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Steam Pressure

Kevin G
Kevin G Member Posts: 4
in THE MAIN WALL
About 20 -25 yrs. ago, I believe it was an I=B=R Seminar for hydronic controls, the instructor stated that the pressetrol should be kept down to the .5# setting. The reason is that a 1/4# of steam will raise five (5) miles. I believed this to be accurate. Gullible??? At that time my only experinece with steam was high pressure and super heated (750 psi).at an oil refinery located in Linden NJ.
Is this info correct?

Kevin G

Comments

  • Tony Conner
    Tony Conner Member Posts: 549
    You Can Neglect...

    ...elevation change when calcuating the flow & pressure drop of compressible fluids - gases. You'll still have the pressure drop for a given flow over say, 200 feet of 2" pipe, but whether the line goes across, or staight up/straight down doesn't matter. The instructor is/was right. Think of a natural gas line feeding the boilers in a penthouse mechanical room in a highrise. If nothing is running in the mech room, the gas pressure at the street level meter station is the same as what shows in the penthouse, maybe 100's feet above you. Steam is the same.

    This is one of the big things that throws people who normally work with hot water heating when they get involved with steam. Many of them think they need to have 12 PSIG in the boiler to push the steam to the rads on the top floor. And in systems with condensate supposed to be returning to the boiler by gravity, we KNOW what happens - and none of it is good.
  • Big Ed
    Big Ed Member Posts: 1,117
    Pressuretrol

    On say a Honeywell P404 , the top set screw and front setting is the Cut-in pressure. It is also important to set the cut-in to .5# as to let the vents reopen for if need be extra venting.....They will stay shut over the valves rated Pressure....
  • Unknown
    Unknown
    Differential Pressure Settings

    Setting the steam pressure on a boiler is a function of overcomingthe resistance of the piping or the pressure drop.

    To settle the question abouit setting system pressure look in the Hoffman Steam System Manual Bulletin TES-181.

    On Page 42 a simple statement is made

    (3) The total pressure drop of the system should not exceed one half the supply pressure when steam and condensate are flowwing in the same direction.

    Simply put if the pressure drop in the steam system is .5 PSIG the operating steam pressure should not exceed 1 PSIG.

    If this instruction is followed the cut out pressure should be 1 PSIG and the differential pressure should be less than .5 PSIG.

    If one is concerned about the vent valves you need to consult the manufactirer and find out where the drop back pressure is for the model vent valve you are using.

    In most cases a system that operates under 2 PSIG and has a set back of .5 PSIG has enough of a drop back to allow the vent valves to cycle.

    The drop back pressure setting of a pressure control is basically used to assure that steam will be present in the piping system when the burner shuts down at the set point.

    All burners have a post purge and prepurge cycle. When air is forced into the boiler the fire box and boiler metal cool. that cooling effect causes the water in the boiler to cool off and steam pressure drops.

    If you set the differential properly you can keep a higher steam pressure in the boiler than the pressure drop of the system. If you do that steam will always flow to all the radiators during the boilers duty cycle.

    Jake

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