Pipa HDPE menggantikan pipa steel

Oleh Rudy Permadi

Rabu, 20 Juni 2012

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Ada kasus menarik, yaitu dipakainya pipa HDPE untuk menggantikan pipa steel di Nuclear power plant, Callaway, USA. Berikut petikannya:

HDPE Replaces Carbon Steel in Safety-Related Pipe System

Jimmy Zhou and Frank Schaaf

Corrosion of steel water pipes in the safety-related piping systems of aging U.S. nuclear power plants is fast becoming a safety concern and a significant operational cost, not to mention an indication of potential future liability for nuclear utilities currently constructing new plants or retrofitting existing sites. According to the Electric Power Research Institute, the physical maintenance of degraded steel water pipe systems, combined with the operational costs of shutting a plant down during repairs, is already costing some nuclear utilities up to $25 million per year.

The problem is particularly sensitive when the water pipe systems in question are safety-related, such as the essential service water (ESW) systems that stand ready to cool a reactor when needed. In these systems, water-cooled secondary heat exchangers are used to maintain public safety and power generation continuity. Lost revenue from a system shutdown, which is likely when an ESW system fails, can be more than $1 million per day for a utility due to the expense of purchasing electricity on the open market to replace what was being generated by the plant.

Carbon steel pipe is the incumbent medium for transporting water to heat exchangers in ESW systems, but alternatives are being explored and, in the case of Ameren’s Callaway Nuclear Power Plant in Fulton, Mo., alternatives are now in operation.

An Alternative to Carbon Steel

AmerenUE, a subsidiary of Ameren Corp., has pioneered the use of polyethylene (PE) as a new alternative to steel pipe ESW systems at its 1,200-MW Callaway plant. Polyethylene material does not corrode, rust, rot, pit, tuberculate, or support biological growth. It also has an outstanding field performance record for more than half a century in water piping systems. Taking advantage of material science advances in bimodal polyethylene resins and a collaborative effort with a resin supplier, a pipe manufacturer, a pipe fittings manufacturer, an engineering contractor, and a primary construction contractor, AmerenUE engineers and construction teams have successfully replaced the carbon steel ESW pipe system at Callaway with a new class of bimodal high-density polyethylene (HDPE) pipe.

2. A nuclear industry first. Installation of an underground essential service water pipe system made from bimodal high-density polyethylene or HDPE at AmerenUE’s Callaway Nuclear Power Plant in Fulton, Mo., was an industry first. Courtesy: Dow Chemical Co.

Prior to the installation at Callaway, PE pipe had not been used for a safety-related ASME Class 3 water pipe application at a nuclear power plant in the U.S. AmerenUE’s use of bimodal HDPE met all design requirements, was validated with extensive test data, and has also received approval from the Nuclear Regulatory Commission (NRC). This use of HDPE in a safety-related water pipe system is a first for the U.S. nuclear power industry and could be instructive for other nuclear utilities grappling with steel water pipe corrosion issues.

The ESW pipe systems at Callaway hold and transport service water drawn from the reservoir pond inside the plant. Since the plant started operations in 1984, the original buried carbon steel water pipe systems have been continually susceptible to corrosion, fouling, rusting, and microbiologically induced corrosion, leading to pipe integrity issues such as pinhole leaks, pitting, and other localized forms of degradation. The detection of even a single pinhole leak meant taking down the offending ESW system for up to 72 hours for repair and sometimes resulted in the plant being shut down entirely. It takes several days to restart a nuclear plant, and problems literally the size of a pinhole were sometimes costing AmerenUE up to $1 million per shutdown.

The tipping point at Callaway was a combination of test results and an internal visual inspection of an ESW pipe in early 2007. Though it had only been in service for 24 years, the unlined water pipe system once again showed signs of significant degradation, including four pinhole leaks and pits. Frustrated at the prospect of yet more disruption and spiraling maintenance costs, the Major Modification Group and mechanical design engineers at Callaway determined that they would start to evaluate alternatives to carbon steel pipe. One immediate option was HDPE, which had already been used at Callaway in non-safety-related water pipe systems such as fire protection piping. The installation of this system gave Callaway engineers valuable experience with HDPE and pipe fusion methods.

HDPE pipe was attractive to AmerenUE at Callaway for several reasons:

• HDPE pipe is leak-free when produced properly, even at joints, which can be as strong and leak-free as the pipe itself through use of the heat fusion joining technique.
• HDPE is corrosion and chemical resistant: It does not rust, rot, pit, corrode, tuberculate, or support biological growth.
• HDPE offers seismic resistance, in that it can safely accommodate repetitive pressure surges above its static pressure rating and is well suited for seismic loading due to its natural flexibility.
• HDPE is easier and more cost-efficient to install than carbon steel.

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