HAZARDS Low Pressure and Vacuum Systems HSE Training Prepared: March 2002

PRESSURE SAFETY Can you get hurt with low pressure? Lets review the details. Force = Pressure multiplied times Area It’s like many small weights sitting on a surface. AND many small weights add 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

EXAMPLE # 1 Force = pressure multiplied by area so: 10 psig 12” X 12” Square Door 10 psig 1,440 Pounds of Force 10 psi pressure on a 12”x12” square surface area (144 square inches) 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. And you can bet the door weighs less than 1,400 lbs -- if suddenly released it goes flying.

EXAMPLE # 2 Force = pressure multiplied by area so: 10 psig 24” X 24” Square Door 10 psig 5,760 Pounds of Force 10 psi pressure on a 24”x24” square surface area (576 square inches) is 5,760 pounds of force! Calculation: 576 X 10= 5,760 This is equivalent to an object that weighs 5,760 lbs.

Again -- if suddenly released it goes flying. EXAMPLE # 3 Force = pressure multiplied by area so: 10 psig 20 in. Manway 3,140 Pounds of Force 10 psi pressure on a 20” manway (314 square inch surface area) is 3,140 pounds of force! Calculation: 314 X 10= 3,140 This is equivalent to an object that weighs 3,140 lbs. And a 20” manway only weighs about 275 lbs. Again -- if suddenly released it goes flying.

Can you get hurt with low pressure? BE AWARE Can you get hurt with low pressure? ABSOLUTELY ! And be especially careful with large surfaces like manways. 1/2 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 the equivalent of 1915 lbs of force on it. And at only 2.8 psi. The door only weighs about 15 pounds -- much less than the 1915 lbs of force on it. Therefore, when it came loose, it slammed open seriously injuring one of our coworkers.

VACUUM SAFETY Atmospheric Pressure The same concepts apply to vacuum However, in vacuum systems the pressure is pushing inward, not outward. The pressure comes from the atmosphere -- we don’t feel it but a tank does when you pull vacuum on it. Atmospheric Pressure Vacuum Atmospheric Pressure (at sea level) is about 14.7 psi, therefore full vacuum is -14.7 psi.

VACUUM SAFETY If a tank is not designed for vacuum, odds are it will be damaged if placed under vacuum. Low pressure storage tanks and railcars are particularly susceptible to damage. This is why those low pressure switches on the suctions of blowers are so important -- you can’t manually shut the blower down fast enough to avoid damage if the blower pulls vacuum on a tank. Not only is the equipment damaged but the hydrocarbon contents may also be released.

Covered Vent This tank collapsed while being pumped out! Painters had covered the vent with plastic sheeting. The steel tank collapsed before the plastic sucked through.

Railcars No match for a closed vent while pumping out the car. Don’t you figure the person standing here got a sinking feeling.

Can’t Happen Here! Or Can It??

Final Thoughts Never underestimate the potential of a low pressure or vacuum condition to cause damage. 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 from striking someone. It doesn’t look like a big hazard -- but under the right conditions it can be!