Understanding thermal transients and how to make SH/RH drains operate effectively

Bob Anderson, principal, Competitive Power Resources Corp, Palmetto, Fla, recognized globally for his HRSG expertise, offered a thermal-transient assessment update. Common issues included small drain pipes, blowdown vessels located above headers, and drainage control not based on condensate detection. Anderson noted that only a few plants have a robust root-cause process for failures related to thermal transients.

Many questions and discussions followed as meeting attendees wrestled with data presented by the speaker and its implications for their plants. Interesting points were made about OEM ramp rates, drum and header thicknesses, numbers of tubes, thermocouples, and other specifics, showing the complexity of thermal transients and the assessments.

Drain control. Anderson next addressed the industry’s ongoing issues with superheater (SH) and reheater (RH) drains and how ultrasonic flowmeters, rigorous system design, and good operating procedures can help mitigate them. Here’s what he suggested/wanted users to be aware of. . .

. . .for startups from 0 psig (simple):

      • Install drain pipes of generous size (gravity head only).

      • Locate the discharge below point of origin.

      • Open all drains before purge to ensure the superheater is empty.

      • Keep drains open during purge.

      • Close drains when pipe temperature downstream of control valve exceeds Tsat + ~80 deg F for 30 seconds.

      • Two more things to remember: (1) Time is available for draining before steam flow begins; (2) a limited quantity of condensate is generated at 0 psig.

. . .and for pressurized startups (complex):

      • More condensate is generated than for cold starts.

      • There is very little time from gas turbine fire to initial steam flow.

      • Drain flow varies greatly with pressure.

      • Opening drains for long periods can result in excessive drum pressure decrease and blowdown system temperature/flow changes.

      • Water detection is critical to minimize steam release during purge. Bear in mind that thermocouples cannot detect water.

Various methods for detecting water were reviewed, including an ultrasonic system using the transit-time technique (illustration). An EPRI prototype drain control system, installed on three HRSGs to date, was then examined.

AHUG fig 5

Current installations cover several pipe sizes and materials plus different configurations. The Flexim Americas Corp fluid detector described by Anderson and shown in the figure calculates signals to correlate the noise ratio of ultrasonic sound waves passing between transducers. DCS logic then controls the drain valves. Development and testing of this system continues.

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