Pre Weld and Postweld (PWHT) Heat Treatment Furnaces

Industrial furnaces are used for pre and post-weld heat treating by welders and fabricators to heat-treat weldments or metallic parts. Industries that require these types of furnaces include pressure vessels and piping, storage tanks, building construction, bridge building, offshore platforms, petrochemical plants, power plants, and oil and gas refineries.

By using furnaces for PWHT,  stress introduced by the welding process is reduced and redistributed through the heating, soaking and cooling the weldment/machined surface. This greatly improves the weld properties. PWHT is most times mandatory in order to comply with welding codes and specifications for the welding of carbon steels, stainless/high alloy steels and work hardened steels.

Fairly sophisticated control systems carefully follow mandatory heating and cooling profiles, as outlined by welding code. Control systems with thermocouples that monitor internal furnace temperature, along with weldment part temperature are usually included. These control systems also include the ability to ramp up to temperature and cool off over specified time intervals. Finally, the use of recorders or data loggers is common for quality control documentation.

Additional benefits of pre and post weld heat treating are:

• Improved ductility and/or harness
• Lower risk of brittle fracture
• Improved metallurgical structure
• Reduction in diffusible hydrogen induced cracking 

Most pre or post weld heat treating furnaces are customized in one way or another, whether it size, total BTU output, energy source (electric, gas, oil), control scheme, door configuration, or having the furnace built on wheels or on rails.

Furnaces can be designed permanently, portable, or field erected. By having the furnace local, or right on-site, companies eliminate the need to send parts out and thus save time and costs associated with shipping. Depending on the scope of work, its not uncommon for the investment made in a fabricated furnace to pay off within the first year.

For more information on any custom built pre heat or post weld heat treat furnaces, contact:

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

Electrostatic Precipitator and Bag House Hopper Heaters Prevent Fly Ash Production Problems

Coal fired power plants must manage fly ash properly.

Coal fired power plants in the United States require the use of electrostatic precipitators or bag houses to filter out very fine fly ash particulate incorporated in the flue gas. The ash is collected as the flue gas passes through filter bags or by large electrodes, and then falls into collection hoppers.

As the hot fly ash cools, condensate on the hopper walls can form. The mixture of dry, sulfur rich fly ash and water is very problematic, so it's very important to keep condensation from forming in the collection hoppers. The mixture of water and fly ash can cause clogging (or “pluggage”) in the throat of the hopper,  and more importantly, the residual sulphur in the flue gas will combine with the condensate to form sulphuric acid. The suluric acid attacks the hopper walls from the inside causing corrosion, weakening the walls and creating significant (and costly) maintenance issues over time.

Fly ash hopper with heaters.

Efficient and continuous fly ash removal is critical for all coal fired power plants. Collection hoppers are an integral part of the removal process. Pluggage or inoperable hoppers are a known issue to engineers and maintenance crews. The constant maintenance and excess of down time seriously hinder a plant’s ability to manage the fly ash production rate. Slower fly ash production means limiting energy production and efficiency. A power plant’s electricity production is directly proportional with its coal combustion rate which, in turn, directly effects the fly ash production rate. Plant maintenance personnel usually attempt to remedy ash system failures in real time by deactivating the affected hopper while continuing to generate power and ash. In some situations (to prevent boilers shutdowns) ash will be dumped on the floor, requiring costly clean up.

The evacuation and management of fly ash is much easier when the ash is kept hot. One of the most common ways to maintain elevated fly ash and hopper temperature are electric hopper heaters attached to the external hopper walls. Hopper heaters play a very important role in the removal of fly ash from precipitators and bag filter hoppers by keeping the hopper temperatures operating above the flue gas acid dew point. The hopper heater’s sole function is to preheat the hopper and the internal environment to prevent the formation of moisture and the resulting clumping of fly ash and development of sulphuric acid.

Flexible heating element used on hopper throat.

Hopper heaters are designed for the dirty, high vibration environment of a power plant. They provide the optimum watt density for proper conduction through the hopper wall and for uniform heating. The are available in square, rectangular, and trapezoidal shapes to accommodate any hopper design. Ancillary flexible heating blankets are available for poke tubes, man-ways, and cylindrical throats.

The use of electric hopper heaters on electrostatic precipitator and bag house fly ash collection systems is a time proven, effective way to prevent condensate formation and the resulting clumping and corrosive acids in hoppers, thus facilitating a better opportunity for uninterrupted fly ash production.

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

www.hotfoilehs.com

HofoilEHS - Your Preferred Source for Pre and Post Weld Heat Treating

HotfoilEHS is a premier manufacturer of pre weld and post weld heat treating equipment. EHS manufactures and sells pre and post-weld heat treating equipment including power consoles, ceramic mat heaters, thermocouple attachment units, pin welders, Brinell testers, Poldi hardness testers, thermocuple wire, insulation, and accessories.

The below video is a little shameless, self promotion.

Power consoles are standardly offered in 6, 9, 12, 18, and 24 zone configurations with a variety of control and recording systems.

Twin heating modules, with and without recorders, and optional ramping controllers are available from EHS.

