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A manhole cover is usually a circular or rectangular plate designed to cover an opening to an underground vault or utility chamber. These covers are typically constructed from materials such as cast iron, steel, or reinforced concrete, providing a durable and weather-resistant seal. Their primary function is to provide access for maintenance and repairs of various utilities, including sewer lines, water pipes, and electrical systems. This access is essential for ensuring that urban infrastructure remains functional and efficient, enabling cities to thrive.

Moreover, gate valves are often preferred for their ability to provide a clear, unobstructed path for fluid flow. When fully opened, the disc of the valve is entirely removed from the flow path, minimizing pressure drops and turbulence. This characteristic is especially critical in applications requiring smooth and efficient fluid transfer, such as in water treatment, oil and gas, and chemical processing.

Scrap steel is melted in a crucible, induction, electric arc or reverberator furnace at 1500° C or 2700°F. The required alloys carbon and silicon, and flux to bind to impurities to create slag are added to the molten iron. The slag is lighter than iron and floats to the top from where it is removed. The molten iron is then poured into a metal ladle with a long handle which is then carried to the sand molds contained in the metal frames and poured through the riser. Green sand has a lower auto-ignition temperature than the temperature of the molten iron allowing the organic material present in the sand to use up all the oxygen present in the mold, preventing oxidation of the manhole cover. Trapped gasses can cause bubbles in the casting.

Cast Iron Manhole Covers
Manhole covers are made with different materials such as cast iron, ductile iron, concrete and in some cases, with glass reinforced plastic (GRP). Cast iron manhole covers are preferred because they are strong, cheap and easy to cast. A heavy manhole cover is essential to ensure that the cover does not get dislodged by pressure from gasses released in the drains and also from sucking pressure of fast moving vehicles. Unpainted cast iron inherently possesses rust resisting properties that make it an excellent choice to ensure best performance and long service life. Though other materials for manhole covers are available, the strength of cast iron cannot be overstated when it comes to resisting earth and pavement pressures.