K. S. Sunder Raj, Power & Energy Systems Services

Feedwater heaters for nuclear plants specified and designed for full-load conditions with all heaters in normal service

Practice conforms to turbine manufacturer’s philosophy of designing turbine

Heaters may be removed from service provided turbine stage loadings do not exceed those under normal conditions

Will depend upon specific piping arrangement (extraction steam and condensate/feedwater)

Feedwater heaters in Figs. 1 to 4 designed for VWO or max. guaranteed heat balance conditions

Fig. 5 shows max. guaranteed heat balance used to specify/design the heaters for cycle in Fig. 1

All heaters extracting normally No tube leaks, no bypasses or emergency

drains in operation Sample specification sheet for heaters 5A, B

(Table 1)

Temperature Rises (TRs) – Heaters 1A, 1B 65.7 °F – Heaters 2A, 2B 34.6 °F – Heaters 3A, 3B 56.1 °F – Heaters 4A, 4B 51.0 °F – Heaters 5A, 5B 54.7 °F

TTDs and DCAs – Heaters 5A, 5B TTD = 5.0 °F DCA = 10.0 °F – Heaters 4A, 4B TTD = 5.0 °F DCA = 10.0 °F – Heaters 3A, 3B TTD = 5.0 °F DCA = 10.0 °F – Heaters 2A, 2B TTD = 5.0 °F DCA = 10.0 °F – Heaters 1A, 1B TTD = 5.0 °F DCA = 10.0 °F

Tube Velocities, V – Heaters 5A, 5B 5.59 fps – Heaters 4A, 4B 6.32 fps – Heaters 3A, 3B 5.42 fps – Heaters 2A, 2B 6.24 fps – Heaters 1A, 1B 4.82 fps

Tube Side Pressure Drop, ∆PT – Heaters 5A, 5B 7.6 psi – Heaters 4A, 4B 9.4 psi – Heaters 3A, 3B 7.9 psi – Heaters 2A, 2B 8.99 psi – Heaters 1A, 1B 8.59 psi

Total design ∆PFw 42.48 psi

Shell Side Pressure Drop, ∆Ps – Heaters 5A, 5B 4.7 psi – Heaters 4A, 4B 4.5 psi – Heaters 3A, 3B 4.5 psi – Heaters 2A, 2B 2.0 psi – Heaters 1A, 1B 1.5 psi

Nuclear plants seldom conduct acceptance tests on their feedwater heaters to verify guarantee.

Parameters commonly monitored: – Terminal temperature differences – Drain cooler approaches – Temperature rises – Heater water level – Drain valve position – Overall pressure drop in condensate/feedwater

string

Permits predicting heater performance for both normal and abnormal conditions

Predicted/calculated values include: – Heater shell pressures – Extraction steam flows – Feedwater/condensate flows – Drain flows – Inlet and outlet temperatures

Predicted/calculated values include: – Pressure drops – Velocities – Heat transferred in condensing and

subcooling zones – Log mean temperature differences – Heat transfer coefficients, etc.

Predicted values may be compared to plant data to confirm/validate tube leaks, plugged tubes, etc.

For each heater in “A” string, effects of plugged tubes simulated

Table 3 shows performance of heaters 5A, 5B for plugging levels from 0% to 15% in steps of 5%.

Following occur: – As tubes are plugged, heat transfer ability and

temperature rise decrease – At constant thermal heat input to cycle, reduction in

feedwater flow due to reduction in final feedwater temperature

Following occur: – Effectiveness of heater 5A decreases and both TTD

and DCA increase – For 10% plugged tubes, TTD increases by 1.0 F

and DCA by 1.7 F. – Generator output decreases by about 0.2 Mwe.

Table 4 shows impact on all heaters when 10% of tubes are plugged in heater 5A.

Reduction of about 1.0 F in overall temperature rise in “A” string

Overall pressure drop in “A” string increases

To summarize effects when tubes are plugged in a heater: – TR across the heater decreases. – TTD and DCA increase. – Effective surface available for heat transfer

decreases and heat transferred to feedwater also decreases.

– Tube side velocity and pressure drop increase. – Overall pressure drop in the heater string containing

the heater with the plugged tubes increases.

To summarize effects when tubes are plugged in a heater:

– Overall temperature rise of feedwater in heater string containing heater with plugged tubes decreases.

– If heater is the highest-pressure heater in the cycle, final feedwater temperature decreases resulting in a slight reduction in total feedwater flow for a constant thermal heat input to the cycle.

– If heater with plugged tubes is other than highest-pressure heater, feedwater temperature entering downstream heater decreases, causing that heater to extract more steam, albeit inefficiently, with a higher TTD and DCA.

