Welding Preheat Basics

welding preheat
Why preheat welds?
Weld preheating is the process of heating the base metal (parts to be welded) to a specific temperature prior to welding. The specific temperature to which the part needs to be heated (before welding) is referred to as the “preheat temperature”.

The area requiring preheat may be the whole (entire) part, or just the area immediately surrounding the weld.

Preheating may continue during the actual welding process, but many times the energy generated from welding will be sufficient to maintain the desired temperature.  The temperature of the weld between the first pass and the last pass is referred to as “interpass temperature”. As long as it can be assured that interpass temperature will not fall below the preheat temperature, continued preheating is usually not required.

There are several key reasons why it's important to preheat before welding. First, a preheated part cools more slowly, which slows the overall cooling rate of the welded part. This improves the metallurgical (crystalline) structure and makes it less prone to cracking. Additionally, hydrogen that may be present immediately after a weld is also released more efficiently, which further reduces the possibility cracking. Preheating also mitigates stress from the shrinkage at the weld joint and nearby metal. Finally, pre-heating reduces the possibility of fracture during fabrication due to brittleness.

Electric welding preheaters, known as "ceramic mat heaters", are rugged and flexible heating elements designed so that they conform uniformly around the weld and surrounding area.  Ceramic mat heaters are normally controlled by a power console that uses thermocouples and electronic controllers to regulate, monitor, and many times record, the preheat temperature profile.

Welding code is the first determinant to whether pre-heating is needed. Welding code carefully specifies the minimum preheat temperature, the soak time, and the welding process. Many criteria are considered by welding codes, all gathered from years of rigorously tested data. This data is accumulated from many sources, including metallurgical science, chemical properties of materials, and radiographic analysis.

Determining whether or not preheating is required should not be taken lightly, as it is critical to the quality of a weld and therefore critical to the performance of a structure. When in doubt, review of industry code or contacting an industry expert, is imperative.

Automatic Heat Treatment Power Consoles

power console
Typical Power Console
(courtesy of HotfoilEHS)
Automatic heat treatment power consoles are used to control various heat treatment processes (i.e. pre-weld, post-weld) by closely controlling the temperature of the item being welded. The power console accurately controls the ramping rate (up and down), the soak temperature, the set point and the time. Power consoles are available from 2 to 24 zones of control. Zone can be used either in the fully automatic or manual mode.

ceramic mat heater
Ceramic Mat Heater
The power console is used to provide power to electric heating elements called ceramic mat heaters. Ceramic mat heaters are constructed of nichrome wire interwoven into ceramic beads which provides electrical insulation and protection. These heaters are quite rugged and conform to curved and irregular shapes.

Thermocouples are used to sense the target temperature and send their signal back to some type of electronic temperature controller, recorder, or combination thereof. The sophistication of the control system can range from simple manual control to fully automatic control with large graphic displays. Recorders are frequently used to document the pre-heat, soak, and post-heat process. Welding integrity depends on precise and accurate control.

Power Console Controller
Recorder used on
welding power console.
Heat treatment power consoles are built on sturdy chassis of steel and depending on ambient conditions, stainless steel. Construction includes wheels and handles for easy relocation and many electrical components for safety and convenience (such as amp meters, indicator lights, cut-off switches, fuses, and alarms).

For more information, contact:

Hotfoil-EHS, Inc.
2960 East State Street Ext.
Hamilton, NJ 08619
Phone # 609.588.0900
Fax # 609.588.8333
Email: dap@hotfoilehs.com

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.

  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. 

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.

Prevent Flash Freezing in Hygroscopic Aggregate, Mined or Quarried Materials with FRP Heaters

FRP heater for freeze prevention
FRP heaters installed
(click for larger view)
When aggregate material with trace amounts of absorbed water comes into contact with very cold metal surfaces, the aggregate can instantaneously freeze, a phenomena know as “flash freezing”. When wet or frozen material encounters steel chutes or hoppers at sub-freezing temperatures, an instantaneous bond is formed. This bond causes immediate and often catastrophic blockage of the hopper and chutes. The bond and resultant blockage can be severe enough to require pneumatic drilling equipment to free up the system. Generally, this a problem for manufacturing and processing plants who utilize conveyors, chutes, and hoppers, such as coal mines, quarries, cement manufacturers, mining facilities, and power plants. Coal, sand, cement, ores, and mined products require a materials handling system that takes into consideration very cold weather and a strategy to prevent flash freezing.

