Ed Karpiel, Ken Webb
Iroquois Pipeline Operating Co.
Shelton, Conn.
Iroquois Gas Transmission System, Shelton, Conn., ha commissioned two corn pressor stations whose design reflects many recently developed engineering innovations and successfully addresses community concerns over noise and air pollution.
These first two station provide incremental firm gas-transportation capacity on the 375-mile system that runs between Canada and Long Island, N.Y. The 11,300-hp station in Wright, N.Y., began operating on Nov. 1, 1993; the 6,960-hp station in Croghan, N.Y., began operating on Dec. 15,1994.
Each includes facility automation for unattended operation, low nitrogen-oxide emissions, noise levels that approximate the ambient rural environment, and unique building design and landscaping designed to reflect each station's rural surroundings.
Both stations occupy previously undeveloped land.
The Wright station maintains service to Dartmouth Power Associates (an electric power generator in Massachusetts) and Niagara Mohawk Power Corp. (a local distribution company in upstate New York) with a total of 65 Mmcfd of firm capacity.
The Croghan station serves Selkirk Cogen Partners L.P. with 55 MMcfd of firm capacity to supply a cogeneration plant in Bethlehem, N.Y. Addition o t e Croghan station has increased the firm contracted volumes transported on Iroquois to 750 MMcfd, including 54 MMcfd to a power generator in Connecticut, contracted for short-haul service.
STATION DESIGN
Planning and design for the compressor stations considered the rural areas of each site, as well as the regulatory requirements for air and noise emissions, for example, in the facilities' permitting process.
William R. Morrison Engineering Services Inc., Toronto, and Colt Engineering Corp., Markham, Ont., provided detailed engineering for both stations. Morrison also performed turbo-compressor equipment selection, flow analysis, and conceptual design.
ATCO Noise Management, Calgary, designed and supplied building acoustic and ventilation. Phenix Environmental Inc., Newtown, Conn., coordinated permitting.
Because the maximum allowable operating pressure (MAOP) of Iroquois' main line is 1,440 psig, the stations were designed to maintain 1,440 psig discharge at the main line, including consideration of future station additions which could increase yard losses, and can accommodate an MAOP of 1,480 psig.
Construction used ANSI Series 600 piping components. During procurement, however, suppliers were verified to ensure all components were rated for working pressure of 1,480 psig or greater.
Yard piping was designed to minimize pressure loss, including specification of large-diameter pipe and components.
The use of low pressure-drop strainers instead of filter separators minimized yard losses upstream of the compressor. That Iroquois transports dry, processed Canadian gas made this efficiency possible.
Design of Croghan high-pressure gas piping required that piping stresses imparted to the compressor nozzles be negligible. The low tolerance for displacement due to mechanical stresses caused the suction and discharge lines to be designed with 360 and 450 elbows, respectively, between the compressor nozzles and the piping entry into the compressor building wall (below grade).
TURBOCOMPRESSOR UNITS
Wright station uses two Solar Turbines Inc. 5,650-hp Centaur 50 model, two shaft, simple-cycle gas turbines, each driving a Solar C337 series/parallel compressor.
The series/parallel compressors were selected to accommodate a wide range of operating conditions, including both high compression ratio and high flow (lower compression ratio) scenarios. The compressor is a single case with valves and passages to permit two or more impellers to operate in either parallel or two-stage, series mode.
These compressors fit well for the Wright application because of the two distinctly different operating conditions that result with one or two-unit operation, given that Wright is 122 miles downstream of Croghan.
When the two units are operating in series, the station maximum compression ratio is 1.88:1; in parallel, 1.4:1.
Croghan uses a single Solar Taurus 60 model 6,960-hp, two shaft, simple-cycle gas turbine driving an IMO Delaval PV-31 axial inlet centrifugal compressor. At Croghan, where operating flexibility is currently less crucial than at Wright, the PV-31 compressor was selected because of its higher efficiency, which translates into lower fuel use and fewer emissions.
The compressor can operate at 85% efficiency at a maximum compression ratio of 1.24:1. Yard piping at Croghan allows series or parallel operation of two units, should a second unit ever be constructed there (Fig. 1).
PIPE SPECIFICATIONS
At both stations, high-pressure gas piping (larger than 16 in. OD) met API 5L with additional Iroquois specifications controlling steel production, plate production, cold expansion of the pipe after forming, and chemical composition of the pipe (for example, 0.1% carbon, 0.01% sulfur, 0.31 maximum carbon equivalent, no boron additions).
Charpy toughness testing was specified to be a minimum of 50 ft-lb at either 25 F. for underground piping, or -50 F. for aboveground pipe, as an average value from three specimens from any one heat.
