| Overview: |
Waterjet
stripping uses the impact force of pressurized water to effectively remove
a variety of coatings ranging from paints, rubbers, and sealants to more
tenacious coatings such as aerospace adhesives and metal flame spray coatings.
These coatings may be removed from many different types of substrates, including
metals, plastics, composites, and concrete. Due to its high versatility,
waterjet stripping has applications in several industries, including automotive,
aerospace, shipbuilding, and construction.
Waterjet stripping
involves the use of water at pressures above 10,000 psi to mechanically
remove coatings. High-pressure pumps force water through specially designed
nozzles that direct the high-velocity stream to impinge upon the coated
substrate. The kinetic energy of the waterjet physically erodes the coating
to expose the underlying substrate surface.
The effectiveness
of waterjet stripping depends on a number of key operating parameters,
including water flow rate, operating pressure, and cross-sectional area
of the nozzle. Other operating parameters that influence coating removal
effectiveness include nozzle standoff distance, angle of incidence, nozzle
rotation speed, and traverse rate (dwell time). By adjusting these parameters,
an optimal removal rate may be achieved while preventing damage to substrate
surfaces.
Waterjet systems
are highly versatile to meet a wide range of coating removal needs. The
selection of a waterjet system depends on the type of coating to be removed;
substrate; desired removal rate and efficiency; and environmental, health,
and safety issues pertaining to the work area. Although most organic coatings
can be removed using pressures of 25,000 psi (170 MPa), lower pressures
may be equally or more effective in some applications. However, higher
pressures may be required for the removal of highly adherent coatings.
Waterjet systems
are classified as high-pressure or ultrahigh-pressure, depending on the
operating pressures employed by the system. High-pressure waterjet systems
employ a stream of water at pressures of 10,000 to 25,000 psi (70 to 170
MPa). Ultrahigh-pressure waterjet systems operate at pressures ranging
from 25,000 to 55,000 psi (170 to 380 MPa). In addition to increased pressures,
ultrahigh-pressure waterjet systems typically use lower volumes of water
(less than 7 gpm) than high-pressure systems.
Waterjet stripping
can be performed using manual or automated equipment. Manual systems include
hand-held lances and manually propelled units for removing coatings from
such items as floors, grates, and vehicles. These systems allow for increased
mobility and flexibility at low capital cost. In high-volume production
applications, automated systems employing robots are often the most efficient
and effective. Robotically manipulated equipment may also be required
in some small-scale applications, since high reaction forces can fatigue
operators of manually controlled waterjet systems.
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| Compliance
Benefit: |
Use of a high or
ultra-high pressure waterjet technologies can decrease the amount of hazardous
waste generated at a facility since solvents are not used and the solid
residue (hazardous) can be separated from the wastewater. The decrease
in hazardous waste helps facilities meet the requirements of waste reduction
under RCRA, 40 CFR 262, and may also help facilities reduce their generator
status and lessen the regulatory burden (e.g. recordkeeping, reporting,
inspections, transportation, accumulation time, emergency prevention and
preparedness, emergency response). If the waterjet unit is a closed-loop
system, this will additionally eliminate water discharge, reduce water
consumption, and concentrate waste for less costly disposal. In a closed
system, a sump pump directs the resulting water/coating mixture to a centrifugal
separator that removes most of the particulate matter. The water then
passes through a series of filters and tanks for further purification
before re-use. The system requires only a small amount of make-up water
to compensate for evaporative losses, but both recycled and make-up water
must be of sufficient purity so as not to introduce sediments or other
impurities that may interfere with the proper functioning of equipment.
In addition to water
and waste reduction, facilities will decrease the amount of solvents on
site and therefore, the possibility that a facility will meet any of the
reporting thresholds of SARA Title III for solvents (40 CFR 300, 355,
370, and 372; and EO 12856) is reduced. Moreover, since waterjet stripping
does not generate any dust or airborne contaminants, the likelihood of
the facility requiring an air permit under 40 CFR 70 and 71 is decreased.
The Compliance Benefits
listed here are only meant to be used as a general guideline and are not
meant to be strictly interpreted. Actual Compliance Benefits will vary
depending on the factors involved, e.g. the amount of workload involved.
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| Materials
Compatibility: |
At optimum operating
conditions, waterjet stripping provides high coating removal efficiency
and removal rates with minimum damage to metals and soft substrates (e.g.,
rubber). Waterjet stripping does not alter the original surface profile
of the substrate, which can be advantageous or disadvantageous, depending
on the application. Waterjet stripping is not recommended for composite
materials (e.g., fiberglass bodies).
Unlike other mechanical
coating removal technologies that may entrap salts or other corrosion
agents, waterjet stripping leaves surfaces clean and unaltered. However,
flash rusting may occur on metal surfaces when using the waterjet process.
This process limitation can be minimized by using de-ionized water, adding
rust inhibitors to the water stream, or by directly applying inhibitors
to the surface after coating removal. However, caution must be taken to
select an inhibitor that does not chemically interfere with the adhesion
of subsequently applied coatings.
