In brief
A mechanical seal seals rotating shafts, for example in pumps and agitators, by means of two sealing faces that slide axially against each other.
Contents
What is Mechanical seal?
The mechanical seal, also known as an axial mechanical face seal, consists of a rotating seal ring that turns with the shaft and a stationary mating ring. Springs press the rings together, and a thin lubricating film forms between the very flat sealing faces. Secondary seals such as O-rings or bellows seal the rings against the shaft and housing. Typical material pairings are carbon graphite, silicon carbide, tungsten carbide or ceramic.
A distinction is made between single and double seals, unbalanced and balanced designs, and cartridge seals, which are installed as a pre-assembled unit. Installation dimensions for common designs are standardised in EN 12756; for pumps in refineries, petrochemicals and chemicals, API 682 often applies.
Selection depends on the medium, pressure, temperature, speed and shaft diameter. Common causes of damage are dry running, solids in the medium, unsuitable materials and installation errors. Leakage at the seal is often the first visible sign of a problem and should be investigated early.
In practice: maintenance and procurement
For a suitable replacement, the pump manufacturer and type, shaft diameter, design and the seal's material code are required. The material code is often stated on the seal or in the pump data sheet.
Synonyms and abbreviations: mechanical face seal · end face seal · cartridge seal (design) · shaft seal (generic term)
Frequently asked questions
Is slight leakage at the mechanical seal normal?
Yes, to a small extent. The lubricating film between the seal faces causes minimal leakage, which with many media escapes as vapour and is barely visible. Noticeable dripping, crystal deposits or an increasing leakage rate, on the other hand, indicate wear, damage or unfavourable operating conditions. Permissible values depend on the seal and application and are given in the manufacturer's specifications.
What is a cartridge seal and what are its advantages?
A cartridge seal is a pre-assembled unit consisting of seal ring, mating ring, springs, secondary seals and shaft sleeve. It is slid onto the shaft and fixed as a whole. This reduces assembly errors, because the fitting dimension and preload are already set, and shortens downtime. In return, it is usually more expensive to buy than a component seal.
What is the difference between a single and a double seal?
A single seal has one pair of seal faces and is sufficient for many non-critical media. A double seal has two pairs, between which a barrier or flush fluid circulates. It is used for toxic, flammable, abrasive or crystallising media, because the product does not escape directly into the environment. A supply system for the barrier fluid is required for this.