The State of the Industry as We See It
BOILERS are inarguably one of the most critical assets in a process plant. They often play an essential role in several downstream processes. They frequently represent 30-50% of a plant’s energy usage. Understandably, managers must ensure that they are well maintained and regularly inspected, running both efficiently and reliably.
Certain aspects of boiler technology have advanced substantially in recent decades. Adoption has been fairly rapid in residential and commercial applications, where it is both economically sensible and physically feasible to replace the entire boiler rather than repairing the existing one, which may not even an option for many lighter-duty class boilers.
On the other hand, adoption of new technologies has been much slower in the industrial process sector. In the USA, roughly half of the boilers in service are more than 25 years old. We regularly see boilers that are 50+ years old, running on antiquated or obsolete components.
We believe there are several reasons for this:
- Complete replacements are large expenses, and physically challenging in process facilities.
- It can be very difficult and costly to manage the downtime required for a replacement.
- Most industrial boilers are designed to be field repaired. (eg. tube replacements)
- Most failures are unanticipated, so the only feasible option at the time is to repair.
- Lack of awareness of new technologies, and the benefits that they provide.
Many process boilers in service are 25 to 50 years old.
HOWEVER, you do not need to replace the boiler to access the benefits of new tech. The fact is, industrial boiler vessel designs have remained mostly unchanged for decades. Almost all major improvements to date are to do with the burner and the controls, which can be upgraded in-situ for a small fraction of the cost of a total replacement, with minimal or no downtime required.
Conventional Burner Technology: Linkage Modulation
Example of a burner with linkage modulation controls.
This is an example of a typical dual-fuel modulating burner on a 25 year old steam boiler. A single “mod motor” simultaneously controls the positions of the air intake damper, gas butterfly valve, and oil control valve via a series of mechanical linkage arms and jackshafts. Shaft lengths and positions are calibrated to adjust the functional range of each control element, as well as their relationships to each other. This is what determines the “air-fuel ratio” during operation.
To the right is a basic “flame safeguard” controller which sequences the start-up, operation, and shutdown phases. It also supervises the flame detector and status of safety limit switches. If an alarm event is triggered, the fuel safety valves will shut closed.
Example of an OEM burner control panel with a digital flame safeguard.
The burner’s firing rate is managed separately by a temperature or pressure controller, most often a mechanical “proportional control” as shown in the photo to the right. This means the firing rate is strictly proportional to the difference between the desired set-point and the measured temperature or pressure.
Example of proportional pressure controls on a steam boiler. Setpoints and differential ranges are adjusted via a screwdriver.
Common Problems With Linkage Modulation
1. Potential Safety Issues
There is no position feedback from the mod motor or any of the connected control elements. Therefore, there is nothing to alert an operator or stop the burner from running if there is a problem with the motor, linkage assembly, valves, or dampers. It is fairly common for linkage to wear out, become loose, deform, and even break. We have seen cases where linkage failure has caused dangerously bad combustion, leading to extreme heat exchanger fouling and hazardous gas emissions.
Linkage is relatively delicate and prone to failure, as shown above.
Example of extreme fouling caused by a linkage failure.
2. Reliability issues
By their nature, most linkage assemblies are relatively delicate and prone to losing accuracy over time due to repetitive movement and exposure to vibration. This results in a lack of repeatability in the air-fuel ratio, a phenomenon known as “hysteresis”. If tune-ups are not performed regularly, this can eventually lead to issues with light-off reliability, loss of flame, and poor combustion.
3. Sub-par efficiency
There are a few reasons why these conventional controls are not very efficient;
- Due to the inherent lack of precision and repeatability, tuning a linkage system must be done conservatively to ensure that enough excess air is available for stable combustion while accounting for hysteresis. As excess air increases, boiler efficiency decreases. The optimal range for excess air is 15-30%.
The ideal balance of efficiency and clean combustion typically occurs around 3% O2.
- The minimum firing rate is generally limited to the ignition setting, even if the burner is capable of running lower after a flame is established. This is because the air + fuel controls cannot operate independently, and the mod motor lacks an independent “ignition position”. On dual-fuel burners, the turndown range is often hampered further. Limited turndown causes more frequent cycling, which wastes fuel and accelerates component wear.
