10 January 2002
This type of condensate return system is referred to as a "three pipe" or a "pumped return" system. If the
steam pressure is sufficiently high, it may be used to force the condensate through the condensate return
system to the central plant as shown in figure 8-1. No accumulator is required for this type system, which is
referred to as a "two pipe" system. Sizing of the lines for both of these systems is presented later in this
e. Condensate Cooling System. Fiberglass reinforced plastic (FRP) piping is usually allowed for most
condensate return systems. Since internal corrosion is a frequent problem in steel condensate lines, FRP
eliminates this problem. However, the FRP materials cannot withstand as high of pressures or temperatures as
steel and often fail when exposed to these conditions. A common temperature in an FRP distribution piping
system where damage will occur is 250 deg. F. Condensate temperatures may exceed 250 deg. F. at the outlet
of steam drip leg traps on steam systems that have pressures greater than 15 psig. In order to use FRP
condensate lines in this case, a condensate cooling system will be employed, as detailed in figure 8-4. In this
system, the high temperature condensate is discharged into a cooling tank where it blends with the system
condensate. The blended condensate is then routed to the condensate main. Note that the detail shows the
FRP (or non-metallic) pipe transitioning to steel inside the valve manhole to avoid burying the transition point.
Also, note that nonmetallic piping will not be allowed in a manhole with high temperature hot water or high
pressure steam systems due to the potential for this pipe being exposed to damaging temperatures within the
manhole if the manhole floods or the carrier pipe on the heating system leaks.
f. Non-metallic Pipe Anchors in Valve Manholes. If anchoring of a non-metallic piping system is
required at the valve manhole wall to comply with the distribution system stress analysis, a typical method is
as detailed in figure 8-5. If the system is to be anchored at both of the valve manhole wall penetrations,
provide adequate piping bends in the manhole to accommodate the expansion between the two anchors.
Steel straps and bolts will be sized to accommodate the axial force of that particular piping layout. These
sizes will be entered on the detail. Also, valve manhole sizes must be large enough to accommodate the
anchors and still allow for maintenance access.
g. Pipe Sizing. Pipe sizing is critical to proper operation of both the steam and the condensate return
(1) Steam. There are several methods to size steam lines. One of the quickest and most popular
methods is using pressure drop versus flow rate charts, which provide steam velocities based on the required
flow and pressure drops. The American Society of Heating, Refrigeration, and Air Conditioning Engineers
(ASHRAE) Fundamentals Handbook, Chapter "Pipe Sizing", is a good source for these steam sizing tables.
Recommended velocities for various system pressure ranges are:
0 - 10 psig, saturated - 1,000 to 4,000 fpm
10 - 50 psig, saturated - 4,000 to 8,000 fpm
50 - 150 psig, saturated - 8,000 to 12,000 fpm
In addition, ensure the total pressure drop in the system will not be excessive. Steam pressure must be high
enough at the end users to meet all special process requirements.
(2) Condensate. As described previously in this chapter, there are basically two types of condensate
return systems used on central heating systems: the two pipe system (which uses steam pressure to force
condensate back to the plant) and the three pipe, or pumped return, system. When sizing lines for these
condensate return systems, table 8-1 will be used for guidance.
h. Steam System Material Selection.
(1) Valves. For high-pressure steam systems (125 psig or greater), valves will be 300-pound class and