Low Pressure and Vacuum Hazards

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.

Documents

Low Pressure and Vacuum Hazards