Transcript Low pressure and vacuum hazards

Low pressure and
vacuum hazards
An lpb toolbox talk
Pressure and Force
Let’s review a few basic principles:
Force = Pressure x Area
this 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!
Example
If Force = Pressure x Area
then, 10psi pressure on a 12”x12” square surface area will
be 1,440 pounds of force.
12”
x 12”
door
12”
X 12”
Square
10 psig
1,440 Pounds of Force
…this is equivalent to an object that weighs 1,440 lbs!
Can you get hurt with
low pressure?
ABSOLUTELY!!!
….and be especially careful with large surfaces
like manways. 1 psi may not even register on the
gauge but it’s enough to send a hatch flying if all
the bolts are removed and the gasket is stuck.
Low pressure hazard
This door had 2.8 psi
behind it, the equivalent
of 1915 lbs of force.
The door only weighs 15
lbs.
So when it came loose, it
slammed open seriously
injuring a worker.
Over-pressured tank
This tank was fitted with:
a high level alarm, which was
accepted, and then forgotten
a pressure control system, which
was out of service
a pressure relief valve with a flame
arrestor which was found to be
blocked
….so when the product was
transferred into the tank, it
overpressured until the roof
ruptured, even though the pressure
was only a few psi over hydrostatic
Vacuum hazards
When the pressure inside the vessel is lower than
atmospheric pressure, the force acts inwards, with
sometimes spectacular results….
What happened?
The tanker was being
steam cleaned and, at
the end of the job, the
hatches were closed.
With no vacuum
breaker fitted, as the
steam condensed, the
tanker imploded…..
Another tanker problem
This tanker was being
pumped out. The hatches
were all closed and the
vacuum breaker failed to
operate.
The problem is not
confined to rail tankers
though…
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 sucked through…
Could this happen
on your site?
How might this happen
on your site?
1 psi on a 24” diameter hatch exerts a force of 450
pounds.
A few bolts left loosely in place will limit the distance
the hatch can move.
Steam condenses to about 1/1700th of its volume 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.
Further reading
Hazards of trapped pressure and
vacuum, BP Process Safety Series,
ISBN 0 85295 475 1, published 2005,
available from www.icheme.org/shop,
price £27.00
See also In brief – corrosion from LPB163, February 2002
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
responsible for the operations to ensure that the regulations
and guidance issued by the authorities are consulted, that an
appropriate risk assessment is carried out and that
appropriate procedures are stipulated and followed.