Control valves plugging

Highly viscous liquid can cause the control valve plug to get stuck up or will not move properly—thus delaying control action. More force will be required here to operate the valve and hence choose the actuator accordingly. 

Three basic forms of nonlinear elements are commonly encountered. First, there is the continuous nonlinear function, such as a pH curve or the characteristic of a control-valve plug. Second is the discontinuous function, typical of saturating types of control elements. Third is the dynamic nonlinearity, whose phase shift and gain vary with signal amplitude, as contrasted to linear dynamic elements, whose phase and gain vary with period. Devices exhibiting hysteresis are members of this category. 

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. 

The effect of a controlled device may not be proportional to its movement. In particular, the shape of valve plugs and the angle of opening of dampers will not give a linear result, and the signal from the controller must take this into account [10, 62]. 

More formal inspections also verily the operation of safety and other controls such as low water and fuel cutoffs, level controls, fusible plugs, pressure gauges, water glasses, gage cocks, stop valves, safety/relief valves, and BD valves and lines. Also, FW pumps, flue and damper arrangements, combustion safeguards, name plate specifications, set pressures, boiler connections, floats, mercury switches, bellows, and other components may be inspected. 

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. 

A valve is a mechanical device that controls the flow of liquids, gases, or loose material through piping systems. There are many types of valves, including butterfly valves, gate valves, plug valves, ball valves, and check valves. 

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... 

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. 

---