BARRIER AND BUFFER FLUID SELECTION

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By current terminology (API 682), a fluid between the two seals in a dual seal at higher pressure than process pressure is a barrier fluid. (A dual seal pressurized this way was formerly called a double seal.)  A barrier fluid completely isolates the pumped process fluid from the environment.  A buffer fluid is instead at lower pressure than pump process pressure. (Dual seals with buffer fluids were previously called tandem seals.)  The AST 80 is double-balanced, so it can be pressurized either way.  Neither barrier nor buffer fluids should be confused with a flush, which is injected directly into the pumpage through the seal gland or the pump's seal chamber.

A barrier or buffer fluid should be
--compatible with the process,
--compatible with the seal materials,
--a good lubricant and heat transfer medium for the seal faces,
--benign to the environment and the workers in the plant.

Some good choices for barrier and buffer fluids:

Water/Ethylene Glycol mixture: Almost as good as water for heat transfer; doesn't freeze in outdoor applications. 50/50 mix by volume is easiest to mix, and gives good freeze protection. Use corrosion-inhibited industrial grade.

Water: Cheap, safe, available, excellent heat transfer characteristics (high specific heat, low viscosity, high thermal conductivity). Don’t use in freezing conditions.

Water/Propylene Glycol mixture: Like water/ethylene glycol, but usable in food applications.

Kerosene or Diesel fuel: Good where an oil is required. Low enough viscosity to flow well and transfer heat, and a good lubricant. Vapor pressure low enough that emissions aren't a problem.

Light mineral and synthetic oils: Generally good. Within synthetics, polyalphaolefin (PAO) based fluids usually better than ester-based. Synthetic oils specifically formulated for use as mechanical seal barrier fluids are available, including grades accepted by FDA and USDA.

Some common, but bad, choices:

Automotive antifreeze: Based on ethylene glycol, but contains additives to prevent corrosion of automotive engine components and to stop small radiator leaks. These additives cause excessive seal face wear.

Uninhibited ethylene glycol: Without corrosion inhibitors, can attack seal parts, notably the nickel binder in tungsten carbide.

Automatic transmission fluid (ATF): Contains additives to increase friction in the bands and clutches in automatic transmissions. These also increase wear and friction in seal faces.

Silicone oils: Inert, but often form glassy particles that abrade and clog seals.