monitoring and maintaining desuperheaters and turbine bypass systems_ccj (2007 outage handbook)

8/14/2019 Monitoring and Maintaining Desuperheaters and Turbine Bypass Systems_CCJ (2007 Outage Handbook)

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COMBINED CYCLE JOURNAL, Third Quarter 2006 OH-53

2007 OUTAGE HANDBOOK SEVERE-SERVICE VALVES

Monitoring and maintaining

desuperheaters andturbine bypass systemsBy Dr Sanjay V Sherikar, PE, CCI-Control Components Inc

Reliable desuperheater sprayand turbine bypass systemsare critical to achieving highefficiency and availability at

combined-cycle plants. These systemsare challenged by demanding operat-ing conditions that often are thecause of unscheduled unit trips, cost-ly valve maintenance, and even morecostly collateral damage to down-stream piping and equipment (Fig 1).

The robustness of the controllogicincluding signal generation as well as of the control valves anddesuperheaters, which are the final

control elements, is a major factorinfluencing the success of these sys-tems.

To mitigate problems, plant per-sonnel must closely monitor main-and reheat-steam desuperheaters(sometimes called attemperators) andsevere-service valves, and implementa rigorous maintenance program forthem. Often, valve specialists areneeded to develop that program andplant outages are required to executethe actual maintenance.

Visible, hidden signsMonitoring severe-service valves has

two aspectsone visible, the otherhidden. The visible aspect is the onethat is readily seen by the operatingstaff. It includes clear-cut questionslike the ones below. If the answer toany of these questions is no, thenthat valve requires attention:n Does the valve open and shut on-

demand normally, without anywork-around procedures?

n Is the process parameter that thevalve is modulating being con-trolled satisfactorily, without anabnormal number of excursions oralarms?

n Does the valve shut-off tightly?The hidden aspect of monitoring

desuperheater spray and turbinebypass valves stems from the pro-cesses that are happening internalto the system, and which impact thereliability and integrity of the pres-sure boundary. To understand thehidden aspect, you must know thecontrol logic, the valves transientperformance, and the severity of theoperating environment.

For example, at some combined-cycle plants the desuperheater sprayvalve is not synchronized with thesteam flow, or the spray-water flowrates are unstable. Either problem

causes quenching of a pressureboundaryspecifically, cold waterspraying on hot pipesresultingin high transient thermal stressesand potentially premature failure.For cycling plants, hidden prob-lems such as this have enormousimportance, yet they typically arenot identified and addressed untilafter actual component failures haveoccurred.

Plant personnel should activelycheck and trend appropriate data toidentify hidden problems. Data thattypically are available directly from

the distributed control system (DCS)or plant historian include upstreampressures and temperatures, demandsignals to the steam valve and thedesuperheater spray valve, andsteam temperatures downstream ofthe desuperheater. However, for com-plete monitoring of the severe-servicevalves, additional data are needed.Most of this instrumentation can beretrofit if it currently is not installedat your plant:n Temperature and pressure imme-

diately upstream of the steam-valve inlet.

n Feedback signal from the steamvalve.

1. Collateral damage from improperly operating desuperheater spray valves can include tube failures in the superheaterand reheater sections (left), header failures in the main and reheat steam systems (right)


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OH-54

2007 OUTAGE HANDBOOK

n Pipe temperatures at key locationsdownstream of the desuperheater.

n Spray-water flow rates.n Signal to the spray-water isolation

valve.n Feedback signal from the water

valve.n Pressure downstream of the steam

valve (if dumping to the condens-er).

n Pressures, using high-quality pres-sure gages, on all valve position-ers.

