Hugh James Graph
WJs have signicant safety margins for pro-
cessing sensitive high-explosives
∇
T
Gradient Technology
Relationship Between Velocity, Diameter, and Density
Conventional WJs cannot achieve the velocities
necessary for secondary explosive initiation, but
explosively driven research waterjets can
Physical Pressure Limits
The maximum pressure a convention-
al WJ can achieve at room temperature
(25°C) is 1 GPa because water freezes
Introduction
Waterjets (WJ) are a non-traditional technology that have evolved from a low pressure civil
and mining engineering tool to a high pressure machining tool over the last 150+ years.
Although still a novelty to many, they have been used for the demilitarization of high-ex-
plosive munitions for the last 95 years.
Later WJ variants using added abrasives, known as abrasive waterjets (AWJ), gave them
the capability of cutting steels and other hard materials. AWJs have been used for the de-
militarization of high-explosive ordnance for almost thirty years. This retrospective over-
view of WJ and AWJ safety analyses was performed to compile the available information
for explosive safety professionals.
Pyrophoric Metals
Only certain metals can create pyrophoric sparks accord-
ing to BRL (Hillstrom, 1973)
Ignition Safety Tests - WJs and AWJs have been tested in amma-
ble gas environments to determine the safety of cutting metals. One safety observation is that small
sparks are occasionally seen when using abrasives in AWJs. Sparks of any nature are of great
concern to safety personnel for both munition demilitarization and DOE facilities deconstruction. In
the rst case, high explosives and propellants are sensitive to ignition by sparks, and in the second
case the radiolysis of water generates large quantities of highly ammable hydrogen and oxygen
gases which can deagrate if ignited. In both cases, the energy required for ignition is extremely low.
Dahn and Reyes (1994) published that very ne TNT has a minimum-ignition-energy (MIE) of only
12 mJ. The ignition energies of hydrocarbons in an oxygen atmosphere have an MIE of only 0.002
mJ, while hydrogen in an oxygen atmosphere has an MIE of only 0.001 mJ according to NFPA 53
(1994).
The results of various empirical and theoretical studies conrm eld experience that AWJs are safe to
cut steels and most other metals around ammable gases, liquids, and solids. Research performed
by Elvin and Fairhurst (1985), Board (1997), Miller (1999), and by Usman (2009) showed that ASJs
and AWJs did not ignite hydrocarbons or hydrogen when cutting common steels. Miller (1999) also
tested AWJ cutting of steel in a hydrocarbon-oxygen atmosphere as well as in a hydrogen-oxygen
atmosphere.
Hillstrom (1973) showed that the chemistry and reac-
tivity of the metal forming the mechanical spark played
a signicant role in the fuel ignition process and only
some nineteen metals were eective pyrophoric mate-
rials. This is reected in Bartknecht (1987) conversion
graphs as some metals are much more capable of ig-
niting ammables than others. In order to be an eec-
tive pyrophoric material, the metal had to produce suf-
cient heat from oxidation and the metal oxide coating
forming on the metal particle had to be able to transmit
that heat to the surrounding combustible gas or sol-
id. Iron and steel were among those metals that were
identied by Hillstrom (1973) as not being sucient-
ly chemically reactive and pyrophoric. Miller’s (1999)
research conrmed that cutting certain exotic metals,
such as zirconium, with an AWJ could ignite hydro-
gen-air mixtures, but only with a low probability, while cutting steel in hydrogen-oxygen atmospheres
would not ignite.
Accidents and Events - There have been several explosions re-
lated to using WJs with energetics:
• Summers (1996) discussed a re during a 103 MPa (15 ksi) washout test at MUST (Missouri
University of Science and Technology) in the 1980s when a graduate student overtightened a
WJ nozzle with a pipe wrench and broke the pipe. When pressurized, the broken nozzle became
a projectile and struck the missile warhead resulting in a re.
• A “pu” (non-propagating ignition) occured during testing of a 114 MPa (16.5 ksi) cryogenic liq-
uid nitrogen (LN2) washout system, according to Spritzer (1999). The cause of the event is still
undetermined (ESD?).
Some of the “reported WJ accidents” are not really related to the actions of the WJ at all. The
following “accidents” are described here for clarication:
• Beaudet (1999) identied that DERA (Defence Engineering and Research Agency) – UK West
Freugh, Scotland, reported an accident in 1995 while mechanically reprocessing explosives
containing abrasive grit from a ASJ cut made three days earlier. This was a processing accident,
not a WJ accident.
• Alliant Techsystems (ATK) had a low-order detonation of a U.S. Navy 8-inch/55-caliber high ca-
pacity projectile at their Elk River, MN, facility in 1996 during mechanical defuzing operations
according to Beaudet (1999). Although ATK had AWJ systems available at the site but elected
to use mechanical defuzing to reduce costs. This event had nothing to do with the WJ system.
• In 2000 Teledyne-Commodore had a re related to their ammonia processing system. The sys-
tem used a WJ converted to pumping anhydrous ammonia, but the re was not related to the
cutting of the M61 rocket but due to a secondary chemical reaction several minutes after being
cut with the ammonia uid according to Beaudet (2001).
