When it comes to boilers, the work — in this case, the work is heat transfer — is done by the boiler tubes. The design type of the boiler (firetube, watertube, etc.) changes the orientation and layout of the boiler but the boiler tubes are still transferring the heat from the fire side to the water side. It is for this reason that boiler tube maintenance and repair are essential to proper boiler operation.

Boiler tubes undergo a tremendous amount of stress due to the large temperature gradients, so the tube design, material, and thickness have all been carefully selected for optimum and efficient operation. The importance of the proper operation, maintenance, and repair cannot be understated as the boiler tubes may be the most crucial component for most boilers. Repairs to boilers built to ASME Boiler & Pressure Vessel Code (BPVC) must be repaired by a National Board certified repair company to ensure compliance with all the required rules and regulations.

At their core, boilers are really just large heat exchangers that take the energy from combustion and pass it into the water on the other side of the internal components (tubes, drums, furnace, etc.).

One of the first questions that come about when looking into boilers is the different types. Two of the main types of boilers are firetube and watertube. The main difference is pointed out in the name but is clarified in the definitions, advantages, and disadvantages below.

A pressure vessel boiler, in which the exhaust gases pass through one or more tubes to transfer heat into the surrounding water of the sealed containment.

Advantages of Firetube Boiler:

• Simple design
• Ease of maintenance and repair
• Lower initial cost
• Higher water volume per boiler horsepower

Disadvantages of Firetube Boiler:

• Lower capacity (Typically up to 2,500 BHP or 86,250 PPH)
• Lower operating pressure range (Typically < 350 PSIG)
• Larger footprint for rated output
• Longer start-up time

A pressure vessel, boiler, in which the exhaust gases travel between small diameter tubes to transfer heat into the water circulating through the tubes between two larger drums. Watertube boilers are more efficient than firetube boilers.

Advantages of Watertube Boiler:

• Smaller footprint for rated output
• Larger steam output capabilities
• Ability to add internal/integral superheater
• Higher operating pressure range (1,000 PSIG+)

Disadvantages of Watertube Boiler:

• Lower water volume per boiler horsepower
• Higher Maintenance Costs for Repairs
• More strict water quality requirements
• Higher initial procurement cost

Boiler tube failures can be caused by many factors, but there are a few common reasons for failure that we see often.

The top reasons we see for boiler tube failures in firetube boilers are:

1. Metal Overheat Rupture: Poor water quality, water side scale
2. Corrosion/Thermal Fatigue: Lack of proper water treatment/heating
3. Tube End Cracking: Tube end extending beyond the tubesheet more than code allowances

The top reasons we see for boiler tube failures in watertube boilers are:

1. Metal Overheat Rupture: Quick start-up or flame impingement
2. Metal Overheat Rupture: Excessive waterside scale or fireside soot
3. Corrosion/Thermal Fatigue: Poor water quality/treatment

Over the years, there have been many boilers that have been “repaired” for extended periods of time by a temporary solution meant to provide enough time to adequately prepare for a boiler shutdown. Many boiler room personnel and boiler repair companies have installed boiler tube plugs in firetube and watertube boilers not realizing the impact on boiler performance and operation.

At Powerhouse, we strongly advise all of our clients to only consider a boiler tube being plugged in an emergency situation when a proper shutdown and repair is not possible but is planned in the near future. In both firetube and watertube boilers, the majority of the heating surface area is accounted for in the boiler tubes. When any boiler tubes are plugged, there is a loss of surface area which reduces efficiency, lowers the output capacity, and creates an additional failure mechanism:

Boiler tube plugs are installed in a ruptured tube which now is filled with high pressure boiler water. These plugs, when not properly installed by a certified repair company, can become dislodged and shoot at a high velocity from the tubesheet doing further damage to your boiler.

Boiler tube plugs in a watertube boiler result in water no longer flowing through the tube, which is what keeps the tube from overheating as the heat from the combustion/flue gas needs to pass through the metal into the water. Depending on the location of the tube, plugging a watertube boiler tube requires slicing the entire length of the tube and installing refractory to protect the exposed metal from overheating and further damage being done to your boiler.

The proper repair of a boiler tube failure must be performed by a National Board certified repair organization (R-Stamp) and in accordance with the appropriate code. The basic steps for boiler tube replacement are as follows:

1. Assess the failure, perform any preliminary testing, and develop an approved repair plan with an Authorized Inspector that will be acceptable to the code and local jurisdiction.
2. Remove the failed tube and determine the probable failure mechanism to assist in operational changes that could prevent future failures.
3. Assess the condition of the tube end connection locations (tube sheets, steam drum, mud drum, etc) and perform any additional NDE required.
4. Installation of the new boiler tube in accordance with the code requirements (rolled, beaded, welded, etc.).
5. Perform a pressure test in accordance with the code requirements and any additional NDE to verify the integrity of the repair.
6. Complete the repair forms and submit them to the National Board and jurisdiction, as required and install the repair plate on the boiler.

Additionally, depending on the tube location, it is possible to repair a watertube boiler tube without replacing it via weld overlay, patching, or section replacement. This is determined on a case-by-case basis but can save considerable time and money, as removing and replacing a single watertube boiler tube is typically very time-consuming and requires sacrificially replacing nearby tubes as well.

Since boiler tubes are the most crucial component for proper heat transfer, retubing (when required) will provide the following benefits:

1. Extend the useful service life of the boiler
2. Restore boiler efficiency
3. Avoid unscheduled downtime from failing boiler tubes