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Singapore generator adds cogeneration to power plant

Singapore’s PowerSeraya has added a cogeneration facility to its original power generation capability in a move towards creating a fully-integrated energy hub. Edward Henry Taylor, Ponnuchamy Veerachamy, Shen Youwang, Shih Chi Lai and Mohd Rafid Bin Ibrahim report on the execution of the project.

Jurong Island in Singapore is the ideal location for PowerSeraya Ltd to pursue its corporate strategy to develop from a pure generator into a fully integrated energy hub. In January 2007 PowerSeraya, with the support of PB Power as the owner’s engineer, embarked on a two phase cogeneration project. In the first phase PowerSeraya’s two existing single-shaft 370 MW Siemens SCC5-4000F combined cycle power (CCP) units were converted to cogeneration, for supplying 130 tonnes/hr of process steam through 2.5 km of high pressure steam pipeline to the end user by 1 September 2009. In the second parallel phase two additional 400 MW Siemens single shaft SCC5-4000F CCP cogeneration units with duct firing capability were installed for commercial operation in mid-2010.

This article will look at some of the key challenges faced during project execution, including the optimal reuse of the plant’s existing facilities and the integration of new equipment. Other aspects of the project covered in this article are the emergency steam dump, venting provisions for safety, steam metering considerations and the dynamic behaviour of the plant when operating in cogeneration mode, when steam is extracted from the multi-unit configuration while connected to the grid.

In mid-2000 PowerSeraya embarked on a corporate strategy of diversifying from its core business of supplying generation to the Singapore Electricity Market as a pure generator to become a fully integrated energy hub. As part of the diversification strategy, PowerSeraya investigated the potential for enhancing the efficiency of its existing power plant assets through their conversion to cogeneration, and for supplying steam to various offtakers in the immediate vicinity of the PowerSeraya site.

Located on Jurong Island, the centre of the Singapore petrochemical industry, PowerSeraya subsequently entered into a steam supply agreement (SSA) with a steam customer on the island. This agreement covers the supply of high-pressure, high-temperature steam and identified potential steam customers for medium and low pressure steam. The Power Division of Parsons Brinckerhoff Pte Ltd (PB Power) was contracted by PowerSeraya as the owner’s engineer for the project.

A view of PowerSeraya’s completed cogeneration project at Jurong Island

PRELIMINARY INVESTIGATIONS

Investigations were carried out to determine the optimal concept for the proposed cogeneration facilities and steam pipe work corridor route, as well as the potential for reusing existing equipment in a cost-effective manner. The investigations considered the reuse of the existing 250 MW steam turbines as well as the associated balance of plant and cooling water facilities and turbine hall. The studies were carried out using established thermodynamic modelling software, supplemented by PB Power’s considerable experience in similar studies.

PB Power’s initial studies found potential to supply steam from the existing CCP units. But they also concluded that risks associated with utilizing the existing steam turbines made installing new power train and associated balance of plant the most cost effective option. The challenges included:

• matching gas turbine and steam turbine performance to achieve optimum efficiencies;

• achieving the transmission code’s requirements for independent generation facilities for the resulting 2 x 2 x 1 arrangement;

• overcoming the major schedule and cost impact of unexpected equipment problems uncovered during the refurbishment of the steam turbines;

• ensuring the ongoing operational reliability of the refurbished steam turbines for a further 25 years of operation.

PB Power also concluded that there was the potential for significant cost benefit in utilizing the existing cooling water facilities and associated equipment of the decommissioned units.

BASIC DESIGN CONCEPT

The proposed cogeneration plant design’s key parameters were:

• meeting the requirements of Singapore’s Energy Market Authority, including those relating to the transmission code and independent generation facilities;

• securing the continuity of cogeneration steam supply under all operating scenarios, encompassing system frequency fluctuat-ions, cogeneration CCP components trips, fuel changeover testing, and tripping interdependence between registered units.

