Danbury, CT – A boiler system functions as a critical component to the continuous operation of a facility. The loss of a boiler can cause disruption of operation and significant loss. Thus, it is important to maintain safe, reliable, and efficient operation while minimizing any downtime of the boiler system.
A boiler system consists of many sub-systems working in harmony, such as the boiler, the burner and its control, boiler control including feed water and draft control, fuel oil handling system (if burning oil is required), water treatment, fuel gas booster system (for areas with low supply gas pressure) etc.These sub-systems are sometimes procured from multiple sources. In order to deliver the safe, reliable and efficient service that the end user expects, it is advantageous to adopt a “full system integration” approach.
A boiler system in general could have many modes of failures. Failures in water level control have serious implications on the longevity of the boiler and in safety (the sudden inrush of feedwater to a baked-dry boiler could lead to a steam explosion). Water treatment failures can decrease the longevity and efficiency of the boiler. Boiler operators need to understand these dangers. Among all sub-systems, the burner system is by far the most sophisticated subsystem in a boiler system. The burner system has many modes of failure that require extensive training and/or experience for the boiler operators to fully understand.
When a boiler system is not delivering satisfactory performance to the end user, it is sometimes difficult to pinpoint the exact cause of the problem. The following example is used to illustrate this difficulty. Sometimes a burner makes a low frequency noise, often called a combustion rumble. The rumble could be a nuisance or discomfort to the operators and residents nearby, or could even cause damage to property. Potential causes of the rumble include, but are not limited to:
- The burner has poor stability at certain firing rates; or the burner’s window of operation is too narrow. This could be related to the design or manufacturing of the burner.
- The air/ fuel ratio is improper due to poor commissioning or lack of maintenance.
- The servos used by the burner control may have poor accuracy or repeatability.
- The linkage between servos and dampers may be loose.
- The system does not have oxygen trim to ensure consistent excess air levels. Any variation in draft, ambient temperature, fuel gas composition, building ventilation (affecting building inside pressure vs. outside ambient pressure), or wind speed blowing on outlet of chimney, can affect the amount of combustion air supplied by the fan.
- Lack of draft control. Severe draft variation may cause the air/fuel ratio to go out of range. This is definitely a challenge if the system does not have an oxygen trim system; it can be a problem even with an oxygen trim if the draft variation is too severe for the oxygen trim to compensate.
- The “acoustic coupling” between the burner and the boiler’s fire chamber and the subsequent space the flue gas flows through.
- The fuel gas booster could surge and cause the gas pressure to oscillate, beyond the pressure regulator’s ability to regulate.
- The boiler room’s ventilation system could be improperly designed. When windows and doors are shut, a significant negative pressure can develop in the boiler room, causing a drop in combustion air supply and air/fuel ratio.
- Fuel gas supply pressure and composition can fluctuate, especially if the fuel gas is from an alternative fuel source, such as land fill gas or, to a lesser degree, digester gas.
- Burner components may not work well together. For example, the gas regulator may be over-sized for the flow rates of the burner.
Problems with the Multiple-Vendor Approach
When the subsystems are procured from many different vendors piece-meal (by the general contractor or the end user) and no engineering firm takes responsibility for integrating these subsystems, it may be difficult to identify the party responsible for correcting the problem. This often results in blame shifting among different parties, ultimately frustration for the end user.
For example: in a piece-meal approach, the burner may be supplied by a burner company, the controls may be supplied by a company that is solely dedicated to burner controls and knows little about the combustion behaviors of the particular burner. The specifications do not call for a draft control or oxygen trim, when in reality one or both of those may be required for the site conditions and requirements. The booster, if there is one for the job, may be supplied by yet another vendor, the commissioning may be done by a contractor, the ventilation system of the boiler room may not have been designed properly to avoid high negative building pressure. The troubleshooting process itself is further complicated by the diverging interests of the different parties involved.
Sole Source Responsibility
The most important advantage of the full system integration approach is that the integrator must accept sole source responsibility. If the burner system does not perform, the integrator is responsible for correcting the problem. There is no blame shifting among different suppliers.
A burner system supplier that adopts the full system integration approach is inclined to build a long term relationship whenever it sells a job. The supplier would look at the specific conditions and requirements of the customer, and look for the best solution tailored for the job instead of chasing the latest trendy requirement in specifications. For example, it may be tempting to ask for a 12:1 or higher turndown from the burner system, but can the non-condensing boiler operate at 12:1 or higher turndown without condensation and corrosion problems? Is 10:1 or 8:1 turndown enough for the job? In another example, does the system require a draft control device to work? Can the burner deliver satisfactory performance without the draft system?
A supplier adopting the full system integration approach would look at total costs of ownership (the fixed costs and the operating costs) for the boiler system, instead of focusing on the fixed costs. In today’s corporate procurement practices, too often the one responsible for buying the boiler system is not the one paying the energy bill, hence there is less incentive to consider the total costs of ownership.
For example, a burner capable of operating at 1.5-2.5% oxygen during the majority of its operation time can lead to significant savings in fuel costs. If a vendor offers a burner system without use of oxygen trim, is the burner operating at consistent excess air levels all year round? Does the lack of oxygen trim mean conservatively high excess air levels? In another example, a fiber mesh burner may be used to meet 9 ppm NOx requirements without FGR, but the additional costs of fuel due to the very high excess air levels (typically 8-9% oxygen dry in flue gas) and the costs of replacing filters and fiber mesh combustion heads need to be factored in when purchasing a burner system. In another example, a burner constructed with flimsy, low grade sheet metals may need frequent service and replacement parts, while a burner constructed out of durable steel can provide years of service beyond the normal warranty periods.
The “full system integration” approach requires an integrator to have in-depth understanding and strong product offerings in all of the following areas:
- Boiler controls. The boiler controls ensure safe and smooth operation (water level control, burner firing rate based on temperature or pressure, draft control if necessary). It should have the capability to manage the lead-lag control of multiple boilers to ensure the boilers are operating at maximum efficiency.
- Fuel oil handling systems (main tank, day tank, pump sets, filtration, leak detection, etc.)
- Burners–especially those designed for both high efficiency and low emissions at the same time. The end user should not be forced to choose between high efficiency and low NOx. High turndown (such as 10:1) helps avoid cycling of the boiler, and low excess air minimizes loss of heat to flue gas. Use of FGR is acceptable, but the incremental costs of running a larger motor due to FGR should be factored in. Advanced designs of burners can achieve mandated NOx emissions with less, little, or no FGR (depending on the NOx levels required).
- Burner controls. The burner must be equipped with the latest Burner Management/ Combustion Control Systems (BMS/CCS) to assure that safety aspects are in accordance with the latest requirements of NFPA 85 and CSD-1. When high efficiency or tight emissions are required, an oxygen trim system should be included, and parallel positioning or fully metered control should be used in lieu of jackshaft. The combustion control and the servos should be designed to modulate the controlled fluids (air, fuel, FGR etc.) in a coordinated manner. For example, if the air servo cannot move fast enough to be in sync with the fuel servo, then the fuel servo needs to be slowed down in modulation, and vice versa.
- Commissioning and maintenance. The burner system is commissioned and maintained by qualified service technicians that are knowledgeable about all the subsystems.
- Technical support and spare parts. These should be available from nearby locations.
Preferred Utilities Manufacturing Corporation has earned a reputation for accepting single-source responsibility. We firmly believe in the advantages of full system integration. Compared to the piece-meal approach, the benefits of full system integration make the choice clear. If you believe the same way, please contact us about your next project.