A double seal, or back-to-back seal, describes two shaft seals mounted back-to-back to provide a higher level of safety and reliability. The term is commonly used in sealing engineering to describe an arrangement with two shaft seals mounted back-to-back for applications where no leakage from the pumped medium to the atmosphere can be accepted.
Between the two seals is a pressurised barrier fluid. The pressure of the barrier fluid is higher than the pump pressure. This means that any leakage will pass from the barrier fluid to the pumped liquid. The barrier fluid pressure must be minimum 2 bar or 10 % above the pump pressure. As the clean barrier fluid has a higher pressure, it also serves as a lubricant for all seal faces.
Grundfos supplies CR multi-stage and NKG end-suction single-stage pumps with back-to-back shaft seal arrangements, making them particularly suitable for viscous and/or abrasive media, for example in industrial processes. The seal arrangement prevents the pumped medium from entering the seal gap and consequently prevents excessive wear.