The New York City Department of Environmental Protection's (NYC-DEP's) North River Water Pollution Control Plant (NRWPCP) processes raw sewage for the western half of Manhattan, from Bank Street to the northern tip of the island. On average, it treats 170 million gal. per day (gpd), with a peak of 340 million gpd during wet weather. The 28-acre facility is situated over the Hudson River, built on a reinforced concrete platform just south of the George Washington Bridge. On its roof, its architectural design incorporates Riverbank State Park, a New York recreational facility with three swimming pools, an amphitheater, an athletic center, a skating rink, a restaurant, and several sports fields. The NRWPCP is one of 14 treatment plants operating in New York City, which, together process more than 1.4 billion gal. of wastewater from homes, businesses, schools, and the streets of the city's five boroughs per day.
Processing Wastewater
Among the many steps required to convert wastewater into a safe, reusable liquid is the processing of sludge, the heavy particulate matter separated from raw sewage. Sludge from primary and secondary treatments consists primarily of water and must be further concentrated and treated before its final removal.
Thickened sludge is placed in an oxygen-free container called a digester and heated to 95F using hot water provided through on-site process boilers. This stimulates the growth of anaerobic bacteria--bacteria that thrive without oxygen--which consume the organic material in the sludge. Methane gas, produced and recovered during the digestion process, is used as fuel for certain facility operations, contributing to plant operating efficiency. After digestion, sludge is dewatered further, producing moist soil-like substances called "bio-solids," which are handled safely and recycled as sustainable and environmentally safe fertilizers and soil nutrient-enrichment products.
System Failures
The NRWPCP was experiencing a combination of failing process-system components. Original process piping used throughout the plant, which primarily was constructed with mechanically coupled joints, was significantly corroded, permitting losses of more than 10,000 gal. of process hot water per day. Besides the high energy losses caused by the process-hot-water losses, the plant routinely was forced into expensive piping-system draindowns to repair or replace corroded sections and equipment components.
Facility issues included secondary-system control failures, which contributed to uncontrolled hot-water delivery to secondary systems and a degradation of process-system water quality caused by fresh-water makeup requirements. This subsequently contributed to premature deterioration of system apparatus, transmission piping, and fatigued low-pressure process-hot-water boilers.
The NYC-DEP's partner in the NRWPCP rehabilitation project is the New York Power Authority (NYPA), which acted as the primary funding source and provided supervisory and management services during project design and construction.
The engineering/construction management firm of DMJM-Harris/AECOM, working in conjunction with the NYPA, the NYC-DEP, the mechanical construction firm of Dynamic Mechanical Contractors Inc. (DMCI), and York-Shipley Global, a division of AESYS Technologies LLC, developed a sustainable solution for providing hot water for the digester process. Initially, with input from the NYC-DEP, considerable effort was expended to identify essential system and equipment components that required rehabilitation and, more importantly, the primary causes of the variables that contributed to the system and component failures. This helped develop a design and scope-of-construction approach that would provide for an efficient, reliable, and sustainable process system.
Departing from the usual design philosophy of employing low-pressure process hot-water boilers for the digester operation, the NYPA and DMJM-Harris/AECOM altered the process configuration to incorporate low-pressure process steam (LPS) with steam-to-hot-water heat exchangers and variable-speed pumps to support the process circuit. This design was intended to permit the boilers to maintain consistent steam volume and pressure regardless of hot-water process load demand, essentially isolating the boilers from the process and ensuring satisfactory system and equipment maintenance, operation, and longevity and an efficient, useful life cycle.
Thermally Induced Stress Cycling
The original design configuration required hot-water boilers to respond directly to varying system load demands of hot-water-to-hot-water heat exchangers. The boilers' fluctuating load response not only created operating inefficiencies, but was the principle cause of premature failure. The varying uncontrolled loads subjected the low-pressure hot-water boilers, which were in service for less than 20 years, to extreme thermally induced stress on the boiler vessel, tube sheets, and firetubes, causing unanticipated and extended boiler shutdowns, adding to facility operating-, maintenance-, and repair-cost burdens and the loss of available excess capacity.
