Designing static equipment in accordance with ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 (ASME VIII DIV.1) requires careful consideration of material toughness, especially for vessels operating in low-temperature environments. Impact testing, typically Charpy V-notch, is a primary method to ensure materials can withstand brittle fracture. However, the code provides certain exemptions and alternative approaches, notably through UCS-68, which can significantly influence manufacturing costs. This article will explore weather using UCS-68 is an economically viable choice that should be accounted for during detailed pressure vessel design.
UCS-68 of ASME VIII DIV.1 offers a crucial provision related to the minimum design metal temperature (MDMT) and impact testing. For certain materials and thicknesses, if post-weld heat treatment (PWHT) is performed without being a Code requirement, a reduction of 17°C (30°F) in the MDMT is permitted without requiring impact testing. This effectively allows manufacturers to avoid the often costly and time-consuming process of impact testing by opting for PWHT. The decision between performing impact tests or applying PWHT boils down to which approach offers the most economic advantage for the fabricator.
The Cost of Impact Testing
Impact testing involves several cost components. Firstly, it necessitates the procurement of additional material for test coupons, which must be representative of the actual vessel material and welding procedures. Secondly, the preparation of these coupons requires specialized machining. Thirdly, the actual testing is performed by accredited laboratories, incurring testing fees. Finally, there’s the administrative burden of managing the testing process, reviewing results, and potentially retesting if initial results fail to meet requirements. For complex vessels with numerous welds or varying material thicknesses, the cumulative cost of impact testing can be substantial. A single Charpy V-notch impact test, including specimen preparation and lab fees, can range from $40 to $100 per specimen, and multiple specimens are required per test lot.
The Cost of Post-Weld Heat Treatment (PWHT)
PWHT, on the other hand, involves heating the entire vessel or a section of it to a specific temperature, holding it for a prescribed time, and then slowly cooling it. The primary costs associated with PWHT include:
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Furnace Time: Utilizing a large PWHT furnace incurs significant costs based on the duration of the heat treatment cycle.
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Energy Consumption: Heating and maintaining the furnace at high temperatures requires a considerable amount of energy.
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Handling and Logistics: Transporting the vessel to and from the PWHT facility, especially for large vessels, adds to the logistical costs.
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Potential for Distortion: While not a direct cost, improper PWHT can lead to distortion, requiring rework and adding to overall expenses.
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Equipment and Consumables: If performed in-house, the capital cost of a furnace or portable heating equipment is substantial. Consumables like insulation and thermocouples also add to the expense.
Which is Cheaper?
While UCS-68 offers the 17°C MDMT reduction as an incentive for PWHT, it is generally found that PWHT is generally not the cheaper option when compared to impact testing for the sole purpose of meeting MDMT requirements. The costs associated with PWHT, particularly for large or complex vessels, tend to outweigh the savings from avoiding impact tests.
Cost Example:
Consider a medium-sized pressure vessel requiring impact testing for three different material heats and two welding procedures, with three specimens per test.
Impact Testing Cost:
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Material for coupons: $200 (estimated)
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Machining of coupons (18 coupons total): $360 (estimated, $20/coupon)
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Laboratory testing (18 specimens at $50/specimen): $900
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Total estimated impact testing cost: $1,460
PWHT Cost for the same vessel:
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Assuming a vessel size that requires 24 hours of furnace time in a commercial facility (includes heating to the desired temperature and cooling back to environment temperature)
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Furnace time and energy consumption: $2,000 – $5,000 (depending on vessel size, weight, and local energy costs). Some sources suggest costs can range from $1 to $3 per square foot, potentially leading to thousands for a medium vessel.
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Logistics (transport to/from facility): $500 – $1,500 (highly variable based on distance and vessel size).
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Potential for rework due to distortion (contingency): $500 – $1,000 (even a small probability adds to the risk).
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Total estimated PWHT cost: $3,000 – $7,500
This example illustrates that even for a relatively straightforward scenario, the operational and logistical costs of PWHT often significantly exceed the costs of performing the necessary impact tests.
The primary drivers making PWHT more expensive include the substantial energy consumption of large furnaces, the significant time the vessel occupies the furnace (which can be a bottleneck in production schedules), and the logistical challenges and costs of moving large components to and from a PWHT facility. Even if a fabricator has in-house PWHT capabilities, the operational costs of running a furnace are considerable. The potential for distortion, requiring costly rework, also adds an element of risk not typically present with impact testing.
Conversely, while impact testing does involve material, machining, and laboratory fees, these costs are often more predictable and scalable. For vessels that do not inherently require PWHT for other metallurgical reasons (e.g., stress relief from thick sections or specific material properties), adding PWHT purely to gain the 17°C MDMT reduction often proves to be a more expensive route. Therefore, a thorough cost-benefit analysis will frequently reveal that, for MDMT compliance alone, performing impact tests is the more economically favourable solution for manufacturers. To sum up, designers should preferably avoid recommending usage of UCS-68 (and avoid PWHT) since it is not the most economically viable choice.
