hrsg steam velocity analysis

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HRSG Steam Velocity Analysis

SE-ACE Innovations Inc., GM revised 6-April-00.

OverviewIn the past, heat-recovery steam generators (HRSGs) primarily served lower pressureindustrial applications to recover heat from chemical or manufacturing processes. Theyoperated at around 500 psig and used softened water as boiler-water makeup.

Today, HRSGs operate at substantially higher pressures and in a variety of configurations.They use sophisticated water treatment systems and play an integral role in the combined-cycle power plant.

No operator would consider running a steam generator over its design pressure without

recertifying the unit in accordance with ASME code. But, are there risks associated withoperating a HRSG below its design pressure, such as sliding pressure operation?

Impact of reduced pressure HRSG operation

Experience with HRSGs reveals that, although a reduced operating pressure does notimmediately compromise the integrity of the pressure vessel, it can cascade into damagingconsequences, such as:

Physics of reduced pressure

Consider the consequences of reduced operating pressure on steam velocity. The lowerdensity of the steam necessarily produces a velocity increase. A second factor is the lowersaturation temperature also lowers the temperature of the tubes. Thus, a HRSG with afixed heat source will actually generate more steam as the cooler tubes draw more heatfrom the flue gas.

1. To quantitatively understand these effects, consider 50-psig steam versus 25-psig

High steam velocities can cause erosion or flow-accelerated corrosion to tubes.The high steam velocity can choke internal circulation. This may lead to tube dryout and

subsequent tube failure.Steam separating capacity may deteriorate due to higher than designed steam velocity.

This can result in solids carryover that can damage downstream equipment, such asturbines, super heaters, and other process equipment.

Valves, such as pegging steam flow control valves and pressure relief valves may beundersized for the actual flow conditions.

Pressure sealing devices, such as elliptical man ways, may leak due to insufficientsealing pressure pressure.


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steam found in the low-pressure (l-p) section. One pound of 50-psig steam occupies6.6 cubic feet, while the 25-psig steam occupies 10.5 cubic feet. This density changeincreases the steam volume by a factor of 1.6.

2. The second factor, lower saturation temperature, increases the rate of steamproduction by a factor of 1.3, or 13,060 lb/hr steam flow at reduced pressure versus

10,000 lb/hr steam flow at 50 psig. Given a constant 400oF exhaust-gas temperature

entering the l-p section, the exhaust-gas temperature exiting is 289 oF at reduced

pressure as compared to 315 oF for 50 psig steam.

Together these two factors increase the steam velocity to more than twice the design value(1.6x1.3=2.08). The risk of flow accelerated corrosion is especially severe for steamgenerators operating below 250 psig.

Case Study

A heat-recovery steam generator (HRSG) experienced tube leaks in its low-pressure (l-p)section after just three years of operation. Testing revealed severe thinning at the upperbends of the low-pressure tubes. Operating at reduced pressure proved to be the rootcause of the flow accelerated corrosion.

This HRSG was not designed for reduced-pressure operation. At actual reduced pressureoperation, thermal performance of the high-pressure and intermediate-pressure sectionswas greater than design. This negatively impacted the low-pressure section because theturbine gas temperature was lower than design in the l-p section.

At design operating conditions, the low-pressure deaerator requires minimal pegging steam,however at reduced pressure operation, the pegging steam control valve was too small tokeep up with the increased demand. This would cause the l-p boiler pressure to decrease,and go as low as 3 psig. Plant operators were not concerned, because the deaerator wasstill performing satisfactorily. What they did not realize was that the steam velocity in the l-ptubes had increased by a factor of two. Extensive repairs to the l-p section upper bendswere required after only three years.

Monitoring Technique

1. Calculate the actual steam velocity in the low-pressure section of the HRSG.

2. Begin calculating velocity when pegging steam control valve is greater than 80%open.

3. Compare actual steam velocity with the steam velocity at design conditions.

4. Send a warning to the operators when actual steam velocity exceeds design by


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140%.

Literature Review

1. Reduced-pressure operation shortens HRSG service life by Robert Krowech,published in Power Magazine, January/February 2000.

2. HRSGs: A different breed of boiler by David Daniels, published in Power Magazine,November/December 1999.

3. Warning: Cycling HRSGs can be dangerous to your health by Michael Pearson,published in Power Magazine, February 1997.

4. Competitive realities change focus of boiler/HRSG design by Cate Jones, publishedin Power Magazine, February 1996.

5. Pasco county cogeneration facility, First-of-a-kind aero turbines boost cogen-plantperformance, published in Power Magazine, April 1994.

hrsg steam velocity analysis

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