Steam Blowing of Boiler & Pipings
PREPARED BY U .N Singh Ravi Kumar
In spite of doing pre-operational chemical cleaning of the circulation system. certain quantity of debris in the form of scales, loose materials, weld spatter, etc will remain in the boiler tubes & the pipe lines especially in the Superheater coils, Reheater coils, Main steam line, Cold and Hot Reheat Lines.
Subsequent steam blowing completes the task of providing a cleaner system for passing steam into the turbine. In other words we attain steam of required quality ,standard by way of chemical cleaning, steam blowing and steam dumping method.
Failure to remove debris may result in increased turbine erosion, blade damage and rapid deterioration of turbine efficiency.
Why Steam Blowing Required ?
Types of Steam Blowing
Puffing Method Continuous Blowing method
Types of Steam BlowingPuffing Method
The system (surfaces) to be cleaned, is first pressurized by steam up to 40-50 kg/cm2 pressure and then releasing of this pressure upto 20-25 kg/cm2 into the atmosphere suddenly causes the cleaning of system internal surfaces, due to high kinetic velocity attained by steam.
Continuous Blowing Method
The surface to be clean is continuously blown by superheated steam for 20-30 min at 50 kg/cm2 with degree of superheat 50 0C parameters with cooling down period of 1.5 hrs.
Circuits Covered with Steam Blowing(Puffing Method)
Drum outlet to Final Super-heater header.
Main steam line up to HP Turbine inlet.
Cold Reheat line up to Re-heater inlet.
Re-heater Coils & Headers.
Hot Reheat Line up to IP turbine inlet.
HP bypass Lines (Partial).
All Auxiliary steam lines.
STAGES & CIRCUITS OF SYSTEM IN PUFFING METHOD
Two Stage Cleaning
1.(a) Drum outlet up to Emergency stop valve (includes Superheater, Main Steam Lines upto H.P Turbine inlet, Cold Reheat Lines)(b) Drum outlet,HP Bypass to CRH .
2.(a) 1b + HRH up to IPESV(Intermediate Pressure Emergency Stop
Valve).(b) 1b + HRH line + LP Bypass .
(c) Main steam line to HTPRDS.(High temp. Pressure Reducing &
Desuperheating System) (d) CRH line to APRDS.
Steam Blowing Process
The boiler is to be lighted up in a normal manner, following start up procedure from Boiler O&M manual.
Boiler drum pressure raised to 40 – 50 kg/sq.cm. Water level in the drum kept at the lowest port
(visible limit) before the start of each steam blow off.
The lines to be blown is charged slowly with steam using the Main Stop Valve bypass lines and warmed up for 30 mins, keeping all the drains in open condition to avoid hammering.
Test Blow at 15 kg/ sq.cm pressure up to 25 kg/ sq.cm to be given first before start of each stage of steam blowing to ensure the system healthiness.
Steam Blowing Process
Close this valve at a pressure of 20-25 kg/sq. cm. Also ensure that, fall in the drum metal temp. is not going to exceeds above 40-45 deg C to avoid thermal stress in the metal.
Limit number of blows per day to 8 with an interval of 1 ½ hours for cooling in addition to overnight cooling.
(Typical Steam blowing starting time at
0600 hrs and end time at 2200hrs)
PRECAUTIONS FOR STEAM BLOWING
Ensure the proper supports of the temporary pipe to take care of all thrust during blowing.
Ensure the direction of steam vent pipe in upward position. This is done to protect against the material impingement on the humans during blowing. If possible entire area of steam blowing is to be cordoned off to prevent any possible fallout of material on Human’s head. Hooter will also be use before the start of the steam blows.
If possible, leave the flange welding of the Control valve with the H.P turbine to prevent any ingress of steam.
Try to declare the steam blowing of any section after the overnight cooling of the pipelines to enhance the system cleanliness.
Completion Criterion for Steam Blowing
Targets plates used initially near pipelines outlet can be of mild steel material but finished stainless steel T.P must be used for declaring the completion of steam blowing.
The final result of the blowing operations can be judged by the absolute number of pittings found on the target plate in the central zone. The piping is considered clean, if there are not more than 5 pittings and shall not have any deformed edges. Besides this, there shall be no pitting in the rim zone.
If Disturbance Factor (D.F) is used for completion criteria,
then calculation for the same must be done for each circuit & its value must be greater than 1.
(D.F is the ratio of the steam flow during steam blowing operation to the Steam flow at TMCR.)
Steam Blowing Image