SUBJECT : The dual seal arrangement 8-8

Dual seals are recommended for a variety of purposes that include:

All of the above are sensible reasons for using two seals in a pump, but In this paper we'll be considering the use of dual seals as an environmental control to prevent the sealed product from either opening the seal faces, or damaging one of the seal components, the two reasons any mechanical seal ever failed.

We can use the dual seals to:

Dual seals can be of either the rotating or stationary version and can be installed in four different configurations.

The fluid that circulates between the seals is called barrier fluid if it is higher than stuffing box pressure. It is called buffer fluid if it is lower than stuffing box pressure. It can be circulated between the two seals by:

The following illustrations describe the rotating version (the spring or springs rotate with the shaft) of these dual seal configurations. You should be aware that a stationary version is also available from any of the major seal companies. You should also consider:

First we will look at the back to back version of a rotating seal:

The rotating back to back version would be your worst possible choice. Here are some of the reasons :

Tandem is the next version. This is the configuration you find in most Oil Refinery applications.

Here are a few comments about this version:

The face to face version is next

This face to face version is a compromise between the "back to back" and the tandem version:

The concentric version is next, but I do not have an illustration to show you. In this version we have one of the seals inside the other sharing a common stationary face. The stationary face is drilled between the rotating faces to allow circulation of the barrier fluid.

The convection tank is a unit you can either purchase or manufacture your self. When a manufacturer supplies this unit, it requires a "Boiler Maker Stamp" and a 600 psi rating, making it very expensive to purchase. You can probably manufacture one for your purposes at a much lower cost.

You have many choices when it comes to your choice of the buffer or barrier fluid:

Now that you have all of the basics under control, we will use a dual seal to solve some of our common application problems:

Use a pressure higher than stuffing box pressure between the seals to:

Use a lower pressure between the seals to:

CAUTION Do not split the pressure between stuffing box and atmospheric pressure. This will put an equal load on both seals and they will wear out at the same rate.

Circulate the correct temperature fluid between the seals, especially when the pump is shut down. You are going to have to make the decision as to what barrier fluid temperature is needed. You can increase the temperature, decrease it or hold it within narrow limits:

Here are a few more considerations:

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