spring and bellows clearances, and concentrate damage on a replaceable part — rarely all at once.

This guide covers how solids destroy a seal, the fluid families, the face and mechanism decisions, the flush plan, and the data an inquiry needs. Mining combines these conditions most severely; the mechanical seal types guide gives the classification.

Damage in abrasive service rarely has one cause; as solids reach places they should not, several processes trigger one another. The gap is microns wide, so one particle disturbs flatness.

On a stripped seal the wear mark — face, sleeve, spring cavity or atmospheric side — identifies the dominant mechanism; see the seal failure and maintenance guide for the symptoms.

• Three-body abrasion: a particle trapped in the gap scores both faces; leakage follows lost flatness.
• Erosion: solids-laden flow cuts the sleeve, seal body and spring area, changing face load.
• Spring and bellows clogging: solids settle between coils and convolutions until axial travel is lost.
• Hang-up on a pusher seal: deposits under the sliding O-ring lock the moving ring.
• Crystallization on the atmospheric side: evaporating leakage leaves salt that scores faces and blocks travel.
• Dry running when the flush stops: the film goes and a hard-hard pair overheats quickly.

Abrasive fluid is not one class; mineral slurry, fiber-laden paper stock and sand-laden wastewater load a seal differently. Concentration, hardness, temperature and chemistry together decide the material and the plan.

In mining, slurry pumps, hydrocyclone feeds, flotation cells and dewatering equipment run in extremely abrasive environments; in pulp and paper, pumps meet high temperature, cellulose fibers and bleaching chemicals.

• Mineral slurry: high concentration of hard, angular particles; abrasion plus erosion.
• Sand-laden water: low concentration, hard grains; settling during stops, dry contact at start-up.
• Pulp and stock: fiber build-up and packing behind the face, plus bleaching chemistry.
• Wastewater sludge and grit chamber discharge: variable solids, fibrous debris, intermittent duty.
• Crystallizing liquors (brine, sugar, caustic): scale on the atmospheric side, crystals on cooling.

The default pair is hard against hard: silicon carbide on both rings minimizes material loss as particles rub the faces. Sintered SSiC is preferred where abrasion governs, reaction-bonded RBSiC where the chemistry allows, as its free silicon phase limits strongly alkaline duty. Where impact and shock dominate, as in grinding and tailings duty, tungsten carbide wins on toughness.

The price of a hard-hard pair is poor dry-running tolerance: with no self-lubricating transfer film, face temperature rises quickly once the film is lost. Such a pair is therefore specified with assured lubrication — uninterrupted clean flush, an atmospheric-side quench, or a dual seal on barrier fluid. Carbon belongs only on the clean side; see the silicon carbide versus tungsten carbide guide.

The elastomer follows the chemistry: EPDM for water-based slurry, FKM for hydrocarbon-bearing slurry, FFKM for aggressive chemistry. In scaling fluids, whether the surface under the O-ring collects deposits matters more than the elastomer class.

The first decision is where the moving parts sit. With the spring unit stationary and the seat rotating, the springs leave the product, and rotation throws solids outward so particles enter the face gap less easily. A tapered or enlarged bore with flow modifiers lets solids and vapor escape.

The second decision protects the spring. Enclosed springs, single-spring arrangements and spring packs outside the product stop solids settling between coils; DR2-S is a balanced industrial seal with protected multiple springs and independent rotation direction. A metal bellows has no spring to clog, but convolutions can fill, so a bellows sits on the clean side or works with a flush.

The third decision is serviceability. A cartridge arrives preset, so nothing is measured in the field and change-out is faster. HUHNSEAL covers single and double cartridge seals for medium and heavy duty.

API 682 names seal support systems by standard plan; in abrasive service two matter most. API Plan 32 injects clean liquid into the seal chamber from an external source, normally through a throat bushing and above chamber pressure, so flow runs outward into the process and solids never reach the faces. The cost is dilution, acceptable where the water balance allows.

API Plan 53B is a dual seal with pressurized barrier fluid, held above seal chamber pressure by a bladder accumulator that keeps gas separated from liquid. Leakage across the inner seal is therefore clean barrier fluid moving into the process. Plan 53B suits lines where dilution is not permitted, clean flush water is unavailable, or the fluid is toxic. Plans 11, 13, 21, 53A and 54 are also used.

A single seal works in abrasive service if a clean flush runs without interruption and dilution is acceptable. If either fails, a dual seal is the better choice.

In a pressurized arrangement the inner faces run on clean barrier fluid and slurry stays at the outside diameter, while the outer seal sits entirely in barrier fluid. This removes three-body abrasion and the dry-running risk of a lost flush; for toxic fluids where no process leakage to atmosphere is permitted, a pressurized arrangement is usually required.

The cost is complexity: a barrier circuit, an accumulator, pressure and level monitoring, space and higher first cost. An unpressurized dual seal with buffer fluid does not dilute the process, but its inner faces stay on the process side.

The data below decide more than the fluid's name. Where they are missing, selection rests on assumptions that come back as early wear.

• Solids concentration, particle size distribution, hardness and grain shape.
• Whether dilution is permitted and the acceptable clean water rate.
• Seal chamber pressure with pump suction and discharge conditions.
• Operating, start-up and post-shutdown temperatures.
• Chemistry: pH, chlorides, bleaching chemicals and hydrocarbon content.
• Continuity and pressure of the clean flush, and any interlock when it stops.
• Start and stop frequency, and settling risk during long standstills.
• Shaft or sleeve diameter, chamber bore and axial length (DIN 24960, EN 12756), and change-out access.

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.

The FUGESCO family covers axial and radial mechanical seals: multiple-spring products with independent direction of rotation, described as a versatile industrial solution. The product data gives 16 bar, 15 m/s and -20 °C to +200 °C, with SiC, carbon, FKM and EPDM. See the Fugesco mining and slurry applications catalog.

HUHNSEAL cartridge seals are defined for medium and heavy duty at 25 bar, 20 m/s and -40 °C to +260 °C; the double cartridge suits a barrier fluid arrangement. DR2-S works to 30 bar and -30 °C to +220 °C. Each figure is that product's own limit, not a promise that peak pressure and temperature coincide.

On the water and wastewater side the Cityline family is used, for example MS-240 Munich, an unbalanced bidirectional single seal with an elastomer bellows, available as a double seal. The full range is on the products page; contact us with your line data for selection support.