Further specific require-ments were:

• initial commercial steam supply of ultra-high pressure (SU) and high pressure steam (SH) to steam customer on 1 September 2009;

• potential for future steam supply to new steam customers by incorporating provisions for a ultra high pressure header (SU), a high pressure header (SH), medium pressure header (SM), and a low pressure header (SL) around 5 bar at 40°C;

• quality of steam supply that meets the requirements of the steam supply agreement;

• duct burner for heat recovery steam generation (HRSG) rather than provision for an auxiliary boiler;

• minimal demolition at the PowerSeraya site;

• reliability of steam supply through having at least two units contributing steam to the steam network at any one time;

• use of steam headers with multiple offtakes with cogeneration generation dictated by steam pressure within the header.

Operation controls

It was essential that the cogeneration integrated steam control system balance power export and steam distribution, meeting grid code stipulations, the buyer’s demand and emergency scenarios.

The steam header pressure was therefore controlled by a pressure regulation valve at the steam extraction point. If several units and/or auxiliary boilers are connected to the steam header, the steam extraction controller selects a lead unit that will control the steam header pressure. The other units contributing steam will follow that unit and respond to the steam off-taker’s varying steam demand as required.

Steam export and metering philosophy

To achieve accurate flow and quality in the cogeneration steam, the steam metering system incorporates:

• purity analyzers for billing and monitoring steam purity;

• utility requirements such as a UPS power supply, instrument air, cooling water and electrical power for the steam metering system;

• steam quality monitoring with conductivity and pH as the main purity measurements, as well as monitoring of ammonia, silica and sodium contents for export steam.

In general, a redundant process measurement concept is followed for steam metering. The steam metering system provided at the owner’s premises will include – at a minimum – pressure, temperature and flow measurements.

These field measurements signals are exchanged with the plant control system for controlling, monitoring and recording purpose and with the owner’s control system for checking measurements. The owner’s steam consumption flow rates decide the requirement of the custody transfer metering (CTM) and supervisory control and data acquisition (SCADA) system for the steam billing system. The normal achievable overall accuracy of the CTM is around 1%. Billing and various reports are generated using an independent SCADA system.

CONTRACTUAL ARRANGEMENT

Two EPC contracts were awarded to a consortium of Siemens and Samsung:

• the interim steam supply agreement (ISSA) covering the conversion of the two existing CCP units to cogeneration to supply steam prior to the completion of the new cogen plant;

• the cogeneration contract, covering the installation of two units of cogeneration CCP and associated steam delivery systems, and subsequently amended to include the supply of two fuel gas compressors.

The two EPC contracts were tied together through an umbrella agreement to ensure that both EPC contractors worked with a common objective of ensuring a commercial supply of steam to the initial steam customer by 1 September 2009 prior to the completion of the new cogen plants.

PHASED IMPLEMENTATION

The project was implemented in three phases.

Phase 1: interim steam supply

The interim steam supply was provided from the existing two Siemens SGT5-4000F combined-cycle single-shaft units, which were modified through the addition of de-aerators for each unit to handle the cogeneration steam supply, complemented by a control and instrumentation upgrade to allow for cogeneration operation and transfer between combined cycle (or power mode) and cogeneration operation.

The cogeneration steam supply was taken directly from the HRSG HP main steam line and transported through a 2.5 km single SU steam pipework to a pressure-reducing station adjacent to the steam customer site – which also provided the SH steam required. Additional water treatment facilities based on RO technology were provided to meet the water demands associated with the cogeneration steam requirements.

Phase 2: new cogeneration plant

The new cogeneration plant comprises two Siemens single-shaft SCC5-4000F combined cycle plants, including a duct-fired triple pressure with reheat cycle HRSG. The units are located at the southern end of the PowerSeraya site and make use of the existing cold water (CW) system of the decommissioned steam units. The plant is controlled by the Siemens SPAA T-3000 control system, which enables full cogeneration operation and interface with the ISSA units.

Phase 3: installation of fuel gas compressors

The installation of two fuel gas compressors was carried out in parallel to the erection of the cogeneration plant, with the final testing and commissioning following the date of readiness for commercial operation.

OVERALL PROJECT PROGRAMME

The steam sales agreement between the steam customer and PowerSeraya required a commercial supply of steam to be available to the steam customer on 1 September, 2009.

The key programme dates for the project were:

• tender preparation, optimization study, issue of tender document and closing of tender over five months, from January to May 2007;

• awarding of the contract in mid-August 2007

• ISSA completion (steam supply to customer) at 25 months after the awarding of the contract;

• completion of new cogen units at 35 months from the awarding of the contract;

• commercial operation from mid-2010.

