While a simple price comparison between new and used boilers may lead to the conclusion that a used boiler is the cost-effective way to go, such a surface-level analysis does not provide information required to make a truly informed decision.
Fire tube boilers have an average useful life of up to 30 years, but only if they are operated correctly, undergo continuous review of their combustion figures, and count on a rigorous control and treatment of their feedwater. The useful life of a fire tube boiler is dramatically shortened when it suffers damages to its furnace and tube sheets because of low water levels. For this reason, it is extremely critical to know how to evaluate the current condition of a boiler before making a used boiler purchase.
Evaluating a Used Boiler
The shell of the boiler, for example, suffers a loss of its required thickness with time. This loss is most acutely observed in the points closest to the steam exit and water entrance due to erosion and internal corrosion. Reduced thickness at any point in the shell of the boiler body means a decline in the pressure at which the boiler can be operated.
One way to determine the maximum allowable working pressure (MAWP) of a used boiler is by measuring wall thickness via ultrasound. This technique is relatively safe and cost-effective, and should be performed on at least 50 points of the boiler shell, including the periphery of the couplings:
- Steam exit
- Water feed
- Bottom blowdowns
- Surface blowdowns
The critical minimum thickness of the boiler shell can thus be detected and based on this measurement, the maximum allowable working pressure of a used boiler can be calculated.
The previous method can be applied only to the shell of the boiler, however this is not the only part of the boiler that can experience wear from use.
The furnace, which is directly exposed to the burner flame during operation, is an even more critical component to the longevity of a used boiler. It is exposed to the greatest amount of material fatigue over time as its steel frame absorbs the expansions and contractions caused by temperature changes (from “off” to “ high flame” to “low flame” to “off” for example).
The material fatigue of the furnace is produced by use over time but can also be caused by a lack of water or overheating; by poor water treatment; or by a defect in the boiler water level control.
With respect to the operation of a used boiler, this depends on the care with which it was run in the past – on the correct operation of its security controls and on the correct (or incorrect) handling of the boiler by its operator.
As discussed previously, a failure caused by low water level produces a tremendous amount of material fatigue in the furnace, thus reducing the material resistance of both the furnace and the tube sheets and considerably shortening the remaining useful life of the boiler. Unfortunately, this type of defect does not result in physical damage to the pressure body of the boiler and is not, therefore, easily detected. This is the same case for small explosions inside the combustion chamber.
The true condition of a used boiler can only be ascertained by its previous owner. Some used boiler salespersons will state that the boiler was hardly ever operated, never had low water levels or explosions, and that its control system is in “almost perfect” condition; unfortunately, this is almost never the case but will be hard to prove.
You get what you pay for, as the saying goes, with used boiler purchases, with “great deals” often turning out to be extremely expensive because of costly repairs. These repairs, moreover, are not recommendable because the pressure body of the boiler will never return to its previous top condition, while changing a tube sheet or furnace causes damage to the old material (via the cutting and welding), further wearing it out and increasing the risk to the future operation of the boiler.
Changing the tubes will not extend the useful life of the other components of the boiler. The tubes represent a part of the material of the boiler that should be changed periodically throughout its useful life. The frequency of the tube changes depends on the gauge of the tubes and the water quality control of the boiler.
The bottom line is it does not matter how a used boiler looks, if it has a new lining and tubes or no. What matters in terms of the useful life of a (used) boiler is the condition of the pressure body – the shell (more specifically its critical thickness), the furnace, and the tube sheets.
Benefits of New Boiler
A new boiler, equipped with a high efficiency burner, is ecological, consuming considerably less fuel than a used boiler. In fact, the acquisition cost of a new boiler is generally amortized by the fuel savings after very little time. The cost of a new boiler will “paid back” in terms of fuel savings, in about a year and a half. After that, the benefit to the owner is pure fuel cost savings over the rest of the useful life of the new boiler.
These impressive savings, coupled with the reduced risk of operational issues and the security of knowing with clarity both the current condition and expected useful life of your capital investment make purchasing a new boiler worth the additional upfront cost.