ESAB PT-36, Instruction Manual

Page: 153 / 436

153
5.1 Introduction
Wear on torch parts is a normal occurrence to plasma cutting. Starting a plasma arc is an erosive process to both
the electrode and nozzle. Regularly scheduled inspection and replacement of PT-36 parts must take place to
maintain cut quality and consistent part size.
hYDrogEN ExplosIoN haZarD.
sEctIoN 5 MaINtENaNcE
DaNgEr
A hazard exists whenever a water table is used for plasma arc cutting operations without
following recommended practices for safe operation. Severe explosions have resulted
from the accumulation of hydrogen beneath the plate being cut. Thousands of dollars
in property damage has been caused by these explosions. Personal injury or death could
result if people are struck by flying debris from the explosion.
The best available information indicates three possible sources of hydrogen in water ta-
bles. Most of the hydrogen is liberated by a fast reaction of molten metal from the kerf
with the water to form metallic oxides. This reaction explains why reactive metals with a
great affinity for oxygen, such as aluminum and magnesium, release greater volumes of
hydrogen during the cut than iron does. Most of this hydrogen will come immediately to
the surface, but some of it will cling to small metallic particles. These particles will settle to
the bottom of the water table and the hydrogen will gradually bubble to the surface.
Hydrogen may also result from slower chemical reactions of cold metal particles with the
water, dissimilar metals, or chemicals in the water table. This hydrogen will also gradually
bubble to the surface.
Finally, hydrogen may come from the plasma gas if H-35 is used. This gas is 35 percent
hydrogen by volume and a total of about 70 cfh of hydrogen will be released.
The hydrogen gas can collect in several places. The most common is in pockets formed by
the plates being cut and slats on the table. Pockets may also be formed in warped plates.
There can also be an accumulation of hydrogen under the slag tray or even in the air reser-
voir. This hydrogen, in the presence of oxygen, can then be ignited by the plasma arc or a
spark from any source. To reduce chances of hydrogen generation and accumulation, and
a consequent explosion, the following practices are recommended:
1. Clean the refuse (particularly fine particles) from the bottom of the table frequently.
Refill the table with clean water.
2. Do not leave plates on the cutting table overnight or over weekends.
3. If water tables have been sitting idle for several hours, vibrate the table in some way
before the first plate is laid in position. This will allow accumulated hydrogen in the
refuse to break loose and dissipate before it is confined by a plate on the table. This
might be accomplished by laying the first plate onto the table with a slight jolt, then
raising it again to permit hydrogen to escape before it is finally positioned.
4. If cutting above water, install fans to circulate air between the plate and the water.
5. If cutting under water, agitate the water under the plate to prevent accumulations of
hydrogen. This can be done by aerating the water using compressed air.
6. The level in the water table can be raised and lowered between cuts to dissipate ac-
cumulated hydrogen.
7. Maintain pH level of the water near 7 (neutral). This should reduce the rate of chemical
reaction between water and metals.