POWER February 2013 - 41

INSTRUMENTATION & CONTROLS
Table 2. Submersion test response time results.
Source: Southern Company
Thermocouple
1/4 inch Type K grounded
3/8 inch Type K grounded
1/2 inch Type E grounded
Response time
309 seconds
323 seconds
249 seconds
Table 3. Submersion test results after the nontapered
thermowell bore was cleaned. Source: Southern Company
Thermocouple
1/4 inch Type K grounded
3/8 inch Type K grounded
1/2 inch Type E grounded
Response time
316 seconds
359 seconds
285 seconds
cleaned bore are greater than for the dirty bore (Figures 4 and 5).
This result is counterintuitive but may be due to surface imperfections
with the dirty bore that created greater contact area between the
thermowell and the thermocouples, thus improving that heat transfer
coefficient, or to unknown and uncontrolled factors in the test
method.
As shown, cleaning the thermowell had very little impact on
the response time of the 1/4-inch thermocouple, likely because
the response time of this thermocouple is largely driven by the
relatively small point of contact area between the thermowell and
thermocouple (arguably, just at the tip). For the 1/2-inch thermocouple,
the response time with the dirty bore was slightly faster
(by approximately 15%) than that observed with the clean bore.
Again, this may be attributable to greater surface contact area provided
by the dirty bore.
One important factor in control system performance is the
response time of the process measurements used in the control
system. In this article, the response time of one common boiler
temperature measurement, the desuperheater outlet, is examined.
Step response tests on an actual unit produced unexpectedly long
response times, which prompted further investigation to identify
the cause. Several issues related to installation of the thermocouple
were identified, but replacing the thermocouple and correcting
the installation issues did not improve the response time. The only
component not replaced was the thermowell, which was a particularly
thick-walled design featuring a nontapered shank. These
thick-walled thermowells were replaced with tapered designs during
the third quarter of 2011, and this replacement resulted in a
much faster closed-loop response time for the desuperheater outlet
temperature control loop.
One of the original straight-shank thermowells was response tested
in a laboratory, and the preliminary results confirm the response
time measured in the plant. Having a fast response time for this loop
is important for improving final steam temperature control for units
that utilize a cascade control scheme.
Though many factors influence the overall response time of
temperature measurement in boiler steam lines, in this particular
case, the thick-walled thermowell was the primary culprit for a
slow response. ■
-Blake Feltman (bafeltma@southernco.com) and John Sorge,
PE (jnsorge@southernco.com) work for Southern Company
Generation in Birmingham, Ala. Cyrus Taft, PE (cwtaft@
taftengineering.com) works for Taft Engineering Inc. in Harriman,
Tenn. This article is based on a paper presented by the authors
at the 2012 ISA POWID Symposium.
February 2013 | POWER
FOR SALE
SIX SETS OF WÄRTSILÄ W20V34DF
DUAL FUEL ENGINES, AUXILIARIES
AND RELATED SERVICES
Actual product may difer visually
Manufacturer: Wärtsilä
Quantity: Six sets
Model: W20V34DF
Hours run: nil
Generator rating: 8.7MW each, 11kV, 50Hz, 750 RPM, IP23, F/B
Type: The engine is 4 stroke, dual fuel (diesel or gas),
trunk piston, turbocharged and intercooled
Equipment condition: Brand new condition, in controlled
storage in Finland since March 2012
Year of manufacture and delivery: 2012
Price: Please contact owner - details below
Technical details of Wärtsilä 20V34DF generating set from
Product catalogue:
Power, electrical:
Heat rate:
Electrical eiciency:
Length:
Width:
Height:
Weight:
# in liquid fuel mode.
Heat rate and electrical eiciency at generator terminals, including engine driven
pumps, ISO 3046 conditions and gas LHV > 28 MJ/Nm3. Tolerance 5%. Power factor
0.8. Gas Methane Number > 80. Nm3 deined at NTP (273.15 K and 101.3 kPa).
More technical details are available from Wärtsilä Product
Catalogue and Wärtsilä 34DF Technology Review at:
www.wartsila.com/en/power-plants/power-generation/
dual-fuel-power-plants
Contact: Atul Garg
Fortescue Metals Group Ltd
Phone: +61-8-9230 1352
Mobile: +61-421 456 200
Email: WartsilaSale@fmgl.com.au
CIRCLE 15 ON READER SERVICE CARD
www.powermag.com
41
8,730 kW
8031 kJ/kWh (8130)#
44.8 % (44.3)#
12,890 mm
3,300 mm
4,400 mm
130 tonnes
http://www.wartsila.com/en/power-plants/power-generation/ http://www.powermag.com
POWER February 2013

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