9 points to differentiate Gas turbine and steam turbine power plant

Hola! power plant professionals, today we will discuss about one of the important topic which is important from interview's perspective. Because while facing interview for professionals who are having experience in both gas turbine operated power plant or combined cycle power plant and steam turbine operated power plant or thermal power plant  first question will be asked is How would you differentiate both from your work experience point of view ? So everyone is having his own observations but I will share by experience of mine,if you find anything to add please share.

Gas turbine or combined cycle power plant :-

1. First of all if you look theoretically gas turbine power plant is working on Brayton Cycle which is gas power cycle i.e. considering gas as a working fluid. 

2. Plant availability factor depends upon unplanned shutdowns and breakdowns in plant and which keeps machine idle for that time .This factor decides availability of machine throughout year and availability factor for Gas turbine power plant is more as compared to thermal power plant i.e. in the range of 80-99%. And it calculated as follows,

Plant availability factor			=	P - S- F
				P

Where,

          P = Total time considered in hours i.e. 8760 hrs for 1 year

          S = Scheduled outage for planned maintenance (hrs)

          F = Forced outage due to breakdown (hrs)

3. Overall cycle efficiency is 40-45% which is better than steam operated thermal power due to operation of duel cycle.

4. Auxiliary power consumption is in the range of 2-5% of total power generation which is comparatively lower than thermal power plant.

5. Heat rate of complete cycle i.e amount of heat required for generation of one unit of power is less as compared to thermal power plant.

6. Start up time of Gas turbine is less as compared to steam cycle power plant.Gas turbine will start within few minutes of time and ready to serve full MW load in short period of time. 

7. Temperature variation of working fluid inside gas turbine is more as combustion of gas takes place inside turbine. 

8. Initial investment for plant setup and time required for completion of erection and commissioning is less .

9. Ability   of  gas turbine is limited to handle multiple fuels, even slight change in fuel specification will cause drop in efficiency, increased maintenance and frequent inspection etc. It handles only gaseous or liquid fuels.

Steam turbine or thermal power plant :-

1. Steam turbine is working on Rankin cycle which is steam power cycle means considering steam as a working fluid.

2. Plant availability factor is less as compared to gas turbine because boiler is always under the threat of problems like tube leakage,clinker formation,fuel handling plant problem in rainy season,etc.And hence it comes to the range of 70-90%.

3. Overall cycle efficiency is less when it is compared with gas turbine.Major factors responsible for lower efficiency are heat loss in steam turbine condenser and wastage of heat from boiler flue gas. Cycle efficiency of steam turbine cycle is 30-36%.

4. Auxiliary power consumption is more in thermal power plants due to operation of power consuming fans,required to create draft inside boiler furnace to handle flue gases.Which is in the range of 9-11% of total power generated.

5. Heat rate of steam turbine is more as compared to gas turbine i.e. heat required per unit of generation is more.

6. Start up time for boiler and steam turbine is more as compared to gas turbine because it is very difficult to increase temperature by considering thermal expansion constraints in boiler and steam turbine.In steam turbine it will cause  high vibrations and uneven differential expansion.

7. Temperature variation in working fluid i.e steam is very less as compared to gas turbine as highly precise temperature controllers or attemperators are provided at boiler steam outlet.

8. Initial investment and time required from erection to operation phase is more as compared to gas turbine because in steam operated power plant boiler and fuel handling plant will take comparatively more time for erection and commissioning.

9. Fuel flexibility is more in utility Boilers because variety of fuels from coal, biomass, wood chips to waste sludge of plant etc could be burnt without any difficulty.

Hope it will help in understanding the power plant cycles from

Important-terms-every-power-plant engineer must know

How-small-ignorance could cause huge damage in APH of boiler

Important-stages-in-commissioning and stabilization of power plant

causes-of-boiler-tube-failure

Most-imporatant-questions-asked-in-boiler-operation-engineer-interview.

