Flexibility and early planning keys to El Centro repowering success
The utility postponed gas turbine selection until signing the
turnkey contract. This and other innovative strategies allowed planners to postpone contract commitments until after licensing approval and drive overall project costs down
By Joseph H. Rowley and Bruce L. Willard, Imperial Irrigation District
With keen forethought, participants in the Imperial Irrigation District?s (IID) El Centro Unit 2 Repowering Project formulated an approach to repowering that provided exceptional flexibility and continuity. As a result, the utility kept the project?s overall costs within the projected $59.5 million budget, and the repowered unit met all performance expectations.
A municipal electric utility, IID serves Imperial County and a portion of Riverside County in Southern California. With a record peak demand of 620 MW, IID is California?s sixth largest electric utility. Gas and oil-fired steam and combined-cycle units, low-head hydroelectric units and peaking gas turbines constitute the utility?s on-system generation resources.
El Centro Unit 2 was originally commissioned in 1952 as a natural gas and residual-oil-fired steam unit with a net rating of 32 MW. The steam turbine-generator is a non-reheat, single cylinder, Allis-Chalmers machine with throttle conditions of 850 pounds per square inch gauge and 900 F. Plans for the repowering called for retiring the existing boiler and replacing it with a new, unfired heat recovery steam generator (HRSG).
IID conducted licensing and project specification development in conjunction with R.W. Beck and Associates. Project licensing began in June 1990 and IID received final regulatory approvals from the California Energy Commission (CEC) and Imperial County Air Pollution Control District 12 months later.
Permit conditions were stringent yet still workable from an operations perspective. These included a NOx emission limit of 9 ppm at 15 percent O2 for natural gas (14 ppm for No. 2 fuel oil) and a fuel oil use limit of 720 hours per year. To achieve the required NOx control, IID proposed steam injection followed by a selective catalytic reduction system using anhydrous ammonia.
Detailed engineering and procurement began in July 1991, and on-site construction began in January 1992. To maintain project continuity and to control costs, IID decided on a turnkey engineering, procurement and construction (EPC) approach to
Flexibility to select the gas turbine after licensing was essential to IID?s repowering approach. Without this flexibility, they faced two choices, both of which were unattractive from IID?s perspective:
1. They could select the gas turbine prior to licensing. This would complicate the purchase contract and would require IID to separate gas turbine procurement from the overall turnkey EPC contract.
2. IID could base licensing on a proxy gas turbine model. However, a proxy model would sway the competitive bidding process in favor of that unit, and selecting a model other than the proxy would require partial re-licensing.
IID avoided these problems by allowing prospective contractors to specify either a General Electric PG7111EA (GE Frame 7EA) or Asea Brown Boveri Type 11N gas turbine as part of the overall project bid. This made gas turbine selection final only upon signing the EPC contract. To support this project approach, IID structured permit applications to include either one of the aforementioned gas turbines. Consequently, IID was able to defer contract commitments until after they had obtained licensing approvals.
Maintaining the flexibility to select the gas turbine after licensing added both work and complexity to the licensing process. For example, energy balances, water balances and dispersion modeling all had to be done twice. Furthermore, IID had to answer many CEC data requests twice, once for each gas turbine.
One disadvantage to turnkey projects is that prospective contractors have a limited amount of time to accomplish the engineering needed to support proposal development. IID and R.W. Beck countered this disadvantage by spending a substantial effort on cycle analysis and site arrangement studies before finalizing the turnkey EPC specification. The specification communicated the results of this work so that proposing contractors could limit their proposal development to options that they knew IID would accept.
Prior to the repowering project, El Centro Unit 2 had one condensate heater, an open deaerating heater and three high-pressure feedwater heaters. The original full load heat balance indicated a throttle flow of 301,000 pounds per hour, extraction flows totaling 85,000 pounds per hour and an exhaust flow of 216,000 pounds per hour. Although the turbine manufacturer indicated that the exhaust annulus would accommodate 301,000 pounds per hour, IID initially chose a more conservative maximum exhaust flow of 250,000 pounds per hour based on hypothetical operation without the top two heaters.
The cycle analysis conducted by IID with R.W. Beck assumed retirement of the existing boiler and installation of a gas turbine and HRSG. This equipment would produce enough steam to load the existing steam turbine-generator to or near its rated capacity. With these assumptions in place, IID and R.W. Beck narrowed the choice of thermodynamic cycles to those that would provide acceptable combined-cycle outputs and heat rates.
The request for proposal (RFP) instructed prospective contractors to propose a specific cycle using the same life-cycle analysis methodology and economic factors that IID would use in the proposal?s evaluation. This process allowed IID to select the optimum cycle based on actual turnkey costs.
