Double block and bleed system

A special double block and bleed system, shown in Figure 12-3, is added to every feed line to a reactor. During normal operating conditions, the block valves are open and the bleed line is closed. When the feed pump is shut off, the block valves are closed and the bleed line is open. 

Figure 12-3 is a relatively simple example of a particularly important application of double block and bleed systems. These systems are also commonly used for reactive intermediates and analyzer systems — anywhere a positive break in a line is desired. 

FIGURE 12.14. Automatic double-block-and-bleed system. 

A double block and bleed system is widely used in the process industries. It uses two closed block valves with a bleed valve between them. The bleed valve can be used to check for leakage through the root isolation valve and/or to route leakage to a safe containment system. The pressure in the section of pipe between the two block valves should be zero. 

Double Block and Bleed Systems Position Proofing Systems... 

Utilize double block and bleed pipe and valving system... 

For a period of time just after startup, the piping upstream of this compressor operated under a vacuum. This upstream piping was equipped with a set of double block and bleed valves to a scrubber that circulated diluted caustic soda. During this unusual startup configuration, moisture was sucked into the system and the wet chlorine completely dissolved the stainless steel ball within the 3-inch (7.6 cm) quarter-turn valves. 

A good oxy/fuel ladle preheater incorporates several safety features. These include double block and bleed, flame supervision, low and high flow interlocks on oxygen and natural gas, a high-temperature interlock and manual reset (see Chapters 9 and 10). The entire system must conform to current NFPA (National Fire Protection Association) safety standards. 

A similar technique, more common with air systems, is to automate the familiar double-block-and-bleed arrangement. The first step in this design might be to replace a simple check valve with a differential-pressure control valve that shuts when the pressure differential reverses. This affords two levels of protection, unless one assumes that the check valve is fundamentally untrustworthy and always allows somebackleakage. Check valves can, of course, be made more reliable by proper installation and by extra measures such as force loading. An elaboration on the diagram above is to add a second valve in the line that closes when the differential pressure is too low or even negative. Finally, the space between the two in-line check valves can be vented for more positive protection. 

FIGURE 37.9 Double block and bleed concept to protect the process system from stream unwanted mixing due to single-fault failure (black = closed valve, white = open valve). 

Double block and bleed isolation can be achieved through the use of any two appropriately located valves in the system, even though the valves may also serve other functions. Actuated isolation valves can be used in double block and bleed arrangements if means are provided to defeat automatic operation of the valves. Single body, integral double block and bleed valves can also be used, particularly in space-restricted areas. 

The key difference between this system and the double block and bleed is that the line connecting the hazardous chemicals to the place of work has a section removed. Therefore there is no way that the chemicals can transit directly to the workers location. The bleed valve is left open so that the workers know if the closed root block valve has started leaking. It can be attached directly to the valve as shown, or it can be attached to the Une leaving the block valve. 

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