FRP heater for freeze prevention
FRP heaters installed
(click for larger view)
An excellent solution to this problem are electric FRP heating panels, a waterproof, dust tight, and vibration resistant heater. FRP heaters are designed to be used in the rugged landscapes where they are required and are very strong and corrosion resistant. They are available with FM approval for use in hazardous areas and are easily customized to conform in shapes and size to virtually and chute or hopper. Furthermore, they are lightweight, can be applied to flat or curved surfaces, and are very easy to install and maintain.

Flash Freeze Animation

Welding Application Note: Demand Pulse Technology Saves Three Weeks Production Time

Demand Pulse technology
welder (courtesy of Aftek EHS)
Demand Pulse is a unique, patented welding process. Because of the low peak current, open butt pipe welds are easier in all positions.

Short arc and pulsed spray both have peak currents in the 375+ amp range, and are at peak many times longer than the Demand Pulse process.

With Demand Pulse, welding current and volts are similar to those seen when “short-arc” welding, but there are NO short-circuits - the arc never goes out, and cold laps are almost impossible.

Demand Pulse allows for the use of very light torches, and still experience extremely long tip life. A 140 amp hand torch will not overheat running 200 amps on overlay work. Its even been reported that some customers only use one tip per week with Demand Pulse,  as opposed to several per day using conventional welders.

The first two pictures below are of an initial job done by a new customer using Demand Pulse MIG. The large skids pictured have (580) open butt, 316 SS pipe welds. The smaller manifold shown has (24) welds, for a grand total of (604) welds.

Only two qualified welders worked on this demanding job. They tested other types of equipment, all with unsatisfactory results. Then, after testing a Demand Pulse system, a good coupon was produced on just the second try, so the customer purchased a system and production started immediately upon delivery.

Astoundingly, the job was completed three weeks ahead of schedule, saving the fabricator 1600 hours of production. Most importantly, there wasn't a single repair required, and all welds passed 100% X-ray to ASME Code.

Large skids with 580 welds, all open butt 316 pipe.

Small manifold with 24 welds.

After experiencing Demand Pulse, this customer commented he'll never use anything else and that the welder paid for itself on the first job.

Below are close-ups of two of the welds on the large skid. The customer insisted on TIG wash on the cap. The TIG cap had 7 repairs - welds using Demand Pulse needed NO repairs.

TIG welds need 7 repairs.

Demand Pulse required 0 repairs.

For more information, contact:

2960 East State Street Ext.
Hamilton, NJ 08619
Phone # 609.588.0900
Fax # 609.588.8333
Email: dap@hotfoilehs.com

Check Out North America's Largest Metal Forming, Fabricating, Welding and Finishing Event

Come visit HotfoilEHS at FABTECH
The annual FABTECH exhibition is happening in Chicago this year on November 9th through 12th, 2015.

FABTECH is sponsored by the the Fabricators & Manufacturers Association (FMA), SME, Precision Metalforming Association (PMA), Chemical Coaters Association International (CCAI), and the American Welding Society (AWS).  There will be 1500 companies exhibiting all kinds of forming & fabricating, tube & pipe, metalform, welding, thermal spray, and finishing equipment.

HotfoilEHS is proud to be exhibiting their power consoles, ceramic mat heaters, welders, GRID systems, transformers and power supplies again this year at Booth N27044.

It's a great event where you can see the latest industry products and developments, and find the tools to  improve productivity, increase profits and discover new solutions to all of your metal forming, fabricating, welding and finishing needs.

For more information on FABTECH, visit http://www.fabtechexpo.com.