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Posted by Tony Conner on July 22, 2002 at 16:57:26:
In Reply to: boiler/line pressure posted by mark on July 22, 2002 at 10:15:07:
Most plants are set up such that there is a pressure transmitter on the header, which controls the burners. For example, it's very common for boilers with 150 PSIG safety valves to feed a header that operates at 125 PSIG. At low loads, the drum and header pressure will be about the same - 125. As the load increases the header pressure falls slightly, and the pressure transmitter tells the burner to fire harder. The only thing that gives you a flow is pressure differential. The higher the differential, the greater the flow, and the higher the velocity. To deliver more steam from the boiler to the header (header pressure remaining constant at 125), then the boiler pressure will have to increase. At high fire, the boiler pressure will be 140 - 145 PSIG. The header is still at 125.
You'll have the same effect in your distribution piping. If you have the same pressure at the far end of the system as you do at the header, then you've got no flow. You in effect have a long, skinny storage tank. In a system with a 125 PSIG header, and a good load, I'd expect to see the pressure at the far end (2 miles away) at something like 115 - 120 PSIG.