• A 2000 lb (907 kg) WWII bomb detonated, according to Hall (2010), while bomb disposal techni-
cians were attempting to put a WJ cutter in place in Göttingen (Germany) on June 1, 2010. The
fuze was believed to be a UK Type 37 chemical-type time delay fuze and detonated as the WJ
was being erected, not during operations.
Safety Studies
The two key questions that explosive safety professionals should ask regarding using
high-pressure WJs on HE ordnance are:
1) Can WJs ever initiate secondary high explosives or propellants?
YES – providing the WJ is suciently fast and is suciently large enough in impact
area. Research by Summers, et al., (1988) has shown this.
2) Is initiating secondary high explosives likely with a commercial unit (rather than a re-
search setup)?
NO – It is extremely unlikely that a commercial WJ can ever initiate secondary high
explosives or propellants. Physics makes it functionally impossible to achieve the ve-
locities or the large enough jet diameters required to detonate secondary high explo-
sives with current commercial units.
Explosive Safety Tests – WJs and AWJs share many similarities
with shaped charge jets and the same mechanics can be used to predict the reaction of explosives
to jet impact. WJs operate at velocities of 0 - 1.5 km/s, as compared to explosive shaped charge
jets which travel at velocities of 1 to 14 km/s. Extensive tests were conducted by LANL (Mader
and Pimbly, 1981) and Summers, et al., (1988) in which they accelerated water using explosive
shaped charges to velocities in excess of ve times the sonic velocity of water in order to achieve
a reaction in secondary high explosives. These tests do not reect the realities of a commercial
WJ system.
First of all, the discharge of water pumped through an orice
is limited by physics to subsonic ows, and supersonic speeds
cannot be achieved by commercial WJ cutting systems due to
choked ow conditions at the orice. Secondly, since water at 1
GPa (147 ksi) freezes at 25°C (77°F) this eectively becomes
the upper pressure limit for commercial WJs. The velocity of
water at this temperature and pressure is approximately 1426
m/s (4680 ft/s), or very close to the published speed of 1496.65
m/s (4910 ft/s) for the sonic velocity of water (McSkimin, 1965).
In addition, research on high explosive initiation has shown
that both velocity and impactor diameter are critical parame-
ters. Slade and Dewey (1957), Roslund, et al., (1974), Field,
et al., (1982), Chan (1985), and Liddard and Roslund (1993) all showed that the impactor’s veloc-
ity (V), diameter (d), and shape (round or at nosed) act together to determine the probability of
the explosive’s initiating. The body of research conrms the predictive equation known as the Ja-
cobs-Roslund equation (Roslund, et al., 1975). High-pressure WJs can only operate at relatively
small (<1 mm) diameters.
Held’s (1987) work focused specically on copper-lined
shaped charge jets and showed a relationship between
the product of a shaped-charge jet’s diameter (d) and
velocity (V) squared (V
2
d) and a specic explosive’s
detonating. Mader and Pimbley (1981) further rened
Held’s equation and showed that a specic relation-
ship for initiation of a given explosive was based on
the product of the density of the jet (ρ) in gm/cm
3
, the
square of the velocity (V
2
) in mm/μs, and the diameter
of the jet (d) in mm, or ρV
2
d.
More recently, James (1996) of AWE-Aldermaston (UK)
developed a criti-
cal energy criterion
based on work per-
formed by Walker and Walsey (1969) at Lawrence Livermore
National Laboratory. The James Criterion establishes a thresh-
old using the critical energy (E
c
) dened by Walker and Wasley
(1969) and the activation energy.
Impact Flash - Another concern safety researchers have noted is that the high
velocity WJ or AWJ impact on a target can create a luminous discharge, known by researchers as
an “impact ash.” The luminescence observed during the impact process has been attributed to
adiabatic compression, superheated ejecta, burning ejecta, or even detonating reaction products
as shown in Bestard and Kocher (2010). All of these concerns focus on the formation of very hot
thermal incandescence that could initiate and propagate an explosive reaction or are the products
of an incipient reaction. It is well known in the safety profession that all of these events are possible
with ever increasing impact force, but WJ can cause the observed “impact ash” even on non-ex-
plosive materials, and the targets show no post-impact heat aected zone.
Prevenslik (2003) showed that shocked argon caused the impact ashes, seen from WJ impacts
on both inert and energetic targets, as all water contains dissolved argon gas unless freshly dis-
tilled. Anbar (1968) showed that water droplets dropped into air-saturated water at velocities as
slow as 5 m/s (16 ft/s) create luminescent impact ashes – equivalent to a drop of only 1.3 m (4.25
ft). Winning and Edgerton (1952) detail the construction of an explosive argon ashlamp for photo-
graphing extremely high speed events. The initiation of a small explosive charge creates a shock
wave in the argon gas, resulting in the intense light. The use of high-intensity explosive light sourc-
es has become standard practice in high-speed photography to generate extremely bright ashes
of light. Sultano (1962), of the U.S. Army’s Ballistic Research Laboratory, specically states that
argon luminescence is a function of the shock pressure and is not due to burning or due to the gas
being heated to thermal incandescence.
