Steamblowing

Purpose of steam blowing

Cleaning and removing the sands, welding slag, high temperature oxide and corrosions inside the main steam pipes and over-heater pipes of the boiler, which have been produced during manufacture, transportation,storage and installation. Prevent pipe explosion of the over-heater, damage of the turbine blades,and other serious accidents. Provide qualified steam for initiating the turbine and ensuring the safety initiation and operation.

Acceptance criteria

The cleaning is successfully completed when the target plate examination fulfills the
following criteria:
No impacts > 0.8 mm in an area of 2500 mm²
Max. 2 impacts >0.4 mm in an area of 2500 mm²
Max.10 impacts >0.2 mm in an area of 2500 mm²
Impacts < 0.2 mm² are well dispersed and nowhere present on concentration.

Diameter indicates maximum corner to corner measurement

STEAM BLOWING

PRINCIPLE

         The activity consist of blow-up the line with steam, discharging to the atmosphere through temporary spools and, if possible, temporary silencers to reduce the noise into acceptable range. Steam blowing utilizes the heat effects and kinetic energy of steam flow.  Heating, blowing and cooling down of the pipe concerned are repeated during steam blowing, this cycle of pipe expansion and contraction together with the flow of steam removes adhering rust, weld slag, spatter and mill scale.

          The results of steam blowing are more effective than the use of kinetic energy only as in air blowing. To obtain good result of cleanness, the steam kinetic energy on internal of steam lines must be higher than that we have during normal operation.

                                                  K =     (m2xV) SB

                                                            (m2xV) MCL

where:

k = cleaning factor

m= steam flow (kg/s)

V= steam specific volume (m3/kg)

SB = Steam Blowing condition

MCL = Max Continuous Load

          For good results during the blow up the factor K must be > 1,25

         The execution of steam blowing proceeds in the sequential order of steam supply common header, unit header, sub-header and individual branch lines.

PRECAUTION

         The following precaution is observed while steam blowing:

1)         Before conducting any steam blowing activity, the following matters are notified on in order to prevent boiler operational upsets during steam blowing caused by BFW consumption and steam demand variations.

- Steam blowing schedule

- Approximate steam consumption and duration

- Actual opening and closing of the steam valve for each steam blow

During steam blowing, it maintains close communications between the user and producer.

2)         Steam traps shall be bypassed, or removed where possible, at first while the upstream connecting lines are steam blown.  As soon as the upstream lines are confirmed clean, the steam traps shall be put into service and the function shall be inspected.

3)         Check pipes are free for the expansion and have sufficient anchorage up to the blow-off point.

4)         Sacrificial valve, such as knife valve or butterfly valve to allow quick opening,  is installed at the start end of the lines, and restriction orifice is installed downstream of such valve if required, in order to alleviate the erosion on the valve.

5)         Steam and debris shall be blown to a safe location.

Temporary lead pipes, vents etc. shall be installed at blow-off points as required to direct steam and debris to safe areas.

6)         The blow off area shall be cordoned off in compliance with site safety regulations.

7)         Exposed piping shall be checked for non-contact with combustible materials such as scaffold boards etc. to prevent the possible outbreak of fire.

8)         Before conducting steam blowing, the pipe is slowly warmed up in order to prevent the occurrence of steam hammer.

9)         In order to ensure effective steam blowing, heating, blowing and cooling shall be cycled by time or temperature monitors.

10)      When the steam line is in the cool-down step or hot condensate remains, the blow-off end must be kept open to prevent a vacuum formation inside the pipe.

11)      Once steam blowing has been successfully completed, the line instrument and line equipment/accessories which are removed prior to steam blowing shall be reinstated and placed in service or otherwise protected to prevent the re-occurrence of rust.

DETAILS FOR IMPLEMENTATION

Planning

(1)      Steam blowing shall be carried out in the sequence from upstream to downstream as follows;

Common Header → Unit Header → Sub header → Branch Lines.

After the steam blow of upstream is completed, the downstream can be started steam blowing.

(2)      Steam blowing route shall be determined and shown on a marked up P&ID.

                  The following items shall be indicated:

                  (a)        Steam blow-off points and its timing for all blows.

                  (b) Major valve positions.

                  (c) Temporary valves, spools, lead pipes, etc.

Preparation

1)   Remove the following and upon completion reinstall:

                  (a)         Safety valves

                  (b)         Check valves

                  (c)  Rupture disks

                  (d) Orifice plates of flow meter elements

                  (e)         In-line mixers/strainers

                  (f)  Spargers

                  (g)         Spray nozzles

                  (h) Control valves except ball/gate type

                  (i)  Self-actuated control valves except ball/gate type

                  (j)  Emergency shut off valves except ball/gate type

               (k)    All other items which could be damaged by the ingress of dirt or restrict the blow path

               (l) Steam traps, where possible

                  Note:  The above equipment/instrument/other accessories, where steam blowing has been finished, can be reinstalled and used for steam blowing of their downstream pipes as required.

2)   All steam traps on lines being blown shall be bypassed at first.

3)   All steam header branch lines are closed when blowing the main header.

4)   All pipe supports related are checked and confirmed its condition.

5)   Target plates is installed and checked its position (if required).

