DUAL SEAL APPLICATIONS D029
There is no question that dual seals cost twice as much as single seals, but at times they’re the only sensible approach to a sealing application.
In the following discussion the term barrier fluid means that the liquid between the dual seals is at a higher pressure than stuffing box pressure. Buffer fluid means that the fluid between the dual seals is at a lower pressure than stuffing box pressure.
Dual seals are used for a variety of purposes that include:
- To prevent a costly product from leaking.
- To prevent a dangerous product from leaking to the atmosphere.
- To prevent a pollutant or fugitive emission from escaping to the atmosphere.
- To prevent damage to the pump, base plate, and surrounding area when the mechanical seal fails, as well as the clean up cost involved.
- To contain a vacuum if a rotating shaft is running through the vessel.
- As a back up seal to prevent costly down time when the first seal wears out or fails. This is an important element in any predictive maintenance program.
- To control the environment outboard the primary seal
- To control the environment at the primary seal faces
Lets look at each of these applications in detail.
- I remember an incident when a mid-western Dupont plant dumped $10,000 worth of Dowtherm heat transfer oil on the ground before anyone discovered the seal leak.
- It doesn’t take very long to put hundreds of gallons of your product on the ground when the seal fails. Most control rooms can tell if the pump is running, but not if it’s leaking. Someone has to see and report the leak, and that’s difficult with many remote pumps.
Dangerous products include:
- Radioactive material.
- High temperature heat transfer fluids that can start a fire if they leak to the atmosphere.
- Many products are considered to be toxic to personnel in the area. Hydrogen sulfide is a good example.
- Cryogenic fluids that can injure personnel.
- High pressure fluids. Many boiler feed pumps and pipeline applications fall into this category.
- Carcinogens (cancer producing chemicals)
- Bacteria laden fluids.
Pollutants or fugitive emissions
- Recent legislation is making the list longer, with substantial fines attached to any violations.
To prevent damage to the pump, baseplate, and surrounding area when the mechanical seal fails, as well as the clean up cost involved.
- Imagine an area that never experienced a seal leak or failure. There would be no rusting of the base plate, the deck would require no refinishing and you would not have to wear hip boots to pass through the area. Some maintenance departments build wooden bridges over the leakage.
To contain a vacuum if a rotating shaft is running through the vessel. In most of these applications the stuffing box see a negative pressure.
- This is a common condition in some mixer applications.
- Any pump that lifts liquid experiences negative pressure.
- Condensate pumps that take they’re suction from a condenser hotwell.
- Heater drain pumps.
As a back up seal to prevent costly down time when the first seal wears out or fails. This is an important element in any predictive maintenance program.
- Batch operations cannot afford a seal failure in the middle of a batch.
- The main propeller shafts on all nuclear submarines are sealed with dual split seals. In the event of a seal failure the second seal allows the submarine to surface to a less hostile environment. This is the same reason a paratrooper wears two parachutes.
- If you do not have a standby pump, you better have a standby seal.
- Some maintenance departments work an eight-hour shift with a utility man watching the store the remaining hours and weekends. The backup seal will allow the equipment to keep running until the regular maintenance crew comes back.
- If the equipment is in an awkward location, and not dangerous, you run both the first and second seals to failure and cut in half the number of times you have to disassemble the equipment. The life of the second seal has the same probability as the first seal.
To control the environment outboard the primary seal
- Cryogenic fluids can freeze moisture on the other side of the seal and interfere with the seal moving forward to compensate for carbon face wear.
- Minute amounts of leakage can cause some fluids to crystallize or solidify at the inside diameter of the seal restricting its movement.
To control the environment at the primary seal faces
- If the pump is run dry the added heat can injure a seal component. The barrier or buffer fluid circulating between the dual seals can keep the seal components within their temperature limits.
- When you seal a gas the buffer fluid between the seals provides the necessary lubrication and cooling. Furnace gas fans are a good example of this application.
- You can run a warm buffer fluid between the two seals to keep the seal faces warm at pump shut down and prevent products from crystallizing or solidifying. A cool buffer fluid can be used to prevent a product from flashing, or in some cases solidifying, at the seal faces.
- To prevent a pressure drop from the stuffing box to atmosphere across the lapped seal faces. This can be necessary :
- To prevent some fluids from flashing or vaporizing.
- In the paper industry to seal Kaoline or any fluid containing solid particles less than one micron in size.
- When sealing ethylene oxide to prevent the fluid from penetrating the dynamic elastomer and exploding out the atmosphere side.
- To prevent some fluids from flashing or vaporizing.
- Do not be tempted to run the dual seals without barrier or buffer fluid. It is the presence of this buffer fluid that will let you know the second seal is working.