Mechanical Seal Types: Classification by Construction, Arrangement and Working Principle

Mechanical seal types are classified by the construction of the seal (component or cartridge), by the arrangement in which it is installed (single or double) and by its working principle (balanced or unbalanced, pusher or bellows, spring design and direction of rotation). These three axes cover every seal type found in the field.

The axes are complementary rather than exclusive: one product can be a cartridge unit, a double arrangement, a balanced design and a multi-spring seal at once. The useful question is not how many types exist, but which option is right on each axis for this duty. For the underlying theory see what is a mechanical seal.

The sections below close with two tables: one by application and one across the product catalog.

A component seal is the classic design in which the rotating unit, the seat and the elastomers are fitted onto the pump one piece at a time. Unit cost is low and the same housing accepts different material combinations, but the installed spring length must be set correctly and the faces kept clean; many early failures start there.

A cartridge mechanical seal delivers the rotating face, the stationary face, the spring set, the shaft sleeve and the gland plate as one factory-assembled, pre-set unit. On site it is only slid onto the shaft, bolted down and freed of its centering clips, so working length is no longer an installer decision and performance becomes repeatable.

Cartridge designs are preferred where downtime is expensive and in double arrangements with barrier fluid; the HUHNSEAL range is a typical example. Component designs keep their place in high-volume automotive and home appliance production, where unit cost decides.

A single mechanical seal separates the process fluid from atmosphere with one pair of faces. It is the most economical answer for clean, lubricating, low-hazard fluids and the most common of the pump seal types. With only one line of defense, fluid lubricity drives service life directly.

A double mechanical seal uses two face pairs with a barrier or buffer fluid between them. On toxic, volatile, crystallizing or non-lubricating media it lubricates the faces and controls emission to atmosphere. API 682 categories give these arrangements a common vocabulary.

Double seals are built in three arrangements - tandem, back-to-back and face-to-face - which differ in the direction the faces point and in whether the interspace fluid sits above or below process pressure.

In our range DR1-D is listed as a double industrial seal rated to 25 bar and 200 °C, while HUHNSEAL provides the same function as a pre-set double cartridge.

• Tandem: buffer fluid below process pressure; primary leakage is collected and monitored.
• Back-to-back: barrier fluid above process pressure; emission to atmosphere is minimized.
• Face-to-face: short axial length, better tolerance of solids-laden fluids.

The balanced and unbalanced distinction describes how much of the process pressure becomes face load, measured as the hydraulically loaded area divided by the face area, which is below one in a balanced seal. In an unbalanced design almost the whole pressure pushes the faces together, so friction heat and wear rise steeply.

In a balanced mechanical seal a step on the shaft or sleeve reduces the hydraulically loaded area, giving lower face load, less friction heat and higher PV capability. High pressure, high surface speed, low-viscosity fluids and media near their boiling point make the balanced type effectively mandatory.

Unbalanced designs are simpler, shorter and cheaper, and stay adequate on low-pressure water and general-purpose pumps. In the product data MS-7 is an unbalanced single seal, IS-1 Istanbul offers balanced and unbalanced variants up to 40 bar, and DR2-S reaches 30 bar with a balanced, protected multi-spring design.

Primary sealing always happens between the two faces. Secondary sealing isolates the rotating unit against the shaft and the stationary unit against the housing; its type sets the temperature limit and dirt tolerance.

In a pusher seal the secondary element slides axially along the shaft to compensate face wear, which allows a wide elastomer choice and high pressure capability. But if the sliding zone crystallizes or collects solids the o-ring hangs up and leakage starts. DRM1-S and Europa are conical-spring pusher designs in the catalog.

In a bellows seal a flexing bellows provides the secondary sealing, so there is no sliding element and no hang-up risk. Elastomer bellows designs are compact and economical and are standard in automotive water pumps, garden pumps and home appliances. FA (Industrial) and 6A represent this group.

A metal bellows seal removes the elastomer from the flexing function, so only the face and static gasket materials set the temperature limit. Heat transfer oils and media that swell elastomers belong here. The MEGASEAL range includes metal bellows balanced single seals to EN 12756, rated up to 220 °C.

The spring produces the preload that holds the faces together at startup and whenever pressure drops; its design also drives axial installation length and dirt tolerance.

A single coil spring uses one strong spring whose large wire cross section resists clogging and corrosion, which suits viscous and solids-laden media. A multi-spring design distributes small springs around the circumference for an even face load on large shafts, but those springs must stay clean or sit out of the process. DR2-S uses protected multi-springs, and DPS is a balanced multi-spring design for food duties.

Wave springs deliver the same preload in a far shorter axial length, suiting narrow chambers and compact cartridges. Conical springs combine single-spring dirt tolerance with a short installation length; DRM1-S, Europa and Simplex belong here. The MEGASEAL MS range uses leaf springs in unbalanced single seals to EN 12756, with MS-7 covering 14 to 200 mm shafts.

A rotation-dependent seal works correctly in one direction only, because of the winding direction of the spring or the geometry of the drive; run the wrong way it can unwind and leak within hours. DRM1-S is listed as rotation-dependent, a feature used deliberately to keep viscous fluids from clogging the seal.

Rotation-independent designs run safely in both directions, a real safety margin where reverse rotation is possible or the installed orientation cannot be guaranteed on site. Most of the catalog - DPS, DR2-S, UWP2, HUHNSEAL and FUGESCO - is rotation-independent.

Confirming shaft rotation before ordering is a small check that becomes expensive at commissioning; see the mechanical seal failure and maintenance guide.

In a split seal every component is made in two halves, so it can be fitted without pulling the pump or the coupling. Large wastewater pumps, agitators and machines with long strip-down times benefit.

A dry gas seal runs without contact: micro-grooves machined into the faces generate a thin gas film. Because a gas film replaces the liquid film, friction heat and power loss are very low; the type was developed for gas compressors, and seal gas cleanliness is critical.

A rotary joint is often confused with a mechanical seal but does a different job: it transfers fluid from a fixed line into a rotating cylinder. It contains a sealing face pair, yet the purpose is fluid transfer, not shaft sealing. HERO rotary joints and Rotary Joint Series 100 belong to this category.

Gland packing is not a seal type but the alternative technology: braided rings compressed by a gland run with a controlled drip. API 622 compliant braids such as GR8800 serve low-emission duties. For a full comparison see mechanical seals versus gland packing.

Once the seal type is fixed, the next decision is the face pair and elastomer combination. Keeping one face softer than the other makes wear progress in a controlled way; two hard faces running together are reserved for abrasive duties.

• SiC/SiC: abrasive and chemically aggressive fluids; high hardness and heat conduction.
• SiC/carbon graphite: clean water and general chemical duty; low friction and leakage.
• WC (tungsten carbide)/carbon graphite: high mechanical loading and thermal shock risk.
• Alumina ceramic/carbon graphite: economical for clean water, lightly loaded duties.
• Elastomers: NBR for mineral oil and cooling water, EPDM for hot water and caustics, FKM for hydrocarbons, FFKM for broad chemical resistance, PTFE for aggressive media.

The table below is a pre-selection tool; the final choice follows from fluid analysis, seal chamber dimensions and duty point. For materials, see the material selection for sealing products guide.

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 table below places the main series of that 189-product catalog on those axes.

For the full list and STEP/DWG files, browse the product catalog.