Double blocks

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

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. 

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. 

Krek, W and Nigg, E. A. (1991b). Mutations of p34c 2 phosphorylation sites induce premature mitotic events in HeLa cells evience for a double block to p34c 2 kinase activation in vertebrates. EMBO J. 10 3331-3341. 

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.

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 two blocks of metal, one at a high temperature and the other at a low temp-eramre Tj [Fig. 6.10(a)] are separated by a perfect heat insulator, and the system as a whole is surrounded by a thermal blanket that permits no transfer of heat in or out, then no change in internal energy can occur with time. If the insulator between the blocks is removed, the hotter block will decrdease in temperature and the cooler one will increase in temperature until the uniform temperamre T2 is reached. This transformation is spontaneous. At/ is zero. But a loss of capacity to perform work has occurred. In the initial state, we could insert a thermocouple lead in the block at Ts and another in that at Tj and obtain electrical work. Insertion of thermocouple leads in the same positions in the double block at T2 cannot generate any work. At the conclusion of the spontaneous transformation without the thermocouple, the internal energy is stiU the same as that at the outset, but it is no longer in a condition where it has the capacity to do work. 

CCSD/EOMCCSD equations defined by Eqs. (58) and (59). These moments are easy to calculate. As implied by Eqs. (58) and (59), their determination requires the explicit consideration of the triples-reference, triples-singles, and triples-doubles blocks of the matrix representing the CCSD/EOMCCSD similarity-transformed Hamiltonian Eq. (13). 

All these methods succeed in accumulating 80-90% of the cells at the Gl/S-interphase and are very suitable for obtaining populations of S-phase cells for study. They are readily scaled up to the level of roller bottles though at this scale the amount of thymidine required for a double block is considerable (0.5 g/roller bottle). Concentrated solutions of thymidine can be made up and sterilised by autoclaving but most of the other solutions should be filter sterilised. 

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. 

As in the earlier sections, bold quantities indicate matrices or vectors representing the corresponding operators and subscripts SS, SD, DS, and DD designate the singles-singles, singles-doubles, doubles-singles, and doubles-doubles blocks of the transformed Hamiltonian matrix. 

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. 

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). 

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