McKenzie’s Boiler Blog

The fact is there are key feature differences between firetube boilers.

The efficiency of a firetube boiler is not a mysterious calculation. High efficiency is the result of tangible design considerations incorporated into the boiler. Reviewing some basic design differences from one boiler to another can provide you with valuable insight on expected efficiency performance. The following design issues should be considered during your boiler evaluation.

Number of boiler passes

The number of boiler passes simply represents the number of times the hot combustion gas travels across the boiler (heat exchanger). A boiler with two passes provides two opportunities for the hot gasses to exchange heat to the water in the boiler. A 4-pass unit provides four opportunities for heat transfer. Many comparisons have been made regarding efficiency and number of boiler passes but, the facts are clear and indisputable. The stack temperature of a 4-pass boiler will be lower than the stack temperature of a similar size 2- or 3-pass boiler operating under similar conditions. The 4-pass will have higher efficiencies and lower fuel costs. This is not an opinion. It is basic heat exchanger physics. The 4-pass design yields higher heat-transfer coefficients.

Many times the lower pass unit will include turbulators or will be tested at less than capacity firing rates to prove lower stack temperatures. Don’t be fooled. Turbulators may help pass an efficiency test but will cost you in maintenance down the road. In fact, you would not need maintenance intensive, boiler tube, add-on devices if the boiler was designed for proper flue gas velocities in the first place. Each boiler pass should be designed with a cross sectional area providing proper flue gas velocity and heat transfer. When it comes to efficiency, the proof is indeed in the passes and in correct heat transfer design.

Burner / boiler compatibility

The term packaged boiler is sometimes used even if a burner manufactured by one vendor is bolted on to a boiler manufactured by a different vendor. Is bolting a “Buy-out” burner on a vessel really a packaged boiler? And more importantly, why does it matter? A true packaged boiler/burner design includes a burner and boiler developed as a single unit, accounting for furnace geometry, radiant and convection heat transfer characteristics, and verified burner performance in the specific boiler package. Development as a truly packaged unit assures the performance of the unit is proven and verified during development.

You can put an engine from one automobile into a different automobile. The car will probably run. It will get you from point “A” to point “B.” But how about performance? Will the car give fuel efficiency and reliable performance for the life of the car? Would you take a long trip where you had to depend on such a car? And if you need service, who will take accountability to repair and guarantee the car?

A boiler provides the same scenario. The buy-out burner will fire the unit. But, will you have capacity, efficiency, turndown, excess air performance and emission performance too? And, who will make sure the unit gives you performance after the initial start-up? Is there even a single accountable manufacturer to make the unit perform in the first place? Buy-out burner packaging can result in lower performance levels and higher start-up and maintenance requirements. It also can cost you money every time you have a problem and the local service people cannot get factory support. You may think you saved money with a buy-out burner package. But did you really?

Repeatable air/ fuel control

The efficiency of the boiler depends on the ability of the burner to provide the proper air to fuel mixture throughout the firing rate, day in and day out, without the need for complex set-up or adjustments. Many burner designs can deliver the required air-to-fuel mix with enough time provided to adjust the burner or for a single test period. The problem is many of these complex linkage designs don’t hold air to fuel settings over time. And, often, they are adjusted at high excess air levels to account for the inconsistency in the burner performance. The fact is you pay for the unit based on the actual ability to operate efficiently. When it comes to choosing the burner, insist on a simple linkage assembly and accessible burner design for true efficiency and real savings.

An additional burner feature to look for is the fan design. Squirrel cage type fans do not provide as reliable air control as a reverse curve fan will provide. Aluminum cast fan design also provides tight tolerances and maximum fan life. Furthermore, register or blade type damper assemblies tend to have limited control of air at low firing conditions and tend to be much less repeatable than radial damper designs. Control of combustion air is critical to burner performance. If the burner cannot provide repeatable air control, again the typical solution is to set the burner up at high excess air levels, resulting in substantial dollars wasted every time you fire the unit. The facts are clear: Reverse fan and radial damper design result in high efficiency and repeatable fuel savings, thus performance paying dividends throughout the life of the boiler.

Heating surface

The heating surface in square feet per boiler horsepower represents, in general terms, how hard the vessel is working. The standard heating surface for a firetube boiler is five square feet per boiler horsepower. How do we know this? Cleaver-Brooks set the standard and provides five square feet as a base design criteria for our firetube products. Proper heating surface means longer boiler life and higher efficiency. Five square feet is the standard.

