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How to decrease startup times by up to 20 minutes at your HRSG plant

As part of normal plant operations, combined cycle power plants around the world use combustible fuels on a daily basis. These fuels help us generate the power we rely on so heavily to maintain our normal standard of living, but they can also present a major hazard if not properly managed within the plant system.

Power plants must cycle combustion turbines through planned shut downs for scheduled maintenance, equipment failures, or lack of energy demand. Once the combustion turbine is restarted, it must typically be purged of any combustible gases to meet the safety standards set by the National Fire Protection Association (NFPA). This purge can lead to system delays resulting in lost revenue and equipment stress.

Plant managers understand the need to follow proper safety protocols, but they also face daily demands of lean operations and increased revenue generation. Instead of compromising on safety, plant managers can take advantage of a Combustion Turbine Purge Credit. With a few strategic system upgrades, a purge credit system can minimize system startup time and provide additional operational benefits while still maintaining high safety standards.

Concerns with normal HRSG purge cycles

As a plant manager, your primary focus is on the safety of your employees and your systems. Following NFPA guidelines for a standard startup purge sequence will certainly help ensure a safe start to your system, but it also creates an operational conflict. Purge cycles can add between 5 and 20 minutes to your startup time, which can cost your plant thousands of dollars in lost revenue.

Although long duration purge sequences will remove hazardous substances from the combustion turbine, they can also compromise your internal system components. The temperature changes caused by normal startup and shutdown cycles can increase system fatigue and shorten the life span of your pressure parts and other sensitive equipment, increasing replacement costs over time.

Since the air used for a purge is much cooler than the exhaust gases flowing through the duct work prior to shutdown, steam collected in the tubes can condense and leave excess water that must be drained before startup.

All of these complications may seem like necessary measures for maintaining NFPA compliance. Fortunately, you can follow NFPA regulations while still mitigating these operational issues by incorporating system upgrades that will qualify for a Combustion Turbine Purge Credit.

What is a Combustion Turbine Purge Credit?

Under NFPA 85 section 8.8.4.6, combustion turbine systems can receive credit for a system purge at shutdown, effectively eliminating the need to purge the system upon the next startup. This purge credit can be established when the system has certain equipment upgrades to the fuel lines and sealing valves. These upgrades ensure that fuel does not leak into the system during a shut down process, which minimizes the fire risk to meet NFPA standards. The airflow through the combustion turbine during a normal shutdown sufficiently removes any residual combustible materials, which allows for a purge credit to be granted for the next startup.

Typical HRSG systems must have a double block valve and single vent valve to comply with NFPA codes for gaseous systems. To implement a combustion turbine purge credit, plants must add at least one additional block valve and one additional vent valve. There must also be instrumentation to monitor for pressure. Once this additional equipment is in place, the integrity of each independent system is verified, which will grant a purge credit to the distributed control system (DCS), allowing the turbine purge to be bypassed during the next startup.

What are the benefits of Combustion Turbine Purge Credit?

If your plant runs on a daily cycle, you can typically expect 150 to 250 starts per year. By implementing a purge credit system, you could save up to 15-20 minutes per startup. Over the course of a year, this could add over 80 run time hours to your system. Decreased startup times can help your plant respond faster to load demand and ensure you meet all production capacity contract requirements.

Purge credits also provide many other benefits, including:

  • Reduced thermal stress and system fatigue on duct work and other major plant components
  • Minimized temperature fluctuations on sensitive HRSG pressure parts
  • Reduced labor costs due to simpler startup procedures and lower system maintenance requirements
  • Decreased condensation pooling and corrosion due to maintaining superheater tubes at higher temperatures
  • Faster ramp rates due to higher pressure levels in the HRSG
  • Overall increases in system safety and reliability
  • Decreased usage of water, fuel, and electricity during startup

Options for adding a purge credit system upgrade to your HRSG

To add the necessary components required for a purge credit, the NFPA provides for two system configurations. Each system has advantages and drawbacks, and the ideal system will depend on plant size, operational needs, and other variables.

Valve Proving Method

To qualify for a purge credit under NFPA 85 Section 8.8.4.6.4 (A), plants can use a valve proving method to validate leak tightness. Continuous monitoring of valve positions and pressures is required, and this system setup qualifies for a maximum credit of 8 days.

Valve Proving Method advantages:

  • Cheaper and faster to install
  • Improved efficiency and cost savings over a traditional system without purge credit
  • Fewer hardware components that require routine maintenance

Valve Proving Method drawbacks:

  • Maximum credit of 8 days before a purge is required
  • Operational cost savings and efficiency is lower than with pressurized pipe section method

Pressurized Pipe Section Method

To qualify for a purge credit under NFPA 85 Section 8.8.4.6.4 (B), plants can use a pressurized pipe section method. In this system, air or inert gas is used to maintain a pressurized section of piping to prevent fuel gas from entering the combustion turbine or duct burners. Continuous monitoring of valve positions and pressures within the pipe sections is required, and this system setup qualifies for purge credit indefinitely, as long as the system pressure is maintained.

Pressurized Pipe Section Method advantages:

  • Maximum operational cost savings and efficiency
  • Greater startup time savings on an annual basis
  • Indefinite startup credit period; no cold starts required unless the system is compromised
  • Best for plants with high generating capacity and highly variable load demands

Pressurized Pipe Section Method drawbacks:

  • Initial system components are more expensive and more complex
  • Requires inert gas or instrument air to maintain the system, increasing costs to end users

Calculating ROI on a Purge Credit System

When considering major system upgrades, you'll likely want to calculate the long-term return on investment for your system. With a purge credit system, the benefits, cost savings, and increased efficiency calculations for your unique situation won't be easy, but our purge credit specialists are here to help.

In addition to helping you calculate ROI for your plant, Forney's experts can assist with every step in the process of a system upgrade, including:

  • Reviewing your plant drawings to determine resource availability
  • Conducting site walk-downs to examine piping layouts
  • Participating in meetings with your controls team
  • Provide an on-site service engineer to program the purge credit logic required
  • Drop in pipe spools and associated instrumentation for the fastest implementation time

Uncertain on your next steps or need help calculating the ROI for your purge credit upgrade? Speak with one of Forney's purge credit specialists today.