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HAZARDSLow Pressure and Vacuum Systems

HSE TrainingPrepared: March 2002

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PRESSURE SAFETY

Can you get hurt with low pressure?

Force = Pressure multiplied times Area

Lets review the details.

Its like many smallweights sitting on a

surface.

AND many small weightsadd up to a big weight.

So at a given pressure,

the LARGER the area

The LARGER the force.The next graphic is slow so give it a few seconds after clicking

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Force = pressure multiplied by area so:

EXAMPLE # 1

10 psi pressure on a 12x12 square surface area (144 squareinches) is 1,440 pounds of force!

Calculation: 144 X 10=1,440 lbs of force.This is equivalent to an object that weighs 1,400 lbs.

12 X 12 Square Door

10 psig

1,440 Pounds of Force

And you can bet the door weighs less than 1,400 lbs -- if suddenly released it goes flying.

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10 psi pressure on a 24x24 square surface area (576 squareinches) is 5,760 pounds of force!

Calculation: 576 X 10= 5,760This is equivalent to an object that weighs 5,760 lbs.

24 X 24 Square Door

10 psig

5,760 Pounds of Force

Force = pressure multiplied by area so:

EXAMPLE # 2

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3,140 Pounds of Force

20 in. Manway

10 psi pressure on a 20 manway (314 square inch surface area) is3,140 pounds of force!

Calculation: 314 X 10= 3,140This is equivalent to an object that weighs 3,140 lbs.

And a 20 manway only weighs about 275 lbs.

Force = pressure multiplied by area so:

EXAMPLE # 3

10 psig

Again -- if suddenly released it goes flying.

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Can you get hurt with low pressure?

ABSOLUTELY !

BE AWARE

And be especially careful with large surfaceslike manways. 1/2 psi may not even register on

the gauge but its enough to send a hatchflying if all the bolts are removed and thegasket is stuck.

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Low Pressure Hazard

This door had theequivalent of 1915 lbsof force on it. And atonly 2.8 psi.

The door only weighsabout 15 pounds --much less than the1915 lbs of force on it.Therefore, when itcame loose, it slammedopen seriously injuringone of our coworkers.

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VACUUM SAFETY

The same conceptsapply to vacuum However, in vacuum

systems the pressure is

pushing inward , notoutward. The pressure comes

from the atmosphere --we dont feel it but atank does when you pullvacuum on it.

Vacuum

Atmospheric Pressure

Atmospheric Pressure (at sea level) is about 14.7 psi,therefore full vacuum is -14.7 psi.

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Covered Vent

This tank collapsed while being pumped out!Painters had covered the vent with plasticsheeting. The steel tank collapsed before theplastic sucked through.

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Railcars

No match for a closed vent while pumping outthe car.

Dont you figure the person standing here gota sinking feeling.

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Cant Happen Here!

Or Can It??

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Final Thoughts

Never underestimate the potential of a lowpressure or vacuum condition to causedamage.

Be especially careful when working around or removing large manways or hatch covers. Make a final check just before the job begins to confirm that ALL

the pressure is bled off -- it only takes one valve leaking through just a little.

Leave a few bolts in (but loose) until the gasket seal is broken. If the system has a little pressure this will keep the manway fromstriking someone.

It doesnt look like a big hazard -- but under the right conditions it can be!