– The cycle output decreases.

For heater 5A, effects of tube leaks simulated Table 5 shows performance of heaters for tube

leaks of 0% (base), 4% and 8% in heater 5A. Following occur:

– 4% tube leak in heater 5A results in feedwater flow increase of 2% in all heaters upstream of 5A in “A” string and like increase in all heaters in “B” string.

– Increased feedwater flow is 2% higher than design flow for heaters

Following occur: – Effects of drain flow due to the tube leak in

heater 5A are felt by all upstream heaters in “A” string.

– Tube leak results in reduction in drain temperature leaving heater 5A and the DCA decreases. The TTD improves slightly.

– Combination of increased feedwater flow and increased drain flows results in increased TTDs and DCAs for all heaters upstream of heater 5A in the “A” string.

Following occur: – Increased feedwater flow results in increased

TTDs and DCAs for all heaters in “B” string. – Overall pressure drop in “B” heater string is higher

than that in “A” string. – For heaters 1 through 4 in each heater string,

differentials in the DCAs between corresponding heaters increase, as the tube leak increases.

– The differentials are greater in the “A” string. – Loss in output for tube leak of 4% in heater 5A is

about 0.34 Mwe and, the loss increases to 0.73 Mwe for tube leak of 8%.

In summary, tube leak in a heater manifests itself as follows: – TTD and DCA of the heater with the

suspected tube leak will decrease. – Feedwater flow to the heaters in all heater

strings will increase. – Pressure drop in the heater strings will

increase. – TTDs and DCAs in the heater string

upstream of the heater with the suspected tube leaks will increase.

In summary, tube leak in a heater manifests itself as follows: – TTDs and DCAs of heaters in the remaining

heater strings will increase. – There will be a differential in drain

temperatures of corresponding heaters in the heater strings.

– The differentials will increase as the tube leak increases and will be greatest in the string containing the heater with the suspected tube leaks.

Corrective action to be taken will depend upon various factors such as: – Location in the cycle of the heater with the

suspected leaks – Magnitude and severity of the leaks – The number of plugged tubes – History of the heater, etc.

Taking a heater out of service needs to be carefully evaluated in terms of impact both on the turbine stages and also upon the remaining heaters.

Amount of load reduction necessary to keep turbine stage loadings to below design depends upon specific arrangement of extraction steam and condensate/feedwater

Guidelines provided by the turbine manufacturer should be followed.

Table 6 and Fig. 7 show results when heater 5A is removed from service

Impact of removing heater 5A from service is as follows: – Final feedwater temperature decreases by 27.0

°F (7.4%), from 367 °F to 340 °F. – At constant thermal heat input to the cycle, total

feedwater flow decreases by 313,751 lb/hr (3.3%), from 9,523,954 lb/hr to 9,210,203 lb/hr.

– Due to the loss of drains normally available from heater 5A, all heaters upstream of heater 5A in the “A” heater string extract more steam to heat the feedwater. Their TTDs increase and their DCAs decrease.

Impact of removing heater 5A from service is as follows: – Extraction steam flow increases by 17% for

heater 4A, 15% for heater 3A, 13% for heater 2A and 11% for heater 1A.

– Decrease in feedwater flow results in less steam being extracted for all heaters in the “B” string. The TTDs decrease.

– Overall temperature rise in the “A” string decreases by about 48.0 °F.

– The cycle output decreases by about 8 Mwe.

Extraneous flows refer to introduction of flows into a heater that was not originally designed to receive these flows.

For example, some plants have provisions in their design to inject flows taken from the condensate/feedwater to subcool the moisture separator and reheater drains to prevent flashing.

These additional flows increase the thermal and hydraulic loading on the heaters and, long-term operation in this mode, could lead to degradation in performance and affect heater reliability.

Extraneous flows may need to be isolated, at least temporarily, if a heater is exhibiting symptoms of tube leaks or other problems.

With plant uprates becoming increasingly common, feedwater heater heaters are being subject to increased thermal and hydraulic loadings.

For example, if the cycle shown in Fig. 1 is uprated from the maximum guaranteed condition, for which it was originally licensed, to the valves-wide-open condition, the feedwater flow increases by about 5%.

Table 7 shows the impact on the performance of the heaters.

HEATER NO.

FW FLOW, LB/HR

PLUGGED TUBES, %

EXT. STM. FLOW, LB/HR

EXT. STM.

PRESS., PSIA

STM. SAT.

