2.4.2 Consideration should be given to installing alarm systems within the
air space of conduit systems which is based on time domain reflectrometer
technology. Such alarm systems can signal the presence of a leak, record the
l o c a t i o n , and log the time of the first sensing of the leak. These alarm
systems can be retrofitted into existing underground heat distribution systems
a s well as installed in new installations,
2.4.3 Infrared Thermography will assess the overall performance of buried
d i s t r i b u t i o n systems by measuring the varying intensities of thermal radiation
identify and record ground surface temperatures. These findings are then
i n t e r p r e t e d by applying knowledge of buried distribution systems to identify
and analyze the results. Thermography is an effective tool for performing
maintenance by identifying problem areas without excavation.
2.4.4 Metallic Casings (Class A Systems). Look for evidence of leakage as
indicated by pending, erosion, or settlement of areas near piping. Weekly
c h e c k s of telltales will indicate the presence of leaks in the inner pipe by
the discharge of steam. T e l l t a l e s are installed in the casing and carried to
aboveground level. When a leaking main is put out of service for some time,
any appreciable leakage will soak the insulation.
( a ) D r y i n g o f I n s u l a t i o n . Systems may become wet because of improper
h a n d l i n g during installation, subsequent inner pipe or casing failure, or
floods . For that reason they must be dryable, a n d the inner pipe insulation
must be capable of withstanding boiling such as would occur with a failure of
t h e inner pipe. To dry the insulation proceed as follows:
(1) Remove the drain and vent plugs and drain any water
accumulation from the conduit.
( 2 ) Place lines in operation. T h i s will apply heat to the
i n s u l a t i o n through the inner piping, helping the drying process.
( 3 ) Circulate pressurized a i r through the conduit airspace. I n j e c t
the air through a vent located at the high end of the run and force the water
out of the lower end. T h e rate of ventilation should not be less than 2 cubic
feet per minute, n o r higher than that producing an air linear v e l o c i t y of 500
f e e t per minute in the airspace. The average air velocity is determined by
dividing the cubic feet per minute at atmospheric pressure by the area of the
ventilated air passages, expressed in square feet. The area i s taken in a
p l a n e at right angles to the centerline of the pipe. In any case, the
required air pressure at the inlet end of the conduit should not be higher
than 20 psig, and the resulting air velocity should not be high enough to
damage the insulation by either impact or erosion.
( 4 ) While drying the insulation, place a cool mirror at the exhaust
p o i n t for a short time at appropriate intervals. Locate the mirror so that it
may show maximum clouding caused by moisture. Shut down the air occasionally
t o allow it to heat up in the conduit.
( 5 ) Continue the drying process for not less than 48 hours. In any
case, do not stop until the mirror shows no visible clouding.