Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

Mechanical Seals 373
Classification of Seals by Arrangement
Seal arrangement is used to describe the design of a particular seal installation
and number of seals used on a pump. Sealing arrangements
may be classified into two groups:
• Single seal installations
* Internally mounted
* Externally mounted
• Multiple seal installations
* Double seals
• Back to back
* Opposed
* Tandem
Single seals are commonly used on most applications. This is the simplest
seal arrangement with the least number of parts. An installation
mounted inside the stuffing box chamber is referred to as an inside
mounted seal (Figure 17-12). Here, liquid in the stuffing box and under
pressure acts with the spring load to keep the faces in contact.
An outside mounted seal refers to a seal mounted outside the stuffing
box (Figure 17-13). Here, if the seal is not balanced for pressure at the
inside diameter of the seal face, the pressure will try to open the seal.
Outside mounted seals are considered only for low pressure applications.
The purpose of an external seal installation is to minimize the effects of
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 seal heads mounted back to back with the
carbon primary rings facing in opposite directions in the stuffing box
chamber (Figure 17-14). A neutral liquid with good lubricating properties,
at higher pressures than the pumpage, is used to cool and lubricate
the seal faces. The inboard seal keeps the liquid being pumped from entering
the stuffing box. Both the inboard and outboard seals prevent the
loss of the neutral lubricating liquid. Circulation for cooling is normally
achieved with an external circulation system and smaller pump. An axial
flow pumping ring and closed system may be considered on low pressure
systems.