Modulating valve

Design arrangements for reduced-temperature secondary systems (sometimes referred to as injection circuits) include fixed provisions for minimum dilution rates. Conventional system-balancing devices with three-port automatic modulating valves to regulate mixed water temperatures and, hence, heat output are used. Automatic safety controls must prevent excessive temperatures occurring in the coil circuits, as floor fabrics or finishes could be damaged very rapidly. 

When using modulating valves for steam, heat emitter output must be based on the steam pressure downstream of the valve, which often has a high-pressure drop across it, even when fully open. 

Use a modulating FW system and modulating valves at steam-use points where possible... 

The graph in Figure 5.37 shows the product loss comparison for a spring valve, a pop/snap action pilot valve and a modulating valve. It is obvious that the product gain of a modulating valve is important (Figure 5.38). 

Operating pressures above 95% of set Preferably use modulating POSRVs or soft-seated valves. The choice for the more expensive modulating pilots depends on system pressure fluctuations. The more fluctuation dose to or over set pressure, the more a modulating valve should be considered. 

The symbol for the control valve in Fig. 8-49 is for a pneumatic modulating valve without a valve positioner. 

The conventional gas burner was connected in series with the heat exchanger (Fig. 2). The gas burner then could assist with initial startup and could supplement the heat exchanger if it became necessary. Both the modulating valve and the gas burner were regulated by the same temperature controller. The incinerator heat exchanger was the primary heat source, and the gas burner was the secondary source. The control system automatically turns off the gas burner when the incinerator heat is sufficient to satisfy the demand. 

Temperature is less easy to control precisely. The temperature in the hold period in autoclaves is generally maintained by modulating valves which open to allow steam entry when the temperature (or pressure, because these valves are more often than not controlled through pressure transducers) begins to drop toward the critical lower limit of the specification. It is generally not possible to control an autoclave to run through a complete hold period at the lower limit of its temperature specification. However, even quite apparently trivial errors in temperature above or below the limit can make significant differences in the amount of lethality delivered. For instance, at a nominal temperature of 121°C, an error of 1 K can increase or decrease the amount of lethality by 25%. 

Engineers select and evaluate process equipment. The selected equipment must be technically sound, rehable, safe and easy to operate, serviceable, economically attractive and environmentally sound, e.g. minimal noise pollution and no mechanical and fire hazards. For example, all pneumatic valves, except for the modulating valves, are air-to-open/ spring-to-close valve type for safe shutdown unless the valve is a divert drain valve as... 

The EFWS can be used to supply feedwater to the steam generators for normal plant cool downs. To accomplish this, the pumps are manually started and the flow rates manually controlled by positioning the flow modulating valves. When the reactor coolant hot-leg temperature is reduced to 350°F (176.7°C) and the RHRS has been activated, the EFWS is secured. The EFWS is not normally used for plant cool down during shutdown because other design provisions contained in the main feedwater system should be used to supply the steam generators in this case. 

The flow-rate of S03/air is controlled by modulating valves linked to a flow sensor, normally an orifice plate. Depending on the required S03-in-air concentration for the sulphonation of different feedstocks, it is necessary to dilute the SO3 from the gas-raising plant with ed process air. The central system for dilution air again employs a flow sensor and modulating valve. 

Solenoid valves are used to play important role in cases of emergency shutdown and are used for actuations. In this connection. Fig. VII/1.1-3 or IX/5.1.3-1 may be referenced. Now for all such cases, there will be fail safe condition of these valves, viz. fail open or fail close and fail lock (last position—mainly for modulating valves). In fail lock condition, regardless of the valve s natural failsafe state, the system makes the valve to lock positioner air inside the valve actuator as shown. However, on account of leakage, this position cannot be held permanently over very long period. Various safe failure conditions of the control valves have been depicted in Fig. EX/... 

Steam condenser, on vacuum side 13 Vacuum modulating valve... 

Modulating valves were not required for the SIFs considered in this example. 

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