Full-transition curves for
both Charpy and dead weight tear-test results were required, with the low-temperature steels to be tested at -50 F., -65 F., and -85 F.
Charpy v-notch impact testing was also required on the pipe weld metal and the heat-affected zone of the pipe seam weld. Minimum absorbed energy was 20 ft-lb at 25 F.
Fittings were purchased using an Iroquois supplement to MSS-SP 75 (MSS = Manufacturers Standardization Society - Standard Practice) with restraints on chemical composition, tensile strength, weld testing, notch toughness requirements, and physical dimensions.
Flanges were purchased to the standard MSS SP-44 specification with additional requirements controlling chemical composition, destructive testing such as Charpy v-notch testing at 25 F. or -50 F. for below-ground and aboveground locations, respectively, and non-destructive examinations by radiography or ultrasonics.
BLENDING IN
Both compressor stations were designed and are currently operated for minimal impact on their immediate surroundings.
The Wright station, one of the first industrial facilities in the town, is on 52 acres of formerly agricultural land and situated to preserve existing farm hedgerows and stone walls.
Painted barn-red and featuring cupolas and a weather vane, the buildings resemble a large farm complex (Fig. 2). At both stations, the exhaust stacks resemble barn silos.
A special landscaping plan, developed in cooperation with federal, state, and town officials, was implemented at Wright to enhance the appearance of the station as part of the rural farm environment. The landscaping effort involved the planting of scores of trees and shrubs in the existing hedgerows, as well as 1,000 daffodils.
In addition, Iroquois has arranged for a local farmer to continue to grow and harvest hay in a portion of the buffer area around the station.
Environmental factors played an even larger role in the situation of the Croghan station than they did at Wright because of the forests and wetlands characteristic of New York's Adirondack Mountains.
Located within 100 acres of a 40-year-old pine plantation, the station was set back 1,500 ft from the road to screen the station as well as reduce noise from the compressor operation (Fig. 3). Although special landscaping is unnecessary at Croghan because of the natural screening provided by the pine plantation, the station buildings were designed to conform to the surrounding environment and, at the local town's request, are clad in forest green siding (Fig. 4). Construction at Croghan required fewer than 11 acres, including the 1,500-ft access road and, under separate authorization, measurement facilities to serve Niagara Mohawk Power Corp. from Iroquois' main line.
Approximately 3 acres have been designated for revegetation. The fenced compressor-station yard area was limited to less than 4 acres, and buffer areas of uncleared pine plantation were preserved to maintain the historical land use of the area.
EMISSIONS CONTROL
Both compressor stations were designed for minimum air pollutant emissions. The Solar Centaur 50 and Taurus 60 turbines produce 42 ppm or less of NOx, using Solar's SoLoNOx combuster system.
Use of the SoLoNOx system facilitated the acquisition of air permits for both stations, allowing each site to be classified by New York State as a minor emissions source.
Also, to facilitate the permit process and to minimize public concerns about visual impacts, the heights of the exhaust stacks at both stations were kept to the minimum permissible under state regulation.
At Wright, dispersion modeling and a request from the local town planning board to minimize height led to a 40-ft stack being used. The auxiliary power unit and boiler for the heating system also came with low NO, combustion equipment.
The Croghan station has a 52.5-ft high exhaust stack, based on air-emissions dispersion modeling that confirms this stack height to be the minimum that conforms to state emissions regulations.
To reduce unburned gas emissions, the stations use electric motors to start the turbines, as opposed to gas-expander starting equipment. This feature was also incorporated to reduce site noise sources: the exhausts of gas expanders are characteristically noisy.
The compressor dry-sea] system permits compressor case and adjacent piping to remain loaded on routine shutdowns, thereby minimizing discharge of unburned gas to the atmosphere.
Air and electric valve operators, which are used extensively, emit no unburned gas to the atmosphere.
NOISE REDUCTION
Both the Wright and Croghan stations operate with significantly less noise than typically required by Federal Energy Regulatory Commission (FERC) certification of such projects.
The Wright station was designed with a target noise limit of 42 db at 300 ft from the turbocompressor buildings, and testing since facility start-up has confirmed this was achieved. The Croghan compressor station design criterion for noise emissions was 40 db at 300 ft.
Working with Iroquois noise consultants Lewis S. Goodfriend & Associates, Morristown, N.J., Morrison Engineering established these goals. Tests conducted in December 1994 indicated that they were achieved.
Noise suppression at the stations was accomplished by abatement of each significant noise source at the respective sites. Such sources included the gas turbine and compressor, the turbine exhaust and intake, high-pressure gas piping, oil cooler, cooling and ventilation fans, and station auxiliary equipment.
