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

1,440 Pounds of Force

12” X 12” Square12” 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 12”x12” square surface area will be 1,440 pounds of force.

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 tankThis tank was fitted with:

a high level alarm, which was accepted, and then forgottena pressure control system, which was out of servicea 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 problemThis 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 happenon 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.