Boiler feedtank systems - Spirax Sarco

1 Boiler feedtank systems2In both cases .. the make-up water has to be softened and treated to prevent scale formation on the Boiler heattransfer surfaces. (This subject is best left to water treatment specialists).Cold water absorbs free oxygen and other gases that are liberated when heated. It is essentialto remove the dissolved oxygen before it is released in the Boiler to prevent corrosion of the boilerand steam in feedwater can be 'driven off' by heating and 'absorbed' by chemical treatment. By heatingthe feedwater typically to 85 C to remove the bulk of the oxygen the amount of scavenging chemicalsrequired can be reduced by up to 75%. This can additionally improve the Boiler efficiency by reducingthe blowdown steam lossCold make-upNolaggingor coverFlash steamfromblowdownVentLaggedandcoveredCon densatereturnColdmake-upWater temperature C0 10 20 30 40 50 60 70 80 90 100 Oxygen content ppm14121086420At worst.

2 Make-up water and flashingcondensate are fed into the tank abovethe water surface. Flash steam with allits heat is lost to atmosphere and thefinal mixture of cold make-up andremaining condensate is colder than itcould best .. both flows, together with blowdownflash steam are distributed throughsparge pipes well below the watersurface. The sparge pipes must be ofstainless steel if they are to last anylength of time and with the necessarysupports and the manufacturing timeinvolved they can be is the enemyand must be eliminatedWhat is a feedtank ?A feedtank , in a steam plant, is normally the meeting place forcold make-up water and condensate larger Boiler plants pressurised deaerators are sometimes installed and live steam is used tobring the feedwater up to above 100 C to 'drive off' the oxygen.

3 This action is normally enhanced bythe steam 'scrubbing' the feedwater . Freed oxygen and other gases are vented with some livesteam to deaerators have to be treated as pressure vessels and with the necessary controls andsafety devices they become expensive. As with most things there is a compromise - Spirax Sarcohave developed a unit for fitting to any feedtank to drive off as much oxygen as possible atatmospheric there an alternative?The Spirax Sarcoflash condensing deaerator head .. all stainless steel of tank Mixing unitCold make-up withremovable spray screenCondensatereturnFlash steamfrom blowdowncan be takento connectionon the samecentre lineat 90 CRecirculating feedwater spray (if fitted)Oxygen and gases to ventThe mixing unit bringstogether all the incomingflows without the problemsand cost of three separatetank connections andsparge systems .

4 It mixesthe cold make-up with itshigh oxygen content withthe flash steam from thecondensate and blowdownheat and other gases areliberated from the cold waterand can be removed from thesystem through a vent beforeentering the further informationplease see page probe protection tubeAccess coverVacuum breakerOverflowTemperaturecontrolDrainBo iler blowdownheat recoveryLive steam injectionSpirax Sarco can supply thecomplete package:-Stainless steel condensing deaerator injection the necessary controls and fittings tocreate an atmospheric deaerator feedwaterspray systemsWhen the condensate return rate is high and thecold make-up flowrate is intermittent it is likely thatvaluable flash steam will be lost through the improve the thermal efficiency and reduce thepossibility of temperature stratification it is oftenworthwhile to take feedwater and pump it to a spraynozzle fitted in the top of the mixing 20% of the feedtank content canbe circulated per hour to provide considerableadditional flash condensing capacity.

5 Only a lowpower pump is benefits Dissolved oxygen entering the Boiler is keptas low as possible. The oxygen content of the feedwater is furtherreduced by thermal dearation. Chemical dosing costs are kept to a minimum. High and steady feedwater temperatureto the Boiler . Long life and minimal attention. Maximum use of the heat in the condensatereturn and from blowdown heat stainless steelis used for all wetted a feedtank becomes an atmospheric 5 Flash condensingdeaerator headAir ventSteaminjectorCondensatereturnLevelin dicatorRecirculatingpumpRecirculated waterto spray headWaterlevelcontrolTemperaturegaugeAir andgasesColdmake-upFeed toboilerdeaeratorVacuum breakerVent head6 TankInternal dimensions mmNominalEmpty weight*capacity(approximate)TypeHWLlitre skgTM-21 2501 2501 2501 950530TM-31 5001 0002 0003 5001 5002 0004 500990TM-62 0001 5002 0006 0001 280TM-82 0002 0002 0008 0001 550TM-122 0002 0003 00012 0002 120TM-162 0002 0004 00016 0002 660TM-202 5002 0004 00020 0003 070TM-302 5002 0006 00030 0004 290* Excluding table below shows the standard range of feedtanks based onmetric plate sizes and indicates the internal dimensionsand the total nominal working capacity will be reduceddepending on the working level and control system empty weights are typical and can be used together with the nominal capacity in kg (litre)to give a total weight to be used for support calculation feedtank rangeMaterialsAll wetted parts are in welded austenitic stainless steel.

