Piston seals or piston rings are used in pneumatic cylinders for fluid sealing and are designed to ensure that pressurized fluid does not leak across the cylinder head as the system pressure pushes the piston down the cylinder bore.
The best sealing capacity of a double-acting cylinder, capable of sealing dynamic pressure from both sides (bidirectional), is achieved by choosing a double-acting seal. A piston design where two single-acting seals on the piston for a double-acting cylinder are used can easily give rise to a breakdown. The reason is that a very high pressure can be trapped between the seals.
Piston seals, both single and double-acting, can be designed for and used with integrated or separate back-up rings and guide rings.