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“How the Right Pot Choice and System Set Up Can Improve Blasting Efficiency”
By David J BarnesElcometer LimitedManchester, UK
Notice: This paper was presented by the author(s) or assigned speakers at the Coatings+ 2020 conference as indicated above. SSPC: The Society for Protective Coatings (“SSPC”) has a worldwide, royalty-free, fully paid up, perpetual, and irrevocable limited license (with the right to sublicense) to do any and all of the following: Publish this paper in the official proceedings for the conference; Record the related presentation on film, tape, disk or other forms of media for sale; Publish the paper or presentation in the Journal of Protective Coatings and Linings; SSPC reserves the right of first publication of the paper or presentation; Distribute printed copies of your presentation on-site to meeting attendees.
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Presented at Coatings+ 2020February 3–February 6, 2020
Long Beach, CA
February 3– 6, 2020 | Long Beach, CA+
HOW THE RIGHT POT CHOICE AND SYSTEM SET UP CAN IMPROVE BLASTING
EFFICIENCY
David J Barnes
Elcometer Limited
Manchester, UK
Abstract
Shot and grit blasting has been around for many years, but are we getting the most from
our systems? This paper looks at the various adjustable parameters that can and do affect the
productivity (yardage), media usage and effectiveness of a blasting operation.
Air flow, pressure and nozzle size all contribute to the efficiency and yardage achievable
by a blast pot. Pot and valve design will contribute both positively and negatively to these
variables but correct set up is imperative for efficient blasting.
Improvement in pot and valve designs can lead to huge gains for the blaster and with some
simple, logical adjustment and/or upgrades the changes recommended here will help you get
the most from your pot.
Introduction
While most people consider shot and sand blasting to be a relatively modern concept, the
first shot blasting machine was actually patented in 1870 by Benjamin Tilghman to clean up
painted and rusted surfaces before making use of the item again. Then, in 1904 Thomas
Pangborn further developed Tilghman’s invention to include compressed air alongside sand
blasting to deep clean metal products. The system operates by propelling the blasting
material, typically metal shot or grit, using either a centrifugal wheel or, more commonly on
portable blast machines, compressed air.
Commonly called sandblasting, abrasive blasting is used principally in our industry to
remove existing coatings and to provide a key for the subsequent re-coating of a metal
surface for protection purposes.
Sand is not used in most countries having been banned in the 1960’s due to the risk of
silicosis in those people in the vicinity of a blasting operation.
The blast media, having removed the existing coating, creates a profile in the metal
surface which increases the surface area of the surface providing a better adhesion of the
applied coating.
A typical blast system consists of a diesel powered air compressor and a blast pot. The
blast pot is filled with the chosen blast media and is pressurised by the compressed air
delivered by the compressor. The media exits the pot and mixes with a flow of pressurised air
to be delivered to the blast nozzle at high speed.
Fig 1 A typical blast set up
The choice of media, the ratio of media to air in the air stream and the air pressure
delivered at the nozzle are major contributors to the efficiency of the blasting operation in
this paper we will discuss the results of some testing we carried out to compare blast pots
from several manufacturers to assess the potential efficiency of the blasting system.
The most crucial element in a blasting system is the pressure that can be delivered at the
nozzle. It has been shown that a loss of 0.07 bar (1 psi) can result in a 1.5% reduction in
productivity. A loss of 1 bar would result in a 21.75% loss in productivity.
Test 1 Procedure
We carried out tests with four pots, ours and a selection of competitors.
Each of the abrasive blast machines was connected in turn to an air compressor set to
generate 17 bar. This was stepped down to 12 bar via a regulator into the air buffer tank and
a further inline regulator was set to 10 Bar ( 145 psi ) feeding the air “bull” hose to supply the
abrasive pots.
Pressures were recorded in PSI using digital pressure needle gauges and latterly converted
to bar for analysis.
Pressures were recorded at the connection of the compressor intake bull hose to the
abrasive machine and at the connection of the nozzle hose to the mixer ‘T’. This effectively
recorded input and output pressures of each machine.
In addition, some pressures were tested using a needle gauge at the nozzle.
Each machine was then timed to reach maximum pressure on the digital gauge and then
depressurisation times to return to zero on the gauge.
The same Elcometer water separator was used on each machine to avoid the results being
skewed by different manufacturer’s separators.
A test was also carried out using abrasive media to see if it affected the charge times or
depressurisation times.
An operational appraisal for each machine was also carried out during the tests.
Results
Fig 2 Pressure loss across a blast pot
From the results it can be seen that the Pot 3 had a loss of 0.07 bar (1psi) the 3 other pots
had losses across the pots ranging from 1.04 bar (15 psi) down to 0.42 bar ( 6 psi).