Consequently, the ashes of light from WJ impact or water cavitation from commercial WJ systems
are of no concern for safety personnel.
Electrostatic Discharge (ESD) - WJs can, unfortunately,
generate electrostatic charges that are potentially hazardous to personnel and harmful to equip-
ment and product as shown in Miller (2001). Several serious res have been attributed to static
discharge during WJ cleaning operations, and static electric arc discharge has been known to
damage composite materials. The WJ industry’s trend toward using higher velocity liquid jets and
higher purity water increases the risk of electric spark generation.
It is a well-known phenomenon in engineering that owing gases and liquids can generate large
amounts of static electricity. The generation of static electricity from owing gases and liquids
even occurs during the operation of items expected to be “safe.” A classic example is the genera-
tion of up to 5,000 volts during the discharge of a carbon dioxide re extinguisher as described by
Petrick (1968). Reif and Hawk (1974) showed that three very large crude oil carriers (VLCC), or
oil-tankers, were destroyed in December 1969 from electrostatic discharge during routine wash-
down by jets of water.
Material Incompatibility - Not all materials are compatible with each other and testing should be performed using a dierential scanning calorimeter (DSC) to deter-
mine the compatibility prior to process acceptance. Miller and Navarro (1996) showed that powdered aluminum, a common additive in military explosives, can react with water. Shidlovskii (1964) rec-
ognized that wet aluminum powder would spontaneously create an exothermic reaction and release hydrogen gas. The most eective control method was developed by Ursenbach and Udy (1962)
who added phosphates to passivate aluminum powders used in slurry high explosives for the commercial blasting industry. Ursenbach and Udy recommended diammonium dihydrogen phosphate,
trisodium phosphate, sodium dihydrogen phosphate, or mono-ammonium dihydrogen phosphate in quantities from 0.1% to 2.0% by weight in water.
Background
1852 - Lt. George McClellan (USACE) invents waterjets for a
military civil engineering operation in Texas
1853 - Anthony Chabot reinvents WJ for hydraulic mining in
California’s Gold Country
1870 - Bvt. Brig. Gen. (ret.) Benjamin C. Tilghman awarded
patent for abrasive waterjet cutter to cut stone and glass
1879 - USACE uses WJ for Mississippi River civil engineering
projects for reshaping the river ow
1923 - Thomas Knight awarded WJ patent to washout HE
projectiles
1932 - Howard Deck and Pasquale DiCosmo patent HE projectile
washout system at Picatinny Arsenal
1933 - Charles Fourness and Charles Pearson develop WJ slitter for the
commercial paper industry
1940s - Ammunition Peculiar Equipment (APE-1300) Explosive Washout
Plant developed to washout HE projectiles
1950s - Thiokol (Redstone Arsenal) develops WJ solid propellant
washout system for recovering rocket motors; Aerojet,
NSWC-Indian Head, and NSWC-Crane quickly follow
1958 - Billie Schwacha (North American Aviation) patents high-pressure
AWJ for cutting exotic metals for the futuristic XB-70 Valkyrie
bomber
1960s - Norman Franz and Eugene Bryan research high-
pressure WJ for cutting wood under a U.S. FPL grant and
restarts interest in high-pressure WJ
1970s - Mohamed Hashish and Gene Yie independently
commercialize Franz’s work into the modern high-pressure WJ and AWJ
systems available today
1980s - Robert Fairhurst develops the abrasive slurry jet (ASJ); David Summers provides
critical safety research on the impact of high-pressure waterjets on high explosives under
contract to NSWC-Crane; Western Area Demil Facility installs South Tower Hardware
Hydraulic Cleaning Systerm for projectile demilitarization; Richard Hanson invents
PAN Disrupter (MK40 MOD 0 Unexploded Ordnance Stando Disrupter
Tool) which shoots a slug of water to disable terrorist bombs
1990s - Paul Miller uses high-pressure AWJ for cutting 172,000 HE
projectiles; performs safety tests on HE at pressure of 1 GPa (147 ksi);
DOE (James Cruchmer) independently conrms research
1997 - DOE (Brett Board) hazards analysis conrms AWJ safe to cut steel
containers holding radioactive haz waste in ammable hydrogen-
air atmosphere
2000 - High-pressure AWJ+WJ projectile
demilitarization line installed at Crane Army
Ammunition Activity (CAAA) with 300,000
projectiles processed to date
BLUF - (Bottom Line Up Front) Waterjets and Abrasive Waterjets have been shown
to be safe for washing out and cutting munitions containing high explosives by theoretical
and empirical testing for almost 100 years
A Retrospective Study on the Safety of Waterjet
(WJ) and Abrasive Waterjet (AWJ) Processing of
High Explosive Ordnance
Paul L. Miller
Gradient Technology, Elk River, MN USA
