Flow reversal prevention

Check Valves These valves are used to prevent reversal of flow. They must be located where flow turbulence or instability does not resmt in chatter (high-frequency opening and closing of the valve) and in systems designed to prevent sudden high-velocity flow reversal which results in slamming upon closure. Many valve manufacturers can provide application aoMce. 

With the introduction of microreactors, transient reactor operations became interesting due to their low internal reactor volume and, thus, fast dynamic behavior. In 1999, Liauw et al. presented a periodically changing flow to prevent coke development on the catalyst and to remove inhibitory reactants in an IMM microchan-nel reactor [58]. This work was preceded by Emig in 1997, of the same group, who presented a fixed-bed reactor with periodically reversed flow [59]. In 2001, Rouge et al. [14] presented the catalytic dehydration of isopropanol in an IMM microreactor. 

Backflow—a flow-reversal, by which liquids get into places they do not belong—can be serious. Here is how to prevent it. 

The pressure differential should be of sufficient magnitude to ensure containment and prevention of flow reversal, but should not be so high as to create turbulence problems. 

For sizes larger than 2 inches, the swing-check valve is most often used. Chattering caused by frequent flow reversals can be stopped by using this type of valve equipped with an outside lever and weight. Checks are used in process pump piping after the discharge nozzles to prevent liquid backup in case of pump failure. 

The atmosphere Both the transfer of heat, the supply and removal of gaseous reactants and the nature of the reactions which occur, or are prevented, depend on the chemical nature of the atmosphere and its flow. Oxidations will occur well in oxygen, less so in air and not at all in argon. Product removal by a fairly rapid gas flow may prevent reverse reactions occurring. 

ABSTRACT In order to analyze the law of airflow catastrophic of side branches induced by gas pressure in upward ventilation after dynamic of outburst disappearing, one dimensional unsteady-state flow momentum equation of gas flow is established. Combined with circuit airflow pressure balance equation, this equation is used in static and dynamic analysis on airflow catastrophic of side branches induced by gas pressure in upward ventilation. The research results show that gas pressure is produced in upward ventilated roadway when gas flowed by when the gas pressure is great enough, the air flow in side branches reverses. Whether the air flow in side branches reverse is affected by their own length and initial velocity. In order to prevent the air flow reversal in the side branches, it is necessary keep the fan normal operating, and avoids adding resistance in external system. The research results may be of important theoretical and practical significance for outburst accident rescue as well as effective prevention of the occurrence of secondary accidents. 

In order to prevent the air flow reversal in the side branches, it is necessary to keep the fan normal operating, and avoid adding resistance in external system. 

Cylinder discharge lines should be equipped with approved check valves to prevent inadvertent contamination of cylinders that are connected to a closed system wh e the possibility of flow reversal exists. Backflow is particularly troublesome in the case of gases used as reactants in a closed system. A cylinder in such a system should be shut off and removed from the system while the pressure remaining in the cylinder is still greater than the pressure in the closed system. If there is a possibility that a cylinder has become contaminated, it should be so labeled and returned to the supplier. 

An alternative periodic reverse-flow process has been proposed that takes advantage of the NH3 storage capacity of the catalyst under unsteady conditions (140,141). In this process, the pellet catalyst is periodically saturated using a > 1. The feed is then switched to a < 1, but NH3 waves creep in the flow direction in a similar way to the propagation of heat waves mentioned above. The NH3 front is maintained within the catalyst bed by periodic flow reversal, thus preventing NH3 slippage. According to the experimental data reported in Reference... 

In the SIA system schematically represented in Figure 7.38 [127], for the determination of ammonium with conductometric detection, the sample and alkaline solution are sequentially aspirated into a single tubular channel and mixed by flow reversal. The formed ammonia permeates through a gas permeable membrane and is collected on a static acceptor solution. The change in conductance of the acceptor solution is related to the concentration of ammonium ions in the sample. The manifold includes an additional valve of the type used for sample injection in FIA that is actuated when required to form a closed loop in the acceptor channel and prevent the liquid from leaving it when the sample is propelled through the donor channel. The detection limit can be lowered from 1.5 to 0.1 mg/1 if preconcentration by successive sample flow reversal is used. 

Nozzle lna-zol n [dimin. of nose] (1683) In injection or transfer molding, the orifice-containing fitment at the delivery end of the injection cylinder or transfer chamber that contacts the mold s sprue bushing and conducts the softened resin into the mold. The nozzle is shaped to form a seal under pressure against the sprue bushing. Its orifice is tapered and sometimes contains a check valve to prevent flow reversal, or an on/off valve to interrupt the flow at any desired point in the molding cycle. 

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