Water Filtration in Power Generation - Delta Pure Filtration in Power Generation By Joseph Bernsley, Business Development Manager, Delta Pure Filtration W. ... lines” EPRI TR-104422

Power | Generation

ater filtration is crucial invarious types of powergeneration systems. This

article provides a basic understandingof some industry terms and concepts in-volving the use of water to generatesteam within various types of powerplants, and explains where, why andhow these power plants use water andwater filters.

THE STEAM POWER CYCLE: BASICA steam power plant is comprised of a

closed loop. The power generation cycleis one of generating steam, sending thesteam to a turbine, cooling the steam back

to water, and then reheating the waterright back up again in a boiler.

Figure 1 shows a simplified version ofthis loop, starting at the boiler, and con-tinuing to the high pressure steam tur-bine, low pressure steam turbine,de-aerating condenser, boiler feed pump,boiler preheat and back to the boileragain. The water, in either a gas or liquidstate, is referred to by different names, de-pending on where it is in the loop. Afterthe boiler, the water is in a gaseous stateand is unsurprisingly called “steam.” Im-mediately after the condenser, when it issignificantly cooled and in a liquid state,it is called “condensate.” After a feed

pump just before the boiler, the liquidwater is called “feedwater” or “boiler feed-water.”

BOILERThe boiler produces pressurized steam

and sends it to a steam turbine where en-ergy released from the expanding steamrotates a turbine. The fuel that is used inthe steam generation plant to heat the liq-uid water to generate steam can be naturalgas, coal, oil, no.2 oil (diesel fuel), ura-nium 235 (in a nuclear system), geother-mal steam, gasified coal, etc.

There are different designs of boilers.A drum boiler will create super-heated

Water Filtration in Power Generation By Joseph Bernsley, Business Development Manager, Delta Pure Filtration

W

steam, while a once through boiler cancreate supercritical “steam.”

TURBINES AND GENERATORSOne is sure to hear the terms “rotor”

and “stator,” and a potential source ofconfusion stems from these terms be-cause steam turbines and electrical gen-erators both have rotors (moving/rotatingparts) and stators (stationary parts). Onemust often discern from context whetherthe terms are referring to a turbine systempart or an electrical generator part.

The use of natural gas as a fuel sourcedoes not, in and of itself, reveal whethera given turbine is a gas turbine or a steamturbine. A turbine is considered a steamturbine when steam is supplying the en-ergy to turn the turbine regardless of thetype of fuel or the source used to produceheat for the steam.

A gas turbine would be one where hotgasses (e.g., from combustion) propel theturbine. Two things enter a combustionchamber for a gas turbine – the fuel (suchas natural gas or diesel fuel), and filtered

compressed air. The power industry oftenrefers to the compressed air filters for thegas turbine as “turbine filters” althoughother fluids and gasses are also filteredand treated. Many power plants have a

combination of turbine types within them– for example, gas turbines and steam tur-bines – and those are called combinedcycle plants.

Figure 1. Simplified Schematic of a Generalized Steam Plant. Note that the feed-water and steam loop is separate from the cooling water loop.

Power | Generation

20 • April 2015 • www.filtnews.com

STEAM TURBINEA “steam turbine” is a series of fan like

blade assemblies positioned serially fromsmallest blade lengths to largest bladelengths along the turbine rotor or shaft.

As high pressure steam enters a nozzleblock at the inlet of the turbine, the steamexpands and causes the turbine blade as-semblies to rotate the turbine shaft. Theturbine shaft is coupled to an electric gen-erator, which consist of a “metal rotor”imbedded with copper coils that are mag-netized by an exciter. The rotor spins in-side of a fixed position “stator” consistingof bundles of copper wire windings. Themagnetic field created by the rotatingmagnetized metal rotor within the copperwires of the stator causes electrons to flowwithin the “stator windings” and electric-ity is produced.

