BEST PRACTICES: Michigan Power

Inspect HRSG boiler drums to prevent failures

Michigan Power BP plant

Challenge. Michigan Power LP (MPLP) is a baseload cogeneration facility. During a major inspection in September 2013, the HP boiler was inspected for flow-accelerated corrosion (FAC) by HRST Inc. Inspectors noticed a rust bloom indication on the lower part of the HP drum head-to-shell weld.

Further investigation revealed multiple circumferential cracks or linear indications running along the head-to-shell weld on the internal surface of the drum. HRST recommended performing NDE on the indications and to consider checking the lower portion of both head welds plus the full circumference of both downcomer nozzles.

The plant hired JANX to mag-particle test the area of interest. It found a linear indication on the lower half of the HP drum, from about the 3 to 7 o’clock position (photo). This area then was inspected ultrasonically. Results: The indication was approximately 1.1 in. deep (the parent metal is 4-in. nominal) and 104 in. long. Inspectors also found a smaller linear indication approximately 3 in. long from the 8 to 9 o’clock position.Michigan Power BP fig

Solution. A boat sample was taken from the HP drum linear indication. It contained a portion of shell base metal, weld metal, and head base metal. Next, Stress Engineering Services Inc and GE Water Technologies performed a metallographic analysis of the boat sample to identify the damage mechanisms that caused the cracking.

Both companies said the drum cracking likely was caused by corrosion fatigue—the combined effect of cyclic stresses and corrosion. There was no evidence of high-temperature damage such as creep.

MPLP doesn’t believe that the indications were the result of cyclic stresses. Data indicated cool-down and warming conditions were well within the manufacturer’s recommendations and the water chemistry for the unit has been exceptional. Cracks in these areas are not common for baseload plants and are more common in cycling plants. The HP-drum linear indication seemed to be a linear crack along the head-to-shell weld.

The linear indication was properly ground out to parent metal, x-rayed, heat treated at 400F as suggested by Deltak, the HRSG manufacturer. The unit was welded by a boiler and mechanical contract company, using an applicable ASME weld procedure and x-rayed to complete the weld process.

Results. JANX x-rayed all head-to-shell welds in the drum; no other indications were identified. After finding the linear indication along the head-to-shell weld in the HP drum, both the HP and LP drums were completely checked with mag particle to make sure there were no indications with either drum.

A hydro test was successfully performed on the HP drum before returning the boiler to service. A preventive maintenance (PM) plan was created to mag particle the HP and LP drums during upcoming minor and major outages.

Recommendation to others: Check all boiler drums with mag particle or dye penetrant during your plant’s first outage and create a PM to check drum welds during future outages—this no matter what the operating regime and pre-commissioning inspection results.

Project participants:

Anthony Peplinski, plant engineer
Dan Cox, EHS specialist

Michigan Power

Owned by Arclight Captial

Operated by Consolidated Asset Management Services

154-MW, gas-fired, 1 × 1 combined-cycle cogeneration facility located in Ludington, Mich

Plant manager: Ken Tomaski