Should we repair or replace this boiler?
Before deciding, consider any demand load changes, and what type boiler is best
By Andy Wales
It goes without saying that steam plays a vital role in chemical and other type processing plants, and that having a reliable, cost-effective and safe boiler is essential to good operations. As a facility ages, it eventually gets to the point where it’s time to choose between upgrading the current boiler or getting a new one.
The existing boiler may very well be in good enough shape to justify repair or upgrade. That doesn’t necessarily mean spending more money on it makes any sense. Stop and consider before concluding that modernization suffices.
First, review in detail your current steam needs, regulatory requirements and fuel costs. In many cases, you’ll find marked changes since the boiler was first installed.
Determine your plant steam profile. If the demand load significantly decreased, the boiler may be running at a very inefficient, low firing rate. Replacing it with a smaller unit or multiple smaller units would save fuel and maintenance costs.
On the other hand, plant load may have increased to the point where the boiler is running at very high firing rates and experiencing increased maintenance costs. In this case, consider adding a new unit that runs more efficiently and keep your current boiler as a backup.
Things to consider
Local air regulatory requirements may have changed, reducing emissions allowed. Your existing boiler may not be grandfathered at its current output levels. If so, a retrofit could be required. If the manufacturer doesn’t offer burner replacement, this might mean replacing the entire unit. It might mean switching from oil to natural gas. A review of local rules with a qualified environmental consultant will clarify what rules you must now meet.
Fuel is by far the biggest expense in running a steam boiler. Check the cost of fuel and the amount used. Typically, efficiency improvement alone doesn’t justify a complete boiler replacement. However, combined with spending otherwise necessary for a burner retrofit or boiler heat exchanger repair, a complete replacement may be the way to go.
On the other hand, economic justification for upgrading with 1) a state-of-the-art programmable logic controller and 2) a servo-based burner-management system, is usually quite apparent. In addition, an economizer is a viable way to boost efficiency.
However, repairing a boiler that’s too small or inefficient isn’t the best use of capital budget. If steam output or pressure is changing, retrofitting generally isn’t possible.
Even when buying a new unit is a go, consider whether it’s worthwhile to repair the old boiler and keep it as a low-cost standby. And don’t automatically assume you should buy the same type of boiler.
Types of boilers
Before choosing a new boiler, it’s important to understand the types of units available. Almost all steam boilers are classified either as “fire tube” or “water tube.”
A fire-tube boiler typically is a horizontal vessel full of water with tubes running through it. The combustion gas flows through the tubes and heats the water around them. The gas can make from one to four passes through the water, depending upon boiler design. Another design point is the type of insulation, either dry-back or wet-back, on the end opposite the burner.
Typically, fire-tube units run up to 1,500 – 2,000 BHP. Larger capacities require a water-tube design.
In water tube boilers, the combustion gas flows around the tubes and water flows in them. Units come in a wide range of configurations such as bent tube, A-style, D-Style and O-style, based on the shape of the tube bundles.
Another major difference between the two kinds of boilers is in how water circulates inside the unit. Most water tube boilers use natural-heat circulation to move water between a vessel at the top, called the steam drum, and a collection manifold at the bottom, called the mud drum.
A third type of boiler, technically a forced-circulation water tube boiler, but commonly referred to as a steam generator, uses a mechanical water pump to control and circulate the internal water flow. Tubes normally are configured in a coil shape, so such units often are called “coil-type” boilers.
When purchasing a boiler consider the following factors:
Costs. Annual fuel cost to run the unit easily can be five times or more the expense to purchase and install it. So, weigh operating costs when comparing prices. The cheapest unit to purchase and install may end up costing you a lot more over a 10-year period.
Startup. If you don’t need steam 24/7, you’ll probably want to shut down the boiler at times. If quick startups are required, consider a steam generator. These efficient boilers can go from startup to steam generating in about 10 minutes.
Efficiency. By and large today’s boilers are much more efficient than ever before. Pay attention to a boiler’s steam efficiency rating, which can range from about 80% to well above 90%.
Fuel supply. Boilers use oil, coal, wood or natural gas. Each fuel has its own benefits and pitfalls. They also can vary greatly as to their efficiency. Local emissions rules may restrict your options. Natural gas usually is the most convenient.
Service/maintenance. A boiler usually operates for a long time. Always carefully consider vendors’ support capabilities when buying a unit.
Safety. Federal and state regulations make boilers much safer than ever before. Check the safety record of any boiler or boiler type before purchase to ensure it meets all safety standards and regulations. Look particularly at whether the unit is certified to the ASME Boiler and Pressure Vessel (BPV) Code. To find out if a boiler you are investigating has the BPV certification, log on to www.asme.org.
Andy Wales is Murrieta, Calif.-based western regional manager for Clayton Industries, Inc. Email him at Andy.firstname.lastname@example.org.