Combustion equipment and power losses–12 hard lessons learned from the blackout of 2003
Normal operations, and then poof, all power gone. No warnings, everything just went dead.
By John R Puskar, PE, Combustion Safety, Inc., Cleveland, OH
Normal operations, and then poof, all power gone. No warnings, everything just went dead. This was reality for nearly 50 million people and every plant with process combustion equipment such as boilers, ovens, and furnaces.
This article discusses safety issues and the economic losses that occurred during the blackout of 2003 related to combustion equipment. It seeks to create an awareness of these issues and others that may be encountered in the future.
Electrical power problems come in many forms but can have the same combustion system impact. They can interrupt fan system airflow, the ability to ignite, and the ability to control combustion processes. In some cases, it could be a brownout and/or voltage sag. In other cases it could be a momentary blip. However it happens, it can be very dangerous and costly.
Twelve combustion equipment/electrical power related issues that must be taken into consideration when it comes to maintaining the integrity and safety of combustion equipment are discussed below. Whether or not you were impacted by the blackout, pay attention and reevaluate your systems to make sure they are not taken off line in an unsafe manner and/or damaged if a blackout or power disruption happens in your area.
Burner management systems
Burner management systems (BMS) can be destroyed from brownouts or voltage sags. Destruction of a burner management system means the equipment is not functional for a few hours even if there is a backup that is still supported by the manufacturer and not obsolete. If the backup is an older, electro-mechanical style, it is possible to be out of service for many days (Fig. 1). Older style, obsolete BMS systems should be upgraded when changed out. This often means a complete rewiring of the control panel and trying to find and order a new BMS in a crisis mode.
Fig. 1. Electro-mechanical controls may be obsolete and can’t be repaired or replaced.
Review BMS systems for obsolescence. (There is a listing of BMS systems that are no longer supported by manufacturers on the web site in the “More info” section.) If the unit is listed, consider upgrading it now. It is also a good time to get a backup BMS system or have ready access to one. Considering the potential for loss, a few thousand dollars for a backup BMS is a great investment.
Loss of cooling for fans, doors, and bearings
Cooling water is sometimes designed into oven doors and for cooling bearings on fans. Sometimes forced draft and induced draft fans on boilers have water-cooled bearings. In some cases this water flow is not monitored. If this water were to be interrupted due to a city, municipal, or booster pump problem it can cause major problems.
Consider an oven door that lost city water flow for cooling. If the door became warped and unable to be opened, it could result in the destruction of a furnace or worse. In some cases plants recirculate water and have city water as a backup.
To avoid this situation, have backup generators to power water pumps. Cooling water must have a backup system and/or some type of flow or temperature alarm. These alarms and backups should be tested regularly.
System failure modes
It is important to know what position each part of the combustion system will go to in case of a power interruption. For example, what position do damper linkages, fuel and/or process-related valves, and boiler feed water valves go to: fully open, the last position, or fully closed?
The wrong position can be catastrophic. Consider the case of an oven ventilation system’s fresh air damper that would fail closed. This could allow an accumulation of flammables in the oven with no chance of escape for a considerable amount of time.
Control systems must be configured to provide for a safe startup after unexpected equipment shut down.
Another issue deals with equipment that restarts automatically when power is restored. Some burner management systems do not provide a lockout circuit for key interlocks that require a manual reset. This process requires a person to physically press the reset button to restore operation. Without a manual reset feature, equipment can instantly restart without warning. This can be extremely dangerous after momentary outages and can damage some components.
Solid fuels in boilers can be a problem when unexpected outages occur. A big pile of coal burning on a stoker grate takes time to burn itself out. If water is not kept in the drums and air kept moving, a lot of damage could occur.
It is prudent practice to have bypass valves around boiler feed water valves so city water can be manually introduced in case of an emergency. It is also a good idea to understand what happens if a fan is lost and to plan an emergency shutdown procedure. Otherwise, smoke and ash can spread throughout the boiler house.
Startups can be challenging
Starting up is always risky and a cause for concern. If shut down instantaneously at high fire with no planning, a firebox could have enough fuel in it to be in the flammable range. All that would be needed is a hot ignition source for an explosion to occur.
In some cases, consider trying to restart only after the firebox has had a chance to cool off. Remember combustion basics: it’s all about fuel, air, and heat as an ignition source.
If power is lost at high fire, it could take four seconds for the fuel valves to close, which is still within the time required by most codes. This could put a lot of unburned fuel in the firebox. Waiting for the firebox to cool could minimize the risk of an explosion.
In some cases, adding purge air to a firebox that already contains fuel and a hot ignition source can create the right conditions for an explosion. Be sure there are well-documented startup/shutdown procedures that consider this circumstance as well as other prestart issues. Visit the web site in the “More info” section for other prestart, walkaround safety issues to consider.
Shutting down in a hurry means that in some cases large oven fans will not get a chance to cool down as the manufacturer intended. Many oven shut down procedures call for ventilation and recirculating fans to continue to operate after flame failures and/or during orderly shutdowns to protect them from overheating and warping.
