Control valves: action, characteristics

There are several aspects of control valves their action, characteristics, and... 

When processes are subject only to slow and small perturbations, conventional feedback PID controllers usually are adequate with set points and instrument characteristics fine-tuned in the field. As an example, two modes of control of a heat exchange process are shown in Figure 3.8 where the objective is to maintain constant outlet temperature by exchanging process heat with a heat transfer medium. Part (a) has a feedback controller which goes into action when a deviation from the preset temperature occurs and attempts to restore the set point. Inevitably some oscillation of the outlet temperature will be generated that will persist for some time and may never die down if perturbations of the inlet condition occur often enough. In the operation of the feedforward control of part (b), the flow rate and temperature of the process input are continually signalled to a computer which then finds the flow rate of heat transfer medium required to maintain constant process outlet temperature and adjusts the flow control valve appropriately. Temperature oscillation amplitude and duration will be much less in this mode. 

A slightly more automated method would be to feed the water to each rotameter from a pressure-controlled header system (Fig. 11.46). The pressure would be measured with a diaphragm seal connected to a transmitter with a filled capillaiy. llie control valve would be a fail-open globe or V-ball valve with equal percentage characteristic. The controller would be reverse-acting with proportional and integral action. This arrangement allows... 

The loss of turbine load is a t5 ical pressurization event The turbine bypass is not credited. The ADS is initiated at 5 s by the ATWS signal of the turbine cmitrol valve quickly closed and reactor power ATWS permissive for 5 s. The calculation results are shown in Fig. 6.49. The pressure increases due to the closure of the turbine control valves. As described in Sect. 6.7.1.3, the inherent characteristics of the Super LWR design make the reactivity insertion and the power increase very small. The peak power is only 104% of the initial value. When the SRVs open, the pressure begins to decrease. After initiating the ADS as the alternative action, the pressure, power, and cladding temperature decrease. The increase in the cladding temperature is about 50°C and the peak pressure is about 26.8 MPa. They are exactly the same as those obtained in the abnormal transient analysis with a reactor scram (see Sect. 6.7.1.3). 

We know that the temperature control of the shower in a bathroom is not so easy. To have a comfortable shower, a hot water valve needs to be manipulated carefully (control action) however, we usually rely on a trial and error action until the proper temperature of a shower is achieved. How can we achieve the most comfortable shower temperature more quickly This is a common problem in the control action of many processes that are controlled by a feedback loop. The difficulty comes from a delay in the response, which naturally exists in any process - in other words, the dynamic characteristics of a process. Therefore, the control action should be determined based on the dynamics of the process. In particular, some bioprocesses are known to have serious delays in response. 

An important issue to be considered at an early stage is whether there are any common oause failures between redundant parts within each layer (for example, between 2 pressure relief valves on the same vessel), between safety layers or between safety layers and the BPCS. An example of this could be where failure of a basic process control system measurement could oause a demand on the safety instrumented system and a device with the same characteristics is used within the safety instrumented system. In such cases it will be necessary to establish if there are oredible failure modes that could cause failure of both devices at the same time. Where a common cause of failure is identified then the following actions can be taken. 

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