Multiple Valve Controls

For positive displacement pumps, a bypass-type control valve should be furnished to set the primary lube system pressure. The valve should be able to maintain system pressure during pump startup and pump transfers, which includes relieving the capacity of one pump, while both are running. The valve should provide stable, constant pressure during these transients. Flow turndown of 8 to 1 is not unusual. Multiple valves in parallel should be used if a single valve is not suitable. The valve should be sized to operate between 10 and 90% of the flow coefficient (Cv). Additional pressure control valves should be furnished as required to pro ide any of the intermediate pressure levels. 

Multiple valves have been constructed on a PDMS valve control layer (see Figure 3.27) [167]. Multilayer soft lithography has been used to generate the valve control layer (4 mm thick for strength) plus a fluid layer (40 pm thick) on PDMS. The small Young modulus ( 750 kPa) of PDMS allows a large deflection (1 mm) to be produced using a small actuation force ( lOOkPa on a 100- x 100-pm valve area). The response time is on the order of 1 ms. Round channels were... 

The Achilles heel of the AIMS system was the Redwood microvalves that were used for positive flow shut-off Failures generally occurred when the system was first powered up, which suggested that power transients were the cause of many of the problems. Specifically, these valves were thermally actuated, so opening one of the SOVs produced a large transient current draw of about one amp, but for less than % second. Adjustments were made in the LabVIEW control program to prevent the simultaneous actuation of multiple valves. In particular, the valves were not allowed to actuate when power was first applied to the chassis. 

The virtual test approach of the furnace was similar to the one used in Example 3 to treat multiple burners with partially premixed turbulent combustion of refinery gas. In actual furnace operation, fuel was adjusted using a valve controlled by the process fluid temperature while air was drawn into the furnace by an induced draft. The damper located at each burner was adjusted until the desired excess oxygen level at the furnace outlet was achieved. Except for the known fuel flow rate, the detailed distribution of fluid flow, temperature, turbulence, and mixture fraction at each burner needed to be defined via a single burner simulation. These profiles... 

The automated radiochemical process is performed in a single functional unit. The instrument design incorporates advanced digital fluid handling techniques with multiple zero dead volume digital syringe pumps and multiple valves for sample and reagent delivery. Comprehensive multithreaded control software was... 

Valves are another occasional source of problems in HVAC control systems. The most common valve failures are sticking, leakage, and failure to close completely. Leakage around the valve stem is easily identified and corrected by replacing the packing. If there is persistent leaking of multiple valves, it is possible that the water supply is dirty and causing premature failures. 

With valve control, the actual and target vacuum is adjusted by influencing the suction rate through valve technology. For this, one or multiple vacuum pumps are operated constantly at a fixed speed. 

Concepts with shared heat exchangers require controllable valves in hotter portions of the loop. In order to share a recuperator and cooler, for Instance, multiple valves are required to operate, as shown in Figure 5, to switch from an operational Brayton to a spare. The number of valves is further increased in this case based on the bearing and alternator cooling arrangement that draws cold gas from the compressor inlet. The hottest of these valves must operate at the turbine exit temperature ( 900 K). 

Interfaces to multiple supplier systems, ranging from tank gauging to loading and valve control systems, will be feasible via internationally accepted communication standards. 

The two principal elements of evaporator control are evaporation rate a.ndproduct concentration. Evaporation rate in single- and multiple-effect evaporators is usually achieved by steam-flow control. Conventional-control instrumentation is used (see Sec. 22), with the added precaution that pressure drop across meter and control valve, which reduces temperature difference available for heat transfer, not be excessive when maximum capacity is desired. Capacity control of thermocompression evaporators depends on the type of compressor positive-displacement compressors can utilize speed control or variations in operating pressure level. Centrifugal machines normally utihze adjustable inlet-guide vanes. Steam jets may have an adjustable spindle in the high-pressure orifice or be arranged as multiple jets that can individually be cut out of the system. 

Use multiple parallel instruments/control valves of different ranges for the for same service... 

Valves are classified by their intended use flow control, pressure control, and direction control. Some valves have multiple functions that fall into more than one classification. 

To maximize the unit s profit, one must operate the unit simultaneously against as many constraints as possible. Examples of these constraints are limits on the air blower, the wet gas compresst>r. reactor/regenerator temperatures, slide valve differentials, etc. The conventional regulatory controllers work only one loop at a time and they do not talk to one another. A skilled operator can push the unit against more than one constraint at a time, but the constraints change often. To operate closer to multiple constraints, a number of refiners have installed an advanced process control (APC) package either within their DCS or in a host computer. 

The qualitative assessment identified multiple opportunities for risk reduction in the feed purification and vaporization areas. These included providing gas detectors in the area, interlocked with automatic isolation valves. Upon detection of a gas release, the system would be shut down, significantly reducing the amount of hydrocarbon that could be released, thus reducing the likelihood of a damaging explosion. It was decided that, with the gas detectors and shutdown controls in place, the frequency of event Scenario 4 would be reduced from a "3" to a "2."... 

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