Vessel design

Pressure vessel design is regulated by strict ASME code requirements. However, there are many variations in vessel design that will still meet the ASME codes. Water circulation, low stress design and accessibility are key criteria for proper pressure vessel design. Specific features to look for include a single tubesheet design. Single tubesheet design provides minimum weldments for low tube sheet stresses and excellent water circulation. In addition to the single tubesheet design, the boiler should include proper tube spacing, cross sectional area sizing in each pass for proper heat transfer, low furnace location, and proper inlet and outlet location. Proper circulation must be incorporated into the design for highest boiler efficiency and longevity. Fully accessible front and rear tube sheets for ease of inspection and low retubing costs are also key design criteria to look for. You will inspect your boiler often, usually every year. Single tube sheet design assures the longest lasting tube sheet and longest tube life. Accessible front and rear heads assure the lowest inspection and re-tubing costs if they occur. Both result in the highest efficiency and lowest possible maintenance costs for your boiler equipment.

When you buy a boiler, you really are putting a down payment on the purchase of steam or hot water. The payments to generate the power are ongoing over the life of the equipment and are driven by fuel-to-steam efficiency and maintenance costs. Even with economical fuel costs, the selection of a high efficiency boiler will result in substantial cost savings. A boiler installation costing $75,000 can easily consume over $400,000 in fuel every year it operates. Selection of a boiler with “designed-in” low maintenance costs and high efficiency can really provide savings and maximize your boiler investment.

Efficiency is only useful if it is repeatable and sustainable over the life of the equipment. Choosing the most efficient boiler is more than just choosing the vendor who is willing to meet a given efficiency value. The burner technology must be proven to be capable of holding the air/fuel ratio year in and year out. Make sure the burner design includes reliable and repeatable features. How do you tell? Ask any boiler technician who has worked on a variety of boiler/burner designs. Burners with high pressure drop design, quality fan and damper design, and simple linkage assemblies are easy to tune and accurately hold the air-to-fuel ratios. Burners with blade or louver damper designs and complex linkage assemblies tend to be harder to set-up over the firing range of the boiler and tend not to accurately hold the air to fuel ratio as the boiler operates.

Why choose the most efficient boiler? Because the dividends paid back each year far outweigh any initial cost savings of a less efficient design. What is the most efficient boiler? One that not only starts up efficiently but continues to operate efficiently year in and year out.

Replace or Repair.

The decision to purchase a new boiler is typically driven by the needed replacement of an old boiler, an expansion of an existing boiler room, or construction of a new boiler room facility.

When considering the replacement of an old boiler, review the following points to make sure you are performing a comprehensive evaluation of your situation.

Maintenance Costs

Review your maintenance costs carefully. The old unit is costing you money in various ways, including emergency maintenance, downtime, major maintenance requirements (past and pending), difficult-to-find and expensive parts requirements, operator time in keeping the unit on-line, and overall vessel, burner, and refractory problems. Many of these costs can be hidden within your overall maintenance budget. You are paying the price for having outdated boiler room equipment. But the costs need to be investigated and totaled.

Boiler performance

New packaged firetube boilers have much higher performance standards than older design units. Turndown, excess air, automatic operation, accurate-repeatable air/fuel ratio burner designs, computer linked combustion controls, low emission technology, and high guaranteed efficiency all are now available on premium designed packaged firetube boilers. The result is low operating costs and automatic power generation for your facility. All cost saving reasons to consider a new packaged firetube boiler.

Fuel Usage

If your old unit is designed to fire low grade fuel oil, or if you need to evaluate propane or any other different fuel capability, review the conversion costs along with existing maintenance, performance, and efficiency issues to see if the time is right to consider a new boiler purchase. Many times an investment is made in an old unit when the costs associated with the next major maintenance requirement will justify a new unit. The result is wasted money on the old unit upgrade.

Efficiency

Your Cleaver-Brooks representative can help check out the efficiency of your old boiler with a simple stack analysis. The data will give you a general idea of the difference between the fuel cost of the existing boiler and a new unit. Based on the results of the stack evaluation, a more comprehensive evaluation of your boiler room requirements should be performed. Boiler size, load characteristics, turndown requirements, back-up requirements, fuel type, control requirements, and emission requirements, all should be evaluated. The result will be an accurate review of the potential savings in fuel, maintenance, and boiler room efficiency that can mean substantial cost improvement for your facility.