The PowerSeraya plant on Jurong Island is situated close to potential steam customers in the country’s petrochemical industry

ERECTION CHALLENGES

The erection process encountered several primary challenges:

1. Establishing ISSA interfaces with two existing CCP Units

The erection of the de-aerator and additional control and instrumentation took place under the PowerSeraya PTW (permission to work) scheme with the existing units remaining in operation. The final connections were made during a major inspection of each of the respective units to minimize the impact on plant availability. Provision was made in the schedule for the subsequent commissioning activities.

2. Erecting cogeneration steam delivery pipe work

One of the main issues resolved during the initial design phase was the most cost-effective process for crossing the busy Seraya Avenue. Local regulations do not permit pipe bridges and therefore an underground crossing had to be considered. An option for pipe jacking had been included at the contract negotiation phase but, following discussions with Jurong Town Corporation (JTC) and the Land Transport Authority (LTA) by the EPC contractor, an alternative of a shallow road crossing for the main steam pipe work with a local road hump was approved.

The requirements of the SS512 standard were followed in the design, construction and operation of the steam pipeline. It was also necessary to obtain letters of no objection from all third parties impacted by the construction, commissioning and testing of the steam line during the implementation phase. This required close co-ordination between the EPC contractor and PowerSeraya in view of the large number of parties impacted along the steam pipeline route.

3. Refurbishment of existing equipment

The use of existing equipment was one of the project’s primary goals. As the tender negotiations offered little time for potential tendereers to carry out a thorough inspection, provision was made for the EPC contractor to make a detailed assessment of the state of the proposed reused equipment within six months of the awarding of the contract. An opportunity to claim for cost variations was specified along with an obligation to maintain the schedule.

Seven refurbishment pack-ages for the steam units covered: the CW intake facility including band screens; cathodic protection for the decommissioned steam plant intake facility; the electro-chlorination plant; 230 kV gas-insulated switchgear; CW pumps and associated culverts; station water tank conversion for demineralized water; and a ventilating system for the CW pump house and electro-chlorination building.

Responsibility for refurbish-ing the CW intake facilities – including a band screen overhaul, screen wash etc. and refurbishment of the ventilation system – was assumed by PowerSeraya, whose maintenance teams carried out the work.

The CW pumps were modified by shaving the impellor to provide the performance required for the cogen plant. Subsequent testing using motor power and head loss within the CW channel confirmed the predicted CW performance against the revised design performance curves.

Refurbishment investigat-ions, which were primarily carried out on behalf of the consortium by the OEMs for the equipment in question, revealed no major surprises and only led to a minor addition to the proposed refurbishment costs.

4. Establishing interfaces with existing equipment

The removal from service of decommissioned steam units created a need to transfer some of the power feeds into the new cogeneration plant. This required very close co-operation between PowerSeraya and the consortium and was handled under the PowerSeraya PTW scheme.

5. Protection of existing equipment

The EPC contractor erected barriers and constructed suitable crossing points to eliminate the risk of damage to the existing GRP CW pipework that ran along the southern boundary of the site. The route of the 230 kV cable along the northern boundary of the site was marked, with temporary barriers set up and construction staff kept informed to safeguard the underground cables. In the immediate vicinity of the existing control room bored piling was also used to remove the risk of piling vibration tripping either of the operating units.

COMMISSIONING AND TESTING OF ISSA AND NEW COGEN UNITS

The steam sales agreement and the target delivery of process steam to the customer required the commissioning phase to be carried out at a very fast pace. The ISSA commissioning phase began in early June 2009 to meet the deadline for providing interim steam to the steam consumer, while commissioning for the two new cogen units started in early November 2009.

Independent engineer testing

Due to the nature of the commonly connected steam supply system from different units, the converted cogen units were subjected to comprehensive independent engineer (IE) testing to confirm compliance with the requirements of the Singapore Transmission Code. The scope of the IE is to identify whether the design opens up a risk of any common failure that could lead to a trip of more than one CCP unit.

After the cogeneration conversion, both steam extractions from existing units were interconnected to a common header for supplying steam to the steam customer. This creates issues of unit independency for power export. Tests therefore had to be carried out to ensure that the design poses no risk of common mode failure. The testing was carried out by an independent engineer, which cannot be the owner’s engineer or one associated with the EPC contractor.

