Dead leg

Dead legs in the sample line must be discharged safely to ensure that the sample will ac tuaUy be representative of the material in the unit. Without blowing down the dead leg, samples taken will be erroneous, as they may be representative of some past operating conditions. If the location is nonroutine, the sample leg may have accumulated debris. The debris could partially or totally block the line. Opening the isolation valve to blow down the line could result in a sudden, uncontrolled release, presenting a hazard to the sample gatherer. 

Environment Near the end of a dead leg. Dispersant, tolyltriazole, silicate 15-25 ppm residual, bromine/chlorine, 1 hr/day... 

Chemical cleaning was considered since small-diameter pipe had been plugged in dead-leg regions. At the time of writing, no decision had been made concerning cleaning. 

Deposition caused by settling of suspended particulate may be reduced by increasing flow. Dead legs, stagnant areas, and other low-flow... 

The inlet was moved to a new position leaving the relief valve on a dead leg."... 

One method of operating a non-chlorinated supply safely is to ensure that the line runs constantly to waste, as do very old drinking fountains. This avoids biological growth that can accumulate in stagnant water. Dead legs in the piping system are always undesirable, especially in such cases. 

As mentioned in Chapter 3, DI water has now replaced distilled water for most laboratory purposes. PVC pipe, which is both inexpensive and easy to install, is normally employed to carry the water to points of usage. It is very important that the lines do not have dead legs where water is allowed to stand, since this could encourage bacterial growth. In cases where sterile water is needed, special steps must be taken. The water may be boiled, which also drives out dissolved gases, or ultraviolet radiation may be employed, a common method in the cosmetics industry. Membrane filters will also effectively remove bacteria. Laboratories needing sterile water often use two bacteria removal systems of... 

Linden, NJ, USA, Refinery, Explosion/Fire Failure of a dead leg piping released vapors 17,500,000... 

Solids handling features have been enhanced by elimination of dead legs use of hydrocyclones to manage solids in the feed preparation steps and in the streams passing to anolyte and catholyte sections of each cell for both agent and energetics... 

No dead leg >6 of the internal diameter of the unused pipe (measured from the axis of the pipe in use)... 

Temporary circulation pumps are often necessary, as permanently installed pumps may be unsuitable or unable to provide the velocity necessary for flushing and cleaning work. Typically the water velocity necessary is 6 to 9 ft/sec (2 to 3 m/sec). Once in motion, debris velocity is virtually independent of particle size, but is clearly a function of water velocity. Relatively high velocities are needed to lift particles up from horizontal pipe surfaces and dirt pockets (dirt collection points at the base of riser pipes), and especially to overcome problems of particle retardation when moving from the horizontal through a vertically upward bend. Special provisions may need to be made to ensure the cleaning of dead-leg areas,... 

When the chiller is bypassed, all pumps should continue to operate to prevent the buildup of biofilms and general contamination in dead-legs and low-flow areas. Conductivity control systems should be employed to maintain COC at 2.0x. Trying to maintain higher levels than this may be fraught with problems in winter and may be impossible to reach in summer. 

Typically, cooling systems that are to be decommissioned or shut down for a period of time should be stored dry. If this is not possible or if there are dead-legs or other water traps, the cooling water should be deconcentrated and conditions adjusted so that the LSI of the water is approximately +0.5. A conventional, multifunctional chemical inhibitor with a strong anticorrosive action should be used, typically at 3 to 5x the normal dose. The objective is to prevent the onset of corrosion. 

In-line instruments or sensors are necessary components for automated processes. For ease of cleaning, sensors should be chosen that directly mount onto vessel nozzles or piping tees with minimum dead leg distances. Also, the instru-... 

Instrumentation will include field instruments used in the manufacturing process and other instruments assoeiated with special tasks, such as that of monitoring laboratory or computer room environmental eonditions. The accessibility of instruments must be such as to permit their eleaning and maintenanee. Siting is also important, and instruments should be installed as close to the point of measurement as possible. The placement of flowmeters in piping dead-legs should be avoided. Careful consideration should also be given to the appropriate position of other instruments sueh as thermometers and thermocouples so that they, too, can fulfill their measurement and eontrol functions. Construction materials that come into direct contact with the pharmaceutical or healthcare production process stream must not contaminate or affect the manufactured product in any way. Instrument lubricants and coolants must not come into contact with in-process product or equipment. The reliability of instruments should also be considered for instance, a pressure transmitter that uses atmospheric pressure as its reference may suffer from poor reliability. A draft calibration schedule may also be prepared. 

The water circulation pump must be of sanitary design and may be equipped either with a single or a double mechanical seal. In the latter case, the purified water from the pump s discharge is used as a seal flush fluid (Fig. 5). If a spare circulation pump is installed for reliability, it is important to ensure that it does not constitute a long dead leg. This can be avoided either by valving it off and draining it free of water... 

Stagnation of water during low or zero production demand is avoided at all costs so as not to promote bacteriological or biological contamination of either circuit. Therefore plants are designed to recirculate water in the polishing loop at least three times the maximum take off rate (m h ) at velocities of 2-3 ms in delivery pipework free of dead legs . 

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