Given a choice regarding three cycle features (gas turbine model, deaeration method and use of condensate heaters), Ebasco Constructors (now Raytheon) proposed the GE Frame 7EA (rated at 84.5 MW at site conditions), an integral deaerator and two condensate heaters. IID selected Ebasco?s proposal which ultimately boosted El Centro?s net rating to 115 MW and lowered its net heat rate to 8,400 Btu/kWhr.
In the early stages of detailed engineering, it became apparent that piping interference under the steam turbine would make it difficult to retain the extraction piping to the two proposed condensate heaters. IID decided to eliminate the two heaters from the cycle and reduced throttle flow accordingly to avoid exceeding the steam turbine exhaust flow limit. Although this change caused a small reduction in output and a corresponding rise in heat rate, it provided several advantages. The space under the steam turbine became roomy and easily accessible, the cycle was simplified substantially from an operations and maintenance perspective, and reliability was enhanced. Also, it was now no longer necessary to retrofit steam turbine water induction prevention (STWIP) measures. IID realized significant capital savings by eliminating the heaters and STWIP measures, and reducing piping, controls and the HRSG?s high-pressure surface area.
In parallel with cycle analysis work, IID developed a document titled OSpecial Instructions from the Owner.O The document conveyed IID?s input to R.W. Beck and Associates and proved useful as a final check on the EPC specification?s content.
The Special Instructions addressed acceptable cycle configurations, defined equipment that IID wanted to remove, replace or retain, and included specific plant features that would enhance operability, maintainability and reliability. It also contained information pertaining to tests, guarantees, documentation and training, and outlined scope boundaries, proposal requirements and proposal evaluation criteria. The Special Instructions were a means of collecting, focusing, reviewing and improving in-house ideas. The project might have lost this valuable input had IID delegated full responsibility for EPC specification development to R.W. Beck and relegated themselves to a review-and-approve role.
High-quality and detail in both the owner?s specification and the selected contractor?s proposal are essential to the success of a lump-sum, turnkey project. Together, both documents formed IID?s turnkey contract with Ebasco Constructors.
IID?s RFP required the proposing contractors to provide a large amount of descriptive engineering information. IID recognized that requiring this detailed information at the proposal stage would place a significant cost/risk burden on the proposing contractors. To alleviate this burden and encourage high-quality proposal development, IID limited the RFP recipients to engineering firms with direct, recent experience with large gas turbines, HRSG?s and electric utility steam turbines.
Gas turbine features
Due to high ambient temperatures at the plant site, IID specified an evaporative cooler for the gas turbine. Dusty conditions at the site also prompted the planners to specify on-line inlet air filter cleaning, on-line and off-line compressor washes, and a totally-enclosed, water-air-cooled generator.
To facilitate routine maintenance, IID specified an oversized, walk-in enclosure for the gas turbine skid. The first-of-a-kind enclosure by GE provides several feet of free space beside and above the engine. Personnel may perform a combustion inspection without dismantling the enclosure, and simply removing the roof allows hot gas path inspections and major overhauls.
Steam turbine bypass
IID specified a steam turbine bypass system because the HRSG produces a substantial fraction of design main steam flow within an hour after gas turbine start. Meanwhile, the steam turbine requires 4.5 hours to reach unrestricted loading from a cold start. To accommodate the lag time in steam turbine start-up, and also to enable uninterrupted gas turbine operation in the event of a steam turbine trip, the turbine bypass system is capable of admitting 100 percent of design main steam flow on a continuous basis. The bypass system has proved to be highly successful in both start-up and trip situations. The system consists of three parts:
1. A steam conditioning valve (10-inch inlet by 24-inch outlet) and a desuperheating water control valve provided by BTG.
2. Ten diameters of 24-inch straight pipe downstream of the steam conditioning valve to accommodate evaporation of desuperheating water.
3. Stainless steel distribution piping inside the condenser, consisting of a 24-inch header and multiple spargers with numerous orifice holes. The condenser, provided by Ecolaire, has design features that prevent direct impingement of steam on the tubes and also protect the turbine exhaust hood.
Replace or retain
IID did not want the project to result in a unit that functioned as though it was part new and part 40 years old. The utility also wanted to avoid a unit that was unnecessarily complex due to the re-use of ill-suited systems or equipment. The desired result was a combined-cycle unit that was effectively new in terms of reliability, operability and maintainability.