GENERATOR STATOR COOLING WATERStator water or stator cooling water is

used as a coolant for the generator stator,which must be kept cool to keep the gen-erator running, but whose metal is subjectto deposits, and whose narrow passagesare subject to plugging. Also, since thegenerator stator is an electrical compo-nent, the water must be kept at very lowconductivity. The stator water cooling cir-cuit may include cartridge filtration, IX(ion exchange) resin, and even gas trans-fer membrane for controlling oxygen lev-els, which have a profound effect on waterchemistry.1

CONDENSERSteam is a condensable gas – a large

volume of steam will condense uponcooling into a much smaller volume ofliquid water (Figure 1, location B).

After the steam exits the turbine it iscondensed back into water condensate ineither a water-cooled vacuum condenser(synonym: surface condenser) or aircooled condenser. Energy transfer effi-ciency is increased when the delta in en-ergy from a source to a heat sink is higher.When steam is condensed in the con-denser, a vacuum is formed and the dif-ferential in energy is increased. Restated,a condenser collapses the steam in orderto create a vacuum that pulls the steam

forward, increasing energy efficiency. The condensed water collects at the

bottom of the condenser in a sump calleda hot well.

Non-condensable gasses such as airmixed in with the steam cause a loss ofenergy efficiency and corrosion; however,the “air off-take section of the de-aeratingcondenser” will remove non-condensablegasses using vacuum produced by somemeans such as by a vacuum pump or asteam ejector. Membrane technologymight be used for sidestream de-aerationat a condensate storage tank.2

Condensate often contains corrosionproducts in the form of iron oxide parti-cles from the plant equipment and solublecontaminants from cooling water ingress(from leaks). When cooled down suffi-ciently, the liquid water condensate can be“polished” or demineralized using ion ex-change resin (IX) to remove dissolvedmetal ions. Without covering every pos-sible permutation or variation, two exam-ples of what one might find are a) IX resinin a “deep bed” format with cartridge fil-ters (such as pleated or string wound) up-stream, and b) resin precoat IX format, inwhich the powdered IX resin (much finerthan a deep bed resin) is precoated onspecial filters called septa. Filter septahold the resin on their surfaces due to dif-ferential pressure created by the waterflowing through the media, and they aredesigned so that the resin can be easily re-placed with new IX resin by backwashingit off and reprecoating the septa with anew precoat of powdered resin.3 Theseprecoat septa are not really filtering thewater themselves, but rather are precoatedwith a flocculated suspension of pow-dered IX resin on their surfaces. Septaoften come in a string�wound, meltblown, or sintered metal mesh format.The ion exchange resin is not only remov-ing ionic species, it is also acting as adepth and surface filter to remove partic-ulate contaminants. The goal is to reducetransport of ions and particulates to thesteam generator where they will plate outon the heat transfer surfaces and reduceefficiency as well as cause under-depositcorrosion.

Instituting condensate polishing obvi-

ously entails capital and operating ex-penses, but there is significant payback interms of reduced maintenance and in-creased heat transfer efficiency. The Elec-tric Power Research Institute (EPRI)published “Condensate Polishing Guide-lines” EPRI TR-104422 in 1996 to suggestwhen to institute condensate polishingtechnology.

The EPRI guidelines suggest institut-ing polishing if any one or more of the fol-lowing situations exist4:

a) The boiler is “once-through” typeb) Brackish water or seawater is used

for cooling waterc) Contaminants enter the steam from

process uses in a cogeneration plantd) Drum pressures are greater than

1800 psi e) Other conditions exist, which

suggest a need to polish

FEEDWATERThe condensate is pumped from the

condenser “hot well” by condensatepumps through the condensate system upto the feedwater pumps where it is nolonger called condensate but instead iscalled boiler feedwater (Figure 1, locationC). In the feedwater system, the water ispreheated before re-entering the boiler(steam generator), and the process ofsteam production starts over again. Somepreheating and deoxygenation occurs ina de-aerator or DA. Some extractionsteam from the turbines is used to preheatthe water and strip out the oxygen, whichis vented from the DA. The DA will gen-erally use trays or spargers to effect oxy-gen separation.