For special projects, EHS can provide mineral insulated (MI) heating circuits designed into a blanket-type layout, using clips to attach the circuits to a wire mesh in a configured pattern. The circuits are CSA and FM Approved for hazardous and non-hazardous areas.

EHS also offers short or long term rentals on power consoles, temperature recorders, and ceramic mat heaters along with complete support and training services.

EHS engineers, designs, and manufactures proven systems to effectively complete any project more efficiently than competitive systems, while staying within budget. Years of application experience and successful installations have produced thousands of happy customers.

Heat Transfer - The Basics

Industrial hopper heaters are
example of conductive heat transfer.

Heat transfer is the movement of heat from one body or substance to another by radiation, conduction, convection, or a combination of these processes.

When heating a pan of water over a gas flame for example, all three forms a heat transfer are taking place. Heat from the flame radiates in all directions. Conduction takes place with the transfer of heat from the burner to the metal pan. This heat transfer is also responsible for making the handle hot after a period of time. Water is heated by the process up convection which is a circular movement caused by heated water rising and cold water falling.

The process of heat transfer also occurs when an object cools. If a mug of hot coffee is left standing on a cold kitchen countertop, its temperature will gradually decrease as heat is lost. The heat energy dissipates by conduction through the mug to the table top by convection as the liquid rises, cools and sinks, and by the radiation of heat into the surrounding air.

One way to conserve the heat a liquid and prevent heat transfer is to place it in a thermos. The use a vacuum chamber with silvered surfaces, along with low conductive materials, can greatly improve the amount of heat or cold that is lost to the surrounding environment. In between the silvered glass walls up a thermos lies a vacuum. In the case of a hot liquid heat transfer by convection through the vacuum is greatly restricted due to the absence have air molecules necessary to facilitate the transfer of heat. The lack of physical contact between the inside and outside walls of the thermos due to this airless space also greatly inhibits the movement of heat by conduction. Heat loss by radiation is prevented by the silvered walls reflecting radiant energy back into the thermos. Some conduction of heat through the stopper in glass can be expected but this too is limited because they are made of materials with very low conductivity. Thus the temperatures are both hot and cold liquids can be maintained by a properly designed thermos that limits the transfer energy through radiation convection and conduction.

Heat capacity is the amount of heat required to change the temperature of an object or substance by one degree Celsius. The heat capacity of water varies depending on its phase. As solid ice, the heat capacity of water is .5 calories per gram for every one degree Celsius, which means it takes half a calorie to raise the temperature of one gram of ice one degree Celsius.

As a liquid, water heat capacity is one calorie per gram for every one degree Celsius, so it takes one calorie of heat energy to raise one gram of water one degree Celsius.

The processes of phase change between solid, liquid, and gas also require a specific amount of heat energy. The amount of energy required to change a liquid into a solid, or a solid into a liquid, is known as heat of fusion. The amount of heat required to change one gram of ice to water is 80 calories. Similarly the heat vaporization is the energy required to transform a liquid into a gas. It requires 540 calories to change one gram of liquid water into a gas. With these values its easy to calculate exactly how many calories of heat energy are required to transform one gram a ice at absolute zero to steam.

To warm 1 gram of ice from -273 degrees Celsius to 0 degrees celsius would be 273 times .5 gram per calorie or about 140 calories. The phase-change of one gram a ice to liquid water requires 80 calories. Then to heat the water from zero degrees Celsius to 100 degrees Celsius with the heat capacity at one calorie per gram, would require 100 calories. The final phase change it one gram a boiling water to steam would require an additional 540 calories. Adding all of these values together yields 860 calories, the amount of heat energy it takes to transform one gram gram of ice at absolute zero to steam.

Watch this video for an illustration of the above:

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. 

Ceramic Mat Heaters for Pre & Post Weld Heat Treating

Ceramic mat heater
for pre and post weld
heat treatment.

Flexible ceramic mat heaters (also known as flexible ceramic pads or FCPs) are designed to provide stress relief for pre and post weld heat treatments for large scale welding requirements, such as those needed for piping fabrication, boilers, pressure vessels, storage tanks, pipeline construction, and mining equipment.

It's very important to pre-heat metal to a specific temperature prior to welding. Pre-heating metals reduces stress in the finished weld,  as well as eliminates moisture and improves the metallic microstructure of the weld material.  For post weld heat treatment (PWHT), use of resistance heaters to apply carefully controlled  cool-down can significantly increase the cross-weld toughness of the heat-affected zone.

Ceramic mat heaters are manufactured using high grade nickel chrome (NiCr) 80/20 wire insulated with interlocking sintered alumina ceramic beads, providing flexibility and convenience for use on curved or flat surfaces. Their design also allows for good contact and high heat transfer. Ceramic mat heaters are intended for repeated use and have considerable mechanical strength and durability. They are capable of achieving temperatures of up to 2050°F.  and are most often are supplied with ceramic insulated leads with either Camlock or Dinse type welder terminations.

The most common voltages used are 40, 60, and 80 volts (as supplied by arc welders) but many custom voltages are available. Special sizes, heating patterns and terminations are also easily accommodated with ceramic mat heaters.

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