Valve sequencing

In certain processes (such as rubber tire manufacture and plastic injection molding operations), it is common for a valve sequencing system to first provide hot steam followed by cold cooling water and thus for cross-contamination to occur. The result may be the stripping of passivated cooling system surfaces by steam and for the condensate to be infiltrated by cooling water. 

Foulkes et al. (1988) have approached the synthesis of operating procedures from a more empirical angle. They have extended the work of Rivas and Rudd (1974) for the synthesis of complex pump and valve sequencing operations, relying on the use of logical propositions (implemented as rule-based expert systems), which capture the various types of constraints imposed on the states of a processing system. 

FIGURE 5.13 Real-time display of valve sequencing. Figure courtesy of Kroungold Analytical (2007). 

Fig. 7.1S Schematic layout of mercury detection system showing valve sequencing for collection, flush and measurement.

Service flow rate Operating pressure Operating temperature Bed pressure drop Backwash flow rate Rinse flow rate Valve sequencing... 

The filters are washed to remove media fines prior to initial start-up as well as during the normal service run when required, as described in Section 4.1. The wash cycle consists of backwash, bed setde and rinse steps. The operating conditions and valve sequencing are given in Section 4.2. It is required that the system is shut down whenever there is a leakage. 

Review valve sequencing for the RO unit given in Table 4.3. 

According to Kentfield (1993), a pulse combustor is a combustion-driven device with self-aspirating feature, and this effect is achieved as a consequence of the internal unsteady flow events. In contrast, a pulsed combustor is a device with cyclic but nonresonant combustion as dictated by wave events. Pulsed combustors usually operate at a much lower than natural frequency, often controlled by an ignition, fuel injection, or a valve sequence. Therefore, valveless or flapper valve combustors fall into category of pulse combustors while mechanically driven valves (e.g., rotary valve) used to control either air or fuel inflow, flue gas outflow, or both should be categorized as pulsed combustors, unless the operation of a mechanical valve is controlled by resonant phenomena in a feedback mode. Such a design is known as a frequency-tunable pulse combustor. 

Although intrinsically contained, the complexity could lead to operator error and automation of the valve sequencing could be beneficial. Jefferis and Schlager applied a quantitative risk assessment technique based on a fault tree analysis to a similar bioreactor sampling valve assembly. Their analysis illustrated the benefits of automation. 

NOTE Typical examples of devices with FPL are smart sensor (for example, pressure transmitter), smart valve, sequence of events controller, dedicated smart alarm box, small data logging systems. 

Application software design and development Architecture To create a software architecture that fulfils the specified requirements for software safety To review and evaluate the requirements placed on the software by the hardware architecture of the SIS 12.4.3 SIS application software safety requirements specification SIS hardware architecture design manuals Description of the architecture design, for example, segregation of application S/W into related process subsystem and SIL(s), for example, recognition of common application S/W modules such as pump or valve sequences... 

A control system has been developed which incorporates valve sequencing for wide range along with compensation for the nonlinear curve. It features a small equal-percentage valve driven by a proportional pH controller. The output of the pH controller also operates a large linear valve through a proportional-plus-reset controller with a dead zone. The system is shown in Fig. 10.15. 

Forepump (vacuum technology) A vacuum pump used to keep the discharge pressure of a high vacuum pump below some critical value. The forepump may also be used as a roughing pump by proper valve sequencing. Also called a Backing pump. See also Roughing pump. 

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