Running into a pilot flame failure or main flame failure may be a result of a damaged, aged, or dirty scanner.  Check your scanners lens so it may provide the best flame signal.  Checking or cleaning this regularly may prevent these failures and make operating your boiler to its full potential a bit more simple. 

To prevent furnace explosions, it is imperative that boiler operators purge the boiler before ignition of the burner. Workers should check the fuel to air ratio, the condition of the draft, and the flame to make sure that it is not too high and not smoky. Ventilation systems should also be inspected and maintained to make sure that combustion gases do not build up in the boiler room.

                         Item                                                                              Item

Gauges, monitors, and indicators - Daily                       Operating control - Annually              

Instrument and equipment settings - Daily                    Low draft, fan, air pressure, and

Firing rate control - Weekly                                                damper position interlocks - Monthly

Flue, vent, stack, or outlet dampers - Monthly             Atomizing air-stream interlock - Annually

Igniter - Weekly                                                                 High and low gas/oil

                                                                                                pressure interlocks - Monthly

Fuel valves pilot and main - Weekly                               High and low oil

                                                                                                temperature interlocks - Monthly

Pilot and main gas or main oil - Annually                       Fuel valve interlock switch - Annually

Combustion safety controls - Weekly                            Purge switch - Annually

Flame Failure - Weekly                                                      Burner position interlock - Annually

Flame signal strength - Weekly                                       Rotary cup interlock - Annually

Pilot turn-down tests - As required/annually                Low fire start interlock - Annually

Refractory hold in - As required/annually                     Automatic changeover

                                                                                               control (duel fuel) - Annually

Low-water fuel cut-off and alarm - Daily/weekly          Safety valves - As required

High limit safety control - Annually                               Inspect burner components - Semiannually 

• Noises during oil burner startup - a “bang” or “puffback” which blows soot into the room through the barometric damper or through other equipment openings: the oil pump may not be shutting down properly at the end of an oil burn cycle, leaking incompletely burned oil into the combustion chamber. That oil ignites at startup causing a potentially dangerous puffback. Immediate service and repair are needed.
• Noises during oil burner startup - a “rumbling” sound (which usually continues all during operation” or a “stumbilng” sound in the combustion chamber probably indicates that the system needs inspection and cleaning very soon. Some noise is normal however, but the normal sounds tend to be more smooth and continuous.
• Noises during oil burner shut-down - a stumbling or rumbling after the oil burner motor has stopped, indicate that oil is continuing to leak into the combustion chamber and risks a dangerous puffback - see “Noises during oil burner startup” above. Immediate service is recommended.
• Noises of shrieks or grinding coming from the electric motor or oil pump on the oil burner mean that immediate service is needed - probably a bearing is failing.
• Startup problems: noises and clues of puff back: if you see flapping at the barometric damper or if you see or hear vibrations in the system, prompt service is needed
• Noises from radiators or heating baseboards:
  • Clanking heating pipes or sharp snapping noises may be heard as a normal consequence of expansion of metals during the heating cycle. These noises can often be eliminated or reduced by careful routing of piping and by allowing room around heating pipes for expansion, but probably not elminated in the case of hot water baseboards.
  • Bubbling or rumbling noises in hot water heating piping can be caused by air in the heating lines. If the amount of air becomes excessive the system may be unable to circulate hot water and extra steps to bleed unwanted air will be required.
  • Hissing sounds such as air escaping from radiators or othe piping where air bleeder valves are installed are normal but should be brief and uncommon. If you constantly hear air hissing from radiator bleed valves double check that you understand what kind of heat you have - hissing from bleeder valves on steam heat radiators as heat is coming up in the building is normal.