Productivity reduction varied across the 4 pots from 1.5 % to 22.5 % based on a 1.5% loss of
productivity for every 1 psi pressure drop at the nozzle. These losses are across the pot and do
not take into account any losses in the blast hose.
These results would appear to vindicate the design of the pipework and remote control
valve (RCV) on pot 3 to improve airflow and reduce the boundary layer effect thus
minimising any pressure losses across the pot.
A major issue in Europe currently is a safety issue regarding pressurisation and more
importantly de-pressurisation times for blast pots. If a pot can be deemed more or less
efficient depending on the pressure drop across it would that mean that a more efficient pot
could be expected to charge and dis-charge more quickly?
0
0.2
0.4
0.6
0.8
1
1.2
1 2 3 4
Pre
ssu
re lo
ss (
Bar
)
Pot Number
In this test the pots were pressurised by depressing a deadman’s handle on a 10 metre
length of blast hose connected to the pot. The time was measured from the moment the
handle was depressed until the pressure in the pressurised ABM had stabilised.
Depressurisation time was measured as the time taken for the pressure at the blast hose
coupling to reach zero from when the deadman’s handle was released.
Fig 3 Time to pressurise the blast pots to 8 bar
The results show that pot 3 was between 0.5 and almost 3 seconds quicker than the other
pots to pressurise. In percentage terms it is up to 50% quicker than the other pots.
The concern with depressurisation is based on accidents whereby the Deadman’s handle
has been released but the pot has remained pressurised for many seconds resulting in injuries
as the “live” blast hose is free to “snake” around.
Fig 4 Time to depressurise the blast pots
0
2
4
6
8
10
12
1 2 3 4
Pre
ssu
risa
tio
n t
ime
in s
ecs
Pot Number
0
2
4
6
8
10
12
1 2 3 4
Dep
ress
uri
sati
on
tim
e in
sec
on
ds
Pot Number
In these tests pot 3 was 2.5 seconds quicker than the next quickest pot and over 5 seconds
quicker than the slowest pot.
The pressure loss across a blast pot, whilst a significant contributor to the overall
efficiency of a blasting system, cannot be used as the sole indicator of blasting efficiency.
The cost of the abrasive and more significantly the cost of cleaning up and removing the used
abrasive from site together with compressor costs contribute greatly to the running cost of an
efficient blast operation. The most efficient blast operations will be a combination of speed of
coverage and amount of blast media used.
Test 2 Procedure
The next set of tests was to evaluate and compare the performance of several pots from
the UK, Europe and the United States against these “efficiency” criteria.
5 pots from different suppliers were set up so that they operated in their optimum
condition, using minimum grit at the best pressure to remove a 2 coat glass flake epoxy from
a steel surface. The coating was applied to give a nominal dry film thickness of 400 microns
(16 mils).
Each pot was tested with three different blast nozzles, Nos. 4, 6, and 8 (¼”, ⅜”, ½”) and at
different compressor pressure settings. The grit valve was set to provide the optimum
(quickest) removal of the coating from the steel.
Pressures were recorded into the pot, out of the pot and at the nozzle. From these numbers
the pressure loss across the pot and along the length of the 90 metre (295 ft) blast hose were
calculated for each nozzle size and pot pressure.
Compressor pressures were set at 8, 10 then 12 bar for all tests, some pots were not
certified for the higher pressures and were therefore only tested to the maximum certified
pressure.
Unfortunately one of the pots failed during these tests as the grit valve liner ruptured hence
the lack of results for pot 5.
Results
0
1
2
3
4
5
6
7
8
8 10 12
Pre
ssu
re lo
ss (
psi
)
Pressure (bar)
Pressure loss across pot (No.8 nozzle)
Pot 1
Pot 2
Pot 3
Pot 4
Pot 5
0
1
2
3
4
5
6
7
8
9
8 10 12
Pre
ssu
re lo
ss (
psi
)
Pressure (bar)
Pressure loss across pot (No.6 nozzle)
Pot 1
Pot 2
Pot 3
Pot 4
Pot 5
Fig 5 Pressure losses across the pots for different nozzle sizes
From Fig 5 it can be seen that the trend for all the pots as a group is to have greater
pressure loss across the pot as the nozzle size increases. The change in pressure provided by
the compressor does not appear to have a consistent effect on the pressure loss across the pot.
When the pressure loss along the blast hose is considered, the results can be seen in Fig 6.
The larger the nozzle size, and in general supply pressure, the greater the losses.
