CHOOSING THE CORRECT MECHANICAL SEAL

The following discussion should help you select the correct type of mechanical seal, install the proper environmental controls, choose the right materials for your application, and install the seal correctly. I have written this course so that you can click on any of the blue underlined words to learn more about that particular subject.

If your plant does not have a set of sealing specifications, I recommend that you learn how to specify the mechanical seal, and not take the seal recommended by the pump or original equipment manufacturer. You know your product better than anyone, and with a little training, plus a lot of common sense, you will be the best qualified to select a proper sealing solution.

Let me mention here at the beginning that mechanical seals fail for only two reasons:

Please keep that information in mind as you go through this course.

We will begin with the type of seal you should be using. Seals fall into many categories:

After choosing the seal type we must select the seal materials. Most of the time you will be choosing three different materials. If you do not know which ones to select, click on the appropriate underlined blue heading:

You must choose seal materials that are chemically compatible with all the fluids that might be passing through the equipment, and that includes any cleaners or solvents that might flush through the lines. This is especially critical with the choice of elastomer (O-ring). The metal parts and face combination are usually compatible with most cleaners and solvents.

The next step is to place your chemical into the proper sealing categories. Keep in mind that most chemicals fall into several sealing categories at the same time. If you are not familiar with these divisions you can click on the following for details :

Click on the following topics for more detailed information about particular sealing problems.

After you have placed your fluid in the proper sealing categories, selecting the correct environmental control comes next. If the equipment is going to operate twenty four hours a day and seven days a week, without shut down, you have a simple sealing problem. But if you are on batch or intermittent service, or if a standby pump is run occasionally then an environmental control will become critical. You need to know about the three most important environmental controls :

Any of these environmental controls will work better if you have installed an oversized stuffing box on your pump. The seal needs lots of radial room to allow centrifugal force to throw solids away from the lapped seal faces and to lessen the generation of heat in the stuffing box. Heat is a major cause of problems with both centrifugal pumps and mechanical seals. Any thing you do that will help to remove heat from the stuffing box will add to the life of your seal and pump bearings.

Check the paper "A few rules of thumb" for a summary of the above with a few additional points.

If you have done all of the above, all that remains is to install the seal correctly, adjust the pump impeller (if it is an open, or semi-open design) and hook up the environmental control you chose to keep the seal functioning. With a little luck you should now get good seal life, but unless you are prepared to do a few pump modifications your seals may last a very long time, but they will never wear out, and last as long as they could.

® Dupont Dow elastomer

 

For information about my CD with over 600 Seal & Pump Subjects explained, click here  

 Link to Mc Nally home page www.mcnallyinstitute.com