Steam blowing – What Is Piping

The steam blowing is one of the initial cleaning operations before starting any power plant or steam lines. Steam Blowing of MS lines, CRH, HRH, SH, RH, HP & LP bypass pipelines of the turbine are carried out in order to remove welding slag, weld bead deposits, loose foreign materials, iron pieces, rust, etc. from the system, generated during manufacturing, transportation & erection prior to turbine operation. The cleaning is accomplished by subjecting the piping systems to heating, blowing steam, and cooling cycles in sufficient number and duration until clean steam is obtained. Effect of Steam Blowing depends on the following factors: Thermal shockRemoval force of steamCleaning force of steam Cleaning Force Ratio or CFR of Steam Blowing The necessity to create in the system, the steam velocity greater than that is possible at MCR condition is obvious. These two velocities are expressed as a ratio “Cleaning factor” or “ Distribution factor” or “Cleaning Force Ratio” denoted by “X” or “K”. CFR (Cleaning Force Ratio) is also known as Cleaning Factor or CF. CFR is an industry-accepted factor that determines the required dynamic pressure. CFR in steam blowing can be defined as the ratio of required dynamic pressure for cleaning to maximum dynamic pressure experienced during system operation. Cleaning Force required (CFR) or distribution Factor Preconditioning for Steam Blowing Chemical cleaning should be completed.SH primary and secondary de-superheater piping and RH’s emergency de-superheater piping ready for operationAll permanent piping & temporary piping insulated and supports/hangers are released with cold settingSilencer should be connected at a temporary pipe exitSoot blowing for APH should be availableMake up for deaerator made readyMotor-Driven BFP with all controls made readyHydraulic test of the following lines completed: Feed LinesMS, HRH, CRH LinesMS to Aux. PRDS LineAll other auxiliary lines identified for steam blowing Sampling system made readyBoiler auxiliaries proved serviceable and ready after a pilot operation like: Fuel oil systemCompressors & Atomizing steam systemStart-up system ( for the continuous system)Coal Mill system (for the continuous system)CHP readinessEconomizer hopper and bottom ash hopper and its evacuation system (for the continuous system) All safety valve disc installed after removing the hydro-static plugin drum(sub-critical), superheaters and reheatersAdequate communication between the control room, boiler, and TG area ensured.Flow nozzle, control valves, and NRV flaps wherever applicable should be not erected before steam blowing and suitable spool pieces are erected. Strainers in the path should be removed.Required number of Target Plates and holder made available Chemical Cleaning Process Boiler Front System Alkaline FlushingMass FlushingHot water RinsingAlkaline FlushingHot DM water Rinsing Main Boiler System Acid Cleaning Super Heater FillingMass FlushingAlkaline FlushingHot DM water RinsingAcid CleaningPassivationACID CLEANING ( BY CITRIC ACID METHOD )will be done by the circulation method for the effectiveness of the cleaning process.Acid cleaning will be followed by PASSIVATION so that the uniform protective coating of GAMMA FERRIC OXIDE is formed on the metal surface and corrosion/oxidation damage to the metal surface is prevented and continue during normal operation by dosing oxygen. The gamma ferric Oxide formed by using the chemical 1-2 % sodium Nitrite(NaNO2) Steam Blowing Procedures Techniques Normally two methods are historically used for steam blowing PUFFING METHODPURGING METHOD / CONTINUOUS BLOW METHOD PUFFING Method of Steam Blowing To give a thermal shock to the contour being purged, to dislodge the scale, etc. Procedure: Raise the boiler pressure to a pre-determined value (40-60 kg/cm2), shut off firing, and at the same time open the quick opening valve(EOTV), thus allowing the steam to escape to atm. with high velocity carrying with it the loose debris. Precautions during puffing method The Pressure drop allowed in the drum is limited to the corresponding saturation temp. change of 40 OC. Scheme of puffing method Steam blowing done in stages Stage-1(a): SH, MSL, ESV, temporary lines from ESV to EOTV, EOTV to CRH line, CRH lines up to boiler end with the temporary exhaust pipe.Tap off lines from CRH to deaerator, auxiliary PRDS, HP heater 6a & 6b, gland sealing, etc. shall remain closed/isolated.Stage 1a endpoint will be concluded by observing the indents on the target plate. Stage- 1(b): SH, MSL, HP bypass interconnection, hand-operated valve mounted in place of HP bypass valve and CRH lines up to Boiler end with temporary exhaust piping.In this stage, 6 to 8 blows will be given through HP bypass lines to ensure the cleanliness of the limb.The boiler MS stop valve will be used for stage 1b. EOTV will be kept closed. Manually Operated Isolation Valves in HP bypass lines will be kept opened fully. Stage-2(a): 1a plus reheater, HRH lines, Interceptor Valve and temporary pipe.CRH line along with attemperator shall be welded with reheater before the start of stage 2a. LP bypass lines shall be blanked during stage 2a. Stage 2a endpoint will be concluded by observing the indents on target plates Stage- 2(b): 2a up to IV + LP bypass lines with the temporary exhaust pipe.Stage 2b blowing will be a parallel blow in the path 2a & 2b. LP Bypass blanks shall be removed for stage 2b. 4 blows will be given through L.P. Bypass line to ensure the cleanliness of the limbs Stage-3: Auxiliary Steam Lines covered in steam blowing are listed below. 