Preparation to Blast Cleaning for Polyurethane Coating   

Prior to blast cleaning, the each pipe shall be inspected by the followings;

• Pipe identification control
  • Pipe shall be accepted as PE coated pipe condition

• The external surface control
  • The pipe internal surface shall be inspected and cleaned to remove all traces of oil, grease and loose deposits. Any visible trace of oil or grease shall be removed using an appropriate non-oily and non-toxic product.

• If pipes have been in contact with marine or industrial atmosphere, they shall be cleaned before abrasive blasting to remove any contamination. Cleaning shall be carried out using an adapted process.

Surface Preparation :

The pipe surface shall be grit/shot blasted to ensure the SA 2 1/2 degree of cleanliness.

The finished pipe surface shall be completely removed all traces of rust and shall be provided a surface average roughness (R_Z) of  50 to 100 microns.

Pipes shall be heated to remove any surface moisture and to ensure more than 3^ºC above the ambient dew point. 

The ambient relative humidity shall not exceed 85 % as measured using a hydrometer and digital thermometer.

Immediately after abrasive blasting, all dust and traces of abrasives shall be removed from the pipe surface by brushing and vacuum cleaning.

Each cleaned pipe shall be inspected for surface defects. Any defect shall be removed.

Ambient Control :

The ambient temperature and ambient relative humidity shall be measured using 

a hydrometer  and digital thermometer.

The ambient relative humidity and ambient temperature shall be recorded to the shift report.

The grit-blasting is not permitted when the ambient relative humidity exceeds 85 %.

The metal temperature shall be 3ºC above the ambient dew point temperature.

Any pipe which has not been coated before degradation of its surface preparation shall be completely re-blasted before coating.

Surface Test and Inspection :

The below tests and inspections shall be performed on blast cleaned pipe:

• Surface profile :

Two methods can be used in measuring surface roughness measurements.          

                    Method 1- By visual comparison gauge

                    Method 2- By a digital profile measuring device.

       Standard grid and shot comparator is compared to the pipe surface. The coinciding segment of the comparator shall be decided.

Surface roughness measured by a digital profile measurement device. Surface roughness shall comply with Rz=50-100 µm

The calibration of equipment shall be performed by reference blocks.               

• Surface cleaning degree :

Standard comparison pictures are used to decide the degree of the surface cleanliness. Surface quality degree shall be Sa 2 ½

• Dust  control :

Residual dust shall comply with acceptable categories of ISO 8502-3 and

specification. 

• Visual inspection :

The surface defects which appearance effect of grit-blasting or other reason shall be checked by visual inspection. In case of any grinding or mechanical reworks, the wall thickness shall be ultrasonically examined and remaining wall thickness shall be met minimum specified requirements.

Pipe external surface shall be cleaned of all dust and foreign matter using clean dry compressed air or vacuum cleaning. Residual dust shall comply with categories of ISO 8502-3 and specification.

COATING APPLICATION

If it shall use to free solvent polyurethane, polyurethane will be approved for potable water.

The solvent-free polyurethane is paint to two components, which cures to a coating with excellent corrosion resistance to fresh water, sea water and underground storage tanks and to abrasion. It is applicable in thick coats by standard heavy duty airless spray equipment, harmless to grain cargo. It use as a lining in potable water tanks and pipelines, where solvent-free materials are required.

If it use for potable water, the surface is preparing to abrasive blasting to min. Sa 2½ with a surface profile corresponding to ISO Comparator Rough Medium (G) and Rz 40-70 blasting profile.  Oil and grease must be removed with suitable detergent, salts and other contaminants by (high pressure) fresh water hosing prior to blasting. After blasting, clean the surface carefully from abrasives and dust.

On old steel surfaces having been exposed to salt water, excessive amounts of salt residues in pitting may call for dry abrasive blasting, high pressure fresh water hosing, drying, and finally, dry abrasive blasting again. Alternatively, water jetting may be used provided the steel surface has already the surface profile as described above.  

Application shall be done with twin feed hot airless spray equipment complying with the instructions recommended and given by the paint manufacturer and shall be done in a manner to prevent sags and runs and to provide adequate thickness in crevices and angles. No thinner should be added. Pot life is 1 min. at 60ºC. Pumping viscosity is achieved at 50-70ºC. The temperature in the mixing unit must be between 65ºC and 75ºC. Nozzle pressure is 15 MPa and nozzle orifice is approx. 0,58-0,81 mm depending on required production speed and DFT

There are two methods as follows.

Fixed boom system consists of :

Fixed boom and pipe cart for linear movement and pipe turning roll on pipe cart for circumferential movement of pipe. The combination of linear and circumferential movements gives spiral movement of pipes.     

Movable boom system consists of :

Boom/boom carriage and pipe turning roll unit. The combination of linear movement of boom and circumferential movement of pipe gives relatively spiral movement for spraying.

The coating shall be free of pinholes, skipped areas, roughness, blistering, cracking and lamination.

The pipes shall wait on waiting bench according to instructions recommended of paint manufacturer for curing.

Cleaning of the spray equipment :

The mixed material will become insoluble within a few minutes after mixing 60ºC. The parts of the spraying equipment containing mixed base and hardener must be cleaned immediately after completion of the job or during any interruption.

Polyurethane Coating Test  :

Surface Cleanliness Comparison

Surface Profile (Roughness)

Dust Contamination

Coating Thickness

Shore Hardness ‘D’

Pinhole Test

Impact Resistance

Pull-Off Adhesion

Manual Holiday Test