Further uncontrolled losses of process hot water, the addition of system fresh-water makeup, and the corrosive nature of fresh water exasperated issues related to oxygen corrosion of transmission-piping systems, boiler-scale formation, oxygen-corrosion pitting, service disruptions, and eventual failure of the hot-water boilers. Although required to maintain boiler water-quality standards, chemical-treatment systems and procedures were compromised, as these systems were unable to support and maintain control parameters because of the elevated losses and fresh-water makeup.
Thermally induced stress cycling is a term that describes a rapid change in boiler-metal temperature brought about by a sudden change in boiler-water temperature attributed to flow rate, entering water vs. leaving medium temperature, firing rate, or the temperature of combustion air entering a boiler.1 It is a normal phenomenon that occurs in all boilers. However, it generally is not a detrimental factor when boilers are operated under controlled cycle frequency and moderate boiler-water temperature variation.
Specifically, water-side thermally induced stress cycling is a condition that results from rapid and extreme changes in boiler-metal temperature primarily caused by reduced entering -water temperature vs. leaving medium temperature or flow rate. Constant cycling--permitting rapid water-temperature reduction in part because of post- and pre-purge air entry--serves to rapidly cool metal and water storage.
Upon demand and restart, boilers must respond rapidly to system load demand. The heat applied to a boiler naturally will cause metal to expand. Resistance to this expansion causes stress to a boiler structure. Boilers are designed to accommodate this stress gradually. However, when flow rate or internal boiler-water temperature changes rapidly, the resulting expansion and contraction "shocks" boiler metal, commonly resulting in vessel and tube-sheet cracks and firetube-attachment failures that weaken the structure.
The NRWPCP process hot-water boilers, working with hot-water-to-hot-water heat exchangers, essentially were exposed directly to the varying process-load demand. Changes in process demand caused the boilers to cycle more frequently than normally desired. As the process hot water and additional fresh-water makeup returned to the boilers, the temperature differential was greater than the design and operating capabilities, fatiguing the boilers.
Abandoning the Hot-Water Concept
The rehabilitation project incorporated three new York-Shipley Global 800-bhp and one new 200-bhp three-pass, water-backed, LPS packaged firetube boilers with Weishaupt tri-fuel (natural gas, No. 2 fuel oil, and digester gas), low-nitrogen-oxide parallel-positioning fuel burners. The boiler plant was supplemented with boiler feedwater/deaerator and condensate receiver/transfer systems, blowdown tanks, and chemical-feed systems.
DMCI installed more than 25,000 linear feet of welded 3- to 14-in. hot-water distribution piping--replacing the primary- and secondary-loop piping of the process system--along with variable-speed pumps, 40-psig digester-gas and 50-psig natural-gas pressure-reducing stations, steam-to-hot-water heat exchangers, hot-water expansion tanks, and process controls, and integrated process-variable monitoring and control into existing facility supervisory-control and data-acquisition systems. Equipment performance was pre-verified through extensive factory-acceptance tests and witnessed by representatives of the design/construction team. On-site maintenance and operational training programs were provided.
The replacement system abandoned the conventional process-hot-water concept and instead relied on an LPS-generation system as the primary source of process heat through steam-to-hot-water heat exchangers. The boilers, individually connected to a main steam-supply header, serve to produce and maintain 10 psig of LPS, regardless of varying process-hot-water-load requirements. The boilers are exposed directly to system steam pressure and maintain a "hot standby" status, ensuring elevated boiler-water temperature and reducing the advent of thermally induced stress conditions.
As process hot-water load demand increases, a programmable lead-lag control system engages the selected lead, second, third, and fourth boilers, respectively, to manage the LPS requirement. As LPS load demand decreases and with process hot-water demand satisfaction, boilers drop offline, relying only on the selected lead boiler to produce and maintain steam set-point parameters.