Armed wi th th ese da ta , pl an tpersonnelwith guidance fromtechnical advisors if necessarycanuncover most of the hidden problemsthat may be affecting severe-servicevalves. For example, spray-waterflow remaining on after the steamvalve is shut means big trouble,because it will cause cold water to

spray on hot pipes giving rise to highthermal stresses which eventuallywill lead to cracks. Similar quenchingof hot pipes will occur if the spray-water valve opens with too long of adelay after the steam valve opens. Ifthe temperature sensor downstreamof the desuperheaters indicates that

the steam is at or below saturationconditions, then you definitely haveliquid water flowing through thosesteam lines, bringing with it a host ofundesirable consequences.

System reviewIf your plant is experiencing prob-lems with desuperheater-spray orturbine-bypass valves, you shouldbegin with a design review, tappingthe talents of the valve OEM or aqualified fluid-system consultant.Instead of repeatedly replacing failedparts in a particular valve, a thor-ough review will help you detect andcorrect big-picture issuessuch asimproper system layout, inadequateinstrumentation, and poorly config-ured controls (Fig 2).

For example, isolation valvesshould back up all spray-watervalves, as a matter of good prac-ticeparticularly on the high-pres-sure steam system. This reduces therisk of water from the spray valve,if it leaks, impinging on hot pipe.Other seemingly minor design fea-

turessuch as the relative eleva-tion of the spray-water valves andthe distance from the spray-watervalves to the spray nozzlesexactan enormous toll on combined-cycleplants, particularly those that cyclefrequently.

A thorough system review canidentify requirements unique to aparticular plant and suggest solutionsthat meet the specific requirements.Doing this early in plant life is best,to limit the accumulation of damage(Fig 3). In fact, the review should bedone during commissioning, sincemany of the excursions from designconditions can be encountered dur-ing initial startup and testing of theplant. Another such review after theplant has been in operation for sometime, perhaps one to three years.

Control logic andinitial valve set-upCorrect set-up of the control logic fordesuperheaters and turbine bypasssystems is important to achievinglong-term reliable performance.Incorrect control logic and impropervalve set-up are the leading causesof problems experienced by these sys-tems. From the standpoint of overall

2. A thorough design review mayuncover big picture causes ofvalve troublesuch as this pipinglayout problem where spray watergets injected directly downstream of

elbows. Resulting problems includenon-uniform steam-velocity profile,water impinging on the pipe wall anddropping out of the flow, and pipecracking

3. Cage from a turbine bypass valvein a combined-cycle plant required to cycle daily exhibits cracking. Accu-mulated damage over many cyclesbetween outages was caused by ther-mal stresses


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OH-55

SEVERE-SERVICE VALVES

plant operation, there are functionaldemands on the systems that mustbe met.

Remember that hardware incor-porated into the systems also hasits own operational requirements such as not allowing spray water toimpinge on hot metal parts. Control

logic must address these require-ments as well. Example: Many plantsstill use a thermocouple downstreamof the turbine bypass system for feed-back to the control system. This cancause two kinds of problems: If thethermocouple is located too far down-stream, its response is very slow;if too near, water in the steam cancause erroneous temperature mea-surement.

In either case, the result is poorcontrol. An enthalpy control algo-rithm eliminates these problemsand is recommended when bypass-ing steam to the condenser; it alsoreduces the potential for inadver-tent quenching of hot pipes down-stream of the steam valves duringbypass operation (Fig 4). Adequateinterlocks between the spray-waterflow and steam flow are necessaryto ensure that the spray water cancome on only when the steam valvesare open.

Most turbine bypass systems aredesigned for steam that is at least 50deg F above saturation at the inlet.This requires adequate of drainage of

condensate formed upstream, as wellas operation of the drains until steamquality is suitable for the bypasssystem. A good design accomplishesthis by locating bypass valves close tomain steam headers; an alternativeis preheating of the valves througha small warming line. Wet steam orcondensate entering the inlet of thesteam bypass valves erodes the seal-ing surfaces of the valve trim. Suchconditions should be avoided to theextent possible.

