![]() ![]() This particular type of control valve uses a large diaphragm and a large spring to move the valve further open with more signal pressure and further closed with less signal pressure. Its purpose is to influence the flow rate of water into the boiler, “throttling” the water flow more or less as determined by controller. The last instrument in this control system is the control valve, operated directly by the air pressure signal output by the controller. Naturally, the controller requires a constant supply of clean, compressed air on which to run, which explains the “A.S.” (Air Supply) tube connecting to it. This means the output of the controller is also a variable air pressure signal, just like the signal output by the level transmitter. As with the transmitter, the controller in this system is pneumatic, operating entirely on compressed air. The controller then generates an output signal telling the control valve to either introduce more or less water into the boiler to maintain the steam drum water level at setpoint. The purpose of this instrument is to compare the level transmitter’s signal against a setpoint value entered by a human operator representing the desired water level in the steam drum. This pneumatic signal is sent to the next instrument in the control system, the level indicating controller, or “LIC”. This is the meaning of the “A.S.” tube (Air Supply) entering the top of the transmitter. Since the transmitter is pneumatic, it must be supplied with a source of clean, compressed air on which to operate. The greater the water level in the drum, the more air pressure output by the level transmitter. In this case, the type of signal is pneumatic: a variable air pressure sent through metal or plastic tubes. The purpose of this device is to sense the water level in the steam drum and report (“transmit”) that measurement to the controller in the form of a signal. The first instrument in this control system is the level transmitter, or “LT”. In this next illustration, you can see the essential elements of a water level control system, showing transmitter, controller, and control valve: If there is too much water in the drum, liquid water may be carried along with the flow of steam, causing problems downstream. If there is not enough water in the drum, the water tubes may run dry and burn through from the heat of the fire. In order to safely and efficiently produce a continuous flow of steam, we must ensure the steam drum never runs too low on water, or too high. An important variable to measure and control in a continuous boiler is the level of water in the “steam drum” (the upper vessel in a water-tube boiler). Making steam continuously, however, is a little more complicated. Anyone who has ever boiled a pot of water for cooking knows how this process works. The process of converting water into steam is quite simple: heat up the water until it boils. ![]() reforming of natural gas into hydrogen and carbon dioxide). a steam engine moving some sort of machine), heating, producing vacuums (through the use of “steam ejectors”), and augmenting chemical processes (e.g. Common uses for steam in industry include doing mechanical work (e.g. Steam boilers are very common in industry, principally because steam power is so useful. ![]()
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