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
8 10 12
Pre
ssu
re lo
ss (
psi
)
Pressure (bar)
Pressure loss across pot (No.4 nozzle)
Pot 1
Pot 2
Pot 3
Pot 4
Pot 5
0
10
20
30
40
50
8 10 12
Pre
ssu
re lo
ss (
psi
)
Pressure (bar)
Pressure loss in 90m hose (No.4 nozzle)
Pot 1
Pot 2
Pot 3
Pot 4
Pot 5
Fig 6 Pressure losses in the hose for different blast nozzles
If we now concentrate on the blast time, as this is seen as a quick comparator of blast
efficiency, we can compare the time taken to blast clean a fixed area and how this changes as
certain parameters change.
The quickest times achieved as one would expect are with the largest nozzle size, an
analogy used by a colleague of mine being “it’s like painting with a wider brush”. The results
shown in Fig 7 bear this out.
0
5
10
15
20
25
30
8 10 12
Pre
ssu
re lo
ss (
psi
)
Pressure (bar)
Pressure loss in 90m hose (No.6 nozzle)
Pot 1
Pot 2
Pot 3
Pot 4
Pot 5
0
10
20
30
40
50
60
8 10 12
Pre
ssu
re lo
ss (
psi
)
Pressure (bar)
Pressure loss in 90m hose (No.8 nozzle)
Pot 1
Pot 2
Pot 3
Pot 4
Pot 5
0
200
400
600
800
1000
1200
1400
1600
4 6 8
Tim
e to
bla
st/m
2(s
ec)
Nozzle size
Blast time for 8 bar pressure
Pot 1
Pot 2
Pot 3
Pot 4
Pot 5
0
200
400
600
800
1000
1200
4 6 8
Tim
e to
bla
st/m
2 (s
ec)
Nozzle size
Blast time for 10 bar pressure
Pot 1
Pot 2
Pot 3
Pot 4
0
100
200
300
400
500
600
700
800
900
1000
4 6 8
Tim
e to
bla
st/m
2(s
ec)
Nozzle size
Blast time for 12 bar pressure
Pot 1
Pot 2
Pot 4
Fig 7 Blast time per square metre for different pressures
We can clearly see from these graphs that the higher the pressure the shorter the time
taken to blast clean a surface and when combined with the larger blast nozzle, the effects are
dramatic. A 25% improvement from 8 to 12 bar with a no.8 nozzle. We can also see in Fig. X
that at 12 bar pressure changing from a no. 4 nozzle to a no. 8 nozzle will reduce the time
taken by at least 50%.
As we discussed earlier the cost of grit usage and subsequent clean-up costs as well as time
taken to blast the surface all contribute to the efficiency, financial and otherwise of the
blasting process. Taking the most commonly quoted pot pressure of 8 bar we can compare
abrasive use to blast a given area.
Fig 8 Abrasive usage per square metre for a given nozzle size
If we then include all other costs, labour, compressor etc then we can calculate the total cost
of blasting.
0
5
10
15
20
25
30
35
40
45
Ab
rasi
ve u
se/m
2(k
g)
Pot
Abrasive usage/m2
Pot 1
Pot 2
Pot 3
Pot 4
Pot 5
Fig 9 Blasting cost per square metre for different pots at 8 bar
Selecting Pot 1 alone we can also see the effect of the cost of blasting when increasing the
blast pot pressure.
Fig 10 Blasting costs for various pressures
Conclusion
Whilst we may think that all blast pots are the same, these tests show that not to be the
case. The design of the pipework, remote control valves and media valves all contribute to
the efficiency of the pot. The ability to precisely control the amount of grit being introduced
into the airstream gives the operator much more control over grit usage and therefore overall
efficiency and hence cost.
When we look at the efficiency of the pot, as well as the amount of grit used, we must also
take into account the pressure requirements and thus energy usage when calculating the true
£0.00
£5.00
£10.00
£15.00
£20.00
£25.00
£30.00
£35.00
£40.00
£45.00
£50.00
Bla
stin
g co
st/m
2
Pot
Blasting cost/m2
Pot 1
Pot 2
Pot 3
Pot 4
Pot 5
£0.00
£5.00
£10.00
£15.00
£20.00
£25.00
£30.00
£35.00
£40.00
Bla
stin
g co
st/m
2
Pressure
Blasting cost at various pressures
8 bar
10 bar
12 bar
cost of blasting. These tests do show that generally the higher the pressure the greater the
efficiency and the lower the blasting costs, an increase in pressure from 8 to 12 bar can
reduce the cost by almost £10 per square metre. It can be said that this may not be a feasible
pressure to use over a long period due to the physical strength of the blaster, but the ability to
use higher pressure pots will enable the blaster to use an efficient pressure at the nozzle over
much greater lengths of blast hose without seeing a loss of efficiency. This is becoming more
important as tighter safety legislation is preventing pots from being located as close to the
blast site.
