Control valves plugged

When pumping dirty water through a small ported control valve (1 inch or less), the valve may easily plug. I can back-flush the valve to blow the dirt out (refer to Fig. 38.5)  

Open drain valve A. (Perhaps routing the drain via a hose to the flare or enclosed sewer might be safer.) 

Water will now flow backward through the control valve. Especially since there will be little or no back-pressure at valve A, I have found this to be a highly effective method to clear small plugged control valves. 

The majority of the process vessels you see in your plant are gravity, vapor-liquid separators. Their main purpose is to settle out droplets of entrained liquid from the up-flowing gas. Factors that affect the settling rate of these droplets are 

All these ideas have been put into one equation, called Stokes law. Nothing against Sir Frederick Stokes, but vapor viscosities are almost always so small that they do not affect settling rates. Also, we never know the particle size distribution of the droplets. There is a more fruitful way to look at the settling tendency of droplets of liquid in an up-flowing vapor stream, as shown in Fig. 39.1. The method states 

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A weigh tank containing chlorosulfonic acid needed to be cleaned to remove salt deposits. The salt deposits precipitated from the material and occasionally plugged the downstream control valve. Since the material was water reactive, heptane was chosen to clean the vessel. Chemists had not anticipated the material would be reactive with heptane. While cleaning the vessel the pressure... 

The control valve is assumed to be fitted with the maximum size plug and seat for its valve body size. 

There are many configurations of plugs and seats ( trim ) that create different control responses. The design specification of control valves is beyond the scope of this book. 

Regular washer-type faucets are hard to control for low water flow, such as required by condensers. For these purposes, needle valves are recommended. They give excellent control, and flow rate does not change as long as the pressure remains constant. The inexpensive plug-type valves are rarely adequate for laboratory water service. 

Inexpensive plug-type valves may be use for hook-ups to appliances that have their own gas flow adjustment. For those that do not, needle valves are recommended for precise control. 

The interface with the process at the other end of the control loop is made by the final control element. In a vast majority of chemical engineering processes the final control element is an automatic control valve which throttles the flow of a manipulated variable. Most control valves consist of a plug on the end of a stem that opens or closes an orifice opening as the stem is raised or lowered. As sketched in Fig. 7.5, the stem is attached to a diaphragm that is driven by changing air pressure above the diaphragm. The force of the air pressure is opposed by a spring. 

The gas plant that was destroyed had been initially commissioned in 1969. It had not undergone a mechanical inspection for six years. Shutdowns were initially scheduled at three-year intervals, but this interval was extended to five years. The plant had not been retrofitted with modern instrumentation. Pen recorders were prevalent in the control room and operating problems had been encountered with several control valves. Three months before the incident, ice plugging in lines caused several process upsets that were not fully investigated. A defective bypass valve had leaked gas to the atmosphere and had been awaiting maintenance up until one day before the accident. 

The most successful method consists of a system of spray nozzles in the oil flow line positioned to that water pumped through the nonla enters the oil in very flne droplets. Immediately following the spray injector is a differential control valve that provides suitable plug and seat surfaces for mechanical shearing along... 

It is positively my experience that the most common reason for pumps cavitation is partial plugging of draw nozzles. This problem is illustrated in Fig. 25.5. This is the side draw-off from a fractionator. Slowly opening the pump s discharge control valve increases flow up to a point. Beyond this point, the pump s discharge pressure and discharge flow become erratically low. It is obvious, then, that the pump is cavitating. 

There are many different types of valve fitted with actuators to form control valves. Valves may be single- or double-ported (Fig. 7.123). With single-ported valves the valve plug is subjected to the total differential force across the valve. Such valves are sensitive to pressure fluctuations and powerful actuator elements are required for large pressure drops. Double-ported valves balance out the pressure differential but it is difficult to obtain complete shut-off. 

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... 

Plug of control valve Once a week Tallow... 

Screw plugs of top bearing Nuts of control valve Ball bearing puller Worm and bowl spindle Ball bearing puller Bowl spindle... 

FIG. 8-66 Post-guided contour plug globe valve with metal seat and raised-face flange end connections. (Courtesy Fisher Controls International LLC.)... 

Class VI in the unbalanced sliding-stem control valve, in turn, is achieved by using the same soft seat required for Class V in the balanced plug valve. The seat load decreases drastically to the same value required for the soft seal in the balanced valve. However, the unbalanced forces may require a significantly larger actuator. 

Now, we shall present an inexpensive means to meter small flows of gas. Accurately controlling and measuring such flowrates is often difficult. Control valves with small trims coupled to differential-pressure transmitters having small orifices (or long capillaries) are prone to plugging and calibration troubles. On the other hand, most mass flowmeters are expensive for small-scale uses. 

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