|Steel mill conditions challenge workers and equipment |
Pouring molten metal from a ladle in a steel mill is dangerous work at best, but when a component in the metal-pouring system malfunctions, the results can be catastrophic — not to mention expensive. Many times, a single component within a system can be the source of trouble.
North American Refractories Co. (NARCO), Cleveland, manufactures state-of-the-art multiple-heat slide gate systems for use in steel-making facilities. Slide gates are refractorylined mechanisms used to control the flow of molten steel. NARCO’s ladle slide gate is available in combinations of fixed and sliding plates. Coil springs apply compressive loading force to the refractory plates to prevent molten metal leakage while still allowing the sliding plate to move. A hydraulic cylinder attached to the sliding plate provides the driving force — as commanded by the operator or a computer — and as the plate moves, holes in the fixed and sliding plates are brought into or out of alignment to allow or block metal flow. Intermediate positions, with partial alignment of plate holes, provide a throttling action and a range of controlled flows at reduced rates.
The system’s hydraulic drive mechanism includes the power unit, control valves, hydraulic cylinder, hoses, and quick-disconnect couplings. The couplings are disconnected when the equipment is moved around the mill.
When some customers reported problems with their slide gates, NARCO discovered that one side of their original quick disconnect’s internal check valves was failing in service. Hydraulic fluid could travel in only one direction, depending on which quick disconnect failed. This prevented the slide gate from moving in one of its two directions — a potentially dangerous and money-losing situation.
NARCO called on several suppliers, including Hansen Coupling, a division of the Tuthill Corp. in Cleveland, to investigate and diagnose the problem. Hansen engineers confirmed NARCO’s suspicion that hydraulic fluid flow surges, occurring at the beginning of each slide gate movement, were overloading the check valve spring retainers in the quick disconnects. The flow surges were beating the spring retainers at the end of the coupling’s check valves against the coupling’s retaining rings.
Eventually the tips of the spring retainers deformed so much they could slip back past the coupling’s retainer ring. Once this happened, the check valve would lock in one direction, preventing the bi-directional hydraulic flow required for proper slide gate throttling control.
|Beefed-up quick-disconnect coupling withstands hydraulic flow surges in demanding steel-mill slide-gate service. |
To solve the problem, Hansen modified one of its pipe-threaded HK-Series hydraulic couplings to meet NARCO’s application requirements. The HK couplings are for systems that require quick 2-way shut-off upon disconnection. Spring-loaded check valves in each coupling half — socket and plug — provide an immediate seal when disconnected. The NARCO application needed 1/2-in. disconnects that operate at 3000 psi and handle 12 gpm. Hansen redesigned the coupling’s spring-activated check valve retainer to meet the pressure and flow surges expected from the slide gate application. Hansen engineers incorporated a circular disc or valve guide, with flow passages to handle flow surges, into the coupling design. This prevents the surges from affecting performance of the coupling during slide gate system operation. Hansen also added a Teflon back-up ring to the female half of the coupling to further enhance resistance to the effects of flow surges.