Important terms every power plant engineer must know about

Evaporation ratio (ER)

It is the ratio of amount of steam generated per unit amount of fuel burned. ER generally considered as a performance evaluation parameter for boiler. Evaporation ratio is depending upon quality of coal used.

Evaporation ratio			Amount of steam generated
		=
			Amount of Fuel burnt

Turbine heat rate

Heat rate is the term used in power plant to express Rankine cycle or thermal efficiency of power plant.Rankine cycles are modified as Reheat Rankine and Regenerative Rankine for increasing efficiency of cycle i.e, to decrease cycle heat rate,less is the heat rate better is the performance.Heat rate is expressed as Kcal/Kwh. Turbine heat rate is the ratio of total energy or enthalpy used in turbine (Kcal) divided by electrical power generated by turbine(Kwh).For extraction cum condensing turbine ,

turbine heat rate is classified as gross turbine heat rate and net turbine heat rate.

Gross turbine heat rate 

Gross turbine heat rate is defined as amount of heat added to water from feed water inlet to economiser up to steam outlet at Main steam stop valve per unit power output of turbine.

Net turbine heat rate

Net turbine heat rate is nearly similar to gross heat rate except in net turbine heat rate power consumption of boiler feed water pump is subtracted from the power output of generator.

Gross Plant heat rate

Gross plant heat rate is the amount of total energy added to boiler by burning coal per unit amount of  power generated by generator. Gross Plant heat rate considers only losses in steam generator and turbine.

Net plant heat rate

Net plant heat rate is same as Gross plant heat  rate except in net plant heat rate we consider amount of auxiliary power consumption of plant. Net plant heat rate considers losses and inefficiencies in auxiliary power consumption of plant.

% Auxiliary power consumption

Auxiliary power consumption is the amount of power consumed by various auxiliary equipments like fans,pump etc.in power plant for generating full capacity of power output from turbine. Generally value of percentage auxiliary power consumption should be 10-12 %.

Specific steam consumption

Amount of steam consumed by turbine per unit of power generated.

Specific fuel consumption

Specific fuel consumption is the amount of fuel consumed by boiler per unit of power generated

Blowdown percentage

Due to continues evaporation of water in boiler TDS level in water increases and to maintain proper amount of TDS level in boiler need to replace some amount of boiler water with fresh water.

Blowdown percentage			Feed water flow - steam generation
		=
			Feed water flow

Excess air percentage

Fuel requires air for combustion, the exact amount of air required for combustion is called as theoretical amount of air required for combustion but there may be possibility of not completing combustion in practical scenario, so for ensuring complete conversion of carbon in to carbon dioxide at the end of  combustion reaction some extra amount of air is supplied is known as Excess air .

Excess air			Oxygen in  flue gas
		=
			21   -   oxygen in flue gas

Where, 21 is the amount of oxygen present in air by weight.


Gross calorific value (GCV)

Amount of heat generated by complete combustion of fuel per kg of fuel burnt.It is expressed in terms of Kcal/kg for solid and liquid fuels and Kcal/m3 for gaseous fuels.



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questions-asked-in-boiler-operation-engineer-interview

How-small-ignorance-could-cause-fire-in-APH-of-boiler-and-preventive-measures

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Easily-understand-mechanisms-and-cause-of-boiler-tube-failure

How small ignorance could cause huge damage in APH of boiler

Hello!! all the power plant professionals,today we will discuss about causes of AHP fire and precautions need to be taken to avoid such mishaps and after that I will share one of the real incident of APH fire which I have witnessed then you will come to know that how it could be disastrous.Function of APH is to increase temperature of air supplied to boiler by recovering waste heat from flue gas to increase overall thermal efficiency of boiler.Installation of APH helps in following ways to boiler;

• Increase in thermal efficiency of boiler as most of the waste heat is absorbed by air from flue gas.
• Increase in temperature of air stabilises ignition and improves combustion efficiency of boiler.
• Decrease in fuel consumption and also make it possible to use low grade fuel.
• Helps in increase in steam generation capacity and gives stability during load fluctuations.

