Dual Mechanical Seals for Safety and Containment (2/3) - Seal Designs

Selecting the seal

There are a number of features within a mechanical seal which can affect its performance and long term reliability. Chesterton summarizes this by its 5 Key Features. 

These 5 key features epitomize good seal design. For simplicity, in the design summaries below we will assume that we already have seals that comprise all of the key design features.

Back to Back Design Summary

The original concept of a dual seal, taking two component mechanical seals and placing them ‘Back-to-Back’ within the stuffing box.




Figure 1: Back To Back Configuration

• Back to Back configurations normally require barrier fluids supplied at higher pressures than the process to operate correctly. As the barrier fluid is circulated at a higher pressure than the process media there is zero risk of leakage to atmosphere under normal operation. Changes in fluid level act as an indication to the condition of the seal, depending on the criticality of the operation a level switch should be installed in the seal support system.
• The outboard seal is carrying the highest pressure differential and should wear out before the inboard seal. Out board seal failure is normally followed by immediate inboard seal failure.
• Designs without dual balance allow the inboard faces to open when a higher than normal pressure in the process occurs and can contaminate the barrier fluid. Dual balance should always be specified for safety and containment as it keeps the faces closed.
• Typically the process media is located at the inside diameter of the inboard faces. In slurry applications centrifugal force will work against the barrier fluid and may cause damage to the seal faces and contaminate the barrier fluid.

Face to Face Design Summary

This design fits, technically, between ‘Back-to-Back’ and ‘Tandem’ as it has some advantages over ‘Back-to-Back’ designs.



     Figure 2: Face to Face Configuration

• This design can be operated in Buffer or Barrier fluid configuration.
• Typically these designs are axially compact and are not located entirely within the stuffing box.
•  Some ‘Face-to-Face’ designs utilize a common stationary component. This is not preferred as heat generated within the stationary component is high. Separate faces should be utilized.
• The outboard seal is carrying the highest pressure differential and should wear out before the inboard seal. Out board seal failure is normally followed by immediate inboard seal failure.
• Typically the process media is located at the outside diameter of the inboard faces. Centrifugal force helps this design to work well in slurries.

Tandem Design Summary

As the name suggests, this design utilizes two seals working in the same orientation. It sometimes referred to as ‘Face-to-Back’. Tandem designs provide the ultimate failure containment when used in conjunction with a properly configured system.



     Figure 3: Tandem Configuration

• Tandem designs are more complex to manufacture as cartridges and so cost a little more.
• Both the inboard and outboard seal have the same pressure capability, meaning that in the event of inboard failure, the outboard seal remains secure.
• This design can be operated in Buffer or Barrier fluid configuration.
• The outboard seal is carrying the highest pressure differential and should wear out before the inboard seal. Out board seal failure does not affect inboard seal operation and can be used as an indication to carry out maintenance.
• Typically the process media is located at the outside diameter of the inboard faces. Centrifugal force helps this design to work well in slurries.

Steven Bullen

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

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

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