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| Economic
Analysis: |
Conventional
abrasive blasting (i.e., steel shot) is commonly used to remove surface
coatings for various types of military vehicles. This process has proven
effective and efficient over the years. Conventional abrasive blasting can
successfully remove coatings in a rather short period of time, however there
is a substantial amount of waste and other environmental concerns related
to this process. Therefore, the National Defense Center for Environmental
Excellence (NDCEE), which is operated by Concurrent Technologies Corporation
(CTC), was tasked by U.S. Army Tank Automotive and Armaments Command (TACOM)
to investigate alternative coatings removal processes that have the potential
to perform as effectively and efficiently as conventional abrasive blasting.
This task specifically focused on two potential coatings removal process
alternatives, high pressure waterjet (HPWJ) and abrasive blasting with alumaglassä.
The objective of
this task was to compare the HPWJ process, abrasive (alumaglass) blasting,
and conventional abrasive blasting for two types of maintenance activities.
The types of activities that were investigated were depot level and field
level. Two depot vehicles (Bradley Fighting Vehicle hull and turret) were
supplied by United Defense Limited Partnership (UDLP) located in Fayette
County, PA, and a third depot level vehicle (M1 Abrams turret) was supplied
by Anniston Army Depot (ANAD) located in Anniston, AL. The field vehicles
[a High Mobility Multipurpose Wheeled Vehicle (HMMWV) and dump truck]
were supplied by 458 Engineer Company, Army Reserves in Johnstown, PA.
The demonstration
results served as the process data for the HPWJ and abrasive blasting
with alumaglass processes. To properly compare the alternatives to the
current blasting process, actual process data for abrasive blasting was
obtained from UDLP, Fayette County, ANAD, and Dynamic Science, Inc. (DSI)
for their respective vehicles. Best engineering judgement, technical assumptions,
and published literature were also used to determine values for any unknown
data.
Economic analysis
revealed that conventional abrasive blasting is the most economical process
for removing coatings from military vehicles for depot level maintenance
activities. Based on a 15-year study period, both the HPWJ and abrasive
blasting with alumaglass processes were found to be economic failures.
Neither process was able to meet a payback within the study period. However,
economic analysis for field level maintenance activities revealed that
replacing abrasive blasting with either a HPWJ or abrasive blasting with
alumaglass process would be more financially beneficial.
A summary of the
financial implications for implementing a waterjet process as a replacement
for abrasive blasting in field level activities, which includes annual
operating cost benefit and capital investment required, is shown in Table
2. The 15-year NPV and IRR, as well as the payback period are also listed
in Table 2.
Assumptions:
- Field level maintenance
activities: strip a HMMWV and a dump truck
- Labor rate: $15/hr
- Operational timeframe:
250 days/yr, one 8-hr shift/day
- Number of vehicles
stripped per year:
- Bradley hull -
48
- Bradley turret
- 90
- M1 Abrams turret
- 90
- HMMWV - 200
- Dump truck - 100
Waterjet Stripping
Process
- Maintenance: 20
hr/yr
- Total labor requirement:
$18,744/yr
- Material costs:
$0
- Utilities costs:
$262/yr electricity, $125/yr compressed air
- Waste disposal
and management costs: $1,570/yr plus $500 water permit every 5 years
- Health and safety
costs: $670/yr
Abrasive Blasting
Process
- Maintenance: 240
hr/yr
- Total labor requirement:
$25,344/yr
- Material (abrasive
and masking) costs: $75,330/yr
- Utilities costs:
$3,557/yr electricity, $371/yr water
- Waste disposal
and management costs: $7,358/yr plus $615 air permit every 5 years
- Health and safety
costs: $1,680/yr
Capital Cost:
The capital cost for high and ultra-high pressure waterjet stripping will
vary considerable depending upon the process configuration required for
the particular stripping application. The capital cost for a high pressure
waterjet system for field level maintenance activities was estimated at
$281,250.
Operational Cost:
Operating costs for waterjet stripping will vary considerable depending
upon the process configuration required for the particular stripping application.
Annual operating costs for the field level waterjet system were estimated
at $21,500, versus $113,700 per year for conventional abrasive blasting.
Payback Period:
The calculated payback period for investment in the equipment/process:
<5 years.
Annual Savings:
The calculated annual savings for waterjet stripping: $82,800 to $110,000,
depending on the vehicle being stripped.
Economic Analysis
Summary:
A summary of the financial implications for implementing a waterjet process
as a replacement for abrasive blasting in field level activities, which
includes annual operating cost benefit and capital investment required,
is shown in the following table.
Financial
Implications of Replacing a
Conventional Abrasive Blasting Process with a Waterjet Process
| |
HMMWV |
Dump Truck |
| Annual
Operational Costs Benefit (per year): |
$82,787 |
$110,049 |
| Investment
Required: |
$281,250
b |
$281,250
b |
| Discounted
Payback Period (year) a : |
4.99 |
3.53 |
| NPV a
: |
$325,547 |
$532,903 |
| IRR a
: |
25.7% |
34.8% |
a This
value was calculated with Pollution Prevention Financial Analysis and
Cost Evaluation System (P2/FINANCE). This software is proprietary and
copyrighted by Tellus Institute of Boston, Massachusetts. A 15-year analysis
and 10% discount rate were assumed.
b This
is based on the cost of the waterjet equipment, installation, and other
contributing one-time fees related to the process.
Economic Analysis
Summary
Annual Savings
for Waterjet Stripping: $82,800 to $110,000
Capital Cost for a High Pressure Waterjet System: $ 281,250
Payback Period for Investment in Equipment/Process: <5 years
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