10 January 2002
to above ground system transitions, all drain points (low points), all below ground valving, all trap
stations, high points for vents of buried systems, and to minimize depth of buried systems.
Distance between valve manholes varies with different applications. However, spacing shall
never exceed 500 feet with Pre-Engineered Underground Heat Distribution Systems or
Prefabricated Underground Heating/Cooling Distribution Systems to minimize excavation when
searching for failures and to minimize effects of a failure. To enhance maintainability, avoid
valve manholes deeper than 6 feet.
a. Manhole internals. Layout of each manhole will be designed on a case by case basis.
(1) Equipment/valve locations. It is important to first layout, to scale, all manhole piping,
insulation, valving (with stems upright 90 degrees or less from vertical), and equipment and then locate the
manhole walls around these appurtenances to ensure adequate manhole size and room for maintenance
personnel. One line diagrams of piping and equipment are unacceptable. See figure 3-1 for a typical
manhole plan. Note that all valve manhole layouts have certain designer requirements in common. The
(a) Provide main line isolation valves in valve manholes to most efficiently minimize outages to
buildings served by the distribution system. When installed, main line isolation valves will be located
downstream of the building's service laterals.
(b) Provide lateral isolation valves within the valve manholes for all laterals runs.
(c) Locate all carrier pipe vents and drains needed within the manhole for proper system
(d) Layout all valve manhole internals (valves and valve stems, pipe w/insulation, access
ladders, isolation flanges, and equipment) to scale to ensure adequate clearance has been provided for
operation and maintenance within the manhole.
(e) Ensure no non-metallic piping is routed in the manholes (i.e., as allowed with chilled water
or condensate return systems) which also serves high temperature mediums that could damage the
non-metallic piping. Damage to non-metallic piping is caused when manholes flood and the hot piping boils
the flood water. Boiling water can exceed the temperature allowables of many nonmetallic piping materials.
Because of this, the designer must transition to steel piping at the manholes (see figure 8-4).
(2) Clearances. Design will provide for clearance around piping and equipment in the manhole in
accordance with table 3-1.
(3) Access Ladders. Access ladders will be required on all valve manholes greater than 3 feet in
depth. Ladders will be welded steel and will consist of uprights and nonslip steps or rungs. Uprights will be not
less than 16 inches apart and steps or rungs will be spaced no greater than 12 inches apart. Ladders will
extend not less than 6 inches from the manhole wall and will be firmly anchored to the wall by steel inserts
spaced not more than three 3 feet apart vertically. All parts of the ladders will be hot -dipped galvanized after
fabrication in conformance with ASTM A 123. The top rung of the ladders shall be not more than 6 inches from
the top of the manhole. A typical valve manhole access ladder detail is shown in figure 3-2.
(4) Insulation. Insulation for valves, fittings, field casing closures, and other piping system
accessories in valve manholes will be of the same types and thicknesses as those provided in the distribution
systems' guide specification. All insulation will be premolded, precut, or job fabricated to fit and will be
removable and reusable. Insulation jackets will be provided for all pipe insulation in manholes and will comply
with the requirements of the particular distribution system guide specification.