MIL-HDBK-1003/7
deaerator be designed for a possible 15 percent increase in the pressure from that shown
on the manufacturer's heat balance.
b) Safety valve manufacturers recommend that a suitable margin be
provided between the maximum operating pressure in a vessel and the set pressure of the
lowest set relief valve. This prevents any undesirable operation of the relief device.
They suggest that this margin be approximately 10 percent above the maximum operating
pressure or 25 psi, whichever is greater.
c) The deaerator design pressure shall be specified with a design
pressure equal to maximum extraction pressure x 1.15 plus allowance for safety valve.
The allowance for safety valve shall equal maximum extraction pressure x 1.15 x 0.1 or
25 psi, whichever is greater. The design pressure should be rounded up to the nearest
even 10 psi.
The maximum allowable working pressure shall be assumed to be equal to
the design pressure.
d) If the deaerator design pressure is 75 psi or greater, the
deaerator shall also be designed for full vacuum (thereby eliminating the need for a
vacuum breaker.)
Deaerator Storage Volume. The deaerator storage volume, elevation, and boiler
8.4.3
feed pump net positive suction head (NPSH) are related as outlined in Rodney S.
Thurston's paper "Design of Suction Piping: Piping and Deaerator Storage Capacity to
Protect Feed Pumps," Journal of Engineering for Power, Volume 83, January 1961, ASME pp
69-73. The boiler feed pump NPSH is calculated using the following equation:
EQUATION:
NPSH = (Pa + Ps - Pv) x (144/D) + hs - f
(11)
where:
Pa = Atmospheric pressure, psia
Ps = Steam pressure in deaerator, psig
Pv = Vapor pressure of boiler feedwater at boiler feed pump
suction, psia
D = Density of boiler feedwater, lb/cu. ft.
hs = Static head between deaerator water level and centerline
of boiler feed pump, ft
f = Friction loss in piping from deaerator to boiler feed pump
suction, ft
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