In actual practice, the user of self-
regulating cable has the option
of selecting from various wattage
per foot cable (generally ranging
from a low of about 3 watts/foot up
to 20 watts/foot) and from either
low temperature or high
temperature range operating
characteristics. Of course, the
material costs of low wattage/low
temperature range cable has the
advantage of being $4 to $5 per foot
less than high wattage/high
temperature cable, however the
installation cost is exactly the same.
In addition, using high wattage/high
temperature range cable has numerous other
benefits that mitigate this material
cost differential quite quickly:
High temperature range cables will not be destroyed if the pipeline is steam-cleaned or if a steam lance is used to remove a blockage.
The full output of the high wattage
cable will be available to prevent freezing or melt-out of the pipeline if there is an insulation breakdown either in local areas or across the complete circuit, thereby reducing process downtime.
The only engineering required
when using high wattage cable is a quick check of manufacturer supplied data to insure that the high wattage cable has sufficient watts/foot to overcome the worstcase heat loss of the pipeline.
This normally results in excess heat capacity being available, but this excess capacity allows the system to compensate for the losses at flanges and valves, where it is more difficult to install enough cable on the pipeline. Then only a rough check of the circuit length for each zone circuit is needed. The actual length is not critical unless the circuit length exceeds the maximum allowable.
By using high wattage/high
temperature cable, only one type of cable need be stocked on site, thereby eliminating selection mistakes during installation. Field installation becomes a matter of mounting only one type of cable on the pipeline, cutting length at either end of the circuit, and connecting to the controller. In addition, savings are achieved from larger volume purchases of a single item and, as pipeline size goes up, the price differential between cable types becomes smaller or disappears.
VTI also recommends that the following procedures be followed in pipeline heating design to insure the most reliable, safe, maintenance-free system:
Use Highest Quality Insulation System
- Polyisocyanurate for temperatures > 250°F
- Perlite expanded calcium sulfate for temperatures < 250°F
- Oversize insulation by one size
- Outside load bearing pipe supports
- All insulation penetrations in lower 180° quadrant (to minimize water penetration into insulation gaps)
Use Highest Integrity Safety System
- Continuous copper braid heating cable
- Overall Flouropolymer jacket heating cable
- Ground fault protection on every circuit