SEALING LIQUIDS THAT COMBINE TO FORM A SOLID COMPOUND SA012
We want to be sure that we do not confuse this application with fluids that set-up or harden because of:
- A change in temperature. Chocolate solidifies when it cools
- A change in pressure. Paint solidifies when you lower the pressure enough to let the solvents evaporate.
- Agitation. Cream becomes butter.
In this application we are talking about liquids that are combined together and then the hardening takes place. If you mix a resin and hardener together you get epoxy glue. Some of the newer coatings and many plastics are made this way
Most of the time the liquids are combined outside the pump to form the compound, so sealing never becomes a problem. We only have to seal the individual fluids and they fall into the convenient categories we list under “Seal application“.
Occasionally we run into a batch mixer application where two or more liquids are mixed and then pumped out of the mixer before the setting-up or hardening takes place. The mixer is then flushed with a solvent of some type to remove the resultant compound and the solvent is then pumped through the system to clean the piping.
The problem occurs with the pump emptying the mixer, because the stuffing box area never gets the proper flushing necessary to prevent the compound from setting up and restricting the seal movement.
The time element takes over and the compound solidifies in the seal components restricting their movement and sometimes it solidifies between the lapped seal faces causing them to break when the pump shaft rotates.
The solution to the problem is obvious. We need a more thorough cleaning of the pump stuffing box area.
Please look at the following illustration:
- Fluids “A” an “B” are added to the mixer and blended together.
- The pump empties the mixer, but some of the compound is left inside and it will solidify unless it is flushed away.
- The solvent flush valve is opened and solvent “C” is added to the mixer. The mixer is filled, agitated and then emptied by the same pump. This action also flushes the compound from the lines.
- There is always some of the compound trapped in the pump stuffing box. Flushing the mixer and lines will not remove it.
The illustration also describes the solution to the problem.
- An inlet line is connected from the solvent flush to the bottom of the pump stuffing box and an outlet line is connected from the top of the stuffing box to the pump discharge.
- The solenoid valve opens when the solvent flush valve opens and mixer flushing begins. This flow provides a constant cleaning of the stuffing while the mixer is filling. Bringing the fluid into the bottom of the stuffing box and out the top is critical to the success of this application. Sometimes it is difficult to install a fitting at the bottom of the box, so get it as close as you can.
- When the solvent flush valve closes, the solenoid valve is wired to close also.
I have some recommendations for the seal that you will be using in this application:
- Installing an oversize stuffing box makes sense.
- This is an instance where using a seal with two hard faces is a sensible choice.
- Because most solvents attack popular O-ring materials, you will probably be using Chemraz or Kalrez® as the dynamic elastomer.
- Select a design with the springs out of the fluid. A metal bellows seal without a dynamic elastomer is another choice that is logical.
- Be sure the fluid in the stuffing box is at the seal outside diameter. It will be a lot easier to flush away.
- If you prefer a dual seal in this application, be sure to use a tandem design with the compound and solvent at the outside diameter of the inner seal.