These are the advantages of APH but when things are going in wrong direction then it could be harmful to both man and machine.

Now causes of APH fire are as follows,

• The important reason for APH fire is continues accumulation of unburnt fuel fines in gaps of tubes or surrounding duct area;when that accumulated unburnt fuel gets favourable conditions for secondary combustion it starts burning in that area and causes APH fire.
• Maintaining low excess air supply for combustion in furnace; which is ultimately responsible for insufficient amount of Oxygen for combustion and resulting in improper combustion in furnace and results in carry over of unburnt fuel along with flue gas to 2^nd  pass duct and will accumulate and may catch fire.
• Due to air ingress from leakage in flue gas passing duct there will be sudden drop in temperature of flue gas and moisture in flue gas gets condensed and water droplets will forms and resulting in stagnant condition for  particles in the flue gas and will get accumulated and when suitable conditions occurs it may catch fire.
• Maintaining improper furnace pressure which will restrict  flue gas flow and causes accumulation of unburnt in APH and surrounding duct and may catch fire when it gets proper resources for burning.
• If ash handling system for APH is not working properly then hoppers will get full and it may catch fire if it gets.

To avoid APH fire precautions need to be taken are as follows;

• Maintain sufficient amount of excess air for complete combustion of fuel in furnace i.e.try to maintain proper O₂ at the time of operation of boiler.
• Find any leakage in duct and arrest them as soon as possible.
• Try to maintain proper negative furnace pressure for free flow of flue gas.
• Check ash handling systems on regular basis.

If suppose there will be fire in APH then ,How to identify it. There are several conditions which are pointing out towards APH fire while normal operation as follows;

• Rise in temperature of air at APH outlet than normal operating conditions.
• Sudden rise in temperature of flue gas after APH.
• Rise in temperature of flue gas at Super heater.
• Increase in opening of steam temperature controlling attemperator control valves.
• Rise in casing plate temperature from outside.May be checked with temp measuring gun.

Now we will discuss about one of such incident of APH fire which I have witnessed.

This incident I have faced when I was commissioning boiler.Before explaining  actual incident I will tell History in brief.So commissioning of boiler was in its last phase i.e steam blowing ,we were giving steam blow downs as per procedure by the gap of 2 hours with targate plate at its position.And finally at 6:00 pm after 77 steam blows we got clear targate plate upon which turbine personal and customer side manger agreed and signed protocols. So now we all have decided to stop boiler at 7:30 pm so that we can wrap up balanced erection work  as soon as possible to start it again for commercial steam generation.So boiler has stopped by shutting fuel feeding of and and told operator to keep all the fans in running condition till all the temperatures inside boiler comes down to around 50 ⁰ C and we left site at 8:00 pm. But in the name of hurry to leave plant operator stop all the fans at 9:00 pm and by mistake left SA fan in running condition and left plant . When next shift person came and he also hasn't checked that APH temp is rising.Now at around 11:45 pm when we reached plant and found that temperature of APH risen so high that APH tubes started melting and outer casing warped due to heat and protective aluminium sheet also melted.And in control room we found only SA fan running ;immediately I have started all the fans and keep them running for cooling of boiler.  

Melted outer Aluminium cladding sheet

Melted APH tubes clicked from menhole

In night we were not able to identify intensity  of damage done and APH outlet temperature found reached to 550 ⁰ C. And after cutting outer casing plate we found the real damage as shown below,

Damaged APH section of boiler

Almost 6000 no of tubes found damaged.After analysis and investigation we come to conclusion that this thing was happened due to accumulation of unburnt fuel in between gap of APH tubes. Amount of moisture in fuel was high because which fuel was not burning properly and carried over to APH section and accumulated over there. After stopping boiler slowly moisture in the fuel got veporized and dry fuel starts burning as all the fans were stopped before sufficient reduction in temperature of boiler. And natural draft created due to ID fan provided air for combustion and unburnt fuel catches fire.But fortunately half of the section saved due SA fan was running because APH was divided in two sections i.e. SA APH and PA APH .As SA fan was running air was carrying heat along with it and saved metal of tubes from getting overheated.As PA fan was not running PA APH tubes got overheated and melted.