If a forced shutdown, with no time for cooling happens, listen for noises and vibrations during start-up. This could mean the fans, bearings or shafts were damaged during an outage. It could also mean the equipment is no longer safe to operate in that condition. It is better to recognize fan damage and stop for an inspection than to start up with an imbalance and have the fan come apart.
Safety switches and interlocks
When things do not operate correctly we rely on safety controls such as high and low gas pressure switches, airflow switches, and flame detectors to keep us safe.
Consider the case of airflow not being correct due to a loss of fan power or voltage during a brownout or a full blackout. In these cases we must have properly functioning airflow switches to ensure that fuel valves do not remain open during these conditions.
In addition, all other safety interlocks need to be tested at least annually by qualified personnel to check if they work at the right set points and conditions. Not having a regular program for testing these devices invites big problems.
Refractory damage vulnerability
When bringing systems back up be sure to check for signs of failed refractory. Some equipment could be prone to refractory failures after an outage because the refractory brick has been thermally cycled (Fig. 2).
Fig. 2. Refractories can crack when thermally cycled.
Refractory failures usually are indicated by glowing hot spots on equipment and/or burned paint or changed surface colors on the outside of equipment walls.
If there are programmable logic controllers (PLC) in the system it may be surprising how things configure during a post power loss startup. The system will most likely recover to whatever was recently stored in the EEPROM or nonvolatile memory. Volatile memory relies on power to the equipment and would be lost during a shutdown. Be aware of this and be sure the post startup configuration of the system is safe.
If there are PLC controls, check to see that the backup memory storage batteries are in good working order. Functioning backup memory could be the difference between safety and a crisis.
Special atmospheres in heat treat ovens
Heat treat operations utilizing special combustible atmospheres are extremely vulnerable to power loss issues. In these cases, get the atmosphere out and safely burned off or a catastrophe can result. This means having an inert gas, such as nitrogen, available to push the atmosphere out in an orderly manner or having a manual burnout procedure in place.
Be sure that all control systems are built and configured to get purging materials to the appropriate area even under power outage conditions.
Casting and smelting facilities need to consider backup power to get molten metal poured and/or kept hot until processed. Certain processes that generate flammable dusts and/or flammable vapors must be able to evacuate the combustibles to facilitate an orderly and safe shutdown.
This could mean that certain “Class A” ovens (those that create flammable atmospheres, such as drying paint) may need to have ventilation fans on emergency power to remove flammable atmospheres so they do not accumulate and become a hazard.
Loss of boiler auxiliaries
Many boiler facilities in the past were built with steam turbine auxiliaries. It was common to find boiler feed water pumps and many forced and induced draft fans running on steam. Today it is rare to see this in most industrial plants. If you must stay on line during power events, revisit the possibility of having steam turbine drives for some auxiliary systems.
Steam turbine drives are something that can also be used to balance steam loads and may even have an economic payback versus emergency generators.
Pressure surges and steam safety relief valves
The sudden loss of power in a major manufacturing facility could mean that control valves for processes using steam suddenly slam shut. This means that a lot of steam is in the system flowing towards the valve when there is suddenly no place to go. Steam is also something that does not instantaneously stop being generated. When this happens safety relief valves open (Fig. 3). If they do not open, process piping can be damaged and people hurt.
Fig. 3. Safety relief valves should be checked annually by certified personnel.
Start a safety relief valve testing program. Safety relief valves need to be lift tested or otherwise verified to operate at least once a year. If you have never tested safety relief valves, seek out some guidance first. You can damage the valves and get hurt if it is done wrong.
So what’s the answer?
There is no one simple answer to the problem of brownouts and blackouts. It is dependent on specific situations and needs. Consider backup power generation for critical processes. Also, remember that emergency generators require routine maintenance and exercise so that they can be reliable and effective when called into duty.
Be aware there are many decisions to be made when considering backup power generation and the form it takes. These decisions include which loads to supply, the kind of fuel, engine size, and whether the backup should be continuous-duty or standby rated.
NFPA 110 (Life Safety Code) identifies two classifications of generator systems, Level I and Level II. Each calls for significantly different design and maintenance criteria. Level I generators are considered life safety generators. They require items such as fuel availability even if the facility’s main fuel is shut off during an emergency.
Another option for emergency power protection is to have an uninterrupted power supply (UPS) for critical control systems. A UPS for control systems or backup generation provides the ability to force controls to certain positions and do an orderly and safe shutdown.
UPSs are commonly used for most computer systems. They work the same way for boiler or combustion equipment controls. Critical controls can usually be run through the UPS at all times so when the power goes out there is no interruption of control system power.
In some cases, very important control systems are operated with compressed air. Compressed air has an advantage; if it is designed properly it can buy time. This additional time can provide a controlled shutdown while the air receiver that stores compressed air slowly empties.
The blackout that plagued the east coast resulted in a lot of disruption. It was also somewhat of a regionalized event. Utility officials say that our nation’s grid system is in shambles and that this could happen again.
Times have changed and will continue to change. We live in a world where electrical grid outages and terrorist incidents are real. It is your responsibility to understand the impact to your facility and to act before it is too late. The 12 issues identified in this article will assist in looking at and determining vulnerabilities through a fresh perspective.
For more information on power and combustion safety services, visit combustionsafety.com or call 888-826-3473. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, email@example.com .