Change 2, 30 December 1991
pressure reducing valve is out of service. Provide a pressure gauge on the
low pressure side. Where steam requirements are relatively large, above
approximately 3,000 pounds/hour (1364 kg/hr), and subject to seasonal
variation, install two reducing valves in parallel, sized to pass 70 percent
and 30 percent of maximum flow. During mild spring and fall weather, set the
large valve at a slightly reduced pressure so that it will remain closed as
long as the smaller valve can supply the demand. During the remainder of the
heating season reverse the valve settings to keep the smaller one closed
except when the larger one is unable to supply the demand.
f) Safety Valves. Provide one or more relief or safety valves on
the low pressure side of each reducing valve in case the piping and/or
equipment on the low pressure side do not meet the requirements of the full
initial pressure. The combined discharge capacity of the relief valves shall
be such that the pressure rating of the lower pressure piping and equipment
will not be exceeded. For special conditions refer to ASME B31.1 and ASHRAE
Handbooks - Systems and Applications.
g) Takeoffs from Mains. Takeoffs from mains to buildings must be
at the top of mains and located at fixed points of the mains, at or near
anchor points. When a branch is short, valves at each takeoff are
unnecessary. Takeoffs shall have valves when the branch is of considerable
length or where several buildings are served. A 45 takeoff is preferred; 90
takeoffs are acceptable. Branch line slope of 1/2 inch (12.6 mm) should be
used for lines less than 10 feet (3.05 m) in length and should be 1/2 inch per
10 feet (3.05 m) on branch lines longer than 10 feet.
Condensate Returns. Condensate returns are preferred if owning and
operating costs of such a system are less than that of using and treating raw
water for makeup. Factors favoring condensate return are: high area
concentration of steam usage; restriction on condensate disposal; high raw
water treatment costs; water treatment space unavailable; high cost of raw
water; and high cost of fuel for feedwater heating. Design considerations are
a) Return Piping. Size condensate trap piping to conform with 30
to 150 psig (206.7 to 1033.5 kPa) steam piping in accordance with Tables 10
and 11 and interpolate these for other pressures.
b) Discharge Piping. Size discharge piping from condensate and
heating pumps in accordance with pump capacities, which may be between one to
three times the capacity of the steam system branch which they serve,
depending on whether continuously or intermittently operated.