With the proliferation of “F”-class gas turbines and HRSGs in recent years,
several problems have been identified with desuperheaters (attemperators).
Problems can occur during startup, transients, and at steady-load conditions.
Some problems (overspray) can cause serious damage with a single occurrence.
Here are a few common problems:
1. Startup overspray. Possible in any HRSG, but more pronounced in “F”
machines because GT exhaust temperatures reach temperatures of 900°F+ much
earlier in startup than traditional turbines. The HRSG is just producing
steam, and steam in the superheaters tries to match exhaust temperature. The
desuperheater sprays heavily to keep steam outlet temperature at the set
point. “Wet” steam sent to the superheater causes damage.
2. High desuperheater demand at low plant loads. When at minimum plant load,
GT exhaust temp is higher than normal with reduced exhaust flow. The result
is increased demand for desuperheater spray, and may result in a “wet” steam
scenario similar to #1. This problem is exacerbated by superheaters that
absorb more heat than intended by the OEM.
3. Reheat bypass/conditioning valve overspray. If reheaters cannot run dry
through startup, HP steam may be conditioned and routed to reheaters. Reheat
conditioning valves may overspray and send “wet” steam to the cold reheat
section of the HRSG.
4. Leaking desuperheater. Some plants aren’t aware they are desuperheating
all the time. Leakage is lost efficiency at normal load, and dangerous at
startup/shutdown conditions.
5. Failed desuperheater nozzle assembly. Several occurrences of failed
desuperheater nozzle assemblies have been discovered in both cyclic and
base-loaded service HRSGs. If a nozzle fails, spray water is not dispersed
for effective mixing and fast evaporation.
6. Plugged nozzles. Nozzles are subject to plugging or sticking open from
even small pieces of debris.
You: OK, I’m worried. My spring outage is approaching, what do I look for?
HRST: There are several tell-tale signs of desuperheater issues:
1. Bent superheater or reheater tubes immediately downstream of the
desuperheater.
2. Steam temp at the desuperheater outlet is within 15°F of the
saturation temp.
3. Steam temp drops across the desuperheater when the control valve
demand is zero.
4. Pitting/wear on the ID of the first pipe elbow downstream of the
desuperheater.
5. Failed desuperheater nozzle assembly and/or plugged nozzles.
You: OK, I have a problem. How do I fix it?
HRST: Nice job…admitting you have a problem is the first step. In some
plants avoidance of problems is a simple matter of improving the controls
and operator education, but in others it can be more complex. Several OEMs
recommend the desuperheater exit steam temp be kept at least 50°F above the
saturation temp. While safe for most designs, that may limit minimum load or
flexibility for rapid transients. Holding values <50°F can be safe in the
right situation.
If you find you need something more than the “Saturation + 50°F” rule, consider
factors that contribute to a safe value: Desuperheater type, orientation, pipe
geometry and straight length up/downstream, water quality & steam purity,
automated valves, instrumentation, etc.
Scott Wambeke, P.E.