Proper drain system for SH, RH panels reduces maintenance of HRSG pressure parts

You’re likely to hear about drain issues at virtually every industry meeting addressing the information needs of HRSG users. Hard to believe, but despite years of industry discussion, equipment specifiers responsible to plant owners and EPC contractors, and the manufacturers of heat-recovery steam generators, often fail to deliver drain systems that meet the expectations of operating personnel.

So it was no surprise that “HRSG Drains” was one of the three presentations in the “F-Class HRSG Spotlight Session offered by engineers from HRST Inc at the 24th annual meeting of the 7F Users Group at the Sheraton Denver Downtown, May 11-15, 2015. Jonathan Aurand, considered by some one of the industry’s young stars, was the presenter and discussion leader.

He said drain issues are most prevalent in the superheater (SH) and reheater (RH) sections—particularly when HRSGs are in cycling service. [link to Also, that each OEM and each design has unique challenges. Resolving persistent drain issues, Aurand continued, often requires a comprehensive understanding of how the system was intended to work, in addition to identification of specific shortfalls. He would not have been a good soldier for the House that Bob Krowech built had he not added “third-party support is often beneficial, specifically for merchant plants without a lot of overhead staff.”

Aurand focused his presentation on the following SH/RH drain issues:

1. Drains undersized and/or in need of automation.

2. Low point in the piping system undrained.

3. Low point in the piping system not drained consistently.

4. Drain movement. Differential expansion amplifies drain lateral displacement and the penetration seal and the hole in the casing must allow for a full range of motion. Other tips Aurand shared with attendees:

        • Properly executed, spring supports can reduce thermal stress on tubes and jumper pipes while also correcting drain movements.

        • Ensure lower headers have bumpers and permit little or no lateral movement.

        • Reconfigure drains to all greater flexibility before piping passes through the casing.

5. Drain bending (expansion overstress).

6. Bypass of saturated or cooler steam.

The speaker said, based on HRST’s surveys of drain systems over the years, here’s how the company’s engineers rank the causes of ineffective draining:

      • Drains on HP SH and RH tube panels of 1 in. diam or smaller for F-class and larger gas turbines.

      • Primary drain valves are not equipped with actuators (manual valves only).

      • Primary drain valves have actuators (pneumatic or motorized), but no water or condensate detection for control.

      • Drains are automated with TE condensate detection, but plant operations has no provision for automatically draining the system in a cold start.

      • Operators have a false sense of security when drain valves are in “auto.”

Here’s what HRSG recommends improve under-performing drain systems:

      • Use drain lines 1½ in. or larger in the HP SH and RH.

      • Automate drains to the greatest extent possible, preferably using ¼-turn ball valves and pneumatic operators.

      • Without automation, Aurand said, operators will forget to open/close drains given all the activity during startup and today’s reduced staffing. If you have an actuator, he continued, there must be a timer integrated into the control system to open the valve. Note that the timing is different for cold, warm, and hot starts. The speaker noted that one plant he know of failed to investigate open/close in logic only to learn later that the drain valves closed on 25 psig during a warm start. . .but there was not logic to open them. Resulting tube failures were thought the outcome of never having properly drained the headers.

      • Condensate detection system must work within site constraints and owner preference for conductivity switches, level transmitter, TE activation, or a newer scheme being developed by HRST using level-transmitter methodology.

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