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Allen-Bradley ControlLogix PLC


PROJECT: 45 MW Biomass Boiler Stability


Completed the design, installation, commissioning, and tuning of improved boiler fuel and air control logic for a 45 MW biomass shaker grate boiler power generator. Previously and since early 2014, it was not uncommon for the 1400 psig boiler to experience pressure swings as great as +-100 psi. Many thought that this was a direct result of the diverse BTU content in their fuel mix. After the completion of the commissioning and tuning portion of the project, the improved control logic design now achieves boiler pressure stability within +-10 psi; even while holding the MW output within +-0.5 MW and not using the turbine for pressure control.

The primary components for this improvement was the implementation of correct control theory and traditional boiler fuel/air process and control logic designs. Prior to the installation of the improved logic/tuning, it was not uncommon for the operators to respond to approximately 100 pressure control associated annunciation alarms. In the first four days of operation with the new logic, not a single pressure control alarm came in!

The boiler fuel and air controls always remain in fully automatic mode with very little need for operator attention.

RESULTS


• Through use of my extensive experience in reverse engineering of control system logic, I found many discrepancies in the departure of traditional fuel/air combustion control theory and stable regulatory analog control theory design.

• Redesigned the fuel/air regulatory analog control logic to more traditional and proven combustion control theory. Also, redesigned the areas of the control logic that I had identified as inherent to causing instabilities.

• Successfully conducted a full boiler tuning program on the upgraded combustion control logic achieving remarkable stability seldom seen on a biomass boiler burner a extremely diverse fuel mix.

PROJECT: Gas Turbine Refurbishment & Turbine/HRSG Controls Upgrade Project


This State Prison was a facility that had a single 3.0 MW Centaur 40 Solar Turbine exhausting heat into its own Heat Recovery Steam Generator (HRSG). Steam produced from the HRSG was used for facility heating, laundry cleaning, kitchen dish washing, and food preparation. The facility has a central power house that also serves as the electrical distribution hub for switchgear control between the gas turbine/generator, the local utility power, and additional high voltage load feeder breakers.

The original system was installed in the mid 1980's and ran almost continuously until the year 2005 at which time the turbine had in excess of 50,000 operating hours. Solar Turbine requires a complete turbine overhaul when the operating hours exceed this standard operating limit. The gas turbine/HRSG system was abandoned and steam was produced out of a single standby package boiler. Due to problems with this package boiler, that at times left the facility without heating steam, The State issued an emergency contract to refurbish the gas turbine, upgrade the turbine controls from the old hardwired relay logic to a PLC controlled system, and upgrade the HRSG controls from the old Bailey single loop controllers. Engineering services were provided to manage the turbine controls subcontractor and to be the primary control system design engineer for the HRSG controls upgrade.

Some of the key features used within the turbine controls were the standard start/stop logic but of special interest was the automated generation load control and the Wet-Nox controls. The engineering service was responsible to coordinate the turbine controls subcontractor on these key features since they did not have much experience with this type of generation in this west coast state. The load controls were design around the strategy of maintaining the paralleled utility power at a set point of approximately 100 kW. Therefore, as the prison facility's load would change, the gas turbine/generator would swing to meet the facility's load and maintain the utility power near it's operator set point.

The State air emission's requirements for this unit specified that for every pound mass of fuel burned an equivalent pound mass of water and was to be injected into the combustion process to maintain exhaust Nox levels within source tested standards. Therefore, engineering service created the control loop design using new process parameters to improve stability within the control loop. The previous Solar Turbine design required constant online operator interaction to maintain the one-to-one ratio of water to fuel. The new Nox controls have achieved zero operator interaction because of this improved design.

The Heat Recovery Steam Generator (HRSG) control system upgrade portion of this project provided an opportunity to install advanced energy anticipation control philosophy for a much needed requirement of stable steam generation. The previous control system was so unstable, the alarm acknowledge push button had been worn off the old controller faceplate! This HRSG configuration also employed the use of a natural gas fired duct burner to supplement the heat energy of the gas turbine exhaust for steam production. The new control system design utilized a similar mode of operation as the original design and was called "single element" control where this "element" was the steam pressure process variable. In addition, the new control logic design utilized a much more anticipatory control philosophy using four elements of process variables to correctly modify the energy demand signal to the diverter valve/duct burner combination in anticipation of process disturbances. The logic allowed the operator to conduct a bumpless "flying swap" between either of these modes. The improved stability was the first this site had seen since the original commissioning of the unit!

Lastly, the engineering service was the control system engineer that correctly identified the requirement to upgrade the existing gas turbine hydraulic starter system to an Allen-Bradley PowerFlex Variable Frequency Drive(VFD), provided the installation design sketches, coordinated the installation construction, determined and programmed the VFD configuration parameters, and successfully started and commissioned this device.

RESULTS


• Successfully designed an innovative and stable multi-mode Plant Master analog control logic for regulation of steam delivery from the HRSG. This logic incorporated predictive control loops using 4 elements of analog process inputs.

• Performed all of the control logic programming for both upgraded HRSG controllers (Plant Master & Drum Level) using Preferred Instruments PC-3 Draw and PC-3 Edit programs. This documentation was a translation of the completed designed simple SAMA logic.

• Provided review of the turbine control system subcontractor's drawing submittals to verify accuracy to the existing electrical design, to the logic design, and input/output list for the new Allen-Bradley ControLogix PLC.

