SUBJECT : A new way of classifying chemicals to assure effective sealing 2-12

The most common question asked by seal salesmen is "what are you sealing?" This is usually followed by asking about shaft size, product, temperature, speed, stuffing box pressure and any other operating conditions they can think of.

The problem with this simplistic approach is that you'd have to have a very large data bank of information to reference a particular problem so as to be able to make a sensible seal recommendation. There's a much more logical approach to the problem that we'll be discussing in the following paragraphs.

A sensible approach to the sealing of various chemicals, mixtures, and compounds would be divided into three parts:

In this paper we'll concentrate on the classification of chemicals and leave the selection of seal materials, types of seals and use of various environmental controls to other papers on this site

A fluid can be classified as either a liquid or a gas, and can be divided into seven categorizes:

1. Fluids sensitive to changes in temperature and/or pressure.

2. Fluids that require two mechanical seals.

3. Non lubricating liquids, gases and solids.

4. Slurries, classified as solids in liquid . The solids may or may not be abrasive.

5. Liquids sensitive to agitation.

6. Liquids that react with each other to form a solid.

7. Lubricating liquids.

We will be investigating each of these categories in detail and learn how they affect the life of a mechanical seal. In other papers, we will learn the detailed methods of sealing each of these problems

Fluids that are sensitive to changes in temperature and/or pressure.

Liquids that require two mechanical seals : These seals are installed with a circulating barrier fluid that can be a "forced circulation", or in many cases a convection system with a "pumping ring." The pressure of the barrier or buffer fluid can be regulated to indicate a failure in either of the mechanical seals allowing time for a pump shut down, isolation and no subsequent loss in the pumping fluid.

Sealing non lubricants.

Slurries, especially abrasive slurries. Clog the seal components and destroy faces like the dry solids mentioned above.

Liquids sensitive to agitation :

Liquids that combine together to form a solid.

We seldom have problems with these liquids in pumps because the blending takes place outside of the pump, but the problem sometimes comes up in mixer applications. You'll note that I have not included anaerobic fluids (they solidify in the absence of air) in any of the categories (super glue is the product that first comes to mind).

Clean, lubricating liquids

Now that leakage is no longer tolerable and product dilution is no longer desirable you must have knowledge of these categories to approach the job of effective sealing. In most cases the fluid you're sealing will fall into several of the above mentioned categories. Using heat transfer oil as an example we note that it falls into the following :

To successfully seal heat transfer oil you'd have to address all of these problems at the same time. As is the case with all slurry applications, you'd also have to recognize the problems with vibration (impeller imbalance), thermal growth, and frequent impeller adjustments.

In addition to handling various chemicals we're often faced with extreme or severe operating conditions. These conditions would include:

In other papers on this site you can learn how to seal each of these fluid categories and learn how to protect the mechanical seal against the affects of these extreme operating conditions.

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