Diesel to gas: Upgrading a combustion turbine

Revamping and modernizing gas turbine plants often involves changing from fuel to natural gas and upgrading the control system. Legacy equipment have to be upgraded in keeping with today’s technology.

Interstate Power and Light Company (IPL) owns and operates three simple cycle combustion turbine generators (CTGs) at its Sutherland Generating Station (SGS) in Marshalltown, Iowa. These three CTGs are similar: Pratt & Whitney FT4 Twin Pac CTGs each capable of generating approximately 52 MW at base conditions (average ambient temperature) when firing fuel oil; these units were commissioned in August 1978.

This article contains excerpts from the paper “Sutherland combustion turbine fuel conversion and controls upgrade project” presented at the Power-Gen International conference in 2017 by Todd Young (HDR), Chad Wall (Alliant Energy) and Aerin Klump (HDR).

IPL’s long-term generation planning called for the installation of a new 2x1 combined cycle plant (Marshalltown Generating Station, or MGS) on adjacent property, coupled with the retirement of the existing Sutherland Unit 1 and 3 coal/gas-fired generating units. Air dispersion modeling and permitting efforts for MGS confirmed that the existing FT4 Twin Pac CTGs would either need to be retired, replaced, or converted to natural gas firing for emissions reasons. IPL reviewed the alternatives and decided to convert the units to natural gas. IPL decided that, in addition to the fuel conversion aspect of this project, they also wanted to increase the availability and reliability of the units through a number of upgrades.

The feasibility study specified two major objectives: 1) determine the condition of the CTG’s and auxiliaries 2) define the scope of the upgrades that would be needed to achieve all of the goals.

For the first major objective of condition assessment, the task was broken down into parts: a generator assessment and assessment of the GG/FT unit. Third party component refurbishment companies were contracted to perform this work. The GG/FT assessment involved inspection with borescope into available ports, inspection at the GG inlet, FT outlet, and removal of the combustion cover as well as all combustion chambers. Coincidentally, one of the FT’s required bearing maintenance during the same time period as the assessment and it was removed from the machine for inspection and repair. The generator assessment involved removal of the generator upper end shields to gain access to the stator end turns and rotor retaining ring areas.

The generator underwent several electrical tests including polarization index testing, meggar testing, winding resistance tests, copper resistance testing, and impedance testing. The results of these evaluations enabled IPL to estimate the cost of the life extension part of the project as well as develop specifications for bidding the work in the contracting phase of the project. To achieve the life extension goal, as well as the conversion to natural gas, a certain amount of upgrades and additional infrastructure were needed to obtain the functionality and availability for the units. IPL studied the following topics in consideration of meeting the project goals: • Available gas conversion components and technology • Demineralized water supply system and water injection equipment • Control system upgrade options • Vibration monitoring system upgrade options • Instrumentation upgrades • Air system upgrades/relocation • Auxiliary electric system upgrades/relocation • New local service building IPL determined that the only viable gas combustion equipment was conventional diffusion flame burners with or without water injection. A DLN burner had been developed by Pratt & Whitney but it was not successfully implemented and also not commercially available.

With this in mind, IPL studied the need for water injection with conventional burners. By examination of the air permit application and the capabilities of the conventional burners, it was determined that water injection would be needed to meet the NOx emission limits. After evaluating the various options described by industry experts, it was determined that the ideal method of achieving gas combustion with water injection was to use a dual fuel burner and inject water through the liquid fuel nozzle while the gas is injected through the gas fuel nozzles. This method had a proven track record with all the companies being evaluated for the performance of the work.

The existing control system installed in 1978 was 36 years old at the beginning of the study. The control system did not utilize computers; rather it used relays and timers to achieve the starting, loading, protection, and shutdown control. This system, although functional, was decades out of date compared to modern control systems. To increase the availability and enhance the remote diagnostic capability, it was decided to upgrade the obsolete original control system. Control systems considered for the upgrade included DCS and PLC type systems. Ultimately, a PLC type system was chosen based on its proven track record with the companies that were considered for the performance of the work. In addition to individual PLC’s, a central PLC was installed to control and supervise all of the new auxiliary equipment. Like the control system, the original vibration system was also obsolete. The system was still functional but did not provide remote monitoring capability nor data analysis capabilities.

IPL explored several different system options and ultimately decided on a Vitec 2110 Monitoring System due to its high compatibility with the chosen PLC control system. The original instrumentation for the unit had been maintained and was still functional an upgrade was desired. To increase availability and remote monitoring capabilities, upgrades to the instrumentation were included. One example of this type of upgrade was to replace certain pressure switches with pressure transmitters.

The starting air system for the unit at the time of the upgrade was not the original equipment. The original equipment included individual air compressors located in enclosures next to each starting air receiver. These had been removed and replaced with a common central air plant located in the retired diesel building. With the demolition of the diesel building being planned, the air plant needed to be relocated. At the same time, the air plant, which was comprised of a low-pressure air compressor, dessicant air dryer and high-pressure air compressor, lacked redundancy. To increase the unit availability, it was desired to add redundancy. IPL devised a plan where a new air plant of the same make and model equipment as the existing units would be installed at a new location and then the existing units would be relocated next to the new units to make a fully redundant air plant. This plan addressed the need to keep the system operational and then have a very short outage to switch over from the old equipment to the new equipment.