July – August 2009 • Volume 39 No. 4

Cure for overlapping control of integrated plant operation page 16

Automated early warning to prevent combustion failures page 20

16 GAS TURBINE WORLD: July - August 2009

CHP plant midlife upgrade for integrated single-point control

Scope of the plant upgrade in-cludes replacing an outdated gas

turbine controller with Ovation tech-nology supplied by Emerson Process Management that will integrate gas turbine and heat recovery plant op-eration from a single-point control system. Project highlights:

Control. Enable plant start-up, monitoring and control from a single location, with a single integrated da-tabase for process data, alarm and operator action.

Analysis. High-speed network will provide 1 ms sequence of events ac-curacy for troubleshooting unsched-uled turbine shutdowns or other events.

Timing. Upgrade will coincide with mechanical work to be per-formed during approximate 8-week gas turbine maintenance outage planned for early 2010.

The combined heat and power (CHP) plant is designed around three 40MW Fr 6B gas turbines, three supplemen-tary-fired heat recovery steam gener-ators (HRSGs) and one 68MW steam turbine, with a total generating capac-ity of 188 MW. The main purpose of the plant is to ensure steam supply to a neighbour-ing nuclear complex; electricity is almost a by-product – which makes operational reliability of paramount importance to the plant owner.

Scope The scope of equipment supply and installation essentially covers one third of the power plant, i.e. replace-ment of the control system covering one gas turbine, one of the three heat recovery steam generators, and the control room. Although an outside service orga-nization is responsible for operation and maintenance, with performance guarantees on steam supply and plant reliability, etc., upgrading the plant’s control automation falls outside of this O&M contract since it represents a strategic modernization investment on the part of the owner. The benefits of the project there-fore must satisfy both the plant own-er, with respect to long-term interests of the CHP plant, and the operator who is required to meet key perfor-mance indicators under terms of his operation and maintenance contract. Emerson’s sales and marketing director for the UK power industry, Loic Charbonneau, points out that op-portunities to improve operation, reli-ability and maintenance at a power plant, (from an automation level) are few and far between – perhaps once every decade.

Approach It generally happens with plants ap-proaching their midlife of operation. “When you build a power plant, gas turbine suppliers will offer a warranty or initial service contract of typically seven years. During this period it is

almost impossible for third parties to propose any type of control modifica-tions or upgrade. “However, when assets enter into 10 or more years of operation, OEM support often becomes a bit patchy so that once equipment is outside the warranty period, power generators start looking to other parties special-ized in turbine control.” Moving away from OEM turbine controls is a big part of the decision-making process. For this CHP proj-ect, the existing gas turbine control system is GE’s Speedtronic Mark IV system, HRSG control is provided by Alstom, and the control room inter-face is a legacy Provox system. One option would be to upgrade each of the separate systems accord-ing to their natural technology evo-lution path. For example, the exist-ing gas turbine controller could have been upgraded to the Mark VI ver-sion; similarly the HRSG and control room could have been upgraded to the latest technology from their re-spective vendors. This offers an easier solution in that each new system would simply duplicate the functionality of the old system, Charbonneau points out, but would not provide the additional op-erating benefits of a single integrated system and related re-engineering im-provements.

Architecture According to Emerson, very few ven-dors are capable of providing single

By Junior Isles

A project is currently underway at a combined heat and power plant in the UK to replace controls for one of three Fr 6B gas turbine and heat recovery steam generator installations with new technology to improve plant operation and reliability.

GAS TURBINE WORLD: July - August 2009 17

system control architecture for all as-sets on site. Such a project requires a sound knowledge of the equipment that is being controlled in order to re-engi-neer the gas turbine control function. In this case, Emerson had to ensure it had all the correct documentation and design data to carry out the project. Obtaining documentation for plants that have been operating for over 10 years can be a challenge. “When you ask a plant owner for the ‘as build’ documentation such as control log-ics, loop narratives, etc, you often discover it has been lost or no longer exists,” comments Charbonneau. Emerson therefore engaged in a joint design study to come up with a new architecture that would suit the purpose and flush out any risk areas from a project execution point of view. A plan was then put in place to mitigate or eliminate risks by de-veloping a set of functional design specifications (FDS). The FDS is an important tool in securing both funding for the project and defining project goals. It serves as the basis for the next phase of any project – the actual equipment sup-ply and engineering contract – and is a good model to follow for any power plant modernization project, says Charbonneau.