This arrangement permits the system to operate at peak efficiency because the boilers are not cycling off and on frequently in response to an uncontrolled varying load. Also, the arrangement minimizes exposure to extreme temperature differentials. A boiler properly operated and maintained at or near a steady-state condition will provide greater useful life, while planned periodic maintenance requirements will be less severe.
As LPS is utilized to support process steam-to-hot-water heat-exchanger demand, resulting condensate is gravity fed to a condensate receiver/transfer system. Boiler feedwater is accommodated through a feedwater/deaerator system, which maintains a 2-psig operating pressure, delivering boiler feedwater at 210 to 215F. As condensate is transferred to the feedwater/deaerator, the now-preheated condensate and subsequent elevated feedwater temperature ensures oxygen removal to prevent scale formation and corrosion. This reduces boiler-water-quality concerns, which are supplemented by a closely monitored and maintained chemical-injection system. Both the condensate-transfer and feedwater systems operate on a continual-recirculation principal with modulating feed valves to ensure that the availability of condensate transfer and boiler feedwater are proportional to the boiler evaporation rates supporting the LPS requirements.
Implementing a Control Strategy
Generally, the failure or inability to maintain a sustained control strategy will lead to an irreversible downward cycle, resulting in system shutdowns; additional operating-, maintenance-,and repair-cost burdens; and the loss of required capacity. The deteriorating conditions experienced at the NRWPCP provided the rehabilitation project's management and design team and constructors the opportunity to employ an unconventional design philosophy and provide for a system-implementation and control strategy that would alleviate future concerns and maintain and provide for the facility's intended service and purpose.
In recognition of the NRWPCP's role as a public health and safety facility, considerable redundancy in system operating capability was accommodated. The importance of the newly adopted design philosophy and its ability to provide a sustainable system and operating control strategies have furthered the facility's ability to ensure effluent sewage processing, regardless of system and component failures or the availability of control parameters.
Specifically, the boiler-control system incorporates a primary plant demand signal (PDS) generated by monitoring all of the system pressure, temperature, flow, and volume variables, which through the lead-lag control system provide an anticipatory response to process hot-water load demand. Additionally, in the event of loss of the PDS, the plant automatically will defer to a steam-pressure transducer designed to compensate for the loss and maintain the required pressure. Further, each boiler has the ability to operate in a local automatic or manual mode, should all primary operating signals be compromised.
Digester gas, a byproduct of the methane-recovery system, is utilized as the primary fuel by the tri-fuel-capable fuel-burning equipment. Natural gas and No. 2 fuel oil are available standby fuels in the event of lost or reduced digester-gas production. With emergencypower-generation capability, the NRWPCP can remain online, providing for its intended purpose under virtually any circumstance.
Project Completed
Given its scope, the project could have taken four to seven years to complete, exceeding the budget and schedule. With the uniquely managed and phased approach employed by the project team, however, there was no interruption of process service during construction. What's more, the team of owner representatives, project managers, designers, prime and sub-contractors, and equipment manufacturers completed the project within two years, six months ahead of an aggressive project schedule, and under the projected budget. The annual energy- and maintenance-cost savings have exceeded the design, construction, performance, and economic expectations and provided an energy-efficient and sustainable benefit to New York City.
Reference
1) Halley, G.M. (1998, winter). Thermally induced stress cycling (thermal shock) in firetube boilers. National Board Bulletin. Available at http://www.nationalboard.org/nationalboard/articles/classics/classic63.aspx
About the Author
The president and chief executive officer of AESYS Technologies LLC, Kevin J. Hoey is the chairman of the American Boiler Manufacturers Association (ABMA) Commercial Systems Group and a member of the ABMA's Board of Directors. He also is a member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers and the American Society of Mechanical Engineers, having served on various technical and pressure-vessel-code committees for both.