Saturday, October 31, 2015

Heat Tracing of Long Pipelines - Part One

Pipelines provide a simple means of transporting materials, liquids, powders, and gasses over sometimes relatively long distances, both efficiently and inexpensively. When thermal insulation alone is not sufficient, then the pipeline needs heating.

There are two factors necessitating heating:
  1. Heat loss compensation – to maintain the pipe at a specific temperature. 
  2. Temperature raising – to elevate the temperature of the pipe and contents in a specific time. 
Materials are heated for many reasons:
  1. To prevent liquids changing state 
  2. To reduce viscosity 
  3. To heat materials in preparation for the next process 
  4. To prevent corrosion 
Electricity offers many advantages. It is clean, easy to install (or repair, if necessary), easy temperature controlled and readily available. Operating costs are low, and on a properly designed and installed system, maintenance is virtually non-existent.

Typically, two types of external heat tracing systems are being used:
  1. Mineral insulated heating cable 
  2. Heating tape 
Heating Cables – Usually of the mineral insulated (mi) type, with a variety of covering. The covering or outer sheath is of metal, i.e. copper, stainless, inconel, cupro nickel, etc. The conductors are usually of low resistance nature. The cables only give point contact, and heat transfer rate and efficiency is low. Vibration is a problem and causes insulant migration. The cables have to operate at higher temperatures to give the needed heat transfer. Heat transfer cement is usually used to assist in heat transfer. M.I. cables are semi-rigid and, once bent into shape or configuration, it is virtually impossible to get the cables straight again. Repeated bending of metal sheathed cable can set up stresses, resulting in stress cracking of the metal sheath.

Heating Tapes – these are the most versatile form of heating. They can be designed for practically any voltage, can be made for single or three phase operation, can be covered with a variety of sheaths for compatibility with the environment, etc. Heating tapes are flexible and thus easy to work with. They can be braided with either stainless steel or nickel plated copper, for hazardous environments or non-metallic pipes.

Usage of copper conductors, or other low resistance metals, is needed for running long lengths of circuits. The length of circuit is governed by temperature of the pipe, voltage supply, type and thickness of thermal insulation, amount of load given out by the tape or heat needed on the pipe, delta ‘t’, etc. Low resistance metals mean that long circuit lengths can be achieved. Long circuits are what is needed to have the voltage supply at one end only. If possible.
Various methods of heating long pipelines by heating tapes are available. It can take the form of one (1) three phase heating tape or three (3) single phase tapes.

Design Consideration
Each pipeline is different and it is doubtful if two projects will ever be the same. Factors governing the designs are:
  1. Temperature Range. This is the final designed maintenance temperature of the pipe. Thought must be given in the initial design to the materials that could be in the pipe during its lifetime. For example, an initial approach may be to pump light oil at a higher maintain temperature. 
  2. Ambient Temperature. Designs must take into account the lowest anticipated ambient temperature. 
  3. Temperature Raising. The design may ask for the temperature of the material to be raised during transit through the pipe. Alternatively, designs may request that the heaters have to have sufficient power to “melt out” a system in the event of prolonged shutdown. The specific heat and gravity of the material needs to be known. 
  4. Voltage. The most convenient voltage supply for the system. 
  5. Needed Info. Length, diameter, thickness and material of the pipeline to be heated. 
  6. Temperature Control. How the pipe is to be temperature controlled and by what means. 
  7. Corrosion Effect. Of materials near to the pipe and heater. 
  8. Materials in the Pipe. If they have a flashpoint, freezing and boiling points. 
  9. Thermal Insulation. Type, thickness, K factor, etc.
The thermal insulation on any project must be known, as this dictates the amount of heat losses and is a most important factor to bear in mind. There are various types:

Polyurethane. Usually used for low temperature applications. It is approximately 95% air and is a good insulator. It is either preformed or can be sprayed on.

Fiberglass. In preformed, half round pipe sections is a common means of thermal insulation.

Mineral Wool. This has similar properties as fiberglass. 

NOTE: In the majority of cases, heat loss tables from heater manufacturers suppliers are based on glass fiber or mineral wool.

Calcium Silicate. Widely used in plants due to its robust, solid qualities. It is not a good insulator and standard heat loss tables have to be raised by 35-40% to accommodate the inefficiency.

With using polyurethane and/or calcium silicate, care must be taken, as they are “hard” materials and do not readily flex. On steam traced lines, either a groove is cut in the insulation to accommodate the tracer, or oversized insulation is used. With mineral wool or fiberglass, it is usually sufficiently flexible to absorb the tracer if the tracer is of small diameter. This factor also has to be borne in mind with a heating cable system. Heating tapes are usually thin in nature and a grooved or oversize insulation is not needed.
To be continued in next post.