Pressure and flow control

Under a large slug condition, the condensate level can go very high. This situation causes severe liquid entrainment into the gas phase. It is a practice to reduce the inlet flow rate when the light liquid level increases substantially at high liquid level, the level control overrides the pressure control (through a low select UY) and starts reducing the inlet flow. 

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Overall, the technical complexity of the Deans switch system is considerably greater than that of a mechanical switching valve and it is accepted that reliability and ease of use is reduced as the system complexity increases. For many compound types, however, the completely non-intrusive nature of the Deans method offers sufficient advantages to justify its application. However, the use of modern electronic pressure and flow controls integrated into the overall computer control of the chromatographic system does now make the operation of Deans switches significantly easier or more reliable than has been reported in its earlier applications. 

To achieve the flow and pressure precision needed for the more demanding HPLC applications, a number of proprietary electropneumatic pressure and flow control loops have been created to ensure repeatable pump performance. 

These include solvent reservoirs and inlet filters, solvent degassing facilities and one or more pumps with associated pressure and flow controls. Most systems are microprocessor or computer controlled enabl.ing parameters to... 

In most practical combustion installations there are two separate parts of the equipment system (1) the burner itself and (2) all of the peripheral equipment necessary to control the burner operation efficiently and safely. The control equipment includes fuel and oxidant pressure and flow controls automatic shutoff controls flame supervision equipment furnace purge equipment and other related devices. The safety issues of the burner are significantly different in character as compared with the safety issues of the control equipment. In many ways, these two safety issues are diametrically opposed. Within the burner, fire is a desired condition, whereas within the control system and surrounding environment, fire is to be avoided entirely. 

Inspect the operation and settings of the pressure and flow control valves... 

In many industrial control problems, notably those involving temperature, pressure, and flow control, measurements of the controlled variable are available, and the manipulated variable is adjusted via a control valve. In feedback control, corrective action is taken regardless of the source of the disturbance. Its chief drawback is that no corrective action is taken until after the controlled variable deviates from the set point. Feedback control may also result in undesirable oscillations in the controlled variable if the controller is not tuned properly, that is, if the adjustable controller parameters are not set at appropriate values. The tuning of the controller can be aided by using a mathematical model of the dynamic process, although an experienced control engineer can use trial-and-error tuning to achieve satisfactory performance in many cases. Next, we discuss the two types of controllers used in most commercial applications. 

Retention-time locking (RTL) is a unique feature available in some GC equipment which has revolutionized gas chromatography by reproducing retention times within hundredths of a minute from one instrument to another. This capability is possible thanks to highly reliable electronic pneumatic control (EPC), (pressure and flow control), good temperature control and high reproducibility of capillary columns. The result is ultimately retention time stability between any GC systems with EPC. 

Some specific design guidance is given for pressure and flow control, gas holder control, burner control, fire and gas detection and process shutdown systems. 

The mobile phase is an inert gas, generally nitrogen or helium, supplied from a cylinder via pressure and flow controls, and passing through purification cartridges before entering the column. 

There are, however, rules of thumb for selecting control valves and matching inherent valve characteristics to common process control loops or processes where the valve pressure drop is fairly constant. The equal percentage characteristic is the most common and is used where variations in pressure drop are expected or in systems where a small percentage of the total system pressure drop is taken across the valve, such as in pressure and flow control. More detailed recommendations are available from control valve vendors, e.g. [15,16]. 

Gas Chromatograph (GC), capable of operating at the conditions given in Table 1, and having a column switching and backflushing system equivalent to Fig. 1. Carrier gas pressure and flow control devices shall be capable... 

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