The selection procedure is an important part of the standard. Many of the problems seen in the field are the result of selecting the wrong type of seal, the wrong materials, or the wrong piping plan. While no procedure can capture every possible consideration, the seal selection procedure does provide a good basis for selecting the correct seal.
The procedure used in the standard was developed to capture selection methods that have proven successful in the field. It is a systematic approach of selecting the seal type, arrangement, and piping plan for a number of common applications. It is not intended to cover every service. It does though, capture the majority of applications seen in a refinery or process industry.
The procedure will always lead to a default solution. In most cases this is seen as the preferable solution. In many cases, though, there are alternative solution. Where alternative solutions are given, they are considered as technical equals by the standard.
The seal selection procedure uses the same basic applications groups used in other parts of the standard. Non-hydrocarbons refers to water, sour water, caustics, amines, crystallizing fluids, and some acids. Non-flashing hydrocarbons refer to hydrocarbons with a vapor pressure less than of 1 bar absolute (14.5 PSIA). These fluids would remain in a liquid phase under ambient conditions. The last group is flashing hydrocarbons which covers liquids with a vapor pressure greater than 1 bar absolute (14.5 PSIA). Some component of these fluids would vaporize under ambient conditions.
Selection Flowchart
The selection procedure flows though a logical sequence of steps. There are three paths trough the procedure based on the application group. Each application group has its own path although all applications groups share some common selection procedures.
The selection procedure has the following steps:
Identify the service or application Select the seal category
At this point, each of the application groups will go to its own sheet to select the seal type
All of the application groups will go to the same sheet to select the seal arrangement
Each application group will go its own sheet to select the piping plan
And finally the buffer or barrier fluid is selected for dual seal applications
We will cover each of these steps in more detail.
Step 1 - Identify the Service
The first step in the process requires the application to be identified. This includes the fluid, temperature, pressure, speed, contaminants, and any other details important to this service.
The application should be classified as a non-hydrocarbon, non-flashing hydrocarbon, or flashing hydrocarbon.
The seal selection process, along with the standard in general, has a range where it is applicable. Examine the application to determine if the seal selection process should be used. The standard does not apply to
applications that have the following conditions.
Over 260ºC (500ºF) or 21 bar (300 PSI) for Category 1 seals Over 400ºC (750ºF) or 41 bar (600 PSI) Category 2 and 3 seals Have surface speeds above 23 m/s (4500 ft/min)
Have vapor pressures over 34 bar (493 PSIA) Contain a high concentration of solids
Have shafts larger than 110mm (4.3 inches) or smaller than 20mm (0.75 inches) Step 2 - Select Seal Category
The next step requires the purchaser to select the seal category. Since the seal category is a sub-specification in the standard, many features of the seal will depend its selection. Important considerations include the intended pump design, the pressure and temperature, the required features, and the required documentation. There is a chart in the seal selection procedure that summarizes the differences between the different categories.
The purchaser is responsible for selecting the required seal category.
After selecting the seal category, the user will select the seal type.
Each application group (non-hydrocarbon, non-flashing hydrocarbon, and flashing hydrocarbon) has its own sheet for selecting the seal type.
The seal type selection is based on a matrix layout. The table illustrated here is a portion of the non-hydrocarbon selection matrix. The user matches the name of the service at the top (in this case water) with the application. The non-hydrocarbon matrix also includes columns for sour water, caustics, amines, crystallizing fluids, and acids.
In each column, the user selects the appropriate temperature and pressure (based on seal category) that matches the actual service. The output of the matrix is the standard seal type. Other options for seal types are listed if applicable. Required special features cover materials or design features for abrasive particles, caustics, amines, ammonia, and H2S services.
Step 4 - Select the Seal Arrangement
The selection of the seal arrangement may be done by the seal OEM, purchaser, or preferably both parties. The selection flowchart directs the user through a series of yes or no questions.
The output is the require seal arrangement.
The questions in the flowchart cover a wide variety of topics including:
Regulations
Emissions considerations
Consequences of leakage on the environment, personal safety, and regulations Experience of the operator and operating practices for the service
The flowchart is filled with many questions that are judgment calls. While the seal OEM may be able to make recommendations based on the technical requirements of the seal, the input from the end user is critical in the selection.
Step 5 - Select the Seal Flush Plan
The next step is to select the required flush plans for the application. Each application group has its own flush plan selection sheet. The user begins the flow chart at a starting point for the selected seal arrangement. The user then goes through a series of decision boxes about the application, fluid, buffer/barrier fluid, temperature, and pump orientation. The output of this sheet is the required piping plan for the application.
In addition to the flowchart, there are a number of notes at the bottom of each sheet that contain important additional details about the selection.
Step 6 - Select the Seal Barrier/Buffer Fluid
For dual seals, the final step in the seal selection procedure gives basic guidelines for the selection of the buffer and barrier fluids. The topics covered include chemical and material compatibility, gas absorption on pressurized seals, recommended viscosity ranges, and environmental and safety considerations. The standard does not actual give specific recommendations to brands or type of fluids. Rather, it gives properties of fluids that have proven to be successful in the field.
After the selection of a barrier fluid, the seal selection procedure is complete.
Tutorials
To better understand the background of the seal selection procedure, Annex A includes extensive tutorials. These give a background on the seal selection procedure as well as guidelines for seals and support systems for
specific applications. The tutorials also give detailed instruction on many of the questions asked in the seal arrangement selection process. Finally, there are a number of tutorials discussing the benefits and
considerations for all of the piping plans.
While these tutorials are not a required part of the seal selection procedure, they should be required reading for any person working with mechanical seals. They provide insightful information based on the experience of many engineers and reliability experts in the process field.