6)   Prior to the commencement of steam blowing, the route shall be checked by Company and Contractor personnel and confirmed at site.

Execution

         Lines for steam blowing are classified into the following groups, dependant on the requirements of the steam user:

         (1)    Lines equipped with a target plate.

         (2)    Lines where repeated blowing to be applied.

         (3)    Lines where once-through steam blow to be applied.

         Hereinafter, outlined procedures are given for the above classifications.

Lines Equipped with a Target Plate

         This method shall be applied to steam turbine inlet line.

         (1)    Warming up

                  Warming up operation shall be carried out by gradually opening the supply valve (initially using the bypass valves) and draining all condensate from the line until dry steam is issuing from the blow point.

         (2)    Blowing

                  Steam blowing shall be done for a period long enough to ensure that dislodged mill scale etc. has sufficient time to exit the pipe.

         (3)    Cooling down

                  Cooling down operation shall be carried out by natural or forced means dependent upon schedule requirements.

         (4)    Repetition

                  Above procedures, (1) ~ (3), shall be repeated three times without target.

         (5)    Target plate installation

                  A target plate shall be installed at the blow-off end immediately after the last steam blowing step.

         (6)    Final blow or Test blow

                  Final blowing shall be done with target plate for 10 minutes after warming up.

         (7)    Target plate inspection

                  Target plate shall be removed and inspected after the final blow.

Steam blowing shall be repeated until the results as per the target blows are acceptable and a certificate of acceptance is signed by the Company.

Lines where Repeated Blowing to be Applied

         The same procedure as described in above (1) through (4) is applicable. Implementation of steam blowing for these lines will be in the order of 10-minute blows to be repeated three times.  However, blowing duration and repetition may be considerably changed according to actual performance.  This method shall be applied to lines in the following service:

         -   Heat Exchanger

         -   Steam Ejector

         -   Atomizer

         -   Desuperheater

         -   Control Valve

         -   Column/Drum/Tank

         -   Sparger

         -   Heater for Tempered Water

         -   Jacket/Coil

         -   Exhaust Steam Lines of steam turbines

         -   Snuffing Steam

         -   Exhaust Steam Line

Lines where Once Through Steam Blowing to be Applied

         Steam blowing is carried out only once.  This method shall be applied to:

         -   Steam Pipe (Header Manifold) for Steam Trace

         -   Hose Station

ACCEPTANCE CRITERIA

Acceptance criteria for steam blowing also differ depend on the steam user classification in previous paragraph.

Lines Equipped with a Target Plate

         A copper or aluminum plate shall be brought into contact with blown-out steam at right angles and the number and degree of flaws formed on the plate due to the impact of foreign materials shall be observed.

         Acceptance criteria for these lines shall be specified by the following or by equipment vendor specification: -

         (1)    The number of flaws observed shall not exceed five per square centimeter.

         (2)    The size of a flaw shall not exceed 0.5 mm.

         Light pot-marks may be acceptable provided that there shall be NO RAISED METAL on any penetration. Steam blowing is deemed complete when the above criteria have been satisfied.

Lines where Repeated Blowing will be applied

         Steam blowing is deemed complete when no dirt or debris is witnessed at the blow-off point.

Lines where Once Through Steam Blowing will be applied

         Steam blowing is deemed complete when a single steam blowing cycle has been carried out.

POST WORK

(1) Pipes concerned are re-installed as per P&ID immediately after steam blowing has been completed or otherwise protected from corrosion and dirt ingress.

(2) All completed blows are documented and recorded on P&ID's.  Target plates are to be retained for evidence.

(3) A reinstatement work through request is conducted.

 

NOISE

During steam blowing noise is one of major HSE concern. The noise produced by a steam blow out point can to be in the region of 130-150 dB PWL. The main concerns coming from noise are as follows:

·      Possibility of hearing damage due to noise received by workers closed to a steam discharging point

·      Noise received at the nearby facilities

The following actions can be taken to avoid the hazards due to noise:

·     Noise source can be barricaded or cordoned off. Minimum distance is given by the noise calculation. In these areas access shall be restricted, regardless of whether the hearing protection is worn.

·     All the people working in the area subject to steam blowing shall use  hear protection equipment such as ear plugs and mufflers (during the steam blowing)

·     Activity can be performed during night time or when there is no personnel working on site (i.e. Sunday)

·      Noise produced can be limited by the use of silencers.

A prediction of the expected sound generated during steam blowing is made here below using the following formulas:

PWL=10 log (1/2 MC²η) +120

While the sound pressure level at a distance r from the source is calculated using this formula:

SPL = PWL – 10 log (4*π*r²)

Where:

SPL :         Sound pressure level

PWL          :         Sound Power Level

r     :         distance from the noise source

M    :         mass flow rate (kg/sec)

C    :         speed of sound of the chocked gas=kRT/Mw

k     :         ratio of specific heats (1.26 for steam)

R    :         Universal gas constant 8.3 *10³

T    :         Temperature (^oK)

Mw  :         Molecular weight

η    :         Acoustic efficiency (assumed 4*10^-3)

The calculations to be made based on noise limit 115 dB (A) (See API EA 7301), and the results give the radius of area where access shall be avoided during the steam blowing. Based on this calculations, we can decided whether to use silencer or not.