Room Pressure Control Systems

Variable air volume (VAV) laboratories are rapidly replacing traditional CAV laboratories as the design standard. These systems are based on fume hoods with face velocity controls. As the users operate the fume hoods, the exhaust volume from the laboratory changes and the supply air volume must adapt to maintain a volume balance and room pressure control. An experienced laboratory ventilation engineer must be consulted to design these systems, because the systems and controls are complex and must be designed, sized, and matched so they operate effectively together. 

In traditional exhaust systems, each fume hood has its own exhaust fan. This arrangement has the following disadvantages and advantages There is no way to dilute the fume hood effluent before release. The possibility of crosscontamination from one fume 

Certain types of fume hoods and exhaust sources, such as perchloric acid hoods, should not be manifolded with other types of fume hood exhausts. In large buildings where the designer wishes to take advantage of the benefits of manifolded exhaust systems but wishes to isolate a few exhaust streams, a combination, or hybrid, of these two types of systems is usually the most prudent and cost-effective alternative. 

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In the on-off device, the sensing element is connected to the spirally wound tube (O) and changes in pressure cause the tube to move such that a lever (P) moves the indicator (L), which indicates the temperature of the system being measured (about 150°F in the figure). Attached to indicator (L) is a contact (A). Placed either side of (L) are moveable arms, (M) and (N), which indicate the minimum and maximum temperature deviation before some corrective action is taken. These two arms are connected to (B) and (C) which also carry electrical contacts. Thus if the temperature drops to 125°F then the contact (A) (on L) makes contact with (B) and presumably results in some form of heat being supplied to the system. On-off systems result in limit cycling, in which the controlled quantity oscillates between the upper and lower limits (many room temperature control systems still work on this principle, and if the upper and lower limit are too far apart one is alternately too hot and then too cold). 

If the regulatoiy control system were perfect, the target could be set exactly equal to the constraint (that is, the target for the pressure controller could be set at the vessel rehef pressure). However, no regulatory control svstem is perfect. Therefore, the value specified for the target must te on the safe side of the constraint, thus giving the control system some elbow room. How much depends on the following ... 

A mismatch between operator procedures and the automatic control system of the reactor (see also Table 17) was the first active failure identified in this scenario. This precursor was still present mainly due to a shortage of people. Literally it was said that the pressure relief valve would open if the wrong value was inserted into the reactor s control system. The second precursor was the failure of the pressure relief valve (see also Table 17), which was not known to the responsible person who decided to ignore the difference between procedures and control system. The pressure relief valve failed, because resins stuck in the valve after it was used for the first time. Consequently the second time the valve was opened it was at a much higher pressure due to the build up of resins in the valve. If this second precursor had not been observed in time by damp on the pipes situated above the pressure relief valve or by the alarms in the control room a possible accident scenario existed. This was especially dangerous as the alarms in the control room are often ignored because of the high incidence of false alarms (see also Table 17), which was the third precursor present. 

The most utilized and reliable process control in the petrofeum and related industries is human observation and surveillance. Local pressure and level gages along with control room instrumentation are provided so that human observation and actions can occur to maintain the proper process conditions. First stage process alarms are provided to alert operators to conditions that they may not have already noticed. Typically when secondary alarm stages are reached, computer control systems employed to automatically implement remedial actions to the process. 

Are vaporizers provided with automatic gas line shutoff valve, downstream pressure-reducing valve, gas flow control valve, temperature control system and interlocks to shut down gas flow on low vaporizer temperature, and appropriate alarms in a continuously manned control room ... 

The original GC control system took the form of a central room which monitors the flowllne6, oil, water, and utility sections, plus a smaller satellite control room monitoring the gas compression and gas conditioning section of the plant. Closed loop process control, such as separator liquid level, pressure, flow and temperature control were handled by local pneumatic analog controllers. The key process variables are displayed in the control room via electronic instrumentation. All the key process and equipment trouble alarms are annunciated m the control rooms, plus the on/off status of key machinery and open/close status of key valves are displayed. 

A central location where instrument leads are short is preferred. In modem facilities with distributed control systems, all units are controlled from a central control room with few operators. Only a few roving operators are available to spot trouble. It is desirable to deep process equipment a minimum of 8 m away from the control room. Any equipment and hydrocarbon-containing equipment should be separated by at least 15 m if possible. Most control rooms are designed with blastproof construction and have emeigency backup power and air conditioning. The room is pressurized to prevent infusion of outside air that may have hydrocarbon content in the explosive range. 

At the end of the run, the product was discharged under N2 flow into a metal bomb which was kept at -73 °C. The product was warmed to room temperature and transferred to the atmospheric pressure weathering system which consisted of two BF3-scrubbers, a solenoid valve and a ten-liter gas collector which was equipped with an automatic pressure controller. Both the weathered liquid and the weathered gas were analyzed on a Scot Pak column. 

FIG. 11 -69 Typical central-station air-conditioning unit and control system. On a rising room wet-bulb temperature, the wet-bulb branch-line air pressure increases through the reverse-acting outdoor-air wet-bulb temperature-limit thermostat Ti to open gradually the maximum outdoor-air damper Di and simultaneously closr return-air damper Dg, then gradually open chilled-water valve Vi- On a rising room dry-bulb temperature, the dry-bulb branch-line air... 

HVAC system design influences architectural layouts with regard to items such as airlock positions, doorways and lobbies. The architectural components have an effect on room pressure differential cascades and cross-contamination control. The prevention of contamination and cross-contamination is an essential design consideration of the HVAC system. In view of these critical aspects, the design of the HVAC system should be considered at the concept design stage of a pharmaceutical manufacturing plant. 

As discussed above, it can be more efficient to have a single computer interfaced with several of these systems. Figure 11 shows a picture of the screen of a computer interfaced with three of these automated atmospheric pressure reactor systems. Each data point represents the time of introduction of the pulse of reactant gas. For room temperature reactions the standard low-pressure reactors shown in Figure 6 can be used with these systems. When temperature control is needed, jacketed versions of these reactors are used with the temperature maintained by a constant temperature recirculating bath. 

Recording elements These are used to provide a visual demonstration of how a chemical process behaves. Usually, the variables recorded are the variables that are directly measured as part of the control system. Various types of recorders (temperature, pressure, flow rate, composition, etc.) can be seen in the control room of a chemical plant, continuously monitoring the behavior of the process. The recent introduction of digital computers in the process control has also expanded the recording opportunities, through video display units (VDUs). 

Ohere are many miscellaneous pressure gauges In the control room vhlch Indicate the pressures at various points In the primary secondary and last ditch coolant systems. Included In these measurements are top of riser pressure crossheader pressures, top of downcomer pressure, control rod coolant pressures, and high tank level and pressure. 

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