The gas turbines and compressors are totally enclosed by Solar acoustic enclosures. Experience indicated total enclosure to be more effective than one which covers only the gas turbine because much of the station noise originates at the pipeline compressor.
The turbine intake air is ducted through two Solar Turbines' air-intake silencers, in series. The exhaust is ducted to a high-performance exhaust silencer.
The unit suction and discharge valves, recycle valve, loading valve, and blowdown valves for each station compressor are within the acoustic compressor buildings to reduce total station noise. All high-pressure gas piping and exhaust ducting within the compressor building are covered with an acoustic insulating jacket.
Lube-oil coolers used with turbocompressors can often be one of the largest contributors of noise. The sources are air-fans, a function of fan speed, and oil pumps which transmit noise to the cooler through the oil piping.
A low-noise oil cooler was part of the Solar Turbines package and is inside a false corner of the compressor building. The interior surfaces of the lube oil cooler area are lined with sound-absorptive material.
Another low-noise feature is an oversized fan with a variable speed motor so the fan provides only what air is required. To assist the cooler and assure that fans stay in the lower speed ranges, Iroquois installed a fuel gas/lube oil heat exchanger to pre-cool the oil using the sub-zero temperature turbine fuel gas which has been regulated from station suction pressures of 800-1,200 psig to 200 psig.
Further noise reduction derives from insulation of the 3-in. OD oil pipes within the cooler enclosure.
The ATCO compressor buildings use assemblies that result in a sound transmission class (STC) of 54. Typically, the wall and roof sections are 8 in. thick and consist of multi-layers of noise-absorbing materials (Fig. 4).
Perforated material lines the inside wall and roof surfaces to permit noise to be absorbed rather than reflected. Layers of mineral wool insulation and a high-density barrier make up the wall cross-section.
The exterior building skin is sheet metal. Acoustically treated, hinged steel equipment doors were provided in addition to mandoors.
At doors and where utility systems and piping penetrate the building walls, gasket material and acoustic caulking minimize noise leakage.
Ventilation air inlets and exhausts use custom-designed parallel baffle silencers 8-10 ft long which extend inside the building to occupy otherwise unused space. At Wright, exhaust vent silencers are located at the roof peaks, disguised as cupolas.
Station auxiliary equipment is located within an acoustically treated room in the control/utility buildings. Auxiliary equipment includes the auxiliary power, compressed air, water/glycol heating, and foam generation. These rooms involve building wall and roof construction similar to that of the compressor buildings.
At both stations, high-pressure gas piping which is not enclosed by the compressor building is placed below-ground. The station suction strainer is aboveground, however, and is jacketed with acoustic insulation.
Other station features contributing to the low-noise design include dry seals that permit retention of compressor-case gas pressure on shutdown, low-noise air compressors, and silenced auxiliary-power-unit exhaust.
IMPLEMENTATION PLAN
To certify facilities to serve the Selkirk cogeneration projects, FERC required pipeline applicants Tennessee Gas Pipeline Co. and Iroquois to prepare implementation plans before beginning construction.
This requirement applied to Iroquois' Croghan compressor station as well as to Tennessee Gas' interconnecting facilities between Wright and the Selkirk plant. FERC specified that the implementation plans required written agency approval.
The plan had to describe how each company would implement environmental mitigation measures required by the certificate.
FERC specified that the plan describe how requirements of the certificate would be incorporated into contract documents, who among company and contractor personnel would receive copies of those contract documents, what training these personnel would receive, which would be responsible for compliance, the procedures and contract penalties the companies would enforce if noncompliance occurred, and the schedule of key preconstruction, construction, and post construction events.
Iroquois prepared a detailed plan that complied with the FERC requirement.
The document detailed the mechanisms by which bidding and contractor selection would communicate all permit requirements to,bidders.
A key feature of the implementation plan was inclusion of the specific contract language which would be used to ensure compliance with each certificate requirement.
CONSTRUCTION CONSIDERATIONS
The prime contractor for construction at Wright, which began in the spring 1993, was Murphy Bros. Inc., East Moline, 111. The prime contractor for construction at Croghan, which began in July 1994, was LaBarge Bros. Co. Inc., Syracuse, N.Y.
At both stations, Iroquois procured material and administered contracts for all aspects of the project, including site development. At Croghan, Iroquois also directed site development and installation of a new main line valve (MLV) on the Iroquois pipeline.
At Wright, Iroquois installed an MLV, branch tees, and side valves during main line construction in 1991 to accommodate the future compressor station, constructed in 1993.
At Croghan, Iroquois installed a fabricated MLV (including tees and side valves) during summer 1994.
Because Iroquois' system consists of a single main line and has a characteristically high load factor (approximately 93.25%), installation of this valve had to be accomplished without interruption of service.