• Designed the full control system logic for the "Wet-Nox Water Injection" for a Solar Turbine Centaur 40. The original logic didn't work. Improved process indications were used combined with new logic to create a very stable and effective control system.

• Completed a successful startup and commissioning of the upgraded control system for the Solar Centaur 40 gas turbine. Complete automatic operation to full load was achieved for all systems including the Wet-Nox system.

• Completed a successful startup and commissioning of the upgraded control system for the HRSG. This included advance 4 element control for the Plant Master switchable to single element on the fly and commissioning the Drum Level controls.

• Completed a conversion of the Solar Turbine Centaur 40 hydraulic starter system to a Variable Frequency Drive (VFD). Provided the electrical design sketches, supervised construction, determined and conducted VFD programming config's, & S/U.

PROJECT: Hydrogen Sulfide Incinerator Replacement for Geothermal Generation


A large number of generating units were installed in the Geysers area around Clear Lake, CA, while PG&E owned these facilities. A by-product of using the steam out of the ground is the off gassing of hydrogen sulfide (H2S) gas which has a strong odor of rotten eggs. While PG&E owned these units, the local air pollution control district implement regulations to control release of H2S gas. PG&E had their Department of Engineering Research build a pilot incinerator at one of the generating site's location. In the year 2000, this pilot incinerator was at least 5 years old and was failing. An independent power producer (IPP) purchased the majority of PG&E's geysers units and also bought a used incinerator of much larger capacity to replace the pilot incinerator. The used incinerator system consisted of a simple antiquated hardwired analog control system using basic control theory to maintain combustion and quenching water flow rates.

The IPP requested that the control system be upgraded to an Allen-Bradley ControlLogix PLC including building the burner management system in a stand-alone ControlLogix PLC. The IPP specified that process safety and stability were a premium due to past challenges from the original prototype incinerator. Therefore, process and control engineering design, testing, construction, and startup services were provided for a completely new and upgraded control system. Included in these services was the development and finalization of the functional process organization for H2S abatement.

RESULTS


• For this DCS upgrade, the field device input/output database was developed implemented and completely managed using Microsoft Access.

• Designed an automated lead/lag motor start for a set of three combustion blowers. The logic automatically chose lead/lag/standby based on operator selected parameters.

• Designed a "simple" versus "complex" analog control logic with cross tracking for combustion air flow based on fuel/air ratio, O2 trim, and cross correction with fuel.

• Built operator graphics for the H2S incineration process and control system. These included standard auto/manual control stations and P&ID indication graphics.

• Conducted final construction inspection, all control system loop checks, systems startup testing, and final commissioning to full operating status of this replacement H2S Incinerator.

PROJECT: Simultaneous Restoration of Two Biomass Power Plants


Two mid 1980's vintage biomass power plants each rated at approximately 12.5 MW's had been shutdown and abandoned in mid year 2010 due to the loss of permanent financing. With the financial challenges prior to these plants being abandoned, the equipment had suffered from over use, lack of maintenance, and make shift repairs with limited site resources. The Allen-Bradley PLC based control systems were one of the casualties of the lack of maintenance and technical expertise to the point where the contract O&M service were unaware of the full extent of the problems.

The initial responsibilities started late in the "refurbishment" process (i.e., four weeks before the scheduled startup date) and included simply fixing some perceived "bugs" in the PLC system, PLC process & control tuning, and startup engineering support. The responsibilities grew to include a full upgrade of the Allen-Bradley system from their mid 1990's version to current hardware and software, rebuilding of many major process control loops, process & control tuning, and full unit engineering startup support which included engineering changes to various process methods.

The combustors for these sites are Energy Products of Idaho (EPI) bubbling fluidized bed combustors providing heat energy to a Zurn 122 kpph at 684 psig & 765F superheated steam boiler. Both sites burned agricultural waste which primarily amounted to chopped up almond orchards during the course of this startup.

The plants were built and placed in operation in 1988 and 1990 but were sold to the second owner in 1992. The facilities were shutdown in 1995 after the power purchase agreements (PPA's)were bought out. The third owners purchased the sites, refurbished the equipment and negotiated new PPA's. This major refurbishment was needed to correct damage for the 12 years of being abandoned the first time. Unfortunately, the control system upgrade was not managed well which resulted in many control loops being operated in manual. During the refurbishment, the sites were sold to the fourth owner which ended up having to fire the general contractor conducting the refurbishment and utilizing the contract O&M service to complete the work. After the startup of this first refurbishment beginning in 2008, the plant only ran to mid year 2010, again, being abandoned. The fifth owner purchased the plants and conducted these current refurbishments.

RESULTS


• Performed a complete control system evaluative assessment for the existing control logic in the PLC's, compared this to the OEM original design functional logic, and BaileyCAD logic. Provided a formal control system improvement program proposal document.

• Created and formally documented a proposed process and control logic upgrade for the boiler master, fuel flow, air flue, gas recirculation, and induced draft fan control logic. This new design included improved process monitoring and control philosophy.

• Found and corrected the preheat burner purge logic. It was clear that past logic changes had been implemented to bypass safety features of the standard purge as defined by NFPA. Implemented the correct changes and successfully commissioned the new logic.

• Corrected and modified a significant number of HMI operator graphics resulting from finding mistakes in the alarm database, the tag database, newly added process instrumentation, and modifications to the control logic.

• Conducted final start up activities for both biomass generation units supporting all control system and process troubleshooting activities. Successfully placed in operation both full power generation facilities ahead of the expected schedule.