Thursday, November 5, 2015

Heat Tracing of Long Pipelines - Part Two


Part Two of Two Part Series

Installation of Heat Tracing


Heating tapes can be either “straight” traced or “spiraled”. Obviously, the easier method is straight traced.

Although heat tape can be supplied in unit lengths of several hundred feet, it is not advisable to have them this long. Long heaters are heavy and hard to handle and, if dropped or mishandled, fall into an uncoiled pile on the ground. To simplify installation and maintenance, medium lengths of heaters should be chosen, i.e. 150-300’-0”. Then, series junction boxes can be used to connect up the lengths of heaters to achieve the total pipe run.

On straight traced applications, the heaters must be secured at approximately 1’0” intervals to prevent sagging of the heater away from the pipe. Contact between heater and pipe is paramount. For heating tapes, securing fiberglass tape or similar should be used.

Junction Boxes
 
Normally on pipelines, there are three (3) types used:
  1. Voltage Supply Box. This is where the client’s supply is brought in and feeds the heating system. 
  2. Series Boxes. This is where “n” number are used to series connect the various lengths of heating means. 
  3. The back end box to connect the heaters in a star or “Y” fashion for three (3) phase applications. 
The boxes should be weatherproof for outdoor locations and suitable for any environmental attack from chemicals, gasses, dusts, etc.

Control of temperature can be achieved by a simple thermostat and contactor method, all the way up to sophisticated control panels. Each system of control must be investigated as to the requirements of the client/engineer for control, monitoring alarm levels, etc.

Repairs – fault finding

Fault finding on long, continuous circuits is very difficult. On uninterrupted runs of 1000’ or more with no series joints, unless there is mechanical damage, a break cannot be easily located. Where there are section lengths of 150-300’, it is easier to find the fault in such a section with standard electrical measuring instruments.

Above/below Ground Locations

The majority of heated pipelines are usually above ground. Some heated lines are below ground and, where such installations exist, records must be kept of the geographical routing, junction boxes, joints, etc. On underground lines, the thermal insulation must be totally waterproof as water tables do exist. Care must be taken on the installation due to the possible dissolved chemicals in the soil, which could attack the total installation.

Records must be kept of all systems, locations, items used, reference numbers of components, etc.

Hotfoil-EHS Design

On all long pipelines, the object is to reduce, to a minimum, the number of voltage supply points. By keeping these to a minimum, the cost of the total project of the heating system is attractive and competitive because it minimizes the electrical conduit and wiring.

Long pipeline systems usually need a three (3) phase voltage supply. Such a supply also offers a balanced three (3) phase load.

There are two (2) ways of achieving the requirements:
  1. A single, three phase heating tape (three foils in one sheath) 
  2. Three, single phase heating tapes (each tape with one foil) 
Although various sheaths can be used on the heating tapes, we have been using rubber. Silicone rubber offers many advantages, such as temperature range and chemical attack resistance.

Systems do not end with just the heating tapes. The junction boxes (series, supply and “Y”), must be provided. Also, the system has to be temperature controlled. For hazardous areas, the heating tape will invariably have to be braided.

Being a project engineering company, Hotfoil can supply all the accessories needed on any system and do all engineering designs, drawings, wiring diagrams, system layout, field supervision, startup services, etc.

Method (a) – One 3 Phase Heating Tape Hotfoil Type HTF – 3P

This system uses a single heating tape with three resistance foils as the heating means. (Sketch 2)

The foils can be of any material depending on the job requirement. As we are concerned with long lines, the foils are usually copper. Copper possesses a low resistivity, 10.3 ohms/c mil-ft. and thus long lengths can be achieved with this low resistance metal conductor.

Calculations are done to determine from loading needed (watts) with a given supply (voltage), the actual resistance of the circuit. This is then translated into the length and cross sectional area of the copper foil.

With the three (3) copper foils suitably spaced apart, they are fed through an extruder and receive a sheathing of silicone rubber. The thickness is dependent on the insulation factor of the project.

The back end of the tape system is taken through the leads to a junction box. On a 3 phase star/”Y” system, the three (3) leads are connected together to form a star point.

The front end of the system is connected to the voltage supply. This has to be a 3 phase supply. Since all three (3) foils are of the same cross sectional area and the same length, the load is balanced evenly over the 3 phases.

Typical systems done so far are:
  • One run of pipe/tape 5,300’ long, one supply point of 600 volts, 3 phase, giving a load of 5 watts per foot of tape/pipe. 
  • One run of pipe/tape 1,400’ long, one supply point of 208 volts, 3 phase, giving a load of 5 watts per foot of tape/pipe. 
  • One run of pipe/tape 7,920’ long, one supply point of 480 volts, 3 phase, giving a load of 7 watts per foot of tape/pipe. 
  • One run of pipe/tape 7,920’ long, one supply point of 480 volts, 3 phase, giving a load of 9 watts per foot of tape/pipe. 
These systems were for freeze protection of steam condensate return lines. The tapes use copper foils with silicone rubber sheaths. These were ideal as the rubber can withstand a 400° F continuous exposure, which the condensate could attain.

Also,
  1. One run of pipe/tape 1,780’ long, one supply point of 480 volts, 3 phase, giving a load of 7 watts per foot of tape/pipe. 
  2. One run of pipe/tape 850’ long, one supply point of 480 volts, 3 phase, giving a load of 7 watts per foot of tape/pipe.
Method (b) – Three Single Phase Heating Tape Hotfoil Type HTF – 1P

This system is basically the same as (a) but each tape is a single phase.

When systems call for high electrical loadings, both on the heating tapes and the pipes, or the pipe/circuit is exceptionally long, the foils must be of a larger cross sectional area. Due to this fact, individual foils are extruded with silicone rubber. (Sketch 3)

Extruded lengths of tape are kept to 100’-150’ due to the weight of the tape and the obtaining of foil in workable lengths.

Junction boxes are used for the series connections, star/“Y” connection and the incoming supply. The heating tapes are straight traced on the pipeline and secured with fiberglass or equal securing tape, every 1’-0”. Note: metal, plastic, nylon or pvc must not be used for securing due to mechanical damage or chemical non-compatibility.

Section lengths of tapes have cold leads, firmly butt spliced to the foils, and with a silicone rubber molding over.

The three (3) tapes are connected in a star/”Y” formation at the back end to achieve a balanced, 3 phase load.

A fourth redundant tape can be installed as a spare. Should any damage occur to one of the three working tapes, the fourth can be connected into the system at the series boxes quickly, and the heat is back on line. This means that the system is 100% operational without removing the thermal insulation or disrupting the system. When the pipeline is off line or shut down for other reasons, the repair of the damaged tape can be effected. This method of four tapes has been more than welcomed on many jobs.

Some projects done are:
  • 6,562’ run of pipe, 12” diameter to raise and maintain at 150°F. Most of the pipe was buried. 
  • 187,000’ of tape for pipes up to 36” diameter to raise temperatures and maintain up to 160°F. 
  • One run of pipeline, 6,853’-0” of 10” diameter, one 3 phase system, 67 KW, to maintain temperatures between 86° and 186° F, hazardous location. 
  • 118,000’of tape on 10” pipe with a total loading of 157 KW to maintain temperatures up to 104°F in a hazardous location.