Seal application, choosing a seal
Is this a mechanical seal application? 15-7
You have three opportunities to do a mechanical seal application:
- You are purchasing a new piece of rotating equipment and you would like it shipped with a mechanical seal that has a reasonable chance of working.
- You are converting a piece of rotating equipment that has been packed with conventional jam packing to a mechanical seal because you want to stop leakage.
- You are troubleshooting a premature mechanical seal failure.
First we will look at the piece of equipment you will be sealing. Sometimes mechanical seal life is directly related to the type of hardware you are sealing:
- A single stage centrifugal pump running at electric motor speeds, is a good seal application candidate.
- A multistage centrifugal pump can work well with mechanical seals if you cross connect the stuffing boxes to equalize the stuffing box pressures and avoid high pressure sealing on one end of the pump.
- Vertical pumps work well with mechanical seals because there is usually plenty of room between the stuffing box and the bearing housing. Be sure to vent the stuffing box to the pump suction to prevent trapping air at the seal faces
- Positive displacement pumps work well with mechanical seals, but there is sometimes a space problem when you try to fit the seal into the stuffing box; especially split seals that mount outside the stuffing box.
- Most positive displacement pumps have the stuffing box mounted on the low-pressure side of the pump, but there are exceptions.
- Some progressive cavity pumps are piped backwards causing a high pressure in the stuffing box.
- Many PD pumps run at slower speeds making sealing easier.
- Submersible pumps sometimes lack the room for a good seal installation. Often, dual seal applications can be converted to a single seal, sealing the bearings in these applications.
- Sleeved bearing equipment can present problems for mechanical seals because of excessive shaft axial and radial movement. You should try to use motion seal designs in these applications.
- Many boiler feed and deep well pumps fall into this category
- Mixers and agitators have lots of shaft displacement problems that can affect seal life. Again motion seals are a logical choice in these applications.
- Many of these mixers and agitators run at slow shaft speeds making sealing easier.
- Bottom entering shafts have real seal problems if there are solids in the liquid.
- Paper mill refiners and similar pieces of equipment also experience excessive axial movement problems.
Constant running equipment is easier to seal than intermittent equipment.
- The sealing liquids tend to stay in a liquid form rather than crystallize, solidify or change state when the equipment shuts down and the fluid changes temperature.
- Breakaway torque is a common cause of premature seal failure, causing the lapped seal faces to open.
Pumps, located inside buildings, are not subject to the changes in weather that affect some liquids you will be sealing.
- Many times the stuffing box has to be kept warm during cold weather to prevent the liquid from becoming viscous or solidifying.
The driver you choose can affect the life of the mechanical seal:
- Electric motors generally work well with mechanical seals
- Gasoline or diesel engines, along with variable speed electric motors, sometimes run at a critical pump speed.
- Steam, water and gas turbines sometimes run at speeds that are to high for some mechanical seals.
- Pulley driven equipment can have problems with shaft displacement.
Does the equipment you will be sealing, fall into the following categories? If so the seal has a good chance of surviving with an environmental control. If your application falls outside these categories you might consider a special seal design:
A temperature range of -40 to 400°F (-40 to 200° C)?
- Cryogenic or cold temperature sealing requires:
- A special cabon/graphite seal face.
- Metal bellows designs to eliminate O-rings and similar rubber elastomers that are sensitive to cold temperatures.
- An environmental control to prevent ice formation outboard the mechanical seal that can interfere with the seal movement.
- High temperature sealing requires:
- The elimination of elastomers in some applications. Metal bellows seals are often a good choice for polymers and monomers.
- Vibration damping.
- Cooling of petroleum and most oil products to prevent coke formation.
- A method of retaining the carbon/graphite seal face to prevent it from falling out of a metal holder.
A pressure range of one Torr to 400 psi (one Torr to 7 bar) in the stuffing box?
- Harder vacuums can out-gas rubber parts casing them to leak.
- Higher pressures can distort some seal faces and extrude some elastomers.
Electric motor speeds or slower?
- High speeds require a lowering of the balance ratio, less spring load at the seal faces, low friction faces, and a stationary design to prevent the lapped faces from separating.
Always consider cleaners, steam and solvents that might circulate in the lines after or between batches. Whatever seal you choose must seal these fluids also.
If you are troubleshooting a seal failure and think you might have an application problem, consider the following:
- Is the same seal running successfully in this same application in another location, in the plant? If it is, you should look at the pump or the seal installation as the problem, not the seal design.
- When did the failure occur? Did something change in the operation? Rubber parts normally fail in five to ten days after being exposed to the incompatible fluid.
- Has anything changed in the operation of this piece of equipment?
- Continuous service has been changed to intermittent service, with frequent starts and stops.
- A different solvent or cleaner is being used to flush out the lines.
- The temperature or concentration of the pumping fluid has changed.
- Is the environmental control operating when the pump is shut down or between batches?
- Does a control valve in the pump discharge sometimes cause the pump to operate off its BEP, causing shaft deflection or vibration?
- On February 18, 2018