题目:Control Valve
Control Valve

This valve controls the expulsion of oil from the spring side of the second gear band servo piston at speeds in the region of 60 km/h. The time period for oil to exhaust then depends upon the governor pressure varying the effective exhaust port restriction. Line pressure oil from the spring side of the second gear band servo piston passes through a passage leading to the 3–2 kickdown valve annular groove and from there to the 2–3 shift valve annular groove. Here some oil exhausts out from a fixed restriction while the remainder passes via a passage to the 3–2 control valve. As the vehicle speed approaches 60 km/h the governor pressure rises sufficiently to force back the 3–2 control valve piston, thus causing the wasted (reduced diameter) part of the control valve to complete the exhaustion of oil.

Valves control the gas flowing into and out of the engine cylinder. The camshaft and valve spring make up the mechanism that lifts and closes the valves. The valve train determines the performance characteristics of four-stroke-cycle engines.

There are two types of valve, inlet and exhaust. Figure 6.1 shows an exhaust valve. An inlet valve has a similar form. The commonly used poppet valve1 is mushroom-shaped. Figure 6.2 illustrates the parts of the valve. A cotter (not shown in Fig. 6.2) which fixes the valve spring retainer to the valve, is inserted into the cotter groove.Alumina valves and seats

corrosion resistant control valve come in many forms: butterfly valve, ball and seat valve, disk-valve, piston-sleeve metering valve, and dart valve, to name but a few. Alumina has been used in many industrial valves. Water faucet valves of the standard disk-on-disk configuration are very common and are discussed in Section 12.2.8. Since they share almost all the same features of pump rotary valves.

Dart valve plugs and seats are a fluid-flow-control component. When used in the mineral processing industry, or in other industries where slurries, or corrosive liquids, or corrosive slurries are flowing, these valve/plug systems need to be highly wear resistant, especially the plug which can be particularly exposed to the flow of the erosive/corrosive fluids. An example of a dart valve and plug is shown in Fig. 12.17. Alumina valves are an increasingly common technology in general.

One revolution of the camshaft gives the amount of valve lift shown in Fig. 6.3. The valve stem moves in the valve guide and also revolves slowly around the stem. The revolving torque is generated by the expansion and contraction of the valve spring.

An engine basically needs one inlet valve and one exhaust valve per cylinder but most modern engines use four valves per cylinder. This multi-valve configuration raises power output, because the increased inlet area gives a higher volume of gas flow. Contemporary five-valve engines use three inlet valves and two exhaust valves to increase trapping efficiency at medium revolutions.

Figure 6.4 summarizes the functions of the valve. The shape of the neck, from the crown to the valve stem, ensures that the gas runs smoothly. The valve typically receives an acceleration of 2000 m/s2 under high temperatures. Valves must be of light weight to allow the rapid reciprocating motion.

With the single seated control valve lowered, the hydraulic pump is applied to bring the bottom plate of the mould to the lower limit. The separator is then lowered into the mould and fed with the shell and the inner core materials. The vibrator is switched on for 5 s to consolidate the content. The space created by consolidation is topped up. The vibrator is switched on again while the separator is extracted from the moulds. The top of the content of mould is flattened, and the mould lid closed and clamped. With the single seated balanced control valve raised, the hydraulic pump is engaged to stress the content to the desired compaction pressure, which was readable on the gauge. The mould lid is opened and with the control valve raised, the block is ejected from the mould.

Typical specimens of hollow SCEB produced with the mechanical kit are shown in Fig. 13.8. The two holes reduced the overall weight of block by 24%. It is also anticipated that the hollowed nature of the block will accommodate any expansion of the inner core material.

Active or passive valves control the flow of samples and reagent through the different steps. Passive valves are able to control fluid movement in a limited way, for example, by allowing flow in one direction through a channel but not in the other one as described above On the other hand, active valves need to be actuated externally using a smart control strategy that typically makes use of sensors to have feedback (Schumacher et al., 2012). Actuation of these valves is very often performed by electrical means, for example, by having a current flow through a copper line and then heating a chamber filled with air that expands and deflects a flexible membrane, which closes a microfluidic channel. Sometimes the deformation of such a membrane is directly performed by using pressurized air coming from an external source, making the valve actuation purely pneumatic instead of electrothermal.

