Evaluating an HP-IP Turbine Replacement

7/31/2019 Evaluating an HP-IP Turbine Replacement

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Evaluating an HP/IPTurbine ReplacementBy Bill Kettenacker, P.E., Scientech

Figure 1 PEPSE TURBINE CYCLE MODEL

Reheat

Main streamGenerator 1

Generator 2

HP turbine IP turbine LP turbine

Condenser

Finalfeedwater

M

any U.S. power producers are stretchingthe useful lives of their existing powerplants because of the favorable economicsas compared to other alternatives.Environmental issues, licensing delays,public outcry and structural costs top

the list of reasons for delaying, cancelingor even considering planned and future generation projects. TheU.S. power plant fleet is aging, as well, with many plants 30 yearsold or older.

To extend the life of these aging plants, power producers arerefurbishing or replacing older systems and equipment to prolonglife, increase efficiency, lower emissions, save money and squeeze thelast ounce of power from these assets.

Because of new technologies developed in recent years, thereplacement equipment being installed is moreefficient than the existing equipment ever was,even when it was brand new. However, how willthis new technology improve plant performance?Just as important, how will it affect the plantsolder systems and components?

To determine how new equipment willperform and interact with the existing olderequipment before any contracts are signed,power producers are using analytical techniquesthat are often beyond those offered byequipment manufacturers. These independentchecks have proven valuable to power producersand have uncovered some surprising results.

As one example of equipment replacementat an older plant, in 2007 a large U.S. utilitysought to replace the high pressure (HP) andintermediate pressure (IP) turbines at one

of its 300 MW coal-fired units to increaseplant efficiency and raise generation withoutburning one more pound of coal, as an

executive of that utility stated.Requests for bid were sent to various turbine manufacturers,

with two manufacturers responding with proposals. To verifyeach vendors performance claim and to determine the newturbines effect on the rest of the unit, the utility used the PEPSE(performance evaluation of power system efficiencies) heat balance

software program, employing a PEPSE simulation model of theunits turbine and boiler systems.

Based on the results of a PEPSE analysis of the proposed newturbines, the turbine manufacturers proposals did indeed meettheir performance guarantees. However, because the new turbines

were more efficient than the existing HP and IP turbines, theyremoved more energy from the steam than the existing HP andIP turbines, leaving less energy available for the downstream lowpressure (LP) turbine. The net effect was an increase in the HP and

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Figure 2 HP/IP PEPSE SUBMODEL

Reheat

HP turbine IP turbine

Main stream

IP turbine output, but a decrease in LPturbine output. While an overall increasein unit output occurred, it was lower thanexpected after factoring in the decreasedoutput of the now energy-starved LPturbine.

Additional PEPSE analyses wereperformed. In one case, rebuilt HP andIP turbines, rather than new turbines,

were analyzed. As expected, the rebuiltturbines were not as efficient and didnot produce as much additional poweras the proposed new turbines, but theircost vs. new turbines was significantlylower. Finally, various additions to boilersurface area in the superheat (primaryand secondary), reheat and economizersections were analyzed using the new

turbines.The utility performed a cost/benefit

analysis based on the PEPSE results.Their three turbine options included:1) do nothing, 2) replace the HP and IPturbines with new turbines and 3) rebuildthe HP and IP turbines. Their variousboiler surface area choices includedsuperheat, reheat and/or economizersurface area additions. The results ofthis cost/benefit analysis encouragedrebuilding the turbines at the next majoroutage rather than replacing with new

equipment. Whats more, the utility willforgo any boiler surface additions.

The UnitThe power generating facility under

study is one unit of a multiple-unitplant. It was designed to produce 310

is a cross-compound arrangement, withsingle-flow HP and IP turbines poweringone 3,600 rpm generator and a double-flow LP turbine powering one 1,800rpm generator. There are seven feedwaterheaters in the cycle.

The ProblemHaving the original HP and IP

turbines from the early 1970s madetheir improvement or replacement a keyissue at the plant. Their deterioratingperformance and increased maintenance

MWe at full load conditions of 2 x 106lb/hr main steam flow, 1,050 F mainsteam temperature, 2,400 psig mainsteam pressure and 1,000 F reheattemperature. This unit first came onlinein the early 1970s.

The boiler continues as its originaldesign built by Combustion Engineering.It burns pulverized coal purchased froma variety of sources, including termcontracts and the spot market.

