Valve dependency

In some cases, power-operated block valves may be substituted for the classic check valve. However, these valves depend on an outside motive... 

Safety Relief Valve - A safety relief valve is an automatic pressure-relieving device suitable for use as either a safety valve or relief valve, depending on application. (In the petroleum industry it is normally used in gas and vapor service or for liquid.) Safety relief valves are classified as "Conventional" or "Balanced", depending upon the effect of back pressure on their performance. 

Valve Arrangement of Auxiliary Feedwater System and Main Steam System during Shutdown - the auxiliary feedwater system has six MO Vs in the flow path to the steam generators and the main steam nonreturn valves are normally closed during shutdown. These valves depend on offsite power and are difficult to manually open. 

B. SIZE. Sizing of control valves is one of the more controversial subjects in process control. The flow rate through a control valve depends on the size of the valve, the pressure drop over the valve, the stem position, and the fluid properties. The design equation for liquids (nonflashing) is... 

The inventory of contained liquid and gas can be calculated between isolation points to determine the amount of material that can be released for pro-cess-related scenarios. The time frame to close manual valves—depending on accessibility, operating conditions, and availability of personnel—is typically 10-30 minutes. For remotely operated valves actuated from the control room, 60-90 seconds is used for isolation. The release rate will decrease based on the pressure decay of the system once the system has been isolated. 

In the electronics indnstry, a Pentinm compnter chip has hundreds of millions of transistors and only a hundred pins in and out. If each transistor had to be addressed individually, it would be impossible to have such a chip. Microfluidic systems are similarly easy to control n fluid lines can be controlled by 21og n control hues. Additionally, the pressure to actuate a valve depends on the width of the control line, so by choosing our pressure carefully, one thinner fluid line can be closed while the wider fluid lines remain open due to insufficient pressure. This idea has been used to develop microfluidic systems that can screen enzymatic libraries and perform in vitro transcription translation of DNA to protein in approximately 30 minutes. 

Safety relief valve (SRV ) Spring-loaded PRV that may be used as either a safety or a relief valve, depending on the application. The SRV works within well-determined operational limits (per PED it is called safety valve). 

Crystal growth was Initiated by diffusion of the salt solutions toward the membrane by opening the two valves using exterior valve handles. Crystals typically appear within 30 sec to 120 sec after opening the valves depending on the Iodide concentrations. Iodide concentrations were chosen to cause crystallization only on the lead(ll) side of the membrane so that photographs would be facilitated. Crystal growth was allowed to proceed for at least 40 hours. 

As with the single check valve, the effectiveness of double check valves depends entirely on their sealing integrity. Should the seals develop leaks, undetected backflow could occur. For this reason, double check valves should only be used where backflow could be objectionable—but not hazardous—and should never be used where backflow could cause a potential health hazard or other serious condition. 

Nonreturn valves are used to prevent backflow of fluid in a process line. They do not normally give an absolute shut-off of the reverse flow. A typical design is shown in Figure 5.9g. Since swing-type check valves depend on gravity to close the valve, care must be taken to orient the valve properly when locating and installing it. 

Figure 3.34 Different conductivity across a spin valve depending on the spin alignment of the ferromagnetic electrodes. An example conducting molecular material, Alq3, is also shown ...

The control valve adjusts the flow rate by changing the size and shape of the flow area offered to the fluid passing through it. There are two main categories of control valve used on process plant globe valves and rotary valves. The flow area in a globe valve is dictated by the position of the valve plug relative to its seat, while the flow area in a rotary valve depends on the... 

The positions of the flow restrictors inside both globe and rotary valves depend on the action of a piston actuator, either directly or through a gear mechanism. As noted in Section 7.2, the stroke of this piston is called the valve travel . We have already given this the symbol, x, with the convention that... 

In a microprocessor-controlled ventilator (Figure 18.8), the electronically actuated valves open from a closed position to allow the flow of blended gases to the patient. The control of flow through each valve depends on the therapist s specification for the mandatory breath. That is, the clinician must specify the following parameters for the delivery of volume-controlled mandatory ventilation breaths (1) respiration rate (2) flow waveform (3) tidal volume (4) oxygen concentration (of the delivered breath) (5) peak flow and (6) PEEP, as shown in the lower left side of Figure 18.8. It is noted that the PEEP selected by the therapist in the mandatory mode is only used for the control of exhalation flow this... 

Some safeguard devices are routinely tested. For example, most plants will bench test the set pressures of their relief valves every 2 years, at least. However, even this program has two potential weaknesses. First, the performance of a relief valve depends not only on its set pressure but also on its capacity, i.e., how quickly it can remove vapors once it is open. It is uncommon for rehef valve capacity to be checked, in spite of its importance. Second, if the rehef valve fails between tests, there is no way of knowing this. (Some safeguards, particularly electronic shutdown systems can test themselves, but mechanical equipment cannot.)... 

Several cautions apply to ratings of valves. The user should discuss this situation with the manufacturer. Nickel valves may have pressure ratings that are low for then-nominal design class. The pressure rating of diaphragm valves depends on size and material of construction. Fluoropolymer-lined plug valves are satisfactory only under certain conditions. 

Selection of Control Valves Depends on the Following Considerations (A typical example is given below for fluid flow control)... 

A pressure/time system usually consists of a reservoir, a dispense valve and a controller. The type of dispense valve depends on whether it needs to be hand-held, stationary or advancing, and if there are standard or high precision requirements. 

Comment A safety relief valve is an automatic pressure-actuated relieving device suitable for use either as a safety valve or a relief valve, depending on the application. Safety relief valves are designed in accordance with ASME Boiler and Pressure Vessel Code Section VIII or Section III for nuclear applications. These valves are general-purpose rehev-ing devices suitable for varied apphcations on compressible and noncompressible fluids. These valves are most commonly used in refineries, oil and gas production facilities, pipelines, paper mills, chemical plants, and other industrial facihties (Fig. 5.190). The following descriptions are for design variations of safety relief valves for specific apphcations. 

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