Saved section of SA APH

So I am suggesting all the power plant professionals that be alert on small signals because it could cause sever damage afterwards.Follow all the instructions given above to prevent such incident to be happened in your plant.

Hope this article will help you all ,feel free to ask and if you have any suggestion always welcome.Please subscribe and follow blog as a motivation for more such important articles.

Important Stages in Commissioning and Stabilization of Power Plant

Hello!! All the power plant professionals .When we talk about any plant it generally has to go through three main phases throughout its lifespan i.e. mechanical Erection of plant, Commissioning and Operation and maintenance of plant. Normally we talk about operation and maintenance of plant but commissioning is also a very important phase as in this phase plant has to start first time, so we need to take special precautions before starting plant. There are various checklists, protocols and SOP’s need to follow in commissioning of plant. So today here we will discuss about various stages involved in commissioning and stabilisation of plant. Here  I have categorised different stages and sequence of performing. Important stages involved in commissioning are as follows,

1. Mechanical erection completion of plant.
2. Hydraulic test of pressure part
3. Duct leakage test on flue gas side
4. No load trail of motors
5. Trail run of rotating equipment
6. Refractory dry out
7. Alkali boil out
8. Safety valve floating
9. Steam Blowing
10. Commercial steam generation
11. Performance guarantee test

So now here we will discuss different stages one by one in detail in terms purpose, care to be taken and procedure in general.

1. Mechanical erection completion of plant

After given clearance from erection team regarding completion of erection, we need to inspect different things from commissioning point of view like quality of weld at tube joints and various channel sections, alignment of weld and water wall panel ,position and tension in various supports provided, room for expansion of tubes and steam line etc as per P&ID .After finding everything satisfying go for next step which is Hydraulic test.

            2. Hydraulic test of pressure part

Hydraulic test in a Boiler is carried out to check leakages of pressure parts and prove the strength of the Boiler at a pressure greater than the working pressure of the boiler, and is carried out at following stages:

•  On completion of erection.
•  On completion of repair of Boiler pressure parts.
•  On completion of Annual overhaul of pressure parts.
• At the request of Authority to full fill the statutory requirement.

Care to be taken

•  Boiler should be at room temperature condition.
•  Water used for hydraulic test must be as near as possible to the temperature of the boiler pressure parts temperature & if there is difference in temperature then it should not be more than 50 deg C.
•  Maintain quality of boiler water as per operation manual or maintain pH between 8.5 to 10.5 & Hydrazine to approximately 200 ppm.
•  Boiler must be hydro tested to 1.5 Times of design pressure only for first time test after completion of erection as per Regulatory requirement.
•  All valves provided on the drains and for instrument isolation shall be completely closed.
•  Vent valves shall be kept open to purge the air pocket while filling and to be closed after the air pockets are purged.
•  Before applying pressure, the unit shall be examined to see that all valves & gasket joints are tight and no leakage is observed.

Procedure

•  The pressure shall be gradually raised to the test pressure with the help of Hydraulic pump preferably 1-1.5 kg/cm2 per minute. The relief valve provided on the leak off line of the pump shall be operated to control the rate of pressure rising. After raising the boiler pressure to test value, the isolation valve at unit inlet shall be closed.
•  The unit shall be held under test pressure for maximum 30 minutes. Quick inspection for leakage/sweating shall be carried out during these 30 minutes. The pressure shall be then reduced to maximum allowable working pressure and maintained at that pressure while the unit is carefully examined for leakage. Depressurise it also with rate of 1-1.5 kg/cm2 per minute
•  Leaks through welds, castings, forging, plate, pipe and tubes excluding gasket joints are not acceptable. Seepage at the test pressure is permitted only at gasket joints.