The current HP, IP and LP turbineswere built by General Electric. The unit


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manufacturers with specificationsrequiring replacement of the HP and IPturbines. Two turbine manufacturers,or original equipment manufacturers(OEMs), responded with quotations andperformance specifications for proposednew turbines. For both respondents,the primary performance specification

was a single heat balance diagram. Thisdiagram showed the new HP and IPturbines. However, the performance ofother components and systems, including

the system boundary conditions,were unchanged from the original1970s-vintage turbine vendor heat

were primary factors contributing to aheightened focus on these turbines. Newturbine technologies offering higherefficiencies, in addition to environmentalissues and rising energy costs, alsocontributed to the decision to exploreoptions for the turbines. A major outagealready planned for 2010 would allowenough lead time to plan any projectinvolving the turbines and sufficient plantdowntime to install the new turbines.

The SolutionIn early 2007, the utility sent a

request for proposals to various turbine

TABLE 2 RESULTS WITH BOILER SURFACEINCREASES, NO TEMPERATURE CONTROL

A B C D E

OEM-1

Only

OEM-1

RH+10%

OEM-1

SSH+10%

OEM-1

PSH+10%

OEM-1

Econ+10%

HP Turbine Power (MW) 92.6 91.8 92.8 93.3 93.0

IP Turbine Power (MW) 82.8 83.0 82.7 82.9 83.0

LP turbine Power (MW) 141.5 141.9 141.4 141.7 141.9

Total Gross Generation* (MW) 312.4 312.1 312.4 313.2 313.3

Plant Heat Rate, Btu/kW-hr 8,935 8,941 8,934 8,911 8,907

* After Generator Losses

RH = reheat, SSH = secondary superheat, PSH = primary superheat, Econ = economizer

TABLE 3 RESULTS WITH BOILER SURFACE INCREASES,NO TEMPERATURE CONTROL

A B C D E

OEM-2

Only

OEM-2

RH+10%

OEM-2

SSH+10%

OEM-2

PSH+10%

OEM-2

Econ+10%








TABLE 1 COMPARISON OF VENDOR RESULTSVS. CURRENT OPERATION

A B C

CurrentOperation

OEM-1 OEM-2

HP Turbine Power (MW) 86.2 92.6 93.8

IP Turbine Power (MW) 78.8 82.8 82.3

LP turbine Power (MW) 146.0 141.5 141.0

Total Gross Generation* (MW) 306.5 312.4 312.6

Plant Heat Rate, Btu/kW-hr 9,104 8,935 8,930


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combined cycles, cogeneration plants and

any other process steam or other fluid

system.PEPSE models of this units turbine

cycle and boiler have existed for years.However, they were modified for this

analysis to account for current plantdata. With PEPSE, these models were

developed and tuned separately, then

combined using one of PEPSEs special

features.A second model was developed for

this studya submodel of the HP andIP turbines only (see Figure 2). This

submodel allowed the input and tuningof each of the two OEMs turbine

performance claims to be analyzed without

influence from the other components andsystems in the cycle. Recall that these

other components and systems were leftunchanged in the OEMs newly proposed

turbine cycle heat balances as comparedto the original heat balance. Then, after

running and tuning the turbines usingthis submodel, the tuning parameters

were transferred to the main turbine

model. The result is a PEPSE model of

the units boiler and turbine cycles tunedto current plant data but using each ofthe OEMs proposed new turbines.

balance diagram. While a heat balance

diagram may be a basis for the turbine

manufacturers guarantees, it does notreflect adequately how the new turbines

wil l perform in the plant today. Componentperformances are usually different,

either through their own deteriorationor replacement over the years. Turbine

cycle boundary conditions such as steamproperties from

the boiler may also

be different forthe same reasons.

Other conditions,for a variety of

reasons, may havealso changed.

Guarantees are onething, but whatsthe real story?

The EvaluationActual turbine

and plant

performance canonly be learned

after the turbines are installed and

tested in the plant. However, a closeapproximation of that performance can be

derived by performing a system analysisusing one of the available heat balance

programs currently on the market. Thisutil ity chose PEPSE because of their long

history with this software program.

PEPSE is a steady-state energy balancesoftware program that calculates the

performance of the plant. This plantanalysis is done using a model that mimics

the actual plant configuration. A modelis developed in a Windows interface

setting by dragging and droppingplant component icons onto the screen

from a component library. This library

contains all the component types found

in any power plant including turbinecycle systems for nuclear plants andfossil plants, fossil boilers, gas turbines,

TABLE 4 RESULTS WITH BOILER SURFACE INCREASES,TEMPERATURE CONTROL

A B C D E

OEM-1

Only

OEM-1

RH+10%

OEM-1

SSH+10%

OEM-1

PSH+10%

OEM-1

Econ+10%








TABLE 5 COMPARISON OF REBUILT HP/IPTURBINES WITH CURRENT OPERATION

A B C

Current

Operation

Rebuilt HP/IP

No Temp Control

Rebuilt HP/IP

Temp Control

HP Turbine Power (MW) 86.2 89.3 90.3

IP Turbine Power (MW) 78.8 79.8 81.1

LP turbine Power (MW) 146.0 144.1 146.3

Total Gross Generation*

(MW)306.5 308.7 313.1

Plant Heat Rate, Btu/kW-hr 9,104 9,042 9,004


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results with temperature control appearin Table 4 (OEM-1 only). Results withtemperature control for OEM-2 show aboutthe same differences when compared withOEM-1 as they did with no temperaturecontrol.

