Virtually all media attention concerning this summer's massive U.S. power blackout focused on the grid. Yet outages can dramatically affect power plant combustion components and create significant safety hazards. Even far smaller events — minor brownouts, voltage sags or momentary blips — can interrupt fan system airflow, the ability to ignite, and the ability to control combustion processes. Whatever the cause, the results can be very costly and extremely dangerous.
John Puskar, P.E., principal of Combustion Safety, Inc. — an engineering firm specializing in boiler and combustion system interlock testing, training, and safety evaluations — identifies a number of combustion equipment/electrical power related issues that must be taken into consideration when it comes to maintaining the integrity and safety of equipment.
Burner Management System Failure Issues — A brownout or voltage sag can destroy a burner management system (BMS). As a result, equipment is not functional for a few hours — assuming a spare is available, not obsolete and still supported by the manufacturer. If the BMS is of an older electro-mechanical design, the plant could be out of service for many days. Old or obsolete BMS systems must be upgraded when changed out. Combustion Safety's website contains a listing of BMS systems no longer supported by manufacturers. "If you have one of the older styles that are no longer manufactured or supported," says Puskar, "consider upgrading it now. It's also a good time to make sure you have a spare BMS system on site or have ready access to one."
Dangers of Equipment that Re-Starts Automatically — Some burner management systems do not provide a lockout circuit for key interlocks that require manual reset. This process requires a person physically pressing 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 Fuel Re-Start Issues — Solid fuels in boilers can present problems when unexpected outages occur. A big pile of coal burning on a stoker grate takes time to burn itself out. "Not enough water in the drums or too much air moving through the boiler firebox can create a mess," says Puskar. That is why it is prudent to have bypass valves around boiler feed water valves so that city water can be manually fed in case of an emergency. It's also a good idea to understand what happens if one or both fans are lost in a balanced draft furnace and plan for an emergency shut down procedure. "If not," he says, "you could end up with smoke, ash, and carbon monoxide throughout the boiler house."
Possible Explosions — An instant shutdown at high fire with no planning could result in a firebox with enough fuel in it to be in the flammable range. Any hot ignition source could cause an explosion. If a plant loses power while a boiler is at high fire, it can take four seconds for the fuel valves to close and still have been operating within the timing required by most codes. This allows for a lot of unburned fuel in the firebox. Waiting for the firebox to cool could minimize the risk of an explosion. "If you head right into purge for a re-light, you provide oxygen in a fuel rich environment that probably already has an ignition source," he says. "The result can be and has been devastating for some sites."
Safety Switches and Interlocks Must Work Correctly — Safety controls such as high- and low-gas pressure switches, airflow switches, and flame detectors keep personnel safe when things are not operating correctly, and during a power outage, they aren't. For example: Correct airflow would not be maintained due to a loss of fan power or voltage during a brownout or blackout. That means airflow switches must operate properly to make sure 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 make sure they work at the right set points and conditions.
Refractory Damage Vulnerability — When systems are brought back into service, it is essential to check for signs of failed refractory. Some equipment could be prone to refractory failures after an outage because the refractory brick is again thermally cycled. Refractory failures are usually indicated by glowing hot spots on equipment and/or burned paint or changed surface colors on the outside of equipment walls.
PLC Controller Issues — During post power loss start-up, operators may be surprised how programmable logic controllers (PLCs) have been configured. The system will most likely recover to whatever was recently stored in the EEPROM or non-volatile memory. Volatile memory relies on power to the equipment and would be lost during a shut-down. "You must be aware of this and make sure that the post start-up configuration of your system is safe," says Puskar. "If you have PLC controls, now might also be a good time to see that your backup memory storage batteries are in good working order. Functioning backup memory could be the difference between safety and a crisis."
Pressure Surges and Steam Safety Relief Valves - Sudden loss of power in a major facility could mean that control valves for processes suddenly slam shut. This means that lots of steam is already in the system flowing toward the valve when, suddenly, there is no place for it to go. Since steam production does not stop instantly, safety relief valves must pop open. If they don't, things fly apart and people die. Puskar says safety relief valves must be lift tested or otherwise verified to operate at least once a year.
"Additional solutions include having backup power for critical processes and/or uninterruptible power supplies (UPS)," says Puskar. "In my 20 years of practice, I have seen the world go from a place where emergency power and power reliability issues were given lip service to where it is now a serious consideration for every client on every job. It's time for everyone to review their situation and learn from what one region of the country experienced. The lessons do not have to be learned the hard way every time by every facility."