           3. Leakage test of furnace or combustor and second pass duct

To make system leak proof, there is need of Leak test inside combustor, air and flue gas path after completion of all mechanical works, included sealing and fin welding. If not done there will be leakages of air and flue gas during operation and which costs in terms of  loss in efficiency of boiler, Load restriction , ID fan overload, cold air ingress.

Care to be taken

• Leakage testing of all ducting is preferably done with smoke.
•  Extra care should be taken while performing  Air pre-heater Leak test
•  Sealing of roof should be examined carefully.
•  Marking should be done at leakage prone area for attending it later.
•  Carry out repair with good engineering practice
•  All fins and seal welding completed as per drawing.
•  All openings such as manholes, SA nozzles, view holes & Instrument pockets are installed and sealed properly.
•  Grid nozzles area to be dummied with help of tape or tarpaulin sheet.
•  Volume to be tested is to be decided by site engineer, based on number of smoke generator machines available and accordingly areas are to be blanked with temporary arrangements.

Procedure

• For leak test first requirement is to seal the area where test will be done.
•  After sealing put smoke generator inside and start the machine.
•  Smoke will spread inside and make full of smoke.
•  If any fan or blower is available and testing area can be pressurized then start it and take adequate pressure.
•  After this check all the area from outside anywhere smoke coming. If not then OK, if any leakage found, repair it in proper manner and again do the leak test.
•  This process will be repeated until no leakage is ensured.
• For every segment, testing area will be decided by site team as per machines availability and sealing of how much area can be done at once.

          4. No load trail of motors

No load trail of motor is to be done after installing motor at position and without coupling keep it in running condition for 4 hrs and note down parameters on hourly basis to test its reliability before taking it in actual operation.

          5. Trail run of rotating equipments

 After completion of no load trail of motors then motors are coupled with respective rotating i.e. pump, fan etc. After coupling equipment stared at lowest minimum load possible and gradually taken to full load and again reduced to lowest possible load. While increasing load parameters like current, rpm, damper opening etc noted down to compare with factory provided data of equipment. This activity is performed to test reliability of whole equipment and after completion of test clearance is given for starting boiler for further activities. 

        6. Refractory dry out (RDO) test

A newly erected boiler, or one on which extensive furnace refractory repairs have been carried out will require to be dried out thoroughly before commissioning the boiler.  The procedure is to allow control drying out of wet brickwork and refractory to the lowest rate of heating possible for avoiding the separation or cracking. Thermal stresses will be produced if heating rate is too rapid. From this it follows that LONGER PERIOD OF LOW TEMPERATURE DRYING CAUSES MORE EVEN HEAT DISTRIBUTION RESULTING IN BETTER REFRACTORY DRY OUT.

If not done cracking or separation of refractory may occur. Pressure parts may face thermal shocks.

Care to be taken

In deciding initial heating temperature and its duration the following factors should be considered:

• Estimated moisture content of walls and refractory.
• Thickness of walls and refractory.
• The distance from heat source.
• Strict follow up of drying curve as per in operation manual.
•  Ensure the availability of adequate feed water as per recommended quality.
•  Quantity of fuel required to be estimated as per the capacity and type of boiler.
•  Ensure the readiness of draught system and required Instrumentation for boiler firing.

Procedure

• Fill boiler with specified water approximately 50 mm below of normal water level is showing in water gauge glass (approx. 1/3 of gauge glass).
•  Open start up vent valve initially 20% and then adjust after lit up as per operating condition
•  Start required drives (Fans, Pumps etc) for combustion and draught control.
•  Lit up the boiler. After fire is established, set air to maintain draft & combustion.
•  Increase slowly the heat input to boiler to raise the pressure & Soaking is to be given as per the heating curve.
•  At a pressure of approximately 2.1 Kg /cm2, the drum air release valves are to be shut.
•  Control the fires & gradually raise furnace temperature and maintain soaking time & temperature as per the heating curve during the activity. The variation in soaking temperature is allowed up to ± 10 deg C.
•  After completion of refractory dry out, stop fans.  Allow boiler to cool down naturally once the fire is extinguished.

