10 January 2002
will have welded ends. Steam and condensate valves at lower pressures will be 150-pound class with welded
ends. Valves on trap stations, including the bypass valve, will be 150-pound class with threaded ends. Shutoff
valves will be gate type.
(2) Fittings. All fittings in the steam distribution system, except as discussed for valves, will be
welded except at equipment, traps, strainers, and items which require frequent removal. These items will be
threaded or flanged.
(3) Piping. Steam and condensate piping will usually be carbon steel conforming to ASTM A 53,
Grade B, Type E or S. Steam piping will be schedule 40. Condensate lines will be schedule 80 as will all
welded piping less than 1-1/2 inches. Condensate lines in a pre-engineered underground heat distribution
system may be FRP as discussed in chapter 5.
i. Field Joints. Radiographic examination of all carrier pipe field joints is required.
8-3. HTHW SYSTEMS.
a. Pipe Sizing. Sizing lines for HTHW systems is similar to any water system, except at high
temperatures water becomes less dense and less viscous, and, therefore, the mass flow rate of the system
must be calculated considering the lower density (usually temperatures are around 400 deg. F for HTHW).
m = q x 60 min/hr x 0.1335 ft 3 /gal x ?
q = system flow in gal/min
m = mass flow rate in lbm/hr
? = density of water at maximum system temperature in lbM/ft 3
Pressure drop versus flow charts may be used to determine line size, such as the chart included in appendix
B. This chart takes into account the characteristics of higher temperature water systems. Recommended
velocities for various HTHW flows are as follows:
Up to 10,000 lbm/hr
1 to 2 feet/sec
10,000 to 30,000 lbm/hr
2 to 3 feet/sec
30,000 to 200,000 lbm/hr
3 to 5 feet/sec
200,000 lbm/hr on up
(use velocity to accommodate 0.50 psi/100 ft., maximum)
b. HTHW System Material Selection
(1) Valves. All valves on HTHW systems will be 300-pound class with welded ends. Shutoff
(isolation) valves will be gate type. Valve packings must be capable of handling the pressures and
temperatures associated with HTHW systems.
(2) Fittings. All fittings on HTHW systems will be welded. The only exceptions will be specialty
equipment such as dielectric flanges used to isolate the piping system from a cathodically protected system.
(3) Piping. HTHW piping will be carbon steel conforming to ASTM A53, Grade B, Type E or S. All
piping will be schedule 40 except for welded pipe less than 1-1/2 inches, which will be schedule 80.