PROJECT: Boiler Controls Upgrade Project

Three Erie City Boilers built in 1959 and each rated at 25,000 lbs/hr flow at 125 psig saturated steam were used to supply the steam energy requirements for a state prison on the west coast. A previous NOx burner and controls upgrade ending in 1998 was successful at meeting new lower stack emission requirements set by the local Air Pollution Control District. The 1998 controls upgrade installed the traditional mechanical jackshaft lever arm control system to position the fuel, air, and gas recirculation flow (GRF) damper. A number of safety, stack emissions, and operational problems arose out of the more traditional jackshaft design. This most recent boiler controls upgrade project corrected the previous deficiencies and provided added benefit.

Engineering provided a large range of services that included identification of the existing process & control deficiencies, establishing the design criteria (listed below), providing a control logic design that met the established design criteria, and overseeing startup and final commissioning.

The project criteria, successfully achieved, is stated below:

1. Safety Criterion:
Improve boiler safety by eliminating furnace over pressure transients.

2. Emissions Criterion:
Maintain stack emissions within current permit limitations under all operating conditions.

3. Efficiency Criterion:
Improve boiler and operating efficiency resulting in a net reduction of total fuel usage and steam plant electrical use.

4. Operating Criterion:
Extend boiler turndown to enable continuous firing at the lowest facility demand level.

To meet all four of the stated criteria, the next level of control sophistication was orchestrated in a design of the control logic philosophy, the controlled field devices, and the microprocessor multi-loop controllers. In summary, the design included upgrading the Plant Master control logic from pure pressure control to steam flow control, upgrading the boiler control system to a “fully metered” system, and increasing the usage of the multi-loop controllers including the addition of controlling boiler flue gas O2.

The final results exceeded expectations for all four project criteria. The most notable achievement being an estimated savings of approximately $140,000 per year in fuel and electricity due to a dramatic improvement in boiler process stability!

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.