Page 410 - Centrifugal Pumps 2E

Mechanical Seals 375

Double seals may be used in an opposed arrangement (Figure 17-15).
Two seals are mounted face to face with the primary sealing rings run-
ning on mating rings supported by a common end plate. The neutral liq-
uid is circulated between the seals at the inside diameter of the seal faces,
The circulation pressure is normally less than the process liquid being
sealed. The inboard seal is similar to a single inside mounted seal and
carries the full differential pressure of the pump stuffing box to the neu-
tral liquid. The outboard seal carries only the pressure of the neutral liq-
uid to atmosphere. Both inboard and outboard primary rings are balanced
to handle pressure at either the outside or inside diameter of the seal faces
without opening. The purpose of this arrangement is to fit a stuffing box
having a smaller confined space than what is possible with back-to-back
double seals. Double seals are normally applied to toxic liquids for plant
safety.
Tandem seals are an arrangement of two single seals mounted in the
same direction (Figure 17-16). The inboard seal carries the full pressure
differential of the process liquid to atmosphere. The outboard seal con-
tains a neutral liquid and creates a buffered zone between the inboard seal
and plant atmosphere. Normally the neutral lubricating liquid is main-
tained at atmospheric pressure. Developed heat at the inboard seal is re-
moved with a seal flush similar to a single seal installation. The liquid in
the outboard seal chamber should be circulated with a pumping ring to
remove unwanted seal heat. Tandem seals are used on toxic or flammable
liquids that require a buffered area or safety zone in the seal installation.
Package seals are an extension of other seal arrangements. A package
seal requires no special measurements prior to seal installation. A pack-
age seal assembly consists of a seal head assembly, mating ring assembly,
and adaptive hardware. Adaptive hardware are those structural parts that
fit a specific seal to a pump. These hardware items are a gland plate,
sleeve, and drive collar. A spacer or spacer clips are provided on most
package seals to properly set the seal faces. The spacer or spacer clips are
removed after the drive collar has been locked to the shaft and the gland
plate bolted to the pump. Examples of package seals are shown in Fig-
ures 17-9, 17-10, and 17-15.

Classification of Seals by Design
Certain design features are considered important and may be used to
describe a seal. These descriptions also form four classification groups.
A seal may be referred to as: