Low-pressure system

As early as 1966, natural gas was available to all of the lower 48 states in the United States. During the period 1967—1990, the U.S. transmission system grew from 362,700 km to 450,800 km. Over this same time period, the distribution mains increased from 867,800 km to 1,347,000 km. As plastic pipe and reUable joining technology became available, the use of plastic pipe expanded to include the distribution of gas in low pressure systems. By 1990, approximately 24% of the U.S. distribution system was based on plastic pipe (1). 

The discovery of chemical N2 fixation under ambient conditions is more compatible with a simple, complementary, low temperature and low pressure system, possibly operated electrochemically and driven by a renewable energy resource (qv), such as solar, wind, or water power, or other off-peak electrical power, located near or in irrigation streams. Such systems might produce and apply ammonia continuously, eg, directly in the rice paddy, or store it as an increasingly concentrated ammoniacal solution for later appHcation. In fact, the Birkeland-Eyde process of N2 oxidation in an electric arc has been... 

Characterize the conditions typical of low-pressure systems, particularly as they relate to ventUation. 

The following table gives suggested maximum velocities in exchanger nozzles. The pressure drop through nozzles should be checked, especially where pressure losses are a problem such as in low pressure systems. 

In the low-pressure systems a shot of material is injected into the mould which, if it did not expand, would give a short shot. However, the expanding gas causes the polymer to fill the mould cavity. One important form of the low-pressure process is the Union Carbide process in which the polymer is fed to and melted in an extruder. It is blended with nitrogen which is fed directly into the extruder. The extruder then feeds the polymer melt into an accumulator which holds it under pressure (14-35 MPa) to prevent premature expansion until a predetermined shot builds up. When this has been obtained a valve opens and the accumulator plunger rams the melt into the mould. At this point the mould is only partially filled but the pressurised gas within the melt allows it to expand. 

High-pressure fluid flows into the low-pressure shell (or tube chaimel if the low-pressure fluid is on the tubeside). The low-pressure volume is represented by differential equations that determine the accumulation of high-pressure fluid within the shell or tube channel. The model determines the pressure inside the shell (or tube channel) based on the accumulation of high-pressure fluid and remaining low pressure fluid. The surrounding low-pressure system model simulates the flow/pressure relationship in the same manner used in water hammer analysis. Low-pressure fluid accumulation, fluid compressibility and pipe expansion are represented by pipe segment symbols. If a relief valve is present, the model must include the spring force and the disk mass inertia. 

Although venting to the atmosphere as described above is preferred, an alternative is to tie into a closed low pressure system, if available. This method may be used in the case of severely toxic fluids. Minimum length vent piping should be used. The effects of any back pressure must be thoroughly examined, since in such a case, superimposed back pressure is additive to the spring force. 

BWRs can more rc.ijily depressurize lo use low-pressure systems... 

The pump gas can be used to fuel the reboiler. The amount of pump gas is normally close to balancing the reboiler fuel gas requirements. The pump gas can also be routed to the facility fuel gas system or to a low-pressure system for compression and sales. If it is not recovered in one of these ways and is just vented locally, the cost of using this type of pump can be very high. 

When the relieving scenarios are defined, assume line sizes, and calculate pressure drop from the vent tip back to each relief valve to assure that the back-pressure is less than or equal to allowable for each scenario. The velocities in the relief piping should be limited to 500 ft/sec, on the high pressure system and 200 ft/sec on the low pressure system. Avoid sonic flow in the relief header because small calculation errors can lead to large pressure drop errors. Velocity at the vent or flare outlet should be between 500 ft/sec and MACH 1 to ensure good dispersion. Sonic velocity is acceptable at the vent tip and may be chosen to impose back-pressure on (he vent scrubber. 

Note that this control valve loss exceeds 60 percent of this system loss, since the valve must take the difference. For other systems where this is not the situation, the system loss must be so adjusted as to assign a value (see earlier section on control valves) of approximately 10 to 20 psi or 25 to 60 percent of the system other than friction losses through the valve. For very low pressure systems, this minimum value of control valve drop may be lowered at the sacrifice of sensitive control. 

Note that this pressure loss does not account for nozzle entrance or exit losses. These losses may be neglected provided velocities are low and no unusual conditions are imposed upon these connections. For low pressure systems, these losses cannot be ignored. 

This compares to assumed value of 0.001 + 0.001 = 0.002 The Kern method is usually easier to handle for pressure systems than for vacuum systems. The recirculation ratio is higher and, therefore, requires more trials to narrow-in on a reasonable value for the low pressure systems. The omission of two-phase flow in pressure drop analysis may be a serious problem in the low pressure system, because a ratio on the high side may result, causing a high hj value. In general, however, for systems from atmospheric pressure and above, the method usually gives conservative results when used within Kern s limitations. 

Thermocouple elements can be incorporated into the flame trap so that a flame that has lit back to the flame trap element and continued to burn there without being quenched can be detected. This sensor can be used to close an upstream safety shut-off valve. Flame traps will be the cause of a significant pressure drop for which allowances must be made in low-pressure systems. 

In plasma-assisted CVD, an electrostatically or electromagnetically induced plasma discharge is carried out in a low pressure system. The result is that the process may be operated at a considerably lower temperature. This has been employed in the deposition of SiOj and Si3N4 in the production of heat-sensitive microelectronic circuits. 

Mixed conditioning has also been used. In this, one part of the circuit (usually the low-pressure system) is dosed conventionally, e.g. with hydrazine/ammonia, whilst the high-pressure system is subjected to oxygenated conditions. 

Low absolute pressure calculations, 129 Low pressure system, 129 American Petroleum Institute, 399 American Society of Mechanical Engineers, 399 API Codes, 399 API oil field separators, 239 API, heat absorbed from fire, 451-453 Babcock steam formula, 103, 107, 108 Back pressure, 404 Effect of, 407, 408 Baffles, lank mixing, 311 Diagrams, 330 Bag filters/separators, 270 Bag materials, 274 Cleaning, 272, 273 Heavy dust loads, 271 Specifications, 271 Temperature range, 271 Bins, silos, hoppers venting, 516 Blast pressure, 496 Blowdown, 404... 

Three regions of the atmosphere are seen to have significant zonal components of flow and thus of advection. The mid-latitude troposphere at the surface tends to exhibit westerly flow (i.e., flow from west to east) on the average. This region contains the familiar high- and low-pressure systems that cause periodicity in mid-latitude weather. Depending on the lifetime of the substances of concern, the motion in these weather systems may be important. 

This describes the geostrophic wind f- 2co sin 4>, where oj is the angular velocity due to the rotation of the Earth and 0 is the latitude). The air moves parallel to the isobars (lines of constant pressure). The geostrophic wind blows counterclockwise around low-pressure systems in the northern hemisphere, clockwise in the southern. 

Fig. 7-5 Surface pressure map (millibars). Fronts are shown by heavy lines. H = high-pressure system L = low-pressure system. Wind directions are shown by arrows wind speeds correspond to the number of bars on the arrow tails.

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