When observing evidence of leaks on a heating boiler, keep these points in mind:

• Even serious leaks may never show up as “wet” spots: A boiler may be leaking but you may see no actual water: during the heating season the boiler may always be hot, causing small leaks on the boiler or on heating piping to simply evaporate. But such leaks will usually be visually very evident: look for a build-up of corrosion, green or white or other colored mineral salts, or look for rust or water stains on the equipment.
• Internal heating boiler leaks: Some critical boiler leaks may be internal and not visible by simple inspection, such as a leak inside the boiler heat exchanger which may pass water into the combustion chamber. A service technician or a skilled home inspector should be able to spot evidence of these leaks.
• Surface rust, light, superficial rusting, is generally repairable. Clean the area and fix the leak when the boiler is next serviced and monitor for any future leaks.
• Exfoliation, or thick flaking rust on any boiler but particularly on a steel heating boiler is very serious, possibly not repairable, and risks loss of the boiler as well as sudden loss of heat in the building.
• Leaks related to temperature or pressure: Some leaks occur only at peak operating temperature–eg at relief valve. On some heating boilers such as some cast-iron units, leaks may occur between boiler sections when the system is cold - on these models some technicians prefer to keep a little heat in the boiler year-round to avoid this problem. Leaks between boiler sections may be repairable but if left unattended can destroy the equipment.

A Catalog of Common Heating System Leak Points - Where to Watch for Heating System Leaks

• Tankless Coil mounting plate - see rust stains below and around plate
• Pipe fittings at face of coil plate - mineral salts
• Leaks around bolt openings - suspect hidden damage
• Leaks between sections of a cast iron boiler
• Leaks at the circulating pump mounting flanges
• Leaks at the boiler temperature/pressure relief valve. This leak may be very dangerous as corrosion from water passing through the valve may prevent its safe operation in an emergency. Prompt expert inspection and repair are needed. Watch for leaks below the valve’s mouth or discharge pipe (a pipe should extend from the relief valve to a few inches from the floor) or watch for corrosion at the tip of the discharge pipe. Gently feel inside the tip of this pipe to see if it’s wet. DO NOT TRY TO TEST or open or operate the relief valve itself.
• Leaks at air bleeder valves - at the boiler or remote where such bleeders are placed on heating piping or baseboards or radiators
• Leaks at radiator control valves
• Leaks at poorly-soldered copper pipe fittings on finned copper baseboard heating systems
• Leaks due to frozen and burst piping or in extreme cases, frozen and burst heating boilers themselves

A relief valve is a valve mechanism for the automatic release of a subtance from a boiler when the pressure or temperature exceeds the presest limit. Safety valves were first used on steam boilers during the industrial revolution. Early boilers without them were prone to accidental explosion.

The earliest and simplest safety valve in 1679 used a weight to hold the pressure of the steam, however, these were easily tampered with or accidentally released. A valve less sensitive used a spring to contain the steam pressure, but these could still be screwed down to increase the pressure beyond design limits. This dangerous practice was sometimes used to marginally increase performance. In 1856 John Ramsbottom invented a tamper-proof spring safety valve which became universal.

The two general types of protection encountered in industry are thermal protection and flow protection.

For liquid-packed vessels, thermal relief valves are generally characterized by the relatively small size of the safety valve necessary to provide protection from excess pressure caused by thermal expansion. In this case a small valve is adequate because most liquids are nearly incompressible, and so a relatively small amount of fluid discharged through the relief valve will produce a substantial reduction in pressure.

Flow protection is characterized by safety valves that are considerably larger than those mounted in thermal protection. They are generally sized for use in situations where significant quantities of gas or high volumes of liquid must be quickly discharged in order to protect the integrity of the vessel or pipeline.

In most countries, industries are legally required to protect pressure vessels and other equipment by using relief valves. Also in most countries, equipment design codes such as those provided by the ASME, API and other organizations like ISO (ISO 4126) must be complied with and those codes include design standards for relief valves.

Temperature/Pressure Relief Valve Testing Advice

This boiler safety device contains mechanical or electronic sensor to moniter the water level inside of the boiler. They are installed on many hydronic heating residential boilers and basically all steam boilers, they are also installed on all comercial boilers of both types. The low water cutoff is a device intended to to turn off electrical power to the oil burner should the water level or pressure in the system fall below a safe level. Low water cutoff valves are installed on all steam boilers, most commercial heating boilers, and some home heating boilers (hydronic or hot water heating systems).

Original low water cut off valve designs used a mechanical float which operated not unlike the float arm in a toilet tank. As water level drops the arm moves down and ultimately trips a mechanical switch that operates an electrical contact to turn the heating system off. Although because they were subject to jamming due to sludge that forms in the steam boiler as water is lost and mineral debris is left behind, they designed a new switch which is electronically controlled.  These newer controls replace the mechanical float switch with a sensor, reducing the chances of a cutoff malfunction.

