Page 97 - Air and gas Drilling Field Guide 3rd Edition

88 CHAPTER 4 Downhole Equipment

FIGURE 4-23. Typical dart type float valve for direct circulation operations (courtesy of
Baker Oil Tools).

The dart type valve is spring activated, which opens to allow the direct circu-
lation flow to pass around the dart. This type of valve provides a more secure shut-
off against high and low pressure back flows. The flapper type valve opens fully
during circulation to provide an unrestricted bore through the valve and closes
when back flow pressure is applied. These valves are used in nearly all deep rotary
air and gas drilling operations. The dart valve is used in the bit sub just above the
drill bit. In practice, at least one flapper valve is placed just above the drill collars
or above a downhole motor. A second is often placed about 1000 ft below the
surface. It is not unusual for a long drill string to have three or four float valves.
The fire stop is a special type of flapper valve. It is essentially an upside-down
float valve. It is usually placed just above the drill bit. These valves have a zinc
ring that holds back a spring-loaded flapper mechanism to allow the compressed
air or gas to be circulated directly from the surface through the inside of the drill
string. Wire line equipment can be run through these valves when the fire stop is
in the normal open position. In the event of a downhole fire, the zinc element
melts, releasing the spring-loaded flapper. This shuts down the flow of air or
gas into the bottom of the well, thus shutting off the source of fuel for the fire.
Fire stop valves are rarely used in present drilling operations.