The feedwater in these large utilitypower plant systems is very hot from pre-heating, and due to high temperatures,this is not where filters are located. Manyfilter companies advertise that their filtersare for boiler feedwater and these can befor smaller commercial/industrial boilerswhere the feedwater is not nearly as hot.In high pressure utility operations filtra-tion and or demineralization takes placein the lower temperature condensate sys-tem, but not in the high temperature feed-water system.5

BOILER MAKEUP WATERThe tasks of initially charging the sys-

tem, and replacing any water loss, (forexample, from boiler blow-down, leaksand evaporative losses) are accomplishedwith “makeup water” (Figure 1, locationA). The volume of makeup water mightbe a couple of percent of the condensatewater flow.6 Makeup water often con-tains impurities based on its source thatshould be removed by RO filtrationand/or ion exchange.

Contaminated water entering theboiler can cause water-and-steam-side-corrosion and deposition issues and re-sults in increased system maintenanceincluding increased need for steam gen-erator blow down, fouled heat exchangesurfaces, scale, corrosion, and water-walltube failures. Dissolved species are de-posited in the boiler upon conversion ofwater to steam, and under certain condi-tions can even be deposited on turbines.7

For this reason, boiler makeup water (butnot boiler feed water) is highly treated.Makeup water is often initially filteredwith back-flushable sand, multimedia or

self�cleaning filter systems. This preventssilt, aquatic life, etc., from entering thesystem. After the removal of macrocont-aminants, the makeup water is furthertreated for soluble ionic contaminantswith membrane systems (such as RO)and/or IX resin. RO is often selected forremoval of colloidal silica. For RO/IX pre-filtration, the water system designers willtake into account water quality throughthe year and select either cartridge filters(1 or 5 micron, often “meltblown” typeand sometimes string-wound type), orMF (microfiltration) or UF (ultrafiltra-tion) membrane filters. UF is often se-lected to control high levels of organics,which may include particulates from lakebottoms, microorganisms and bacteria,and virus size particles.

COOLING WATER / COOLING TOWERSA power plant can be located near a

river or large body of water if water-cooled condensers are used, or in aridareas with minimal water if air-cooledcondensers are used. For all steam powerplant designs, it is important to have a

cooling component to condense thesteam (Figure 1, locations D and E).

Cooling towers take advantage of theremarkable way a body of water is cooleddown when a portion of the water evap-orates off from it. Air moving across waterin a cooling tower can be at a muchhigher temperature than the water, yet thewater will cool down. The top of the cool-ing tower will generally have spray noz-zles (filtration can help to prevent thesenozzles from plugging) and water willcascade downward across a fill or pack-ing, as air flows upward. The air carriesaway heat and gas phase water vapor,which rise up out of the cooling tower.The gaseous water vapor condenses toform harmless fog that can sometimes beseen from miles away. Actual waterdroplets that are carried upward are called“drift” – drift is not always so harmless.For example, drift can carry, salts, partic-ulate matter, or chemical additives intothe surrounding communities.8 Anotherpotential contaminant in drift is organ-isms capable of causing disease (such asLegionella). Water treatment, including

www.filtnews.com • April 2015 • 21

Power | Generation

disinfection and “drift eliminators” makea power plant a good neighbor.

EPRI (Electric Power Research Insti-tute) lists three treatment applications forcooling tower water in a power plant:9

1. Pre-treatment of makeup water (to remove chemical contaminants and heavy metals, soften, adjust pH, reduce silica, TDS and TSS)

2. Sidestream treatment of recirculating water(to soften, reduce silica, and reduce TSS)

3. Post treatment of cooling tower blow down water – (to reduce volume)

Especially in arid regions, makeupwater for cooling towers (or boilers)might be derived from non- potable watersuch as from a POTW (publically ownedtreatment works, or a sewage plant), fromindustrial wastewater, or from some other“degraded” water source. In such cases,the treatment used would need to be tai-lored to incoming water quality, with bestefforts made to accommodate anticipatedvariations/ranges. Such treatment mightinclude UF and RO filtration. Other po-tential pre-treatment methods include: airstripping, activated carbon, biologicaltreatment, ion exchange, precipitation10,and there are no doubt others. Puttingwastewater to work is called “recycling”,

“re-use,” “beneficial use,” etc. Please referto Figure 1, location D.