         7. Alkali Boil Out

To remove oil, grease and other foreign particle a newly erected boiler, or one on which extensive repairs to pressure parts have been carried out, may have an oil or grease film on internal areas which is impossible to remove fully by manual cleaning. Since oil and grease have a very low rate of heat transfer, tubes contaminated by these, can be overheated when subjected to high temperatures. This will lead to blistering or burning of the tube metal with consequent risk of rupture. Chemical cleaning of boiler pressure parts is carried out to overcome the above problems. If not done then Oil, Grease has very low heat transfer coefficient, if not removed they can hindrance in local heat transfer thus tube failure may occur.

Care to be taken

• Chemicals used must never be poured in solid form, but at first to be dissolved in water and then poured in to the boiler system.
• Alkali boil out is carried in Two stages : Atmospheric boil out at low pressure for pre cleaning followed by pressure boil out at 75% of working pressure or 40 kg/cm2 whichever is less.
• The solution is formulated by dissolving the following proportionate quantities for Atmospheric boil out.

     0.05% Na2PO4.7H2O  (500 PPM disodium phosphate)

        0.1% Na3PO4.12H2O  (1000 PPM tri sodium phosphate)

                                                Surfactant (Washing powder) 10 to 20 PPM to reduce surface tension.

• The  atmospheric boil out is carried for 12 hours giving blow downs from all header and drum drains every two hours.
• The solution is formulated by dissolving the following proportionate quantities for pressure boil out

     0.05% Na2PO4.7H2O  (500 PPM disodium phosphate)

        0.05% Na3PO4.12H2O  (500 PPM tri sodium phosphate)

                      Surfactant (Washing powder) 10 to 20 PPM to reduce surface tension

Risk: if chemical poured and feed pump got failed or any heavy leakage occurred or any other interruption occurred, then you have to maintain furnace temperature more than 200⁰C in any case, otherwise chemical to be drained out.

•  Pressure boil out is declared complete when oil level in sample falls Below 5 ppm or after a minimum of 12 hours.

       Final inspection after chemical boil out: 

• After rinsing, drain the boiler completely; cut the inspection caps of all the bottom headers (as per engineering instruction).
• When boiler has cooled down open all manhole doors to allow inspection of all internal surfaces.
• Open the steam drum & water drum manholes, clean drum with wire brush and DM water and remove the sediments, accumulated dirt and sludge (if any).
• Examine all internal baffles and check the tightness of all fittings, also the internal pipe work etc.
• After completion of the rinsing & cleaning of all bottom headers, inspect all bottom headers for any unwanted particle with help of light/torch and clearance can be given for end cap welding.
• If water gauge glass has got fouled in boiling out, these must be dismantled and cleaned as per maker’s instructions.
• Re-weld the end caps on headers as per the recommended procedures.
• Examine all external heating surfaces and gas passes of boiler for excessive soot deposits resulting from low firing rates employed in boiling out. Clean manually the soot deposits.

       Care to be taken 

•       Required temperature should be maintained through out the process at all stages.
•       If in any case interruption occurred and activity can not be carried out then acid cleaning circuit to be flushed properly with DM water after chemical draining immediately.
•       Its effluent should be drained at proper place like effluent pond or drain trench.
•       During acid cleaning upto 1^st stage passivation it should be ensured that the system always remained filled with solution, if there is any partial draining required simultaneously DM water should be filled up, so that any open air cannot be contact with boiler tube surface.
•       Monitor the return lines temperature by temperature gauge to ensure the flow through each circuit.

              8. Safety valve floating

      

       Setting the safety valves to the designed set pressures before allowing the boiler to go for commercial steaming.

      Set Pressure

      This is the pressure at which the valve begins to lift from the valve seat.

      Full lift pressure

      This is the pressure at which the valve reaches its fully open position. 

      Closing Pressure

      Closing pressure is the pressure at which the valve reseats.

      

      Blow down %

      The blow down is the difference between the set pressure and the closing pressure expressed as a percentage of the set pressure e.g.