3a) Main steam line to Aux PRDS3b) CRH to Deaerator3c) CRH to HPH-6. All auxiliary steam lines will be steam blown by continuous blowing method Stage-4: The following Aux steam lines, connected to deaerator are steam blown using auxiliary steam from Aux PRDS header. 4a)PRDS to Deaerator.4b)Extraction 4 to4c) PRDS to Gland steam header Continuous Steam Blowing Procedure The initial procedure is same as puffing method except:Continuous firing till the completion of steam blowing. No need to shut off the firing during blowing.Maintain constant pressure during the blow Recommended blowing parameters for continuous steam blowing Dynamic steam pressure = 3.5 MPaMS temp = 420(not to exceed)HRH temp = 480( not to exceed)Steam flow = 845 TPHCorresponding Drum pr. = 40 KscFurnace load ≈ 39% Method of Continuous Steam blowing Set the Temporary purging valve, by-pass valve, and drain valve and granulating device behind the temporary pipesCheck the tightness, support, and expansion of the temporary systemSection by section rinse of condensate water piping, feedwater piping and boiler through the start-up system to CW systemCirculation begins when Fe+ in the water of the main feedwater pipe and separator outlet will be less than 100 ppm.Maintain the circulation flow or start-up flow at a minimum set valueStart oil firing and raise the temp and press. according to the cold start-up.rise rate of water wall = less than 2OC / minrise rate of main steam = 4 ~ 5 OC / minBefore MS press reaches 1.0 Mpa, open the bypass valve of the temporary purging valve to warm the piping with all water drain valves of the system open. -The blowing system divided into two parts A) Preliminary Steam BlowingB) Final Steam Blowing Preliminary Cleaning PURPOSE- Primary cleaning out sundries and bulky grain deposited in the RH and main steam systemTo ensure the fastness of supports and hangers with proper expansionKnow well about the operation property of oil burning system, condensate water, and feed water systems Preliminary Steam Blowing Procedure Start the oil firing with max. rating of 15%Control the gas temperature at furnace outlet below 500 OC (max.538 OC )Raise the SH outlet press to 1.6 – 1.8 Mpa with steam temperature around 350 OC.Open the temporary purging valve for 15-20 min to blow in MS systemMaintain K around 0.5.SH & RH desuperheater water system also cleaned Final Steam Blowing Precedure Main steam and reheated steam system are purged in seriesStart 2-3 coal mills when oil burner hits the rated firing rate of 15%Increase the SH outlet press. to 3.5 Mpa.Maintain the MS temp < 420 OC.Keep HRH steam temp around 480 OC.When the steam line blowing parameter reaches, open the temporary purging valve gradually and increase the fuel and feed water volume to sustain parameter stability.Establish the MS flow around 40% of the MCR (max.- 50%)Ensure CFR (K) should be 1.25 to 1.3 for MS and 1.05 to 1.1 for reheated steamBlow under this operating condition for 20 to 30 minGradually reduce coal firing and close the purge valve at 0.5MPa.Remove the target plate and check itBy estimation, the target may satisfy the requirements after 15 to 20 times in-series purgingWhen the steam line temp is above the sat. temp of MS press, close all drains, and Open the boiler MSSV fully.Maintain the press. through controlled firingInsert a reference targetCheck the CFRAllow running for 30-60 min Target Plate in steam Blowing Generally Stainless steel panelTarget plate set at the first, sixth, ninth, twelfth purging, thereafter set a target for each purging until purging results qualified.Width about 8% of the steam vent tubes inner diameter(ID) and length equal to the IDBrinell hardness < 90Steam velocity – 258 m/secTarget plates to be introduced just before steam blowing/light up—to be removed soon after blowing is completed. Online Target Plate Change Arrangement Debris Filter Steam Blowing Completion Criteria At least 2 continuous target plates should not have ORIFICE GRANULARITY on the target shall be no larger than 0.8 mm.CFR should be 1.25 to 1.4of an orifice, granularity shall not surpass 08 nos. Advantages of Steam Blowing Required less time for completion of the total processLess time required to normalize the system for final light-up to synchronizationThis reduces the reactionary forces on the temporary pipesStresses on the boiler system are lower Comparison between Puffing & Continuous Method of Steam Blowing PUFFING METHODCONTINUOUS METHOD More time required for complete steam blowing due to stage-wise blowing(8-10 days) More time required for stage wise temporary pipe erection and shifting of blowing device No mill required Quality of cleanliness is better than a continuous process Thermal shock is the driving force of cleaning More thermal stress on tube material and sudden loading on supports Repeated light-up and shutdown There is a time gap between the blows to make-up DM water System normalization time after steam blowing is more Silencer use is optional Less time required for completion (3-4 days) Less time required as only valves to be opened for different systems Minimum 02 nos. of mill required The quality of cleanliness is slightly less than Puffing. Steam velocity or Removal force is the driving force Less thermal stress on tube material Light-up only once at the beginning of the steam blowing DM water make-up to the system during steam blowing is a challenge System normalization time after steam blowing is less. Silencer use is compulsory. Difference Between Puffing and Continuous method of Steam Blowing Calculation of Steam Blowing Thrust force at the end of Pipe Exit The thrust force (F) of the jet at the pipe exit is estimated as