Double-block valve

Connections to equipment are typically 50 mm and 80 mm for process vessels and exchangers, according to the size of the equipment. Each connection includes an accessible block valve. Double block valves are provided if required. A check valve should be included if overpressure or other hazard could result from reverse flow during simultaneous drainage from more than one vessel. Individual connections from the equipment are made into the top of the drain header. 

Before this particular start-up, a double block valve arrangement was installed in the bottom discharge line. It was not realized, however, that the second valve created a dead space between the two block valves and that water was trapped between them. 

If a tight shut-off is necessary for maintenance and repair, double-block valves are used. A bleed valve is installed between the block valve to break down the pressure and for leakage detection. 

In those conflgurations shown above requiring a vent line, it is almost always necessary to install one or more block valves so as to help set the vent gas flow and for isolation purposes. If the solids are erosive, one must limit the velocity within the vent line to acceptable levels (generally under 30 m/s) so as not to erode the vent line piping. Vent line elbows, valves and any flow restriction orifice that may be present are especially vulnerable to erosive attack. Special erosion resistant, double-block valves may be necessary under highly erosive conditions. 

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. 

The OSHA investigation13 found that (1) no process hazard analysis had been performed in the polyethylene plant, and as a result, many serious safety deficiencies were ignored or overlooked (2) the single-block (DEMCO) valve on the settling leg was not designed to fail to a safe closed position when the air failed (3) rather than relying on a single-block valve, a double-... 

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. 

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. 

Are remote-operated isolation valves now needed Are double-block-... 

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. 

The only application in which a throttling control valve must shut off tightly is when it is required to do double duty as a remote-controlled block valve. There are a number of instances where this is a legitimate requirement. For example, there are turbine bypass valves that need to conserve energy when closed, but open quickly and throttle when the turbine is tripped off the line. 

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. 

Utility stations and permanent connections to services as utility air, water, steam, nitrogen, etc., should be equipped with double block and bleeds and check valves (if there is a remote opportunity for reverse flow) [28]. 

Does the design provide for isolation by valve blocking, spools, double blocks and bleeds, flanges, and flushing connections ... 

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