What is a Heat Recovery Steam Generator (HRSG)?
Generally pronounced her-sig in US industry.
HRSG systems are used to recover energy that would otherwise be lost from thermal oxidizers, gas turbines or other heat generating processes. By recovering this energy in a HRSG, we can produce steam, which can be used for other purposes.
Generally, HRSG’s are considered ASME Section I – Power Boilers and are considered boilers by AHJ, Authority Having Jurisdiction.
Large HRSG systems are extremely common in peak energy projects or energy efficient combined cycle power projects around the world. They are also commonly used in the Middle East for desalination plants. In the Midwest, HRSG systems are very common in ethanol plants downstream of thermal oxidizers.
Large universities commonly use HRSG systems as a way to recover energy from diesel generators or gas turbines which provide power for their campus. This recovered energy is then used to heat spaces, or provide hot water to residences. This application is known as Combined Heat and Power (CHP).
HRSG Maintenance and Service
Heat Recovery Steam Generation systems can be very large and complex systems which require a detailed maintenance and service plan. An effective maintenance strategy involves working with plant personnel. Start by identifying what has been upgraded in the past, as well as what the goals are for the future. You will want to consider known issues, failure history, available upgrades and field inspection reports during the HRSG maintenance planning process.
Conversely, Heat Recovery Steam Generators can be a simple packaged watertube or packaged firetube. Maintenance and service are very similar to other industrial use boilers. See our maintenance checklist for more information.
In HRSG systems, similar to other boiler systems, it is generally advised to use 3rd party experts. Water chemistry professionals and boiler maintenance and repair contractors can inspect, analyze and help manage the system operations. This will lead to better suggestions for changes in operating practices.
Hot Maintenance and Cold Maintenance
Once a plan is set, maintenance can begin. There are two primary types of maintenance on a HRSG System. Hot Inspection and maintenance are things that can be done while the system is in operation and therefore “hot”. Cold Inspection and maintenance can only be performed when the system is taken offline which allows the unit to cool and be drained. These inspections are typically required every year in the United States by the AHJ, but can vary slightly by jurisdiction.
Maintenance and inspections can provide invaluable insight into how the unit is aging and its overall reliability. The internal condition of a boiler or HRSG cannot be determined by external inspection. Internal inspection is the only way to know if scaling, pitting or other damage is occurring. If found then operating practices and water treatment need to be adjusted accordingly.
HRSG Maintenance and Service Benefits
The exceptionally high cost of an unexpected outage with equipment this large makes the befits of service significant. Inspected HRSG systems thoroughly to help prevent outages before they occur. Some common issues to look for during maintenance and inspection are:
- Tube thinning, pitting, or corrosion
- Mis-alignment of sootblowers
- Piping system failures and leaks Broken pipe support equipment (springs, constant supports, hangers, rollers, snubbers, etc)
- Leaking valve packing
- Leaking plant air or instrument air
- Piping or duct expansion joint failure or distortion
- Drum internals failures (loose or broken separators, spargers, or vortex breakers)
- Pressure safety valve certification/inspection
- Gas side baffle problems such as loose, overheated, warped or burned out
- Attemperator loop solids buildup (superheated units)
- Faulty desuperheater sensors
- Creep rupture (High temperature units)
- Ammonia Grid Failures (Units with SCR)
Note: A major challenge for HRSG users is to find and repair a tube leak while it is still a minor problem. Approximately half of all such leaks occur in the economizer section, and are caused by fatigue, corrosion-fatigue, or freeze damage (Deltak LLC)
Cleaning is often part of a HRSG maintenance plan but can come in many forms for a HRSG system. Gas side cleaning removes and dislodges deposits and corroded material within the tube bundles. It is important that when cleaning, special care is taken to protect the tube bundles. Obviously, it is advantageous to schedule this cold maintenance during plant outages. A clean system will often result in reduced back pressure and a decrease in stack temperature as a result of more heat absorption. In systems with a diesel generator or gas turbine, this may also mean more power from generator.
High Pressure Water Blasting, grit blasting, CO2 (dry ice) blast, and pressure wave cleaning are 4 common types of cleaning technology on the market. Each with their own set of benefits, the right choice for a plant is often decided on a case by case basis. In the Midwest, dry ice blasting is very common in ethanol plants to remove ash and debris from the HRSG heating surface.
Following the cleaning, all debris should be removed from system. This can often be several tons of debris, depending on the condition of the heat recovery steam generator.
Need help with HRSG maintenance, service or repair? Reach out to Rasmussen Mechanical for expert advice and service. Call us at 1-800-237-3141, email firstname.lastname@example.org, chat with a support agent, or contact us online. We would be happy to help you out.