      Blow down % = Set pressure-Closing pressure x100

                                              Set pressure

      And is usually in order of 2 ½   to 4 ½ .

      

      Sequence of setting  

       Drum 1-  Drum 2 - Superheater 

    

     

      Care to be taken

• Safety valve exhaust pipe support to be checked
• Silencer support to be checked
•  Safety valve flange bolts  proper tightening with metallic gasket
•  Drip pan drain line connection
•  safety valve body vent and drain to be extended upto safe zone
• Drip pan pipe peripheral gap checking for expansion purpose
•  Drip pan top cover plate to be kept free
• Safety valve cold assembly to be checked
• Prior to floating boiler maximum pressure to be raised to check thermal expansion in all area of Boiler.
• Hand popping to be done before safety valve floating
• Check Start up vent control valve to be smooth in operation for safety valve floating. 

      Setting the safety Valves

  

      This operation involves increasing and decreasing heat input to the boiler, to raise and lower the steam pressure. The set pressure and closing pressure of each valve is checked and adjusted in turn from the highest-pressure valve first to the lowest pressure last.

• Steam flow through super heater must be maintained and controlled at an   adequate rate through operation of super heater drain / startup vent valve
•  Raise steam pressure slowly to set pressure for first safety valve.
•  Raise the boiler pressure upto set pressure/full lift pressure. When the valve pops, cut off fire and open start up vent valve. Restore drum level.
•  When the sound of escaping steam is heard note pressure reading on calibrated gauge. Note pressure again when full lift point is reached.
•  Allow pressure to fall and note pressure at which safety valve resets (Closing pressure).
• The set pressure is adjusted by either tightening or loosening the adjusting nut. Tightening of the nut increases the set pressure and vice versa.
•  If noted pressures are not as specified, then adjustments must be made and the process of checking repeated until correct reading is obtained.
•  During the test a close watch must be kept on drum water level, as steam will     be lost at each lift of the safety valves as well as through the continuous discharge from the super heater drain/startup vent.  Control the feed to   compensate for these losses.
•  Repeat test for each safety valve in descending order of set pressures. 

The valve, which was already set, should be left ungaged.

 image source:google                                    Safety valve

      9. Steam Blowing

       The Purpose of steam blowing is to de-scale and remove foreign materials from steam pipelines leading to turbine in order to avoid damage to turbine blades from such material in the course of normal operation. If not done then Damage to turbine blades from such material in the course of normal operation.

Principle

• Steam Blowing is carried out by Puffing Method for main steam line to Turbine. 
•  Steam Blowing is carried out by Continuous blow Method for low and medium pressure lines to process plants/ Deaerator.
•  Dislodge rust/scales from pipe work by thermal shocks.
•  High momentum of expanding steam in the pipe work purges out the loosened material.
•  It is required to create a higher momentum during steam blowing than possible during operation of the unit. This is applicable for all steam piping leading to turbine.
•  Care to be taken to terminate the discharge to a safer place
•  Steam blowing completion criteria to be discussed & decided as per the application of steam.  

            Care to be taken

•       Steam blowing temporary arrangement with MOV operated sacrificing valve and target plate assembly to be erected with proper support.

•       Main Steam line supports to be checked.

•       All drain lines to be checked for not choked and clear during line hydro test

•       Attemperator control valve to be made ready

•      Sacrificing valve ,MSSV and its bypass operation to be checked from DCS.

•   Steam blowing procedure and target plate acceptance criteria to be finalized with all concerned.

•       Feed pump and feed water control station should be ready for normal operation

                    Drum level and pressure transmitters to be properly calibrated.

