Application

The Philosophy of Seal Selection

My Great-Great-Great Grandfather wrote a book entitled “The Philosophy of Religion”. I suspect that I am one of the few to read it and also that he might not appreciate a book entitled “The Philosophy of Seal Selection”.  Since this is not really a book, perhaps the title would have been acceptable to him.

What is seal selection? As used herein, “seal selection” includes not only selecting a seal type/design for a given service but also includes matching the seal to the equipment as well as choosing the arrangement, configuration and piping plan.  In other words, selecting a complete system to meet the requirements.

There must be at least as many seal selection guides as there are seal engineers but here is my attempt to consolidate them. There are

  • Service Based Selections
  • Local Preferences and Experience
  • Fluid Based Selections
  • Fluid/Service Group Based Selections
  • Scientific Method
  • Product Spec Sheets

Of course, the reality is that all of the above methods and considerations, whether directly or indirectly, are used to make a seal selection. Even so, it is useful to consider these methods separately for purposes of discussion.

Service based selections

When an end user asks for a seal selection guide, he is likely to be thinking in terms of point solutions for his particular services — especially if his plant is small. Therefore, the request might be for specific services such as vacuum bottoms pumps, crude oil pipelines, propane pipelines, ethylene product pumps, cooling water pumps, etc.  Some application guides provide such service based selections.  For example, an API 682 Type A, Arrangement 3 fitted with carbon/silicon carbide faces, peroxide cured perfluoroelastomer, Super Duplex metallurgy and Plan 53A for the Desalter Effluent pump in a crude oil distillation unit.

The advantage of the Service Based Selection method is that it can be done in great detail with few inputs and can be based on existing application guidelines. The disadvantage is that the number of point solutions is essentially infinite.

Local preferences and experience

In many ways, local preferences and experience – whether end user or local service center – are essentially point solutions to specific services. Local preferences can be considered product rationalization on the local level.  Inventory, training and experience may be the driving forces behind local preferences.

Fluid Based Selections

A variation of Service Based Selection considers only the fluid instead of the service. For example, propane would always be sealed in the same manner whether the process service was a reflux pump in a refinery light ends unit or a propane pipeline.  Obviously, additional data – especially pressure and temperature – is needed in order to make consistent selections based solely on the fluid.

Fluid/Service Group Based Selections

Seal selection in API 682 is organized around fluid groups with subdivisions for typical services and variations. The API 682 fluid groups are

  • Water
  • Sour water
  • Caustics
  • Amines
  • Acids
  • Hydrocarbons

Some end users also take this approach but include additional groups or considerations. For example, some end users extend the pressure range of API 682 and includes variations based on hydrogen sulfide content.

Seal selection by Fluid/Service Group is probably the most common method.

Scientific Method

Computationally complex modelling methods such as Finite Element analysis are not normally used for seal selection because of the time required to develop the model. In addition to time, mathematical models may not consider metallurgy, elastomers or piping plan.  In many ways, this “scientific” method is really a fluid based method.

Product Spec Sheets

A manufacturer’s product spec sheet often provides a simple means for checking to see if a particular sealing product is good for the application under consideration. Unfortunately, there is considerable variation in the basis and organization of the spec sheets.

Many of the spec sheets include the following information:

  • An abbreviated list of typical applications
  • Pressure and temperature range.
  • Procedure for determining dynamic pressure rating
  • Hydrostatic test pressure limits
  • Special considerations
  • Dimensional data
  • Standard and optional materials

This is essentially a trial and error procedure. Candidate seals are quickly selected using only the summary page of the spec sheet.  Final selection is made based on the details within the spec sheet.

Machine based selections

In a way, machine based seal selection is essentially service based seal selection. Heavy duty pumps are expected to use heavy duty seals and vice versa.  “API” pumps use “API” seals.  “Pipeline” pumps use “Pipeline” seals.  Small general purpose pumps may be required to be used in many different fluids without modification and also to be inexpensive.