A double block bleed valve is a necessity in the oil and gas industry. Being able to isolate the downstream side of a pipe from the upstream side is a must when equipment is being maintained. The typical valve consists of two globe, gate or needle valves separated with a cavity. The cavity is vented by another valve which can be opened; the pressure trapped between the two main valves is safely vented. In the industry, these double valves are also being used with a suitable quill for chemical injection.
Originally the process of double block and bleed was accomplished using individual valves and a spool between them. This arrangement, although workable, took up a great deal of space and the assembly was heavy. As more and more oil and gas exploration and extraction moved offshore, the need for more compact and lighter equipment became necessary. The result is the double block bleed valve, an integral device which includes two valves and a bleed in a much smaller and lighter package.
As the arrangement eliminates flanges and bolts from the line, it also eliminates leak paths that can go to atmosphere which in turn are potential hazards. The smaller sizes of the valve can easily reduce the weight by 60 percent and the time to install the valve by 70 percent. In an industry that values time, this reduction is significant.
A typical application for double block bleed valves is in the petroleum industry where a common pipe delivers different materials to different storage facilities. In many cases the same pipeline is used to deliver fuel oil and gasoline to their respective storage tanks. In the past, the common product pipeline was fitted with four standard gate valves and two bleed valves. When the fluid carried was fuel oil, the two gate valves and the bleed valve on the gasoline side were shut and of course the opposite was done when the fluid was fuel oil. With integral double bleed valves, the four independent valves and spool are eliminated in lieu of two compact valves with the bleed in a cavity within the valve body, the overall length of the assembly can be reduced by perhaps 75 percent.
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