- The limitations of a basic proportional pressure/temperature controller can also cause excessive cycling, because it can’t be optimized for the system’s load profile.
The effect of burner cycling due to poor control and inadequate turndown.
Modern Burner Technology: Parallel Positioning
In industrial boiler applications today, most burner manufacturers have stopped using linkage systems, and have moved on to some form of parallel positioning or “linkage-less” control platform.
This concept involves a single digital controller, or combination of controllers known as a “burner management system” combined with two or more servo motors which control the air, gas, and oil flow independently from each other. These elements are controlled electronically by a digital fuel-air ratio controller (FARC), with each control motor sending a live feedback signal back to the FARC. This unlocks incredibly fine control resolution and excellent repeatability, so tuning can be done precisely to maximize safety, reliability, and efficiency. A dedicated ignition point allows us to push the low-fire point lower, increasing the turndown range. These are the key factors that contribute to typical fuel savings of 5-10% or more, compared to a linkage system.
Example of a modern dual-fuel burner with “linkage-less” air and fuel controls.
Certain FARC systems like the Siemens LMV52 can also include VFD control, oxygen trim, FGR, and be integrated within a single control unit that also handles flame safeguard functions and PID pressure/temperature control. These options can further reduce energy consumption, reduce emissions, and increase safety.
The oxygen sensor is installed in the flue venting near the boiler.
Oxygen trim is a closed-loop feedback system compatible with certain parallel positioning platforms. Similar to the system used in modern internal combustion engines, an oxygen sensor installed in the exhaust stream provides continuous feedback to the fuel-air ratio controller, where the measured oxygen content is an indicator of combustion performance. If the reading is higher or lower than the target value, the controller will automatically adjust or “trim” the airflow rate to correct the error. This feature typically increases overall efficiency by 1-2%.
Oxygen trim allows the burner to automatically compensate for changes in combustion air that can result from seasonal temperature swings, variations in room pressure, or physical restrictions in the airflow path. It also serves as a safety warning system, triggering an alarm if the O2 content breaches an upper or lower limit, giving staff immediate indication that a problem may exist.
Example of monitoring capabilities with a digital burner management system with O2 trim.
Our Upgrade Process
CCI assesses each case individually to help the client choose the best solution. In cases where the main burner components are obsolete, worn out or damaged, a full burner replacement with a factory-fitted control system is recommended. This is also the case if they want to add a backup fuel system. If the burner is still in good shape and/or incompatible with 3rd party replacements, a control system retrofit is usually the more sensible approach.
Covers Up to 75% of Upgrade Cost
As a FortisBC Point-Of-Sale Partner, we cover 75% of the total upgrade cost for any FortisBC customers, up to a maximum amount of $49,000 per boiler. This offer applies to any existing steam boilers which are being upgraded to a parallel positioning control system with oxygen trim.
Example: If the total upgrade cost is $52,000, the net cost is only $13,000 (you save $39,000!) CCI is reimbursed on the customer’s behalf, so our customers never pay the full amount.
Note: This rebate offer may be subject to change in 2021. See FortisBC’s webpage for more information.
Control System Retrofits
Below before and after images are an example of a control system retrofit completed by CCI.
Burner panel before CCI upgrade.
In this method,
- The original main burner housing, blower/air assembly and combustion head remain in place, while we replace the mod motor and old linkage assembly with dedicated actuators.
- We install oxygen and temperature sensors in the flue stack, along with an ambient temperature sensor. This enables oxygen trim control and continuous efficiency monitoring.
- We install a digital gas flow meter which provides real-time fuel consumption data.
- On firetube boilers, we install an immersion temperature probe beneath the waterline of the boiler. This enables the “automatic cold start” function which prevents thermal shock.
- The existing flame safeguard and control panel are replaced with a CCI Custom Panel which contains the Siemens LMV52 digital burner management system and HMI.
Interior of custom panel completed by CCI.
The typical all-in cost for this upgrade package is $52-60k, however our rebate customers only pay $13-15k, resulting in a typical ROI of 1-2 years. Check out this case study to see how our customer was able to reduce fuel costs, increase their reliability, and improve boiler performance.