A kelly hose is a piece of mining equipment. Specially, it refers to a piece of equipment used in the mining of field or semi-field resources. Such as oil and natural gas. The main purpose of a kelly hose is to allow the drill string to be raised and lowered at the same time that drill fuild is being pumped through it. This is important, as drilll fuild is critical to be mining process.

In most case, a kelly hose is classified as a large-diameter hose, This means that the inside diameter is usually between 3 and 5 inches (about 7.6cm and 12.7cm). This wide diameter allows for a significant rate of flow and reduces the likelihood of a blockage occurring in the hose.

The kelly hose must be able to withstand large amounts of pressure. This applies primarily to the pressure of the fuild flowing through the hose. For this reason, it is often made of highly durable material and is generally reinforced with steel.

In a derrick or drilling rig, the kelly hose connects the standpipe, which is the rigid metal shaft that delivers the mining fluid, to the swivel, which is the piece that supports the weight of and controls the rotation of the drill string. Its purpose is to provide a flexible drilling fluid conduit, as a rigid conduit would be unable to move with the swivel and would therefore disallow movement of the drill string and, subsequently, the bit.

The drilling fluid, sometimes called drilling mud, carried by a kelly hose is critical to operations in several ways. It keeps the bit cool, which helps reduce friction and failure. It also cleans the bit and carries away drill cuttings so they cannot damage the drill assembly. Some varieties are used for additional purposes, such as preventing corrosion and providing hydrostatic pressure. Drilling fluid is not necessarily fluid, but may, in fact, be a solid, liquid, gas or other combined form.

The kelly hose is so named because of its connection with the kelly, the actual mechanical piece that ejects the drilling fluid over the drill string. It may alternatively be called a mud hose or a rotary hose. 

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