Widely Open Position

Results of studies to determine the effect of stator vane control on acceleration are shown in Figures 4-67 and 4-68. The first control step shown in Figure 4-67 is to have the stator vanes normally on flow control with a 2 sec lag in the flow control. On reaching trip, the stator vanes drive to the wide open position after a 1 sec lag. The vanes move at design speed. 

The trip operates at approximately 4.25 see, but no effeet is seen until the 1 see lag in the trip aetuation of the vanes has passed. Thus, it is after the 1 see lag that the vanes start driving to the wide open position at the design aetuation rate. The result is a sharp inerease in air flow and blower horsepower. Note that this flow is going into the regenerator sinee no SNORK valve has been ineluded in the ealeu-lations. The aeeeleration reaehes a peak at about 6.5 see and blower horsepower peaks at about 8.25 see, with the vanes fully open at 8.5 see. Sometime over 30 see the horsepower of the blower and expander will equalize. If no other aetion results they will stabilize. 

Control Valve - A control valve with a limit stop to restrict the maximum opening is not normally acceptable as a means of limiting the capacity of a pressurizing path, since the stop may later be removed or the valve changed. Credit for the limiting capacity of a control valve in the wide open position may be taken only if all of the following conditions apply ... 

If there is a bypass around the control valve, downstream equipment must be protected so that its pressure would not exceed the 1.5 Times Design Pressure Rule, considering that the control valve is in the wide open position, and the bypass 50% open. 

Figure 10-13 illustrates the flow versus valve stem travel characteristic for various typical valve trim functions (Fisher Controls, 1987). The quick opening characteristic provides the maximum change in flow rate at low opening or stem travel, with a fairly linear relationship. As the valve approaches the wide open position, the change in flow with travel approaches zero. This is best suited for on-off control but is... 

Where the safety device consists of an additional regulator that is associated with or functions in combination with one or more regulators in a series arrangement to control or limit the pressure in a piping system, suitable checks shall be made. These checks shall be conducted to determine that the equipment will operate in a satisfactory manner to prevent any pressure in excess of the established MAOP of the system, should any one of the associated regulators malfunction or remain in the wide-open position. 

A. ACTION OF THE VALVE. Valves are designed to either fail in the wide-open position or completely shut. Which action is appropriate depends on the effect of the manipulated variable on the process. For example, if the valve is handling steam or fuel, you would want the flow to be cut off in an emergency, i.e., you want the valve to fail shut. If the valve is handling cooling water to a reactor, you... 

Safety shutdown valves, which are normally wide open and operate infrequently, are expected to respond to a safety trip command reliably and without fault. To achieve the level of reliability required in this application, the safety valve must be periodically tested to ensure positive operation under safety trip conditions. To test the operation of the shutdown system without disturbing the process, the traditional method is to physically lock the valve stem in the wide-open position and then to electrically operate the pneumatic shutdown solenoid valve. Observing that the pneumatic solenoid valve has properly vented the actuator pressure to zero, the actuator is seen as capable of applying sufficient spring force to close the valve, and a positive safety valve test is indicated. The... 

Greg Shinskey1 presents a practical discussion of some of the advantages and problems of this control structure. The use of a valve position control (VPC) structure is recommended. A reactor temperature controller sets the coolant valve position. Then a VPC looks at the position of the coolant valve and adjusts the flowrate of the reactor feed to keep the coolant valve near its wide open position. 

If valve V4 starts to approach the wide-open position, the valve on the spill air line (V5) should be opened enough to keep V4 about 90 percent open. 

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