The main condenser, all piping systems (except the circulating water conduits), all pumps (except the circulating water pumps) and the entire control system, including local instruments, were replaced. The new control system employs GE Mark V gas turbine controls and a Bailey Infi 90 distributed control system. IID retained the steam turbine-generator but decided to rewind the generator field and stator. Planners also retained the cooling tower (replaced 10 years ago) and circulating water system as well as the steam jet air ejector and instrument/service air compressors. Finally, IID kept much of the old auxiliary power system, although with a reduced role. That system now feeds only the cooling tower fans, condensate pumps and minor equipment. With a large fraction of the total project cost tied up in major equipment (gas turbine, HRSG, etc.), replacement and upgrades of balance-of-plant equipment did not strongly influence total project cost on a percentage basis.
Start-up can substantially age or damage a new plant if operators do not take necessary precautions. Ebasco and IID developed detailed start-up procedures and followed them carefully. The contractor and utility initially fired the gas turbine in February 1993, and completed start-up of the combined cycle in June 1993.
Another essential factor in a successful start-up is simply understanding the plant. Teaming the contractor with utility personnel brings to bear this plant-specific expertise during start-up, and teaming is especially important in the case of a repowering project. New equipment (installed by the contractor) and old equipment (operated by the utility) must be combined into a single, integrated, start-up process. The successful startup of the El Centro Unit 2 was attributable to an effective IID/Ebasco team. IID maintained continuity of information and expertise by keeping the same personnel thoroughly involved in the project from beginning to end. END
Joseph H. Rowley is assistant manager of the IID Power Department. His responsibilities include IID?s existing generating facilities, design and construction of new facilities, and planning for future resource and transmission needs. Rowley was project manager/engineer of the El Centro Unit 2 Repowering from initial concept through licensing, design and construction. Prior to this, Rowley was employed by San Diego Gas & Electric. He holds a bachelor?s degree in chemical engineering from Brigham Young University and P.gif. registration in mechanical engineering.
Bruce L. Willard is a mechanical engineer in the IID Power Generation Section. He was in charge of startup of the El Centro Unit 2 Repowering and participated in engineering and construction. Prior to this, Willard worked with the independent power producers Mission Operations and Maintenance and Oxbow Power Services. Willard?s background in plant operations includes geothermal, biomass and marine propulsion. He holds a bachelor?s degree in mechanical engineering from the Oregon Institute of Technology.
Repowering IID?s El Centro Unit 2 boosted its net rating from 32 MW to 115 MW and lowered its net heat rate to 8,400 Btu/kWhr.
Negotiating permit conditions
The success of IID?s licensing effort is attributed to two factors. First, the repowering project had a sound technical basis in terms of both justification of need and mitigation of environmental impacts. Second, IID treated the licensing process as being primarily a technical negotiation rather than a regulatory proceeding. IID found the following guidelines to be essential to the successful negotiation of permit conditions:
Y Avoid creating an adversarial tone in workshops and hearings.
Y Foster mutual respect with the regulatory agency staff, recognizing that the licensing process will take months or years.
Y Create common ground by applying the fact that the public utility and the regulatory agency are both chartered to work in the public interest. Workshops are the best venue for finding common ground, but the participants must maintain a working atmosphere.
Y Avoid creating a formal atmosphere and turning workshops into pseudo-hearings. The mere presence of the proponent?s attorney can create a formal and even adversarial atmosphere.
Y Keep in mind that licensing is a negotiation process. Avoid placing too much emphasis on procedural matters that may waste energy or limit either party?s flexibility.
Y Be familiar with rules, regulations and established precedent that may limit the negotiating flexibility available to the regulatory agency staff.
Y Get to know the individuals with whom you are negotiating. Are they familiar with your project? What previous projects have they licensed? Do they have knowledge of established precedent? Do they have the authority to approve the settlement of a given issue? If not, are positions they favor typically supported by the ultimate decision makers?
Y Let the regulatory agency staff know who the negotiator is on the utility side of the table. Don?t confuse the situation with multiple voices. Strictly limit the number of utility personnel attending workshops and make sure the ones that do attend understand the details of issues, analysis, etc. The utility?s negotiator must have the authority to approve compromises on the spot.
Y The utility negotiator must have more than a superficial understanding of technical issues. Recognize that licensing negotiations often turn on technical issues that may seem esoteric or obscure. Know your own negotiating constraints and formulate fallback positions prior to workshops and hearings. Be prepared to make concessions, but be sure they are being made in return for favorable recommendations or rulings.
Y Be creative and open-minded in offering solutions that will meet both regulatory agency requirements and project needs. Recognize that the project proponent has more flexibility in suggesting solutions than does the regulatory agency staff.
Most of the old control panels were removed, making room for the new distributed control system. Included in the control room are operator stations (center), engineering work station (right) and processor cabinets (right background).