Dual Mechanical Seals for Safety and Containment (1/3) - Introduction

Dual mechanical seals are recommended by both pump and seal manufacturers for a variety of reasons, many of which include;

• Preventing an expensive product being lost
• Preventing a hazardous product reaching the atmosphere
• Preventing pollution of the environment
• Utilizing the outboard seal backup seal to allow planned repair and overhaul of the equipment when the inboard seal has reached the end of its life or failed

The list above is not exhaustive, but has the primary reasons covered. To understand the application of dual seals for safety and containment it is essential that the user understands the types of dual seals that are available and how to properly apply them with their associated support systems.

Dual seals and their support systems can be used to;

• Control the operating temperature of the seal. (Up or Down)
• Reduce the amount of pressure drop occurring across the seal faces.
• Provide lubrication to the seal faces.
• Provide clean fluid to the seal faces.
• Isolate the mechanical seal from the atmospheric conditions.

Rotary and stationary versions of Dual mechanical seals are widely available and we’ll look at 3 typical configurations.

• Back to Back
• Face to Face
• Tandem

Dual seals require fluid exchange between the inboard and outboard seal faces in order to operate properly. There are two operating modes for this fluid exchange; Barrier Fluid, Fluid at a higher pressure than the media being sealed, or Buffer Fluid, Fluid at a lower pressure than the media being sealed.

The fluid supporting the dual seal must be circulated in order to optimize seal life. This can be achieved in the following ways;

• Thermosyphoning - A principle which employs natural convection to circulate fluid to and from a Seal and Tank System. Heat generated by the rotation of the seal causes the fluid to rise up the return pipe and the cooled volume of fluid in the tank feeds fresh fluid to the seal.
• Pumping Rings – Many but not all of todays dual Cartridge seals feature a device which encourages fluid circulation.
• External Sources – Fluid fed from an external source can be that of a mains water system or a closed loop tank system with pump. For mains fed systems it is essential that a check valve is installed to prevent contamination of the source.

Steven Bullen

Rotating Equipment Segment Manager, EMEA at A. W. Chesterton Company

Vice Chairman, Mechanical Seals European Sealing AssociationSteven.Bullen@chesterton.com