Gas-lubricated seals; for pumps too?
Original article date: September 1998
Gas-lubricated mechanical seals are a modern service-proven alternative for sealing gaseous or liquid media in machine components with rotating shafts. JOSEF NOSOWICZ ANFDREAS SCHRUFER and ULRICH RID of Feodor Burgmann explain the fundamentals.
In the compressor sector high performance gas-lubricated seals can replace throttle rings labyrinth seals and oil-lubricated mechanical seals. Low leakage rates wear-free operation and an extremely low level of power consumption are key characteristics of gas-lubricated seals and the reasons for their high efficiency.
The advantages of the gas-lubricated seal can be applied just as easily in pumps and agitators. However where double seal arrangements are operated they must use a suitable buffer gas supply system. Sealing of fans blowers steam turbines and use as safety seals for liquid-pressurised mechanical seals are other applications.
In its basic design a gas-lubricated seal resembles a conventional balanced mechanical seal. The seal consists of a spring-loaded seal face and a seat. While one ring is designed to be stationary the other rotates together with the shaft. A distinction is made between seals in which springs are on the rotor (flexible rotor designs) and those in which the springs are behind the stationary seal face (flexible stator design). The seal face and the seat are sealed from the housing and shaft respectively by secondary seals. Silicon carbide silicon nitride tungsten carbide or carbon are the materials used to manufacture the seal face and the seat. The flat faces of the seal face and seat form the sliding faces. Gas grooves measuring just a few microns in depth are machined into one of the sliding faces.
Unlike liquid-lubricated mechanical seals a gas-lubricated seal uses a stable film of gas between the sliding faces to keep the faces apart. Complete separation of the sliding faces during operation is absolutely essential because contrary to liquid-lubricated mechanical seals a gas-lubricated seal would be unable to dissipate the heat of friction. Unlubricated operation with contact would soon lead to the destruction of the sliding faces. The sealing gap attained by the sliding faces measures no more than a few microns.. A highly stable film of gas is created by gas grooves worked into the one sealing face thus enabling the seal to work safely. The gas medium flows through the sealing gap and is relieved. A small gap width means that leakage rates are kept to a minimum.
When the machine is at a standstill the axially moveable seal face is pressed by springs against the sliding face of the seat. Parting of the two sliding faces is brought about by differential pressure and the rotation of the shaft. For the two sealing faces to be separated with zero pressurisation they would have to be moved at a relative speed of about 0.5m/s. This is called aerodynamic lift-off. With a sufficiently high pressure differential it is possible however to reduce the loft-off speed considerably or indeed to make it completely unnecessary. This is called aerostatic lift-off.
Seals with a rotating seal face (flexible rotor designs) are generally used only in the low to medium speed range on account of the rotating springs. Seal faces of staitonary (flexible stator) design are used for high speeds. Gases and solid-free liquids are sealed by gas-lubricated seals featuring external pressurisation. Seals for media-containing solids are open to the product side in order to prevent deposits and are equipped with rotating seats and internal pressurisation in order to stop particles from getting into the sealing gap.
Feodor Burgmann Dichtungswerke has published a 76-page reference book on gas-lubricated seals ,well worth reading.
- Feodor Burgmann
- Ellen Neubarth
- 00 49 81 7123 1453
September 1998