Valve maintenanceEven if the design review and day-to-day operation reveal no major prob-lems, all severe-service valves shouldstill receive periodic inspection andminor maintenance. This includesmeasuring the critical dimensions onall the parts, and changing all of thesoft goods (the seals). Plants shouldhave at least one set of soft goods pervalve on-hand at any given time. Ingeneral, seals should never be reusedsince they are designed to take a per-manent set when assembled as partof their functioning.

If valve inspection reveals minordamage to the trim component, itlikely can be cleaned up onsite. If the

damage is heavy, trim-componentrepair might still be possible, but itsbest to replace with new trim. Foreach set of identical valves, have atleast one set of spare trim parts (seatring, plug, and cage) on-hand. Theseparts are not off-the-shelf items, andtheir lead times can be significant.

Outage work. If severe-servicevalve problems are encountered,and their root causes are identified,major repairs can begin. Unless theproblem is traced to signal genera-tion or the external control element(valve positioner plus accessories),the valves must be completely dis-mantled. Typically this can only bedone during an outage, though itusually requires attention weeks ormonths before that event to allow forprocurement of materials and sched-uling of personnel.

If the problem stems from faultysensors or poor calibration, then theseissues should be addressed first. If itrelates to operation when the valveis open, then response of the valve

based on its feedback position shouldbe compared to the demand signal.This determines whether the prob-lem is caused by signal generation orby the valve itself.

If the valve leaks when shut, acouple of items require checking.First, be sure the full actuator loadis being applied for valve shut-off,using the pressure gages on the posi-tioner. If full actuator load is notbeing applied, is it because of limitswitches, or improper calibration, orsome other reason?

If the problem is noticed because ofa long-term trendsuch as the plugposition keeps getting lower at thesame maximum flow conditionthenthere could be damage to valve inter-nals. If the plug-lift progressivelygets higher over time for a given flowcondition, the trim could be blockedby foreign material. Either situa-

4. Pipe cracking downstream of ahot-reheat bypass valve was causedby repeated thermal shocks attributedto poor control of spray water flow


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OH-56 COMBINED CYCLE JOURNAL, Third Quarter 2006

tion requires disassembly for furthertrouble-shooting.

When planning your next outage,allow five days for disassembly andreassembly of the desuperheaterspray and turbine bypass valves. Thisincludes time required for the valvesto cool down. The window may be lon-ger if there are unique constraints such as constricted workspace, lackof lifting equipment, and so on. Theteardown and repair of severe-ser-vice valves requires specially trainedpersonnel, because it involves highlyengineered equipment in which toler-ances and precision are particularlyimportant. This task is best carriedout by the OEMs specialists.

As part of your outage planning,ensure that the recommended sparesare on-hand. If not, order immedi-ately; many of the engineered com-ponents are not stock items and youdo not want to extend the outagewaiting for spare parts, or incur expe-diting charges that could have beenavoided through better planning.

Replacement needed? Whenrepairs become too expensive, orunderperformance of the systemresults in unacceptable penalties, con-sideration must be given to upgradingthe troublesome valve. Once the rootcause of the problem, and any specialsystem requirements, are identified,

it often is possible to modify valveinternals to address the issues of con-

cern (Fig 5).When upgrades cannot adequatelyaddress the root cause of the prob-lem, replacement is the only practicalsolution. Improper selection and siz-ing of severe-service valves has beena common problem in combined-cycleplants and many of these valves havehad to be replacedan expensive andtime-consuming taskafter only afew years of service. CCJ OH

5. Erosion of sealing surface on avalve plug was caused by wet steam.Bypass valve was located too farfrom the main steam header and pre-warming was not provided. Propersurface treatment can improve resis-tance to droplet erosion

CCIs unique Valve Doctor approach andapplication knowledge providelong term solutions for attemperatorproblems in combined cycle power plants .

www.ccivalve.com 949.858.1877

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monitoring and maintaining desuperheaters and turbine bypass systems_ccj (2007 outage handbook)

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