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Setting the operating steam pressure for a steam system is a function of overcomming the the system pressure drop.
Steam systems have varrying pressure drops. It depends on the size of the pipe, amount of pipe and fittings in the system. All of the above must be translated into feet of pipe and converted to a finite pressure drop.
How to calculate a pressure drop in a steam system is shown in Hoffmans Steam Heating Systems Design Manual and Engeneering Data Bulletin TES-181.
If you look at Table 3 on page 41 you will find that system pressure drops range from 116 PSI or 1 OZ. thru 2 PSI.
So for this exercize we will use a 1/2 PSI as the pressure drop.
Read on page 42, where 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 flow in the same direction.
This means that the steam pressure should be about twice the pressure drop.
If the pressure drop is .5 PSI the operating pressure should not exceed 1 PSI.
If you use a pressure control that has a differential setting you would set the differmtial at .4 PSI or less.
Why .4 or less?
Burners have post purge and prepurge cycles. Ambient air from the boiler room is forced into the combustion area of the boiler to clear hot flue gasses before the burner fires.
The air forced into the boiler has a cooling effect on the water and steam in the boiler. This causes steam pressure drops quickly.
The proper differential setting permits us to keep steam pressure above the pressure drop of the piping system.
By keeping the steam pressure above the pressure drop of the steam system steam will flow to the radiators. When the boiler comes on line steam is still flowing to the radiators, the burner produces more steam and the steam pressure increases.
The differential setting assures that steam, uninteruppted, will flow during the boilers duty cycle.
The whole excersize is an attempt to maximize the energy used in an economical manner.
As to protecting the vent valves and allowing for a drop back where the vents can cycle, this concern is moot, because, in a properly set up steam system where the proper vent valves are installed the drop back pressure is well within the limits of a properly set operating pressure.
If one is realy concerned about the vent valves you need to get in touch with the manufactirer and by vent valve model find out what the drop back pressure needs to be.
Additionally, you should be more concerned that the vent valve chosen is within the operating pressure parameters of the steam system they are used on.
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