Block and Bleed

This system prevents the reactor contents from siphoning back into the monomer storage vessel, even if the block valves leak. This prevents an unexpected chemical reaction in the storage tank. 

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

The valve was operated by compressed air, and the two air hoses, one to open the valve and one to close it, were connected up the wrong way around. The two connectors should have been different in size or design so that this could not occur. In addition, they were not disconnected, and a lockout device on the valve—a mechanical stop—had been removed. It is also bad practice to carry out work on equipment isolated from hot flammable gas under pressure by a single isolation valve. The take-off branch should have been slip-plated, and double block and bleed valves should have been provided so the slip-plate could be inserted safely (Figure 1-1), [16, 17]. 

The correct way to light a furnace (hot or cold) that bums gas or burns light oil is to start with a positive isolation, such as a slip-plate, in the fuel line. Other positive isolations are disconnected hoses, lutes filled with water (if the fuel is gas at low pressure), and double block and bleed valves closed valves without a bleed are not sufficient. Then ... 

A service that is used intermittently should be connected to process equipment by a hose, which is disconnected when not in use, or by double block and bleed valves. If a hose is used, it should be provided with a vent so that it can be depressured before it is disconnected. 

A boiler had been shut down for the repair of a forced draft fan. A blind was not installed in the fuel gas line, nor apparently was a double block and bleed in the fuel line utilized. Gas leaked into the firebox during the repair period and was not removed. A severe explosion occurred during the attempt to light of. 

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 12-3 A double block and bleed arrangement used to prevent reactant from entering reactor vessel.

Pressure gauges shall have block and bleed valves. 

Pressure control valve Check vale Block and bleed valve... 

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 %-in. connection is provided on the pump discharge, upstream of the block and check valves, and it is connected to the nearest flare header to vent fuel gas and/or hydrocarbon vapor during commissioning. A double block and bleed are provided on the discharge to avoid problems of freezing in cryogenic service. 

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. 

FIGURE 5.11-2 Schemes Tor control of columa pressure (a) pressure control for na atmospheric column (vent bleed to atmosphere) (b) split ranga valves in a block and bleed arrangement (vent bleed to vacuum) . (c) hot vapor bypass pressure control and (d) flooded condeaser pressure control. 

Confirm that the furnace fuel lines are completely isolated from the furnace (either by disconnection, blind flange, or a double block and bleed valve assembly). 

Reestablish the main burner fuel supply (either by cormecting the main fuel line, removing a blind flange, or reversing fhe double block and bleed valve assembly) to provide a fuel source to the furnace. 

A Double Block and Bleed means two block valves with a bleed valve located between them. Bleed-off valving for pressure instrumentation on hot lines is provided with a double block and bleed valve. These valves and the bleed piping for hot services should not be less than 1/2 in. or 3/4 in. in size. They are arranged so that a rod may be driven up the valve and fitting. Many refiners still believe in the tried-and-true method of using double block and bleed valves for bubble-tight service. 

Examples of troublesome reverse flow into columns that were shut down are in Sec. 11.4 (also see item 14 in Sec. 11.6). Blinding, double block and bleed, and line disconnection are used to prevent backflow where lines are operated intermittently or when a piece of equipment is shut down. Check valves (double check valves when high reliability is required), good piping and operating practices, trips, and alarms are commonly used for prevention of undesirable backflow in continuously operating lines. 

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

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