Classification Of Seals By Arrangement

Sealing arrangements can be classified into two groups:

1. Single seal installations a. Internally mounted b. Externally mounted

2. Multiple seal installations a. Double seals b. Tandem seals

Single seals are used in most applications. This is the simplest seal arrangement with the least number of parts. An installation can be referred to as inside-mounted or outside-mounted, depending on whether the seal is mounted inside or outside the seal chamber (see Figure 13). The most common installation is an inside-mounted seal. Here the liquid under pressure acts with the spring load to keep the seal faces in contact.

Outside-mounted seals are considered to be used for low-pressure applications since both seal faces, the primary ring and mating ring, are put in tension. This limits the pressure capability of the seal. An external seal installation is used to minimize corrosion that might occur if the metal parts of the seal were directly exposed to the liquid being sealed. Multiple seals are used in applications requiring

• A neutral liquid for lubrication

• Improved corrosion resistance

• A buffered area for plant safety

Double seals consist of two single seals back to back, with the primary rings facing in opposite directions in the seal chamber. The neutral liquid, at a pressure higher than that of the liquid being pumped, lubricates the seal faces (see Figure 14). The inboard seal keeps the liquid being pumped from entering the seal chamber. Both inboard and outboard seals prevent the loss of neutral lubricating liquid.

FIGURE 13 Single seal installations: a) outside mounted, b) inside mounted

FIGURE 13 Single seal installations: a) outside mounted, b) inside mounted

FIGURE 14 Double Seals

Double seals can be used in an opposed arrangement. Two seals are mounted face to face, with the primary sealing rings rotating on a common mating ring (see Figure 15). In this case, the neutral liquid is circulated between the seals at a pressure lower than that of the process fluid. This pressure is limited since the outboard seal faces are in tension. The inboard seal is similar to a single inside-mounted seal and carries the full differential pressure of the seal chamber to the neutral liquid. The outboard seal carries only the pressure of the neutral liquid to the atmosphere. The purpose of this arrangement is to fit a seal installation having a shorter axial length than is possible with back-to-back double seals and still form a buffered area for plant safety.

Tandem seals are arranged with two single seals mounted in the same direction (see Figure 16). The outboard seal and neutral liquid create a buffer zone between the liquid being pumped and the atmosphere. Normally, the pressure differential from the liquid

FIGURE 15 Opposed double seals

BYPASS r— NEUTRAL

LIQUID \ CIRCULATION

BYPASS r— NEUTRAL

LIQUID \ CIRCULATION

FIGURE 16 Tandem seals

being sealed and atmosphere is taken across the inboard seal, while the neutral lubricating liquid is at atmosphere pressure. This arrangement can also be used as a method to break down the pressure on high-pressure applications. For example, the pressure difference across each seal can be half the fluid pressure being sealed. The liquid in the outboard seal chamber may be circulated to remove seal heat. Tandem seals are used on toxic or flammable liquids, which require a buffered or safety zone.

Package or cartridge seals are an extension of other seal arrangements. A package seal requires no special measurements prior to seal installation. For a single seal, the seal package consists of the gland plate, sleeve, and drive collar (see Figure 17). A spacer is provided on most package seals to properly set the seal faces. The spacer is removed after the drive collar has been locked to the shaft and the gland plate bolted to the pump.

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