Automatic, self-cleaning screens, orback�washable screens, can be used toclean up surface water and ground waterparticulate debris as well as macro foulingfrom aquatic life.

According to EPRI, “Filtration is usedto limit suspended matter in the coolingwater. Water is directed from the returnline on the hot side of the cooling circuit,[and] fed through filters.”11 In some util-ity applications, approximately 5% of therecirculation may be filtered as a side-stream, but a large utility cooling towersystem can run unfiltered due to the lo-gistics of handling large volumes of solidsand the very high cost.12 Among the filtertypes found at cooling towers generallyare self�cleaning filter systems, cyclonic(centrifugal) separators, disc filters, sandfilters, multi-media filters, bag filters orcartridge filters.

Not all water based cooling is per-formed with cooling towers. For example,once-through cooling will put the hotwater directly into a body of water suchas a river. In the U.S., the breakdown isapproximately as follows: cooling towers52%, once-through cooling 33%, coolingpond 13%, and dry cooling 2%.13 Theability to get cool water in once-throughcooling will greatly impact the efficiencyof the power plant – in fact, plant effi-

ciency will increase in the winter and di-minish in the summer. The downside ofusing a cool river is that when we depositthe warmer water directly back into theriver, this may have an environmental im-pact and is referred to as “temperaturepollution.” To avoid this impact on theenvironment, many modern plants aredesigned with cooling towers or air-cooled condensers.

NUCLEAR POWER WATER FILTRATIONThis section will touch on some of the

peculiarities of how water is employedand treated in a nuclear power plant. Re-ferring frequently to Figure 2 should helpto clarify the points in this section.

Water is crucial for a number of func-tions within a nuclear power plant –some of these functions are identical orvery similar to the usage of water in a fos-sil fuel burning power plant – such as theuse of water to create steam to drive a tur-bine, and the use of water in a coolingtower, while some uses of water are spe-cialized.

Water is crucial for controlling tem-perature in the core (where the nuclearfuel resides) in many reactor designs – inother words, water is a coolant. Water isalso a “moderator” that slows neutrons –this moderation or slowing of the neu-trons is necessary for desired fission re-actions to occur. Water in nuclearsystems is produced at near “theoreticalwater quality levels” (i.e., ultrapure waterat the lowest achievable conductivity lev-els) and then specific additives are in-jected into the feedwater or the reactor tocontrol corrosion or deposition.14

One may also hear about boron, forexample, boric acid can be added to water,and boron is important as an impurity oradditive because it can control pH inPWR reactors and absorbs neutrons.Boron is used in a complementary waywith control rods.15

Another difference from conventionalpower plants is the presence of hydrogengas in water. Discussing hydrogen in de-tail is beyond the scope of this paper, butsuffice it to say for now that hydrogen isboth used and carefully managed in nu-clear power systems.


Figure 2. Simplified Schematic of Generalized Light Water Reactors

LIGHT WATER REACTORS, PWRS, BWRSA light water reactor uses ordinary

water (as opposed to heavy water, whichhas a higher percentage of water withdeuterium) that is highly purified. Whilethere are many kinds of nuclear reactordesigns, in keeping with our focus onwater, we can briefly go over two types oflight water reactors: the PWR (pressur-ized water reactor) and the BWR (boilingwater reactor). See Figure 2.

