|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.
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
Water: Cheap, safe, available, excellent
heat transfer characteristics (high specific heat, low viscosity, high thermal
conductivity). Dont 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
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.