Steam Blowing by Puffing method:

• Lit up Boiler as per procedure and raise the pressure upto 15 kg/cm2.First blow will be given at 15 kg/cm2 by opening sacrificing valve as quick as possible and after completion of blow check the blowing line and its support. 2-3 blows initially will be given on 15-20 kg/cm2. After these initial blows raise the pressure upto 75% working pressure or 40 kg/cm2 whichever maximum.
•  Maintain Drum level to 50 % of gauge glass & then open gradually the Main Steam Stop Valve fully and charge the line to the inlet of the closed sacrificing valve. Minimize the firing rate to maintain the drum level. The super heater drain should now be closed.
•  Open the sacrificing valve as fast as possible and keep it open till the boiler pressure falls down to 40 % of the working pressure or a pressure drop of 10 kg/cm2 whichever is lower . Now close the sacrificing valve. Once the sacrificing valve close completely, close Main steam stop valve. Open the drain line valves to depressurize the line. After depressurizing the line, open the sacrificing valve fully to cool down the line.
•  Maintain at least 2 hour time gap between two consecutive steam blows.
•  Repeat the above steam blowing exercise several times as long as you could visually observe the steam blowing is clear. Now, isolate MSSV and insert soft metal target plate in the holder provided in temporary piping. Increase Boiler Pressure up to 75 % of its working Pressure, limited to a maximum of 40 kg/cm2. Repeat the blowing one time with the target plate.  Close MSSV and remove target plate. If target plate is not clean, repeat steam-blowing cycle for 3 to 4 times without target plate. Then fix target plate in its predetermined position and then again give steam blows till the target plate is found clear. Target plate will be checked as per the acceptance criteria and upon acceptance, the steam blowing is said to be completed. 

         Steam Blowing by continuous method:

•  Lit up Boiler as per procedure and raise the pressure upto 15 kg/cm2.First blow will be given at 15 kg/cm2 by opening sacrificing valve as quick as possible and after blow check the blowing line and its support. 2-3 blows initially will be given on 15-20 kg/cm2. After these initial blows raise the pressure upto 75% working pressure or 40 kg/cm2 whichever maximum.
•  Maintain Drum level to 55 % of gauge glass & then open gradually the Main Steam Stop valve fully and charge the line to the inlet of the line to be blown.
•  Open gradually the valve at the inlet of the line to be blown keeping a close watch on drum level and firing rate to maintain the pressure. Open the valve till the flow is achieved equivalent to the design flow of this line. Boiler main steam flow meter can be utilized to monitor the flow. If the flow is below 30% MCR of the boiler flow, keep the start up vent open to maintain at least 30% flow thru the boiler. Keep this condition for about ONE hour and then close the valve for allowing the line to cool down.
•  Maintain at least 2 hour time gap between two consecutive steam blows.
•  Repeat the above steam blowing exercise several times as long as you could visually observe the steam blowing is clear. Now, isolate MSSV and insert soft metal target plate in the holder provided for temporary piping. Increase Boiler Pressure up to 75 % of its working Pressure, limited to a maximum of 40 kg/cm2. Repeat the blowing one time with the target plate.  Close MSSV and remove target plate. If target plate is not clean, repeat steam-blowing cycle for 3 to 4 times without target plate. Then fix target plate in its predetermined position and then again give steam blows till the target plate is found clear. Target plate will be checked as per the acceptance criteria and upon acceptance, the steam blowing is said to be completed.

             

             10. Commercial steam generation:

              After steam blowing completed and target plate clearance given by turbine or customer personal then we are ready to generate steam on commercial basis as per requirement.

             11. Performance guarantee test

• PG test is not only limited to the efficiency test of the boiler, PG test is type of protocol regarding various parameters decided at the time of contract finalization.
• PG test Procedure is prepared as per contract and the Code Requirement.
• Have a pre test MOM with Client. Jointly calibrate the instruments
• Conduct the test at site i.e. run plant at full capacity for 4 hrs continuously and noting down parameters after every half an hour.
• Signed log sheets after completion of test.
• Need to prepare a report and submit to client along with a contractual letter

         After completion of PG test satisfactorily ,plant is completely handed over to client and he      is responsible for further operation of plant.

Hope we have discussed enough on commissioning of power plant. And for any quires feel free to contact and suggestions always welcome.