The results of the boiler surface area studywere inconclusive relative to whether theturbine replacement was feasible. One moreanalysis was performed with the PEPSEmodel, this time looking at rebuilt HPand IP turbines rather than new turbines.Rebuilt turbines represent a lower cost/lower return alternative when compared tonew turbines.

Turbine performance characteristicsbased on the utilitys best estimate ofrebuilt turbines (83.5 percent efficiency forthe HP turbine, 90 percent efficiency for

the IP turbine) were used in the PEPSEmodel. Just using rebuilt turbines (noboiler surface changes), the PEPSE modelresults showed that the HP and IP outputcollectively increased by about 4 MW andthe LP output decreased by about 2 MW.This resulted in a net output gain of about2 MW. These results are shown in Table5, column B alongside current operationsshown in column A.

Finally, this PEPSE analysis continuedusing these rebuilt turbines and includingtemperature control to the original design

values of 1,050 F main steam temperatureand 1,000 F reheat temperature. Theseresults are presented in Table 5, column C.

The DecisionWith all the performance numbers in

and all the feasible scenarios analyzed, whatremained was to put dollars to the numbersto determine the best cost/benefit. Lookingat upfront expenses, replacing the HPand IP turbines costs about $12 million.Rebuilding the turbines instead of replacingthem runs about $1.5 million. Doingnothing costs $0.

But what about later? Here the paybacksare reversed, with the new turbine optionpaying back the most, doing nothing payingthe least and the rebuild somewhere in themiddle.

The utilitys decision after careful anddetailed analysis was to rebuild the turbinesduring the next major outage. No additionalboiler surface will be added.

Author: Bill Kettenacker, P.E., is thePEPSE Project Manager at Scientech, adivision of Curtiss-Wright Flow Control

Service Corp. He is an original author andmajor contributor to the development of thePEPSE software. He has over 30 years of

The ResultsResults of the PEPSE analysis showed

that the replacement turbines for bothOEMs behaved as proposed. However, thenew turbines removed much more energyfrom the steam than the original turbines,thus energy-starving the LP turbine. Thiscaused the LP turbine output to decrease.The net effect for both OEMs compared tocurrent plant operation was an increase inthe HP and IP generation of about 11 MW,but a decrease in the LP generation of about5 MW. This gave a net increase of about 6MW. These results, showing such a smallnet increase in the total plant output, causedthe turbine replacement project to lookmuch less attractive than originally thought.Results are summarized in Table 1, columnsB and C for the two turbine manufacturers

compared to current operations in columnA. In an attempt to keep the turbinereplacement project alive, various boilermodifications were suggested by the utilityand investigated using PEPSE. The mostpractical involved increasing the surfacearea of some of the boiler sections. This

would possibly allow more energy transferfrom the boiler to the turbine cycle throughmain steam and reheat, potentially makingup for the energy lost going to the LPturbine. The PEPSE model was again used,this time to investigate increases in boiler

superheat (both primary and secondary),reheat and economizer surface along withthe turbine replacement. This involvedadding rows of tubes to these boiler sectionsin the PEPSE boiler model. Additions of 5percent to 25 percent were investigated. As apractical matter, an increase of 25 percent is

virtually impossible due to space limitationsin the boiler backpass. A practical limit of10 percent was considered more realisticand used as the basis for comparisons.Each boiler sections tube row increase wasanalyzed separately; that is, no two sections

were increased at the same time.Two separate versions of the boiler tube

row increase analysis were performed.The first was with no superheat or reheattemperature control; that is, the fuel flowremained constant at its current plantoperating value at 100 percent load. Second,the fuel flow was adjusted to maintaintemperature control at the original design

values of 1,050 F main steam temperatureand 1,000 F reheat temperature.

The results of this boiler surface PEPSEanalysis using the proposed new turbinesshowed very small increases in generation

over that with just the new turbines alone,about 1 MW or less. The results with notemperature control are presented in Table

continued from page 36


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Evaluating an HP-IP Turbine Replacement

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