TEMP., F

DRAIN FLOW IN,

LB/HR

DRAIN FLOW OUT, LB/HR

CZ FW TEMP. IN, F

CZ FW TEMP. OUT, F

CZ TR, F

SZ FW TEMP. IN, F

SZ FW TEMP. OUT, F

SZ DRAIN TEMP. OUT, F

SZ TR, F

TOTAL TR, F TTD, F DCA, F

TUBE VELOCITY,

FPS

HTR. TUBE

DP, PSI

SZ SHELL DP, PSI

CZ HEAT TRANSFER,

BTU/HR

SZ HEAT TRANSFER,

BTU/HR

TOTAL HEAT

TRANSFER BTU/HR

5A 5,000,614 0 320,841 182.08 374.02 588,752 909,593 322.64 368.51 45.88 313.40 322.64 324.15 9.24 55.12 5.50 10.75 5.87 8.38 5.14 2.398E+08 4.773E+07 2.875E+085B 5,000,614 0 320,841 182.08 374.02 588,752 909,593 322.64 368.51 45.88 313.40 322.64 324.15 9.24 55.12 5.50 10.75 5.87 8.38 5.14 2.398E+08 4.773E+07 2.875E+084A 5,000,614 0 236,382 88.27 318.91 909,764 1,146,145 272.58 313.40 40.82 261.82 272.58 272.57 10.76 51.58 5.51 10.75 6.64 10.37 4.95 2.091E+08 5.467E+07 2.637E+084B 5,000,614 0 236,382 88.27 318.91 909,764 1,146,145 272.58 313.40 40.82 261.82 272.58 272.57 10.76 51.58 5.51 10.75 6.64 10.37 4.95 2.091E+08 5.467E+07 2.637E+083A 5,000,614 0 267,088 40.03 267.30 1,146,256 1,413,344 219.85 261.82 41.97 205.21 219.85 215.96 14.64 56.61 5.48 10.75 5.69 8.71 4.97 2.120E+08 7.355E+07 2.856E+083B 5,000,614 0 267,088 40.03 267.30 1,146,256 1,413,344 219.85 261.82 41.97 205.21 219.85 215.96 14.64 56.61 5.48 10.75 5.69 8.71 4.97 2.120E+08 7.355E+07 2.856E+082A 5,000,614 0 194,807 14.31 210.65 1,413,425 1,608,233 179.67 205.21 25.54 169.98 179.67 180.69 9.69 35.23 5.44 10.71 6.55 9.91 2.22 1.279E+08 4.843E+07 1.764E+082B 5,000,614 0 194,807 14.31 210.65 1,413,425 1,608,233 179.67 205.21 25.54 169.98 179.67 180.69 9.69 35.23 5.44 10.71 6.55 9.91 2.22 1.279E+08 4.843E+07 1.764E+081A 5,000,614 0 303,518 6.81 175.60 1,608,260 1,911,777 125.58 169.98 44.40 101.37 125.58 112.55 24.21 44.40 5.62 11.18 5.06 9.47 1.68 2.216E+08 1.206E+08 2.216E+081B 5,000,614 0 303,518 6.81 175.60 1,608,260 1,911,777 125.58 169.98 44.40 101.37 125.58 112.55 24.21 44.40 5.62 11.18 5.06 9.47 1.68 2.216E+08 1.206E+08 2.216E+08

Total Feedwater Temperature Rise in LP String "A" Heaters = 242.93 FTotal Feedwater Temperature Rise in LP String "B" Heaters = 242.93 F

Pressure Drop in LP String "A" Heaters = 46.85 PsiPressure Drop in LP String "B" Heaters = 46.85 Psi

NOTES:

1 The split in feedwater flows between the "A" and "B" condensate strings was assumed to be 50/50.2 The calculated condensate flow of 5,000,614 lb/hr through each heater string is approximately 5.02% higher than the design flow of 4,761,656 lb/hr for each string.

Effects of plant uprates on the original heaters need to be considered carefully, especially if there is no design margin.

Besides the impact of the increased feedwater flow, other factors that need to be considered include the potential effects of plugged tubes, tube leaks, heaters taken out of service and, extraneous flows.

Performance modeling tools can be invaluable in evaluating all these effects as part of the uprate studies and in specifying new or replacement heaters, as necessary.

Performance monitoring programs for feedwater heaters coupled with performance modeling could be invaluable in not only maintaining optimum performance levels but also in diagnosing potential or suspected problems with the heaters and planning for appropriate corrective action.

The effects of plugged tubes, tube leaks, heaters out of service, extraneous flows and plant uprates can be predicted using performance modeling tools and plant performance data can help to confirm/validate predictions

Besides monitoring the TTD, DCA, TR, heater water level and, drain valve position for each heater in each heater string, the differentials between the TTD, DCA, TR and drain temperature for corresponding heaters in multiple heater strings should also be monitored.

If provisions exist, the overall pressure drop in the heater string should also be monitored.