Control Flow Direction

Valves are integral components in piping systems they are the primary method of controlling the flow, pressure, and direction of the fluid. Valves may be required to 

In addition to the low or no dead volume feature, valves should also be fully drainable and of fast actuation especially if they are used for the gradient formation. In the past, there were Gemii, Saunders, and ITT nowadays new vendors offer designs that feature improved chromatographic performance that is, NOVA-SEPTIC, Furon, Robolux (Burkert), and Swagelok. 

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Compatibility with Personnel Expectations Compatibility refers to the degree of similarity between the direction of physical movement of a control or an instrument indicator and the worker s expectations. Many errors are due to the fact that the operation of the controls or the layout of the displays is incompatible with population stereotypes. For instance, on a control panel it is customary to increase the value of a parameter by turning the appropriate switch clockwise and reduce its value by turning it coimterclockwise. (Note that this stereotype is the opposite for controls which control flow directly, e.g., valves.) If such a stereotype is violated, errors may occur. Although such errors may be recoverable in the short run, under the stress of a process transient they may lead to serious consequences. 

As mentioned, the expanded-bed operation requires two pumps one for pumping the feedstock, the wash, and the elution solutions on the column, and the other for lowering the flow adaptor. To control flow direction, four-way valves and two-way valves are needed. Manual and automated control systems are available for operating an expanded-bed column. 

A check valve (see Figure 5-5) is a type of automatic valve designed to control flow direction and prevent possibie contamination or damage to equipment. The check valve will prevent backflow as long as the device is operating properly. Check valves come in a variety of designs and applications. Typicai designs include ... 

Pinch The industrial equivalent of controlling flow bv pinching a soda straw is the pinch valve, Mves of this tvpe use fabric-reinforced elastomer sleeves that completely isolate the process fluid from the metal parts in the valve. The valve is actuated bv applying air pressure directly to the outside of the sleeve, causing it to contract or pinch. Another method is to pinch the sleeve with a linear actuator with a specially attached foot. Pinch valves are used extensively for corrosive material service and erosive sliirrv senice. This type of valve is used in applications with pressure drops up to 10 bar (145 psi),... 

Column Bottom Temperature. The bottom temperature is often controlled on the reboiler outlet line with a control valve in the heating medium line. The control point can also be on a bottom section tray. Care must be exercised in location of the temperature control point. It is recommended, especially for large columns, that a cascade arrangement be used. The recommended scheme has a complete flow recorder/controller (FRC) in the heating medium line including orifice and control valve. The set point of this FRC is manipulated by the temperature recorder/controller (TRC). This eliminates the TRC from manipulating the control valve directly (recall that temperature is the most difficult parameter to control). This makes for smoother control for normal operations. Also, it is handy for startup to be able to uncouple the TRC and run the reboiler on FRC for a period. 

The firing controls that best ensure an air-rich mixture are often referred to as metering type controls, because gas flow and air flow are metered, thus the fuel-air ratio is controlled. The fuel-air ratio is the most important factor for safe, economical firing, so it is better to control it directly. Do not settle for low budget controllers that... 

Auxiliary air cupboards have many problems, most of which have been reported in the literature. A main problem that does not receive much attention is coordination of the flow into the opening controlled by the exhaust with the supply flow directed down immediately above the opening. This includes the complex and simultaneous relationships between velocities, flow rates, flow widths, flow directions, flow stability, turbulence, and temperatures. To this should be added the same problems that exist for normal fume cupboards, such as necessary exhaust flow rate and velocity, flow pattern inside the cupboard, working procedures, and the influence of people on the flow pattern outside and into the opening. The auxiliary air outlet may also be a source of noise. 

In air ducts, the measurement of the local air vekxiity is used to determine the flow rate in the duct. The duct flow is usually more stable and the flow direction under better control than in the room space. Different types of disturbances in the ductwork, such as bends, tees, or dampers, will influence the nature of the flow and cause swirl and other problems in velocity measurement. 

Piping, manifolds, switching valves and controls to direct and control the flow of gases according to the process requirements. 

Valves have two main functions in a pipeline to control the amount of flow, or to stop the flow completely. There are many different types but the most commonly used are the gate valve and the globe valve. The gate valve contains a disk that slides at right angles to the flow direction. This type of valve is used primarily for on-off control of a liquid flow. Because small lateral adjustments of the disk cause e.xtreme changes in the flow cross-sectional area, this type of valve is not suitable for adjusting flow rates. 

Valves are classified by their intended use flow control, pressure control, and direction control. Some valves have multiple functions that fall into more than one classification. 

Figure 7-4 Overall material balance control in direction opposite to flow.

If a solution forms part of an electrochemical cell, the potential of the cell, the current flowing through it and its resistance are all determined by the chemical composition of the solution. Quantitative and qualitative information can thus be obtained by measuring one or more of these electrical properties under controlled conditions. Direct measurements can be made in which sample solutions are compared with standards alternatively, the changes in an electrical property during the course of a titration can be followed to enable the equivalence point to be detected. Before considering the individual electrochemical techniques, some fundamental aspects of electrochemistry will be summarized in this section. 

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