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8/3/2019 Low Pressure and Vacuum Hazards
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Low pressure and vacuum
hazards
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Pressure and Force
Lets review a few basic
principles:
Force = Pressure x Areathis is like many small weights
sitting on an area
the larger the area, the greater the force
the weights may be small, but if the area is large,
they will add up to a big force!
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Example
1,440 Pounds of Force
12 X 12 Square
12 x 12 door
10 psig
If Force = Pressure x Area
this is equivalent to an object that weighs 1,440 lbs!
then, 10psi pressure on a 12x12 square surface area will
be 1,440 pounds of force.
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Can you get hurt with low
pressure?
ABSOLUTELY!!!
.and be especially careful with large surfaces
like manways.
1 psi may not even register on thegauge but its enough to send a hatch flying if all
the bolts are removed and the gasket is stuck.
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Low pressure hazard
This door had 2.8 psi
behind it, the
equivalent of 1915lbs of force.
The door only
weighs 15 lbs.
So when it came
loose, it slammed
open seriously
injuring a worker.
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Over-pressured tankThis tank was fitted with:
a high level alarm, which wasaccepted, and then forgotten
a pressure control system,
which was out of servicea pressure relief valve with aflame arrestor which wasfound to be blocked
.so when the product was
transferred into the tank, itoverpressured until the roofruptured, even though thepressure was only a few psiover hydrostatic
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Vacuum hazards
When the pressure inside the vessel is
lower than atmospheric pressure, the
force acts inwards, with sometimes
spectacular results.
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What happened?
The tanker was
being steam
cleaned and, atthe end of the job,
the hatches were
closed.
With no vacuumbreaker fitted, as
the steam
condensed, the
tanker
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Another tanker problem
This tanker was being
pumped out. The
hatches were all
closed and thevacuum breaker failed
to operate.
The problem is not
confined to rail
tankers though
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Covered vent
A tank was being painted and the painters
had covered the vent with plastic sheeting.
When operations started to empty the
tank, it collapsed before the plastic suckedthrough
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Could this happen
on your site?
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How might this happen
on your site?1 psi on a 24 diameter hatch exerts a force of450 pounds.
A few bolts left loosely in place will limit the
distance the hatch can move.Steam condenses to about 1/1700th of itsvolume as it cools.
It sometimes takes a vacuum of only a few
inches of water to collapse a tank.Be aware of blockages and trapped pressure.
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Disclaimer
Whilst the Institution of Chemical Engineers has
made every effort to ensure the accuracy of the
information contained in this training presentation
it remains the responsibility of those responsiblefor the operations to ensure that the regulations
and guidance issued by the authorities are
consulted, that an appropriate risk assessment iscarried out and that appropriate procedures are
stipulated and followed.