Estimating Abrasive Consumption Rates OMG!

Abrasive Consumption Rates in Blast Operations can be highly variable – and hard to estimate unless several conditions are known.  This Blog will attempt to highlight the factors that affect Abrasive Consumption Rates (ACR) – and provide suggested Rate ranges – based on those factors.

First, let’s assume that we have well balanced setup in our Blast Operation:

• We have around 25% more compressed air than is needed – given our setup.
• Our compressed air is sufficiently dry – with more than minimal Driers in the system.
• That our Nozzle is new – or not worn out.
• That our Hose lengths are minimized – both compressed Air Supply & Blast Hoses – with maximum & optimum ID’s for both Air and Blast hoses.
• We can maintain a minimum of 100 psi Nozzle pressure with the dried compressed air supply.
• Coal Slag Abrasive being used as a baseline media for comparison purposes.
• Most importantly, we have the Abrasive Metering Valve tuned to the optimum level – not too much Abrasive – and not little – leaned out to the point when we just start to lose Production.

Abrasive Consumption Rate (ACR) for new steel is a well-defined Rate – depending on a couple factors.

• For new steel – uncoated – we have a known, narrow range of Consumption Rates as follows. For tight mill scale (no rust – SSPC Rust Grade A):
• Coal Slag Fine 30×60: 3 to 5 lbs/sf
• Crushed Glass Fine 40×70: 3 to 5 lbs/sf
• Copper Slag – HI Performance 30×60: 2 to 4 lbs/sf
• Staurolite: 2 to 3 lbs/sf
• Garnet 30×60 or 80: 2 to 3 lbs/sf
• For new steel – uncoated – with complete Rust Bloom (SSPC Rust Grade C or D) – the Rust Bloom will allow for the lowest Consumption Rate regardless of type of media:
• Coal Slag Fine 30×60: 2 to 4 lbs/sf
• Crushed Glass Fine 40×70: 2 to 4 lbs/sf
• Copper Slag – HI Performance 30×60: 5 to 3 lbs/sf
• Staurolite: 5 to 3 lbs/sf
• Garnet 30×60 or 80: 5 to 3 lbs/sf

For steel that is coated, the Abrasive Consumption Range is much more variable – and will be dependent on the following:

• Type of coating – for example, from easiest removal to hardest – and estimated ACR:
  • Old, thin Alkyd Enamel system (approx 15 to 30% higher ACR than bare steel rate)
  • Old, thin Epoxy system (approx 20 to 40% higher ACR than bare steel rate)
  • Old, thin – less than 15 mils DFT – Epoxy / Urethane system (approx 20 to 40% higher ACR than bare steel rate)
  • Acrylic DTM system (more rubbery) – or any thermoplastic system (approx 40 to 100% higher ACR than bare steel rate)
  • Thick Epoxy system – greater than 20 mils DFT (approx 50 to 200% higher ACR than bare steel rate)
  • Thick Urethane Elastomeric system (e.g., Polibrid type or equal) – greater than 25 mils DFT (approx 200 to 400% higher ACR than bare steel rate)
  • Thermal spray systems (approx 200 to 500% higher ACR than bare steel rate)
  • Glass or fiber-reinforced Epoxy, Polyester or Vinyl Ester linings (approx 300 to 500% higher ACR than bare steel rate)

The reason for the variability of ACR for coated steel has to do with the physics and chemistry of various cured films.  Simply put, some paint films are more difficult to remove than others.

• Alkyd films cure by oxidation – and continue to oxidize throughout their lifetime – and as result become more brittle over time – thus more easily fractured and removed via Abrasive Blasting.
• Thin film Epoxy systems are more resilient and tighter films – and do not oxidize as readily – so are more difficult to remove.
• Epoxy / Urethane systems are even more stable and flexible than straight epoxy systems – and therefore require a bit higher ACR.
• Acrylics – and other thermoplastic films – tend to “fluidize” – or “liquify” when subjected to Abrasive Blasting due to the generated heat at point of impact – which some Blasters describe as the film “rolling up” when blasting – rather than fracturing. Thus, the elevated ACR range – as removal is slower.
• Thick Epoxy systems – > 20 mils DFT – are more tightly cross-linked, harder and more impact-resistant than systems above – and require more intense Blasting – thus raising the typical ACR.
• Thick Urethane Elastomerics – some of the most difficult to remove due tremendous cohesive strength the Elastomer – and the resilience of the cured film – where abrasives tend to “bounce off” of the film. Sometimes Waterjetting is recommended directed at the bond plane between Elastomer and steel substrate to remove in Elastomer in sheets.  In any case, the ACR range is significantly higher by comparison should dry blasting be employed.
• Thermal Spray – see Thick Urethane Elastomeric explanation above – only more difficult to remove.
• Glass or fiber-reinforced Epoxy, Polyester or Vinyl Ester linings – can be the most difficult to remove due to the physical characteristics of the cured systems. Extremely hard, abrasion resistant, cohesive and adherent make these films potentially the highest ACR for removal.

Please note that Staurolite – due to it’s sub-angular (rounded) shape – is NOT recommended for coating removal – only for bare steel – where it is a top-performer.

In conclusion, when estimating ACR – Abrasive Consumption Rate – for any project, information will be key – knowing the following will narrow down the anticipated ACR:

1. New Steel – mill scale, Rust Bloom or pitted?
2. If coated, then investigate to learn;
   1. Type of existing coating
   2. Film build
   3. Overall film integrity – flexible vs brittle, adhesion, friability, cohesive strength, etc.

Knowing these factors will allow for more accurate estimation of ACR – and Production Rate – and a more realistic and profitable project.

As always, please contact TCR Blast Abrasives with any questions or discussions.