St. Paul, MN -Plastic couplings made from fluoroplastics are desirable in applications involving aggressive chemicals and deionized water because of the material's chemical inertness. The trade-off, however, is that fluoroplastics have low strength, which limits working pressures to as low as 35 psi, depending on the size of the coupling.
Although use of metal in couplings is possible, corrosion in and around the fluid flow path presents a contamination threat to process fluids. "Springs made from metal that are coated with a fluoropolymer or made from a fluoropolymer material typically close the coupling shut-off valves," says Thomas Braun, a mechanical engineer and co-patent holder of the new ChemQuik(TM)CQN Series couplings from Colder Products Co. "The end result is that the first spring's coating either wears, cracks, or eventually permeates; becomes corroded; and contaminates the fluid. The other spring is low force and susceptible to physical creep resulting in valve leakage."
In the CQN coupling, fluid flows along the path indicated. Reducing the valve seal's annular face area (a mere 0.08 square inche for the 3/8-inch size coupling) reduces the force required to separate the coupling halves.
The CQN locates fluoropolymer-encapsulated springs outside of the flow path, away from ultra-pure and corrosive fluids, which provides a double-layer of protection and eliminates the contamination risk. On the insert side of the coupling, fluid flows around the edges. On the body side, fluid flows through the middle.
Braun knew that coupling separation force is directly proportional to the small annular area of the valve's seal upon which the fluid pressure acts. The force increases as the size of the annular area increases and must be carried by the latching mechanism holding the two coupling halves together. The CQN design minimizes the annular area of the valve's seal that is exposed to system pressure. "If you want to develop a coupling that incorporates an easy-to-use thumb latch, then you must isolate the system pressure from the force acting on the thumb latch," says Braun.
When uncoupled, fluid system pressure acts on the small annular area balancing the forces so that the couplings remain in the closed position in a fail-safe manner. "Upon connection, this area is isolated and effectively balanced during the joining action of the two coupling halves," says Braun. The system pressure provides no significant separating force, except that which is applied to the small annular face area of the valves. This force, along with the spring bias, is what separates the coupling halves when the thumb latch is released. A typical 1.00-inch coupling at 80 psig with poppet valve design would have up to 192 lbs of force acting on the latching mechanism trying to separate the coupling halves, but a 1.00-inch CQN coupling would have only 14 lbs of force acting on the latching mechanism.
Unlike poppet-style valves, the CQN's pressure-balanced design allows for connection and disconnection while under pressure with virtually no spillage. "The amount of spillage is directly related to the amount of dead volume that is exposed to the system pressure at the instant the flow path is closed