Similarly, both cogen units were subjected to an even more comprehensive set of independent engineer testing as the cogen units are new plant and have the potential to operate with any combination of all four units, even though the existing units were used for an interim steam supply. The converted cogen units underwent 23 IE tests and the two new cogeneration units were subjected to 65 IE tests.

Transmission code test and power system operator test

Registering the unit as a cogeneration plant for exporting process steam and power to the Singapore Transmission System required

the units to undergo tests defined in the Transmission Code, which include:

• Automatic governor control (AGC) test, carried out through loading up and down by pulses sent by the Energy Market Authority (EMA) control system at various rates up to the maximum permissible ramp rate. The load achieved for each test within the stipulated period determines whether the machine can meet the AGC requirements.

• Spinning reserve test, which utilizes the software simulation circuit built into the gas turbine controller. The stipulated programmed test signal is injected into the gas turbine (GT) controller to ‘fool’ the controller that there is a drop in grid frequency so that the GT controller will respond accordingly to pick up load.

• Maximum capacity test, which loads the machine continuously at base load for eight hours.

STEAM CONTROL AND DYNAMIC RESPONSE

The process steam system consists of a steam header that collects the extracted steam of all four units and routes them to the steam customer. It has two pressure stages: the ultra high pressure stage (SU) and the high pressure stage (SH). The control system for the process steam is designed so that export steam can be taken from any unit.

Due to the need to ensure a continuous and uninterrupted supply of steam to the steam customer, the control scheme is designed to provide flexibility in steam supply and also to enable steam transfer from one unit to the other with minimal transient effect.

The keys scenarios that were considered in the control design and optimization included:

• In the case of a single unit tripping, the other cogen units are capable of quickly supplying the lost process steam to ensure a stable steam supply to the steam customer.

• If steam is being supplied from existing units, this would result in a reduction in steam turbine load. However, the optimization of the control scheme enables a predetermined load increment to be imposed to the load set point to allow the running unit to maintain or even pick up the spinning reserve while still maintaining the steam flow at the steam customer terminal point.

• If steam is being supplied from the new cogen units, where duct firing is available, the steam dynamic behaviour is somewhat different. Under the same scenario of a one-unit trip, the duct burner will cut in automatically after the unit trip to compensate for the loss of steam supply from the tripped unit. This enables the steam supply and the frequency response margin to be maintained.

• In the case of the steam customer’s plant tripping, the steam customer’s process will trigger a sudden increase in the process steam demand. But the steam flow limitation function in the control system will curb the steam flow and prevent disruption of the cogen units’ operation.

OPERATING EXPERIENCE

Since starting commercial operation, the units have been operated continuously without major issues. In operating and taking over the plant PowerSeraya has faced no difficulties relating to the new cogen units, which reflects training conducted by the contractor both locally and overseas.

During the commissioning phases, new operators at the steam plant underwent on the job training under the supervision of the contractor’s supervisor. Existing operators from the established units were integrated with the new operators to accelerate their training programme. The similarities between the control systems at the old and new units enabled existing operators to master the operation of the new cogen CCP very swiftly.

CONCLUSIONS

The converted cogeneration facilities and new cogen-eration facilities have been tested comprehensively in full compliance with contract and regulatory requirements.

Close co-operation and support between the owner, the owner’s engineer and the EPC contractor has played a key role in overcoming the numerous challenges faced during the plant erection and commissioning.

Early and regular communication between the owner and the Energy Market Authority’s Power System Operator (PSO) throughout the project implementation is essential to ensure that EMA-PSO requirements are fully understood and met.

Edward Henry Taylor is Engineering Director, PB Power Asia, Singapore. Email: taylore@pbworld.com

Ponnuchamy Veerachamy is Manager Thermal Generation, and Shen Youwang is the Senior Generation Engineer, both with PB Power Asia, Singapore.

Shih Chi Lai is the Head of Projects while Mohd Rafid Bin Ibrahim is the Operations Manager, both with PowerSeraya Limited, Singapore.

This article is based on a paper delivered to the POWER-GEN Asia 2010 conference

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