Valve mode

Virtually all devices that use controllable MR fluids operate in a valvemode, direct-shear mode, or a combination of these two modes. Diagrams of the basic valve and direct-shear modes are shown in Fig. 6.76. Examples of valve-mode devices include dampers, and shock absorbers. Examples of direct shear-mode devices include clutches, brakes, chucking and locking devices, and some dampers. 

Fig. 6.76. Two modes of MR fluid operation a valve-mode, b direct-shear mode...

The pressure drop developed by a valve-mode device can be divided into two components, the pressure AP due to the fluid viscosity and APmr due to the magnetic field-induced yield. These pressures may be approximated by [131,157,158] ... 

An example of a simple valve-mode device is the RD-1005-3 linear damper by Lord Corporation shown in Fig. 6.77 [152]. As in the vast majority of all commercial MR fluid dampers, these dampers have an internal, axi-symmetric valve with an annular flow path. In this case the damper is a single-ended, mono-tube style having an internal rod volmne accumulator pressurized with nitrogen. As indicated in the Fig. 6.77, downward motion of the piston causes MR fluid to flow up through the annular flow channel. Application of current to the coil creates a magnetic field that interacts with the MR fluid in two regions where the magnetic flux crosses the flow channel. 

The range of force control that is possible with a valve-mode MR fluid damper is illustrated in Fig. 6.78. Here the force/velocity character that is typical of a passive hydraulic damper is compared to the range of forces possible with a MR damper. With appropriate control based on displacement, velocity or acceleration, any force profile between the upper and lower bounds can be realized. Unlike passive viscous dampers, with the MR damper it is easy to achieve large force at very low speed. 

In this expression, forces are in N (or torques in Nm), speed v in m/s (or rad/s) and 14iin in m . The constant a equals 1 for direct-shear devices, while for valve-mode devices it has a value of approximately 2. This approximation is valid for any MR flmd having that is on the order... 

For valve-mode devices the estimated miiurnmn active flmd volmne of... 

Retroactive Effects on the Power Electronics. The operating modes of electrorheological actuators are classified into three types flow mode (or valve mode), shear mode and squeeze mode. The former two modes do not exhibit any retroactive effects, whereas the squeeze mode can generate high voltages when the distance between the capacitor plates are altered quickly. When an actuator is operated in squeeze mode, extra precautions must be taken to protect the amplifier from damage. 

In the valve mode to control the flow rate of particulate solids... 

In the valve mode of operation, the solids flow rate through the nonmechanical device is controlled by the amount of aeration gas added to it. The most common types of nonmechanical valves are the L-valve and the J-valve. These devices are shown schematically in Fig. 19. The primary difference between these devices are their shapes and the directions in which they discharge solids. Both devices operate on the same principle. It is harder to fabricate a smooth 180-degree bend for a typical J-valve. Therefore the J-valve can be approximated and configured more simply by the geometry shown in Fig. 19C. 

A nonmechanical device operating in the valve mode is always located at the bottom of an underflow standpipe operating in moving packed bed flow. The standpipe is usually fed by a hopper, which can either be fluidized or nonfluidized. Knowlton and Hirsan (1978) and Knowlton et al. (1978) have shown that the operation of a nonmechanical valve is dependent upon the pressure balance and the geometry of the system. 

An EMEA table contains a series of columns for the equipment reference number, the name of the piece of equipment, a description of the equipment type, configuration, service characteristics, etc, which may impact the fadure modes and/or effects, and aflst of the fadure modes. Table 2 provides a Hst of representative fadure modes for valves, pumps, and heat exchangers. The last column of the EMEA table is reserved for a description of the immediate and ultimate effects of each of the fadure modes on other equipment and the system. 

In.strument air failure. The consequences of the loss of instrument air should be evaluated in coujuuc tiou with the failure mode of the control valve ac tuators. It should not be assumed that the correct air failure response will occur on these control valves, as some valves may stick in their last operating position. 

In controlled venting operation, the quench tank pressure is maintained at a desired level by a pressure controller/control valve system or pressure rehef valve. This mode of operation is used when the discharge mixture bubble point is close to or below the maximum ambient temperature, and it is desired to hmit the maximum quench tank pressure. 

Fig. 1.4. Miniature boiler fittings mode from brass a water-level gauge, a steam valve, a pressure gauge, and a feed-water injector. Brass is so easy to machine that it is good for intricate ports like these.

Installation of a shut-off valve in the expander exhaust line prevents baekflow during expander isolation (Figure 6-37). Although the need for this valve may not be immediately obvious, it ean be very bene-fieial if the expander is operated for an extended period in the windmilling mode. Often this type of operation is not antieipated during the initial evaluation of valve requirements, but beeomes neeessary at some later date. For example, it is useful when it is deemed prudent... 

The foregoing comparison of different valve arrangements for both full main air blower trains and TPG trains emphasizes its importance. The range of desired regenerator control, expected modes of operation, and system constraints all influence the choice of valve aiTangements. The selected arrangement depends on safety consciousness, cost considerations, and desired process flexibility. 

In die original system eonfiguration, die hot flue gas leaving die regenerator was expanded in die double slide valve and orifiee ehamber to atmospherie pressure, and dien passed via die waste heat boiler to die main staek. This mode of operation remains possible following die expander retrofit. 

To prevent surges, a well-trained operator would put the controller in manual mode and freeze the valve in an open position. This stops the control loop oscillations and decreases the compressor discharge resistance, thus breaking the surge cycle. Unfortunately, the operator has no way of knowing how much to open the valve and, subsequently, how much to close it. 

In most applieations, the valve should be designed so that in the event of an air supply loss, it defaults to the open position. This failure default mode will proteet the eompressor by allowing full reeyele. 

Check valves are required in the piping system at any point where backflow of gas after a shutdown has the ability to restart the compressor, running it backwards or, for that matter, even in the normal direction. Reverse rotation is totally bad, as many components of the various compressor types are not designed for reverse rotation, and there is some possibility, generally remote, that the compressor could reach a destructive over speed. Forward rotation is bad primarily because the intent was to stop the compressor, and it is now operating out of control. This is a problem, particularly if the shutdown was caused by a compressor failure indication, and the need to stop was to prevent further damage. In this mode, it is unlikely that the compressor can attain an overspeed condition. An application with a high potential for backflow is the parallel operation of two or more compressors. 

An important application of a rupture disc device is at the inlet of a pressure relief valve. The sizing of the pressure relief valve or rupture disc device combination requires that the pressure relief valve first be sized to meet the required relieving capacity. The normal size of the rupture disc device installed at the inlet of the pressure relief valve must be equal to or greater than the nominal size of the inlet connection of the valve to permit sufficient flow capacity and valve performance. The failure modes of rupture discs are [40] ... 

Remember that the failure position of a valve refers to its failure mode if there is a utility failure. A valve can mechanically fail in any position it is possible for a fail closed valve to get stuck in the open position. When doing a process hazard analysis it is important to consider all possible failure positions of a valve, and not only the failure position resulting from utility failure. 

Table 5.1-3 Motor-Operated Valve Failure Modes...

Failure of power or controls to the valve (generally related to the seismic capacity of the cable trays, control room, and emergency power). These failure modes are analyzed as failures of separate systems linked to the equipment since they are not related to the specific piece of equipment (i.e., a motor-operated valve) and are common to all active equipment. 

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