Stroking the valve

Trip Valves The trip valve is part of a system that is used where a specific valve action (i.e., fail up, Fail down, or lock in last position) is required when pneumatic supply pressure to the control valve falls befow a preset level. Trip systems are used primarily on springless piston ac tuators requiring fail-open or fail-closed acrion. An air storage or Volume tank and a check v ve are used with the trip valve to provide power to stroke the valve when supply pressure is lost. Trip valves are designed with hysteresis around the trip point to avoid instabihty when the trip pressure and the reset pressure settings are too close to the same value. 

Also located in the control room is the manual-automatic switching hardware (or software). During start-up or under abnormal conditions, the plant operator may want to be able to set the position of the control valve himself instead of having the controller position it. A switch is usually provided on the control panel or in the computer system as sketched in Fig. 7.2. In the manual position the operator can stroke the valve by changing a knob (a pressure regula-... 

Take the case of a process dominated by dead time, settling out about 1 hr after a disturbance. If the sampling Interval were set for best performance, the control vtllvc would be repositioned only once an hour. And if the valve program called for a maximum Increment of 25 percent per sample, 4 hr would be needed tO fully stroke the valve. From the viewpoint that an Important function of automatic controls Is to react to an emergency condition within the plant, a 4-hr valve stroke Is Intoler able. For this reason, seldom will sample Intervals greater than a minute be encountered. 

Control valves have a tendency to stick. Even though the valve is shown to be moving on the panel screen, it may not be moving in the field. This is easy to check. The stem of the valve should be shiny and bright. This indicates that the stem has been traveling up and down recently. If the stem is covered with dirt, ask the panel operator to "stroke" the valve. This means he will fully open the valve and then fully close the valve. The valve stem has a position indicator. You can observe in the field if the valve opens and closes to its full extent. 

The expander inlet valves are modeled using the exaet Cg versus position eharaeteristies supplied by the valve vendor. The speeified valve stroking times ean be varied as part of the analysis. A eontroller positions the valves to hold eonstant regenerator pressure. 

The control valves have a full-stroke actuating time of 600 msec thus, in 6 msec the valves are actuated through 1 % of their full stroke. Only controllers with a TPET of less than 3 msec are able to meet the strict requirements of this application. 

Pump stroking but not pumping. Check the valves to see if they are seating properly. 

As the piston starts its suction stroke, the gas that remains in the cylinder in the fixed and added clearance areas expands until the pressure in the cylinder is equal to the pressure in the line outside of the cylinder. The greater the clearance, the longer it takes for the suction valves to open and the less new gas enters the cylinder. Therefore, less gas will be compressed as cylinder clearance is increased. 

In Figure 2 you can see that from position 1 to 2 the piston moves down the cylinder on the intake stroke as it is filled with turbocharger boost air higher than atmospheric pressure, as indicated in line PI. Depending on the valve timing, actual inlet valve closure will control the degree of trapped cylinder air... 

Newcomen engines continued to operate, especially in collieries, almost to the end of the eighteenth centuiy. Yet they, too, were wasteful. The way was open for a great advance, James Watt s invention in 1768 of the sepai atc condenser. Next to the driving cylinder stood another cylinder, exhausted and permanently cold. When the inoincnt for the down-stroke came, a valve opened, the steam rushed over and condensed the valve then closed and the cycle was repeated. The driving cylinder stayed permanently hot. 

Condition 1 Figure 12-12. Start of the compression stroke. The cylinder is full of gas at suction pressure and essentially suction temperature (neglecting valve loss). The piston moves during compression toward condition (2) with suction and discharge valves closed. 

This is the total volume remaining in the cylinder at the end of the piston stroke. This consists of the volume between the end of the piston and the cylinder head, in the valve ports and the volume in the suction valve guards and the discharge valve seats. See Figures 12-12,12-17A, and 12-17B. 

The wave form associated with the fundamental frequencies is primarily the result of the pulse produced by the stroke of the compressor piston, which is, in turn, modified by the action of the intake or discharge valve. In most cases the wave form is shaped by valve action and is partially modified by the characteristics of the piping downstream of the valve. The chief disturbing frequencies lie in the range of 4— 100 cycles/sec. 

The four-stroke engine requires four piston strokes (or two crankshaft revolutions) for each cycle. In a downward stroke, the intake valve is opened and the combustible mixture is brought into the cylinder. In an upward stroke the fuel-air mixture is compressed and ignited near the top of the stroke. This forces the piston downward (to provide power). In the next upward stroke the exhaust valve is opened and the spent gases are forced from the cylinder. In the following downward stroke, the cycle is repeated with the opening of the intake valve. 

During the discharge stroke, the inlet check valve is closed. Condensate draining from the steam space then fills the inlet piping. Unless the piping is sufficiently large or contains a receiver section (reservoir), condensate could back up into the steam space being drained. 

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