Boiler operation Engineer: 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,

Mechanical erection completion of plant.
Hydraulic test of pressure part
Duct leakage test on flue gas side
No load trail of motors
Trail run of rotating equipment
Refractory dry out
Alkali boil out
Safety valve floating
Steam Blowing
Commercial steam generation
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