6 This material has proved to be idealfor most Boiler feedtank applications and is offered as the best option stainless steel can be adversely affected by the combination of exceptional chloride levels,high temperatures, and stress. Any possibility of this unusual condition should be discussedwith your water treatment steel channels are used externally on all tank sides and bases. (TM-2 has no side channels).Data dossierA detailed drawing of each tank, as built, is supplied with a materials list and a hydrostatic test conditions and fittingsAn access cover and lifting eyes are fitted as standard. All other connectionsand fittings to suit are screwed sockets BSP up to 50 mm (2")and flanged to BS 4504 PN16 for larger flanged connections (apart from the drain) stand off 150 mm to facilitate installation details see separate flash condensing deaerator headRecirculating feedwater spray systemsMixing unitTwo rubbergasketsTop of tankImmersion tubeA " BSP AV13 air vent andVB14 vacuum breakershould be fitted to the top of the systemMaterialsconsists of.

7 -BodyInternalsTwo ball valvesCast steelStainless steelStrainerCast ironStainless steelPumpCast ironStainless steelSpray nozzleStainless steelStainless steelFor full details see separate literatureA flash condensing deaerator head consists ofthree parts, the mixing unit with a PN16 or Class 150mounting flange, the immersion tube with a platetop flange, and two gaskets. The plate flangeis sandwiched between the mixing unit and amating flange welded to the top of the SpiraxSarco feedtank to act as a stiffener. All partsare manufactured from weldable austeniticstainless steel and held in place with stainlesssteel nuts and flash condensing deaerator head is easilyfitted to other head selectionWe recommend that selection of a deaeratorhead is carried out in consultation with yourlocal Spirax Sarco units are selected to suit the Boiler total steamgeneration rate.

8 Immersion tubes (fully described inseparate literature) are selected to suit the diameterof the deaerator head and to be slightly shorter thanthe depth of the tank. Two gaskets are selected tosuit the mixing unit diameter and flanges. Mountingflanges are specified as either PN16 or Class steamMixingTank depthgenerationunit1 2501 5002 0002 500rate kg /hsize Mixing unit / immersion tube selection5 000DN150MU150MU150MU150IT-950IT-1 200IT- 160010 000DN200MU200MU200MU200MU200IT-950IT-120 0IT-1600IT-210020 000DN250MU250MU250MU250IT-1200IT-1600IT- 210030 000DN300MU300MU300MU300IT-1200IT-1600IT- 210050 000DN400MU400MU400MU400IT-1200IT-1600IT- 2100* Mounting flange to BS 4504 feedwatercontentsspray systemsSizelitre (kg)Designation Speed setting BSP< 3 000 RFS111"3 000 to 6 000 RFS121"6 000 to 8 000 RFS131"8 000 to 10 000 RFS221 "10 000 to 30 000 RFS231 "Example.

9 For a Boiler with a totalsteam generation rateof 15 000 kg / h and a2 000 mm deep feedtankflanged BS 4504 PN16,Selection would be:MU250 mixing unit (DN250),an IT-1600 immersion tube,and 2 off 250 mm designation:1 off MU 250 PN16;1 off IT 250-1 600 PN16;2 off 250 mm gaskets of the products may not be available in certain Issue 5piihOther controls and fittingsSteam injection systemsSpirax Sarco steam injectors are available in three sizes to cover most requirements. They ensurequiet but vigorous mixing of steam and feedwater to drive off the dissolved oxygen content down toapproximately 2 ppm. This action minimises the amount of oxygen scavenging chemicals temperature of the feedwater is maintained at a high steady level for smooth Boiler level controlsThe design and operation of water softening plant calls for full flowrate and never a trickle.

10 The flow istherefore normally controlled by switching a pump or valve on and off. This may suit the softener but isnot ideal for a number of reasons. Heat from Boiler blowdown is not so easily recovered, flash steam inthe condensate return is more difficult to condense and it is difficult to achieve an even feedwatertemperature. Ideally an intermediate softened water storage tank should be used and the flow from itto the main feedtank controlled smoothly using a Spirax Sarco modulating control valve headsThe Spirax Sarco stainless steel vent head is designed to 'fit and forget'. It will ensure the safe ventingof any excess flash steam from the feedtank and the separation of any entrained fittingsMost other requirements for level and temperature indication, valve and other ancillaries can bemet from the Spirax Sarco design serviceSpirax Sarco engineers can assist with the design of new feedwater systems , or survey old onesthat need replacing or updating.