The open tank and multi hole single seated control valve arrangement (Fig. 4) used here together with the 3% cavitation criterion in Fig. 2 is considered to be an industry-based and reliable method for determination of NPSHR in the pump best efficiency region, ISO [4]. More elaborate closed vacuum tank arrangements are used by pump manufacturers to establish NPSHR-curves for water. The measured NPSHR-values obtained here for water (Fig. 5) were about 10% larger than values from the GIW-pump curves. This means that the slurry NPSHR-results in Fig. 5 were about 1.5 times the water values from the pump curves. The scatter may represent the increased cavitation intensity of flow disturbances in an open tank system when compared to a closed tank arrangement.

Experimental closed tank results for sands with average particle sizes of 0.18 and 0.5 mm in pumps with impeller diameters of 0.35 and 0.6 m, respectively, were reported by Herbich [5]. Slurry densities were up to about 1400 kg/m3. It was found that the NPSHR-values (expressed in m of slurry) were similar to the water values, independent of the slurry density. Similar results were also reported by Herbich [5] and Ladouani et al. [6] for non-settling clay-silt slurries with densities of up to 1300 kg/m3 in pumps with impeller diametres less than 0.275 m. Ladouani et al. [6] used an open-tank loop arrangement. Detailed inspection of their data indicates that the independence of the slurry density on NPSHR was limited to flow rates smaller than about 70% of the best efficiency point (BEP). With larger flow rates, NPSHR increased with increasing slurry densities, giving values from 1 to about 2 times the water values in the BEP-region.

The results obtained here were for flow rates close to BEP. Field NPSHR results agreed reasonably well with the laboratory data for the same type of pump pumping phosphate (Fig. 1) at about 500 rpm for flow rates of about 75% of BEP, Addie et al. [7]. In practice, it is therefore reasonable to assume that the laboratory NPSHR-results obtained here are applicable for the flow rate region where most slurry pumps operate today (0.75 to 1.0 of BEP).

Balancing (Fig. 12.15(a and b)) With the compressed air passing to the brake actuator chambers, air pressure is built up beneath the upper and lower pistons. Eventually the upthrust created by this air pressure equals the downward spring force; the pistons and valve carrier lift and the inlet valves close, thus interrupting the compressed air supply to the brake actuators. At the same time, the exhaust valves remain closed. The valves are then in a balanced condition with equal force above an below the upper piston and with equal air pressure being held in both halves of the brake line circuits.

Pushing the treadle down still further applies an additional force on top of the graduating spring. There will be a corresponding increase in the air pressure delivered and a new point of balance will be reached.

Removing some of the effort on the foot treadle reduces the force on top of the graduating spring. The pistons and valve carrier will then lift due to the air pressure and piston return springs. When this occurs the inlet valves remain closed and the exhaust valves open to exhausting air pressure from the brake actuators until a state of balance is obtained at lower pressure.

Releasing brakes (Fig. 12.15(b)) Removing the driver's force from the treadle allows the upper and lower piston and the valve carrier to rise to the highest position. This initially causes the inlet/exhaust valves to close their inlet seats, but with further upward movement of the pistons and valve assembly both exhaust valves open. Air from both brake circuits will therefore quickly escape to the atmosphere thus fully releasing the brakes.

It’s difficult to obtain the flow of sequential valves due to lack of measurement data. The main steam is separated through the four valves and then enter into corresponding group of nozzles. The only flow data we can get from measurement is the main steam. Although there are various formulas to calculate the theoretical flow of valves, flow characteristics of valves are required to obtain the actual flow. However, the flow characteristics of sequential valves are not available through experiment. So a calculation method rely on operation data is necessary.


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