Overall by monitoring boiler water level and turning off the oil or gas or electric heat source to the heating boiler should water level drop too low in the steam boiler, this important safety device prevents damage to the boiler should the system lose its water.

If mechanical low water cut off’s are used they need to be flushed to remove sediment that could prevent the cutoff from working. Since steam heating systems are constantly using water, losing some of it as water vapor venting at steam radiators, and regaining water as the automatic (or on some systems manual) water feeder replaces water in the system, these systems tend to produce sediment at the boiler.

If sediment collects in the low water cutoff valve it could prevent the valve’s internal float from falling as water level in the steam boiler drops, thus preventing the valve from safely shutting down the boiler should water level fall to an unsafe level. For this reason the low water cutoff valve needs to be flushed regularly, often once a week.

Steam Boiler Low Water Cutout Valve Flushing Procedure

When we flush a steam boiler or a hydronic heating boiler low water cutoff valve we:

• Place a 5-gallon bucket under the end of the flush valve drain pipe, being careful not to bang into piping and maybe cause a leak

• Open and shut the flush valve several times, opening it briefly - just a few seconds, then closing it each time. This avoids a large surge of cold water entering (and possibly damaging) a hot steam boiler, and it also seems to help stir up and remove sediment from within the low water cutoff valve.

• Watch for clear water: After flushing the low water cut off valve several times, when we see that clear water is coming out of the boiler, the job has been done. But notice: often if you hold the flush valve open longer than just a few seconds, you may see clear water coming from the boiler.

• Check for clear flush-water once more: But if you close the flush valve and open it again you may see new brown sludge water coming out of the valve again! So when we think we’ve cleaned out the low water cutoff valve by this flush procedure, we close the valve, wait about 15 or 20 seconds, and then try it once more. If we see that the first water coming out of the valve on this last try is clear too, then we figure we’ve flushed out the valve successfully.

• If an automatic water feed valve is installed: you’ll hear new water flowing into the boiler just after each time you open and flush the low water cut off valve. That’s another reason to do this flush procedure gradually - so we don’t damage the boiler by filling it with lots of cold water when the boiler is itself very hot. 

• If only a manual water makeup feed valve is installed: you’ll need to add water to the steam boiler, filling it back to the “full” line marked on or behind the sight glass. It’s best to add water to the steam boiler slowly to reduce the chances of cracking (more of a risk with cast iron boilers).

• Usually the total volume of water flushed is 2-4 gallons.

• Dump the flushed boiler water down a building drain or into another approved outdoor area where some rust and sludge won’t contaminate anything, but to not dump hot water into a cold sink or toilet - it may crack.

Safety warning: Be careful, when a steam boiler has been running, water coming out of the low water cutoff flush valve is hot and can scald a bystander.

Boiler systems are designed for safety and efficiency. The boiler operator is the key to safe boiler operations. Having knowledge about boiler systems and maintenance can ensure years of safe, reliable service.

History has shown that without proper operation and maintenance, boiler conditions and safety deteriorate causing potential hazards due to neglect and misunderstanding. Routine maintenance is well within the ability of most boiler operators. Boiler tune up and repairs, however, are best left to trained professionals. Understanding when to turn to qualified professionals for assistance is one of the operator’s responsibilities and can save time and money. Some of the areas where trained professionals are needed are:

• Leaking safety and or safety relief valves
• Feed water to boiler
• Steam leaks (steam systems)
• High stack temperatures (excess of 350ºF)
• Insufficient heat for building
• Condensate dripping down stack or out the front of the boiler
• Constantly resetting of controllers and safety devices

Boiler accidents can occur when the boiler is allowed to operate without adequate water in the boiler. Proper functioning low water cutoffs are essential to prevent these types of accidents. Boiler damage can run from severe buckling and deforming of the boiler to complete meltdown or potential boiler explosions.

Another type of boiler accident and the most lethal is excessive pressure. These accidents occur when the boiler can no longer contain the excessive pressure allowed to build in the boiler. Excessive pressure accidents, even in small boilers, have been known to completely destroy a building.

Fuel related accidents usually occur when there is a failure to purge combustible gases from the firebox before ignition is attempted. Leaking fuel valves can also be the cause of these accidents. If the operator notices any gas odor, the boiler should be shut down and the fuel supplier notified immediately.

“Never bypass safety devices with jumper wires to restart your boiler. Unintended ignition of unburned combustion gases in the fire box is possible.”