A pressurized water reactor (PWR) hasthree or four separate water loops16 andheat flows from one to the other in thisorder – using three water systems in thisillustration: 1) water that flows throughthe reactor core but is not allowed to boildue to pressurization, 2) a steam genera-tor (heat exchanger) loop that transfersthe heat from the pressurized reactorwater loop to the feed water and generatessteam that drives the turbine, 3) a con-denser cooling water loop. In this PWR il-lustration, the water/steam that goesthrough the turbine is once removed fromthe water that goes through the core, and the water that goes through the cooling tower is twice removed from the water

Delta Pure Filtration in Ashland, Virginia, manufactures cartridge filters, includ-ing melt blown and string wound type filters for RO pre-filtration. Pictured is thenew FUSION™ series filter, which is part melt blown and part string wound type.



that goes through the core.In a BWR, water flows through the re-

actor core and is converted to steam,which in turn drives a turbine. The waterin the reactor vessel flows around and upthrough a nuclear core of zirconium alloycladded fuel rods that contain the nuclearfuel pellets. The steam in the BWR is ra-dioactive and therefore the steam turbine,condensate and feedwater cycles are al-ways contaminated with radioactive ma-terials and shielded for personnelprotection.17 In a BWR the water/steamthat goes through the turbine is the samewater that goes through the core. Thewater that goes through the coolingtower is once removed from the waterthat goes through the core.

Just as there is polishing with ion ex-change in a conventional steam turbineprocess, there is also water deionizationin light water reactors. Water must besufficiently cooled before it can betreated. In cases where precoat septa areused, the septa can be of a polymeric

string wrapped or melt blown design orsintered metal mesh design depending onapplication. IX resin used for a reactorwater cleanup system will be radioactiveand will need special handling.

BWRs will have condensate polishing,while some PWRs do.18

In a BWR, approximately 1% of reac-tor water is treated in a side-stream filter/demineralizer to continually polish andmaintain high reactor water quality. Thissystem is referred to as the RWCU - Re-actor Water Cleanup System.19

In a PWR, the pressurized water thatgoes through the core is treated onlywhen “let down,” i.e., depressurized andcooled off and treated with filtration andIX in a process called the Chemical andVolume Control System or CVCS.

SPENT FUEL POOLAnother place where water is filtered

is the spent fuel pool. The spent fuelpool is a storage pool for spent fuel rods,and although the fuel is substantially

used up, the rods continue to give offheat and must be kept cool. The spentfuel pool is generally on-site within thereactor building plant. This fuel poolwater is recirculated, filtered and deion-ized as well. Used filters and spent resinmust be handled as radioactive waste.

CONTINGENCIESA strategy in contingency venting

in the event of a core melt accidentuses “water filters” to scrub contain-ment gas and capture radioactive ma-terials before releasing the gas toatmosphere.20 Generally, some mech-anism is needed to capture watermist – and this might use demisterscomprised of sintered metal fibermedia. Water plays a vital role inplant safety and accident contin-gency planning.

COOLING TOWERSNuclear power plants also have

cooling towers. Some lay people see

Power | Generation

water vapor rising up out of a coolingtower and may have concerns – butthe cooling tower water that createsthe vapor is not the same water thatcomes in contact with the fuel rods –it is a separate cooling water loop.

OTHER APPLICATIONSOne supplier of filtration to the

industry lists potential additionalwater filtration applications thathave not yet been mentioned here,including: steam generator blowdown, laundry rinse clean up, pumpseal injection, radioactive wastecleanup, and ECC (emergency corecooling) strainers.21

CONCLUDING THOUGHTSIn order to help with future chal-

lenges in power generation, the filterindustry needs to educate itself aboutpower plants and how they operate,and in turn educate the power indus-try about available filtration tech-

nologies – and keep innovating. Thispaper is intended for people just be-ginning that journey.

Some of the themes that can beseen in water filtration for powergeneration are:

Water scarcity: How to make useof waste water or recycled water in-stead of using drinking watersources? Also, are there effectiveways to reduce the use of water?

Efficiency: How to increase ther-modynamic efficiency of the plant,such as by protecting heat exchangesurfaces, or by using novel coolingand heat transfer methods?

