Choose a barrier fluid by what your process can safely accept first, then by the temperatures the fluid will see at the seal, from a cold standby to full running heat. Most duties then fit one of three families:
- Water: when the process accepts water and the barrier neither freezes nor runs hot.
- Water-glycol mixture: when water would freeze.
- Low-viscosity oil: when the seal runs too hot for water, or when water must stay out of the process.
Between the seals of a double mechanical seal, the fluid keeps the process away from the atmosphere, lubricates the seal faces and carries frictional heat to a cooler. If the fluid between your seals is unpressurized, it is a buffer fluid rather than a barrier fluid, and process fluid leaks into it instead of the other way round.
Properties of a Good Barrier Fluid
A good barrier fluid is harmless if it leaks into your process, and it keeps a workable viscosity at every temperature the seal sees. The properties that separate one fluid from another:
- Acceptable in the process: small amounts leak inward in normal running and much more after an inner seal failure, so the fluid must not react with the product, contaminate it, or upset downstream processing.
- Viscosity high enough when hot: the fluid has to hold a film between the seal faces at running temperature.
- Viscosity low enough when cold: the fluid has to circulate at start-up and on standby, when an outdoor seal pot sits at ambient temperature, as well as in normal running.
- Viscosity low enough for carbon faces: a thick fluid sheared between a carbon face and a hard face can blister the carbon.
- Good heat transfer: the fluid carries frictional heat from the faces to the cooler, and water does this better than oils.
- Safe to handle: low toxicity, no flammability and no regulated volatile compounds.
Water, Glycol, Oil and Other Barrier Fluids
Most double seals run on water, a water-glycol mixture, or a mineral or synthetic oil.
The Fluid Sealing Association’s table of typical buffer and barrier fluids gives each fluid a suggested pump temperature range. Set your coldest standby temperature and your hottest running temperature against these ranges before choosing a family.
Water
Water suits a double seal as a barrier fluid when the process can accept small amounts of it and the pump temperature stays between 5 °C and 70 °C (40 °F and 160 °F), running and on standby.
In food, pharmaceutical and biotech plants, condensate is the most common barrier fluid; where even water is unwelcome, use a food-grade oil instead.
Water rusts carbon steel, so the barrier system has to be built from non-rusting materials. A plant water supply must be clean as well, free of rust, dirt and treatment chemicals such as descalers, rust inhibitors and biocides.
Water can run once-through from a clean plant supply, or circulate in a closed system such as an API Plan 53A seal pot. API 682 recommends holding a pressurized barrier at least 1.4 bar (20 psi) above seal chamber pressure.
If an API Plan 54 unit circulates the water, check its pump. Many Plan 54 systems use positive displacement pumps with rubbing or sliding parts designed to run on a lubricating fluid, and running them on water can greatly reduce the system’s reliability.
Water-Glycol Mixtures
Use a water-glycol mixture when plain water would freeze, typically in a seal pot that sits outdoors or on a standby pump in winter, and you still want most of water’s heat transfer. Water-glycol barrier fluids can be used down to about -29 °C (-20 °F); colder than that, use an alcohol.
Ethylene glycol is the most common glycol in barrier fluids, usually as a 50/50 mixture with water. Propylene glycol performs about the same, is non-toxic and comes in food grade, so it is the one to use when leakage reaches a food or pharmaceutical stream or where local rules restrict ethylene glycol.
Use pure, uninhibited glycol. Automotive antifreeze carries additives that can come out of solution and wear the seal faces, so it does not belong in a barrier system.
Oils
An oil is the better barrier fluid when the seal runs hotter than water or glycol can handle, or when water is unwelcome in the process.
Keep the oil’s viscosity below that of ISO Grade 32. If the duty needs a thicker fluid, change the faces to a hard-versus-hard pair such as silicon carbide against silicon carbide.
Among oils, paraffinic oils generally perform better than naphthenic ones, and synthetic oils better again. Synthetic fluids made specifically as barrier fluids perform best but cost the most. Turbine oils and automatic transmission fluids have a weaker record.
Other Barrier Fluids
Alcohols, kerosene or diesel, heat transfer fluids and fluorinated oils are used where water, glycol and oil do not fit, each for one kind of duty:
- Alcohols: for services colder than water-glycol can reach. Methanol was the older choice; propanol covers a broader temperature range and lubricates better at ambient temperature.
- Kerosene and diesel: both work well with seal faces, but both are flammable.
- Heat transfer fluids: for very high temperatures, with the risk that they decompose and form coke, a hard carbon deposit.
- Fluorinated oils (PCTFE): for processes that react with ordinary hydrocarbon oils, such as those run with Lewis-acid catalysts.