Operation The decision to retrofit the CHP plant was driven by a number of issues, the most important of which was to im-prove operations. Currently, the gas turbine startup procedure calls for operations to be executed in both the control room and electrical room. The new control system will allow automatic start-up from just the control room. With multiple control systems each performing different functions, alarms and events must now be viewed from two separate locations. Charbonneau notes that this is “not ideal and cer-tainly not best practice.” For instance, the existing setup of multiple systems makes it almost impossible to conform to the EE-MUA191 standard for handling mul-

tiple alarms that dictates how many alarms an operator should be faced with per hour, per shift, etc. According to Charbonneau, having a single integrated database for pro-cess data, alarm and operator action is one of the key benefits of a single system. It is also extremely useful for diagnosing the cause of abnormal events which currently requires look-ing at data collected by three separate systems.

Troubleshooting Ideally, power plant operators should be able to analyze almost instanta-neous time synchronization data, at the 1 ms level. Instead, says Charbon-neau, most existing systems are typi-cally limited at around 250 to 500 ms accuracy. An awful lot of events can take place in 0.5 seconds when a turbine goes into shutdown mode. To provide better diagnostic data, he says, the

new system will provide 1 ms se-quence of events accuracy. From a technical point of view, overlapping control systems require multiple interface communication links between the various systems. These interfaces may or may not be redundant. For plants built in the 1980s, they are often based on low-throughput serial links. Today’s systems are now able to offer consistent, predictable through-put, with no black-box interfaces to any system components, all operating on a single network with 100Mbits per second capacity. Using a single uniform network for all control system components – from process controllers to workstations – is a big advantage. Historically, there would be one network for control-lers and I/O subsystems plus a bridge made up of servers to link to another network that connects with the opera-tor and engineering workstations.

Combined heat and power station. The retrofit project will replace multiple overlap-ping plant controls with an integrated gas turbine and supplementary fired HRSG monitoring and control system that also will monitor generator synchronizer functions and interface with an existing continuous emissions monitoring system.

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Redundancy On top of it all, the two separate net-works are usually proprietary systems supplied by different vendors. The new system in contrast offers a net-work that is fast, open, reliable and redundant. With respect to the CHP project, Charbonneau notes that GE’s control system philosophy is based on triple modular redundancy (TMR) while Emerson’s Ovation concept is based on dual redundancy. “The fact the customer is mov-ing from TMR to dual redundancy while expecting higher reliability is

sending a very strong signal to the marketplace,” he says. “Many would find the project quite unique in this aspect.” Most power plants, especially those more than five years old, have separate systems to perform gas tur-bine control, steam turbine control and boiler control. In the new archi-tecture, a single Ovation system will be used for all applications.

TimetableWaiting for an urgent obsolescence issue to arise before putting a control system migration strategy into place

is not a good option for any plant owner. In this case, the CHP plant upgrade project was planned well in advance – between 18 to 24 months – to coin-cide with a planned maintenance and overhaul shutdown. The controls retrofit will take place during the approximately eight-week scheduled outage of one of the three gas turbines. Importantly, the upgrade will not require an extension of the mechanical outage. Commissioning of the new control system is expected to take place dur-ing the Spring of 2010.

320MW Termocandelaria plant in Colombia.Emerson and Mitsubishi Power Systems Americas (MPSA) jointly converted this plant’s two Westinghouse 501F gas tur-bines from ‘gas only’ to dual fuel operation by migrating the existing gas turbine controls from a distributed control design to the company’s Ovation expert control system. Scope of MPSA supply included gas and oil fuel systems (featuring Emerson control valves and instrumen-tation); balance of plant water and oil treatment equipment; water-injected diffusion flame combustors (fuel nozzles, combustor baskets and transition pieces). The retrofit also included MPSA’s new dual fuel system which incorporates a duplex liquid fuel nozzle that eliminates the need for a flow divider and improves startup

and low-load particulate emissions. Purge air systems and Mitsubishi’s high energy ignition system were also applied to assure high starting reliability. Automatic startup on either gas or oil to reliably gen-erate requested power on demand allows Termocandelaria to maximize generating revenue and also avoid financial pen-alties that would be incurred if it were not able to fulfill regula-tory obligations with the Colombian Energy System. “Startup reliability has been greatly enhanced with the dual fuel capability now in place,” stresses Miguel Angel Perez Ghisays, plant manager. “The significance of this can-not be underestimated, as the plant’s ability to be a reliable source of back-up power plays an important role in maintain-ing grid stability.”

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