CEMP-E
TI 810-32
10 January 2002
CHAPTER 5
PRE-ENGINEERED UNDERGROUND HEAT DISTRIBUTION SYSTEM
5-1. GENERAL. Unlike Heat Distribution Systems in Concrete Trenches, which are totally designed by
the designer, Pre-engineered Underground Heat Distribution Systems are designed by the system
manufacturer. These pre-engineered systems are factory fabricated in lengths, which are transported
to the site for field assembly. Other types of systems and materials are continuously being evaluated
and will be included in guide specifications and this manual when deemed acceptable. There are two
types of these systems. The DDT type is allowed for severe, bad and moderate site conditions. In
severe sites allow only drainable-dryable-testable type systems. In bad and moderate sites allow DDT
and water spread limiting systems. These
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\ site /1/ conditions (or classifications) are described in
detail in the guide specifications. Although the manufacturer is responsible for the pre-engineered
system design, the project designer also has design responsibilities which include establishing the site,
soil, and groundwater conditions, pipe sizes, proposed routing (including construction limits) estimated
length, elevations, profiles of the system along with existing and finished earth surfaces and
obstructions
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\ within /1/ 8m (25 feet) of the system centerline including adjacent or crossing utilities.
The project designer also provides information on location and design of manholes and entrances to
buildings and manholes.
5-2. MANUFACTURER'S RESPONSIBILITY.
a. Pre-engineered Underground Heat Distribution System design. The manufacturer is responsible
for the Pre-engineered Underground Heat Distribution System. This responsibility includes any or all of
the following: insulation types, guided and anchor supports, end seals, casing and piping joint closure,
casing type and thickness, and carrier pipe depending on
the type of Pre-engineered Underground
Heat Distribution System provided. There are two types of Pre-Engineered Underground Heat Distribution
Systems. The drainable-dryable-testable system is a factory fabricated system, which includes a water-
tight outer protective casing of steel, an air space, and an insulated carrier steel pipe. Casing drains and
vents
are provided in end plates, which are installed in valve manholes. DDT systems can be used for
any heating medium including HTHW, high and low pressure steam, and condensate ret urn and in any
site condition (severe, bad or moderate). The water spread limiting
systems is also a factory fabricated
system, which includes an outer protective casing and
an insulated carrier pipe. The system is
fabricated in sections, which are independent from each other. Ground water or condensate, which leaks
from or into one section, cannot travel into the next section. Field-assembly of the sections requires no
welding; the sections are pushed together and are sealed with a system of couplings and seals. WSL
systems can
be used only in bad and moderate site conditions. The designer must determine the
site conditions before a system type is selected. The site conditions will be considered severe, bad or
moderate based on the site investigation results. For DDT and WSL systems, steam and condensate
lines must always be installed in separate casings, as shown for a DDT system in figure 5-1, due to the
corrosion problems associated with condensate return systems. Water systems may use just one
casing for both supply and return pipes as detailed for a DDT system in figure 5-2, although installing the
pipes in separate casings is preferred by most users because it is less difficult to isolate leaks. The tops
of the casings will typically be buried between 2 and 6 feet below grade. However, note that excessive
burial depths increase the installation and repair costs and should be avoided where possible.
b. Expansion compensation. The manufacturer is responsible for the system expansion
compensation. A detailed stress analysis will be submitted for review as part of the contract requirements.
The manufacturer will normally make use of expansion loops and bends to absorb system expansion in DDT
systems. For WSL systems, field joints may be used to accommodate expansion. Except in rare
instances, expansion joints will not be permitted.
5-1
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