Control valves operation, closed position

Why did the control board show the control valve was closed when it must have actually been open It turns out that there is no way for the control room operator to get confirmation that the valve has actually closed after he commands it closed. The valve was not equipped with a valve stem position monitor, so the control room operator only knows that a signal has gone to the valve for it to close but not whether it has actually done so. The operators in many accidents, including Three Mile Island, have been confused about the actual position of valves due to similar designs. 

Control valves only work properly within a certain range of positions. When a control valve is mostly dosed (perhaps 5 to 10 percent open), opening it a little bit more will increase flow a lot. When a control valve is mostly open (perhaps 80 to 90 percent), opening it a lot more will hardly increase the flow at all. To summarize, a control valve can only respond to a change in position in a linear fashion, between 20 to 80 percent open. To avoid the control valve operating in the non-linear, mostly closed position, the Soviet instrument engineer installed a small control valve that has a giant AP at 50 percent open. 

In order to hold a cylinder upon which external forces are being applied firmly in position, it is not enough just to block the working connections of the directional valve. Leakage in this operational position leads to a decrease in the load. In order to avoid this, unlocking non-retum valves are used. With these valves, the closed position can be overridden by the control of the valve cone. Throughflow which was blocked in one direction is thus permitted. Both types of non-retum valves are illustrated in Figure 5.13. 

The expander is now ready to be put into operation. Open the compressor inlet valve and expander discharge valve simultaneously. Then open the compressor discharge valve. Make sure the variable nozzles are in the closed position. Push the start button on the control panel to open the inlet shutdown valve. 

Remote Depressuring - A pilot operated valve is sufficiently positive in action to be used as a depressuring device. By using a hand valve, a control valve or a solenoid valve to exhaust the piston chamber, one can open the pilot-operated valve and close it at pressures below its set point from any remote location, without affecting its operation as a pressure relief valve. 

In one plant, the operating procedures required that valve A should be placed into the "manual closed position." The process worker misinterpreted this information and instead of placing the valve controller in the manual position, he closed the block valve manually and deprived the plant of an essential feed. 

The catalyst preparation area is positioned between the two polyethylene production units with 60 feet separating each one. The aluminum alkyls storage canopy and isopentane horizontal storage tank are located at a remote area at an approximate distance of 250 feet away from the production and utility areas. The isopentane is transported to the catalyst preparation area through a 3-inch pipeline. A remote actuated isolation valve on this supply line that fails closed is located at the isopentane storage tank. This control valve and an associated isopentane feed pump are managed by the operator in the control room. 

Figure 17.4 shows a centrifugal pump, driven by a steam turbine. The correct operating speed for the pump and turbine is that speed that puts the process-control valve in a mostly open, but still controllable, position. As we slow the turbine to force open the process-control valve, the turbine s governor valve will close. Steam flow to the turbine will decline in accordance with fan laws ... 

When the control valve downstream of a pump is operating in a mostly closed position, the upstream pump is a good candidate to have its impeller trimmed. Sometimes, the pressure drop across a control valve is so huge (>100 psi) that it makes a roaring sound. The energy represented by this wasteful AP is coming from the electricity supplied to the pump s motor. Hence, trimming the impeller also reduces wear because of erosion in downstream control valves. 

Bowl does not close although control valve is turned to position 3 Check 1. That tank contains water 2. That cock in conduit from tank is open 3. Control valve 310-1 4. Paring disc device 310-2 5. Control means of bowl Nozzle C15 in bowl body. Plugs C21 in operating slide 300-4... 

Section I of the ASME code for fired boilers requires that the PSVs reach their full lift at a pressure not greater than 3% over their set point. Section VIII of the ASME code for unfired vessels does not provide a blowdown requirement, and the industrial practice is about 7%, which means that the normal operating pressure must be under 93% of set pressure. The position of the adjustable ring on the PSV nozzle controls the blowdown. This position establishes a secondary orifice area as the valve opens and closes. 

The following are the pre-startup procedures (a) Check operation of the chlorine pressure reducing valve (PRV) by turning ON the power switch on the sludge oxidation unit control panel. Turn the chlorine valve switch to the OPEN position, observe operation of valve actuator (b) turn switch to the CLOSED position and (c) adjust time delay relays according to the manufacturer s instruction manual. 

Fig. 14 Schematic of a pilot-operated valve in the closed position. (Reproduced by kind permission of Tyco Valves Controls.) (View this art in...

As an example, consider a real accident report on a chemical overflow from a tank, which injured several workers in the vicinity [118]. The control room operator issued an instruction to open a valve to start the flow of liquid into the tank. The flow meter did not indicate a flow, so the control room operator asked an outside operator to check the manual valves near the tank to see if they were closed. The control room operator believed that the valves were normally left in an open position to facilitate conducting the operation remotely. The tank level at this time was 7.2 feet. 

The outside operator checked and found the manual valves at the tank open. The outside operator also saw no indication of flow on the flow meter and made an effort to visually verify that there was no flow. He then began to open and close the valves manually to try to fix the problem. He reported to the control room operator that he heard a clunk that may have cleared an obstruction, and the control room operator tried opening the valve remotely again. Both operators stUl saw no flow on the flow meter. The outside operator at this time got a call to deal with a problem in a different part of the plant and left. He did not make another attempt to visually verify if there was flow. The control room operator left the valve in the closed position. In retrospect, it appears that the tank level at this time was approximately 77 feet. 

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