Environment: How to reduceemissions, lessen thermal pollution,and diminish discharge?

Safety: How to prevent accidents,reduce exposure to dangerous mate-rials, and better deal with varioustypes of hazardous waste?

Resources: How to prolong theavailability of finite resources, and/or

reduce our dependency on them?Affordability: How to maintain and

expand the availability of affordablepower, cope with volatile fuel pricesand supply, reduce maintenance andoperating costs, and deal with capitalcost issues for new plants?

ABOUT THE AUTHORJoseph (Jay) Bernsley is Business De-

velopment Manager at Delta Pure Filtra-tion, a manufacturer of RO pre-filtercartridges, melt blown, string wound,and carbon filters based in Ashland, Va.With almost 30 years of experience in fil-tration, the writer acknowledges theinput of Philip D’Angelo of JoDAN Tech-nologies, Ltd. and Thomas Poschmannof Scinor Water America.

For more information contact:Jay BernsleyTel: 1- 804-798-2888Email: [email protected]: www.deltapure.com

FN


References

1. David G. Daniels,“Forgotten Water: Stator Cooling Water Chemistry,” Power, 12/15/2007. www.powermag.com/forgotten-water-stator-cooling-water-chemistry/?pagenum=2 2. Interview of industry consultant.3. Interview of industry consultant.4. Condensate Polishing Guidelines” EPRI TR-104422, 1996 (open Literature).5. Multiple industry sources – by personal interview and/or emails. 6. http://en.wikipedia.org/wiki/Thermal_power_station 7. “Purolite Technical Bulletin, Condensate Purification”, 1999, (marketing literature).http://purolite.com.br/downloads/Brochuras/Condensate%20Polisher.pdf 8. www.epa.gov/ttnchie1/ap42/ch13/final/c13s04.pdf 9.http://mydocs.epri.com/docs/PublicMeetingMaterials/0712/watertreatment_RFI_Final.pdf 10. “Use of Degraded Water Sources as Cooling Water in Power Plants”, Consultant Report for theCalifornia Energy Commission P500-03-110, October 2003, Prepared by Electric Power Research Institute.11.http://mydocs.epri.com/docs/PublicMeetingMaterials/0712/watertreatment_RFI_Final.pdf12. Based on interview of a water treatment consultant. Also, a Chemical Engineer from a majormidwestern utility said that their plants do not use this type of cooling water loop filtration, althoughit has been discussed.13. Anna Delgado Martin, Water Footprint of Electric Power Generation: Modeling Its Use and Ana-lyzing Options for a Water Scarce Future, MIT Masters Thesis, May 11, 2012; (which in turn refer-ences EIA 2008, UCS 2001). https://sequestration.mit.edu/pdf/AnnaDelgado_Thesis_June2012.pdf 14. Based on interview of industry consultant.15. The Westinghouse Pressurized Water Reactor Nuclear Power Plant, 1984.www4.ncsu.edu/~doster/NE405/Manuals/PWR_Manual.pdf 16. The Westinghouse Pressurized Water Reactor Nuclear Power Plant,1984.www4.ncsu.edu/~doster/NE405/Manuals/PWR_Manual.pdf 17. Based on interview of water treatment consultant.18. Based on interview of water treatment consultant.19. Based on interview of water treatment consultant.20. Filtered Containment Venting System brochure, Areva. http://www.areva.com/globaloffer/liblo-cal/docs/Brochures/AREVA_Filtered-Containment-Venting-System_vEN.pdf 21. Nuclear Filtration and Systems brochure, CCI Thermal Technologies, Inc.http://enertech.cwfc.com/brandproducts/spokes/PDF/3Lfilters/3Lfilters_engineered-filtration-sys-tems.pdf

Water Filtration in Power Generation - Delta Pure Filtration in Power Generation By Joseph Bernsley, Business Development Manager, Delta Pure Filtration W. ... lines” EPRI TR-104422

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