HOW THE RIGHT POT CHOICE AND SYSTEM SET UP CAN IMPROVE
BLASTING EFFICIENCYDavid J Barnes
Group Technical ManagerElcometer Limited
Manchester, UK
Abstract
• Shot and grit blasting has been around for many years, but are we getting the most from our systems?
• Air flow, pressure and nozzle size all contribute to the efficiency and yardage achievable by a blast pot. Pot and valve design will contribute both positively and negatively to these variables but correct set up is imperative for efficient blasting.
• Improvement in pot and valve designs can lead to huge gains for the blaster and with some simple, logical adjustment and/or upgrades the changes recommended here will help you get the most from your pot.
Blast system
• A typical blast system consists of a diesel powered air compressor and a blast pot.
• The blast pot is filled with blast media and pressurised by the compressed air.
• The media exits the pot, mixes with a flow of pressurised air to be delivered to the blast nozzle at high speed.
Blast system
• The choice of media, the ratio of media to air in the air stream and the air pressure delivered at the nozzle are major contributors to the efficiency of the blasting operation
Blast system
• The most crucial element in a blasting system is the pressure that can be delivered at the nozzle.
• It is a widely held belief that a loss of 0.07 bar (1 psi) can result in a 1.5% reduction in productivity.
• A loss of 1 bar (14.5 psi) would result in a 21.75% loss in productivity.
Test parameters
• We carried out tests with four pots, ours and a selection of competitors. – Each blast pot connected to an air compressor set to
generate 17 bar (247 psi). – Stepped down to 12 bar (174 psi) via a regulator into the
air receiver– Inline regulator was set to 10 Bar ( 145 psi ) feeding the air
“bull” hose to supply the abrasive pots.
Test parameters
• Pressures were recorded;– At the bull hose/abrasive machine connection – At the blast hose/mixer ‘T’ connection.
This effectively recorded input and output pressures of each machine.
Test parameters
• Time recorded to reach maximum pressure on the digital gauge
• Times to return to zero on the gauge were also recorded.
Test parameters
• Same water separator was used on each machine to avoid the results being skewed by different manufacturer’s separators.
• A test was also carried out using abrasive media to see if it affected the charge times or depressurisation times.
Results
0
0.2
0.4
0.6
0.8
1
1.2
1 2 3 4
Pres
sure
loss
(Bar
)
Pot Number
• Pot 3 had a loss of 0.07 bar (1psi)
• The 3 other pots had losses across the pots from 1.04 bar (15 psi) down to 0.42 bar ( 6 psi).
• Productivity reduction varied from 1.5 % to 22.5 % based on a 1.5% loss of productivity for every 1 psi pressure drop at the nozzle.
Results
• These results would appear to vindicate the design of the pipework and remote control valve (RCV) on pot 3 to improve airflow and reduce the boundary layer effect thus minimising any pressure losses across the pot.
0
0.2
0.4
0.6
0.8
1
1.2
1 2 3 4
Pres
sure
loss
(Bar
)
Pot Number
Safety test
• A major issue in Europe currently is a safety issue regarding pressurisation and more importantly de-pressurisation times for blast pots.
• If a pot can be deemed more or less efficient depending on the pressure drop across it, would that mean that a more efficient pot could be expected to charge and dis-charge more quickly?
Safety test
• A 10 metre length of blast hose was connected to the pot. • The time was measured from the moment the deadman’s
handle was depressed until the pressure in the pressurised ABM had stabilised.
• Time was also measured as the time taken for the pressure at the blast hose coupling to reach zero from when the deadman’s handle was released.
Safety test
0
2
4
6
8
10
12
1 2 3 4
Pres
suris
atio
n tim
e in
secs
Pot Number
• Pot 3 was between 0.5 and almost 3 seconds quicker than the other pots to pressurise.
• In percentage terms it is up to 50% quicker than the other pots.
Safety test
0
2
4
6
8
10
12
1 2 3 4
Depr
essu
risat
ion
time
in se
cond
s
Pot Number
The concern with depressurisation is based on accidents whereby the Deadman’s handle has been released but the pot has remained pressurised for many seconds resulting in injuries as the “live” blast hose is free to “snake” around.
Overall system efficiency
• The pressure loss across a blast pot cannot be used as the sole indicator of blasting efficiency.
• The cost of the abrasive, the cost of cleaning up and removing the used abrasive from site and compressor costs contribute greatly.
