Leaking valve cause of tube-to-header failures

Inspecting heat-recovery steam generators (HRSG) often is like detective work, HRST Inc’s Daniel E Acosta, PE, told the editors. Given a mystery issue, you need clues to help solve the case. Like a seasoned sleuth, the equipment inspector must know how to distinguish tell-tale signs and use them to identify the culprit. While not all criminal minds or HRSG problems are created equal, a few basic forms of investigation can go a long way in helping to assess a situation, Acosta said.

Aside from the usual visual inspection of HRSG components, valuable information can be obtained by analyzing selected DCS data, the boiler expert continued. Sometimes, a quick review of key points can prove extremely beneficial. This form of investigation helps identify and diagnose a variety of issues—such as performance shortfall, desuperheater overspray, and control-valve leakage.

A recent example where this type of data review proved useful was at a relatively new plant (less than one year of operation) that had experienced several tube-to-header failures in its reheater panels (harps). HRST was charged with solving the mystery of the recurring failures.

First step was a visual inspection of the affected HRSG. At the time of shutdown, plant personnel were not aware of any new active leaks. Yet immediately after opening the internal access doors leading to the crawl space where the reheater panels are located, staff noted puddles of water on the floor—right below where the previous failures had occurred.

Inspectors checked the tube-to-header connections and found a few crack indications near tubes that had been plugged previously. Leaks were confirmed by introducing water into the affected panels. Knowledge of related problems at other plants having a similar HRSG design led engineers to collect DCS startup and shutdown data from several key spots in the area where failures were occurring. A cursory review of the data gathered revealed several common problems.

Desuperheater hunting, or rapid changes in desuperheater spray flow, was widespread. Hunting commonly leads to thermal fatigue of the spray-nozzle assembly in probe-style attemperators. Graphing the data facilitates problem identification: Desuperheater hunting was immediately noticeable in the Fig 1 chart profiling a startup.

Note, too, that the steam temperature immediately downstream of the reheat desuperheater came near—and in one case even reached—the saturated steam temperature. If possible, steam temperature downstream of the desuperheater should be at least 50 deg F above saturation at all times, Acosta said, and, as a general rule, should never approach within 30 deg F of saturation. When steam temperature downstream of the desuperheater falls below this value, the risk of overspray (unevaporated spray water quenching the pipe and the next stage of tubes) is high. These problems were consistent with the failures currently taking place.

 HRST 1

In base-load operation (Fig 2) data revealed a temperature difference of about 72 deg F between the steam inlet to the desuperheater and the outlet—even after spray flow was shut off. This indicated the desuperheater control valve was leaking spray water into the reheat steam line.

Lastly, operating data obtained for the desuperheater control valve showed it never opened above 40% and operated at less than 20% open most of the time it was in use. This indicated the valve was not sized correctly for this application and a smaller control valve was needed. The plant subsequently replaced its desuperheater control valves with ones better suited to the service.

In addition to the issues described in the foregoing example, other problems that can be identified with a review of DCS data, Acosta said, include the following:

    • Feedwater control/economizer shocking. Are there large fluctuations in steady-state operation? Does the flow start/stop during HRSG startup? Are the drums being topped-off at night, thereby contributing to metal fatigue?

    • Automated superheater and reheater drains. Do drains open at the correct times and remain open long enough?

    • Drum level. Do the various level indicators agree?

    • Drum-temperature ramp rate (can be derived from pressure). Are drum nozzles and manways suffering fatigue damage?

    • Overall performance. How do the pinch point, approach point, and stack temperature compare to OEM design values?

    • Gas-side pressure drop. Is there a continuous build-up of debris that’s slowly increasing backpressure? Is it time to clean the modules?

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