Iroquois utilized stopple and bypass techniques to install the MLV assembly. Two 12-in. hot taps, approximately 400 ft apart, were made and a 12-in. bypass line installed parallel to the 30-in. line so that flow was maintained while the stopples were installed.
A 70-ft section of the 30-in. line was cut out, permitting the MLV assembly to be lowered and welded into place. All stoppling and welding to the valve assembly took 2 1/2 days.
At both stations, gas piping butt welds were made using low-hydrogen processes. High-pressure gas piping was hydrostatically tested at 2,220 psig.
Where high-pressure gas piping is buried (at all locations except the suction strainer), continuously poured concrete mudmats support it to reduce piping stresses.
Belowground pipe was factory-coated with fusion-bonded epoxy. A urethane-epoxy coating was used where belowground piping was field coated.
Every high-pressure gas piping butt weld was nondestructively tested with radiography. All high-pressure gas piping socket welds were non-destructively examined with dye penetrant.
Both of Iroquois' compressor stations involved construction of a compressor building for each turbocompressor unit and a single control/utility building.
The buildings consist of pre-engineered steel frames supporting the ATCO acoustic roof and wall assemblies. The control/utility buildings include an office area, a station control room, garage and shop bays, and an auxiliary equipment room.
REMOTE OPERATIONS
Both stations have been designed for unattended, remote operation with microprocessor-based control and two-way satellite communication with Iroquois' corporate office in Shelton and Tennessee Gas Pipeline Co.'s gas-control center in Hockley, Tex. Tennessee is under contract to Iroquois to operate its system.
Instrumentation can detect failure of any station's primary operating system or auxiliary system and operator errors. The instrumentation will detect any equipment which is not in a ready state.
A Waukesha engine-generator auxiliary power unit, station electric power transfer switch, and uninterruptible power supply system allow the plant to continue operating through failure of commercial power at the site.
Following a short power loss incurred during transition to backup power, the station allows automatic restart of plant equipment. Certain safety-related conditions or alarms require equipment to be reset on site for further operation.
The plant control panel at each compressor station controls key operations of each plant, including fire-safety systems, foaming, building ventilation, high-pressure gas valving and relief, fuel-gas control, power-gas control, compressed-air system operation, electrical power monitoring, auxiliary power unit operation, plant security, and the plant heating system.
Safety controls are failsafe. In the event of power loss in a circuit or failure of the control system, the equipment and/or plant is shut down to its safe state.
The plant control panel also automates starts and stops of the turbocompressor units and performs process control. The unit control panel and plant control panel both use programmable logic controllers (PLCS) and display panels provided by Allen Bradley.
The plant control panel is networked with on-site turbocompressor control panels and the main line supervisory control and data acquisition (scada) system.
Through the network, plant and unit control panel information is also available remotely. Control panels operate independently with hardwired backup signals, allowing station operation to continue should the on-site network fail.
Through Iroquois' scada system, system-wide (main line and compressor stations) operating data displays can be accessed from either compressor station. Three independent communication paths provide access to the scada system: VSAT (very small aperture terminal) satellite, dial-up, and leased line.
Should total communication fail, the plant is capable of stand-alone operation with an operator on site.
Fig. 5 (73586 bytes) shows one of the Croghan compressor station operating displays as can be viewed at any scada system network terminal including Iroquois' headquarters office.
Fig. 6 (94799 bytes) shows the complete Iroquois pipeline system including the Wright and Croghan compressor stations as viewed at any scada system terminal.
Scada is an OASyS system from Valmet Automation, Calgary. Iroquois uses Sun work stations with SunOS, a version of the UNIX operating system.
Compressor surge control uses an annubar flow-measurement device. At the Croghan station, ultrasonic flow meters are also used to measure flows (unit and station). This equipment will later become the primary surge control flow-measurement equipment.
All flow information is available remotely through the scada system. The stations utilize electronic measurement equipment to monitor fuel-gas flow including use of a vortex shedding meter.
The stations utilize ultra-violet infra-red (UV-IR) fire-detection systems with sensors located at several locations in the compressor building. Gas detectors and heat detectors are also placed in the compressor building as well as the auxiliary room of the control/utility building.
Each compressor building is served by a foam-generating system which can completely fill a compressor building in 3 minutes upon activation by the fire-detection system. The foam-distribution system is fabricated of stainless steel pipe and components to provide the highest degree of reliability.
The turbocompressor enclosures are also equipped with UV-IR fire-detection devices and automated CO2 fire-extinguishing capability.
Cameras provide visual monitoring of the compressor stations for security and safety. These cameras can be monitored locally as well as from the Corporate Office.