• The most efficient blast operations will be a combination of speed of coverage and amount of blast media used.
Overall system efficiency
• 5 pots from different suppliers • Using minimum grit at the best pressure to remove a 3 coat
glass flake epoxy from a steel surface. • Tested with three different blast nozzles, Nos. 4, 6, and 8 (¼”,
⅜”, ½”)
Overall system efficiency
• Pressures were recorded into the pot, out of the pot and at the nozzle.
• The pressure loss across the pot and along the length of the 90 metre (295 ft) blast hose were calculated for each nozzle size and pot pressure.
• Compressor pressures were set at 8, 10 then 12 bar (116, 145, 174 psi) for all tests, some pots were not certified for the higher pressures and were therefore only tested to the maximum certified pressure.
Results
012345678
8 10 12
Pres
sure
loss
(psi
)
Pressure (bar)
Pressure loss across pot (No.8 nozzle)
Pot 1
Pot 2
Pot 3
Pot 4
Pot 5 0
2
4
6
8
10
8 10 12
Pres
sure
loss
(psi
)
Pressure (bar)
Pressure loss across pot (No.6 nozzle)
Pot 1
Pot 2
Pot 3
Pot 4
Pot 5
0
1
2
3
4
5
8 10 12
Pres
sure
loss
(psi
)
Pressure (bar)
Pressure loss across pot (No.4 nozzle)
Pot 1
Pot 2
Pot 3
Pot 4
Pot 5
Results
0
10
20
30
40
50
8 10 12
Pres
sure
loss
(psi
)
Pressure (bar)
Pressure loss in 90m hose (No.4 nozzle)
Pot 1
Pot 2
Pot 3
Pot 4
Pot 5
0
5
10
15
20
25
30
8 10 12
Pres
sure
loss
(psi
)
Pressure (bar)
Pressure loss in 90m hose (No.6 nozzle)
Pot 1
Pot 2
Pot 3
Pot 4
Pot 50
10
20
30
40
50
60
8 10 12
Pres
sure
loss
(psi
)
Pressure (bar)
Pressure loss in 90m hose (No.8 nozzle)
Pot 1
Pot 2
Pot 3
Pot 4
Pot 5
Results
0200400600800
1000120014001600
4 6 8
Tim
e to
bla
st/m
2(s
ec)
Nozzle size
Blast time for 8 bar pressure
Pot 1
Pot 2
Pot 3
Pot 4
Pot 50
200
400
600
800
1000
1200
4 6 8
Tim
e to
bla
st/m
2 (s
ec)
Nozzle size
Blast time for 10 bar pressure
Pot 1
Pot 2
Pot 3
Pot 4
0100200300400500600700800900
1000
4 6 8
Tim
e to
bla
st/m
2(s
ec)
Nozzle size
Blast time for 12 bar pressure
Pot 1
Pot 2
Pot 4
Results
0
5
10
15
20
25
30
35
40
45
Abra
sive
use
/m2
(kg)
Pot
Abrasive usage/m2
Pot 1
Pot 2
Pot 3
Pot 4
Pot 5
Results
£0.00
£5.00
£10.00
£15.00
£20.00
£25.00
£30.00
£35.00
£40.00
£45.00
£50.00
Blas
ting
cost
/m2
Pot
Blasting cost/m2
Pot 1
Pot 2
Pot 3
Pot 4
Pot 5
Results
£0.00
£5.00
£10.00
£15.00
£20.00
£25.00
£30.00
£35.00
£40.00
Blas
ting
cost
/m2
Pressure
Blasting cost at various pressures
8 bar
10 bar
12 bar
Conclusion
• Whilst we may think that all blast pots are the same, these tests show that not to be the case.
• The design of the pipework, remote control valves and media valves all contribute to the efficiency of the pot.
• The ability to precisely control the amount of grit being introduced into the airstream gives the operator much more control over grit usage and therefore overall efficiency and hence cost.
Conclusion
• When we look at the efficiency of the pot, as well as the amount of grit used, we must also take into account the pressure requirements and thus energy usage when calculating the true cost of blasting
• Generally the higher the pressure the greater the efficiency and the lower the blasting costs
• An increase in pressure from 8 to 12 bar (116 to 174 psi) can reduce the cost by almost £10 per square metre.
Conclusion
• It may not be a feasible pressure to use over a long period due to the physical strength of the blaster, but the ability to use higher pressure pots will enable the blaster to use an efficient pressure at the nozzle over much greater lengths of blast hose without seeing a loss of efficiency.
• This is becoming more important as tighter safety legislation is preventing pots from being located as close to the blast site.
