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Pressure, Pushing Down On Me


Everyone knows what an explosion is, but what about its opposite, an implosion?  An explosion occurs when a large amount of energy is released into a small volume in a very short time, but what happens in an implosion?  Put simply, an implosion is the opposite of an explosion, matter and energy collapse inward and all implosions are caused by some form of pressure acting from the outside on an object.  If that pressure is greater than the pressure within the object, without adequate support, the object will collapse.  This is a real risk in process industries (and submarines!).

Tanks and vessels that carry fluids at normal temperatures and pressures are generally designed to withstand the outward pressure that the fluid within them exerts on their walls.  Their construction is usually cylindrical or ellipsoidal, which helps to distribute the hydrostatic forces evenly, and allows these structures to withstand internal pressures much larger than the vacuums needed to collapse them.  However, such tanks and vessels often collapse when they are emptied without proper venting.  It is also possible to collapse a vented tank, if it is filled with steam and cooled rapidly.  This is by far the most common method of creating an accidental vacuum inside a tank.  If a tank is filled with water vapour (e.g. during steam cleaning), the vapour displaces most of the air out of the tank.  If the tank is then sealed, the vapour condenses back to water as it cools.  A small volume of water is left inside the tank instead of a large volume of water vapour (water occupies approximately 1/1600th of the volume of steam).  This decreases the internal pressure and brings about the risk of a catastrophic collapse.

The implosion of a milk tanker occurred during the ’wash out’ procedure, approximately 15 minutes after it was internally steam cleaned.

But why does the tank collapse? Typically, the pressure both inside and outside the tank is the same, approximately 105 Newtons per square meter, or 14.7 pounds per square inch.  However, with the air in the tank removed, atmospheric pressure pushes on the external surfaces of the tank, creating an unbalanced situation (shown below).  At this point, the structural integrity of the tank wall prevents it from collapsing.  If the tank wall is thick enough, it will remain stable under a full vacuum.  However, if the tank wall is too thin, or there is either a defect or dent in the tank wall, when the even distribution of pressure is lost, stresses will be concentrated at localised points leading to a rapid escalation of the collapse.

A full tank where the pressures are balanced compared to the forces acting upon a tank under a vacuum.

An interesting test that demonstrates this can be completed at home, with the correct safety precautions in place, of course.  If you take a metal can (the ‘tank’) with a screw-on cap, add a small volume of water, which you then bring to the boil, the steam generated displaces the air within the can.  Then, if you screw on the cap, sealing the can from the atmosphere, after just a few minutes, the vapour condenses as it cools to room temperature and the pressure in the can falls.  Eventually, the can implodes!

But beyond steam, what else can cause the implosion of a tank?  This is where mechanical problems and human agency come into play.

Even in the smallest of tank applications, it is imperative that a serviceable vent is installed.  If the rate of air admittance into a tank is lower than the rate at which product is being discharged from the tank, a vacuum will be created, which could cause the tank to collapse.  Hawkins has investigated several instances where tanks have collapsed due to inadequate venting during pumping operations.  A typical example is a slurry tanker, which is used on farms to distribute fertiliser onto fields. 

An imploded slurry tanker. The incident occurred during the filling operation.
An imploded slurry tanker. The incident occurred during the filling operation.

The risk of tanker implosion occurs during the filling operation when a vacuum system is used, which creates an atmospheric pressure difference to either fill or empty the tanker.  By creating a vacuum (depression) in the tank, the slurry can be sucked in.  When spreading, the principle is reversed: the tank is pressurised by the pump, which allows the slurry to be expelled.  In regular operation, if the pressure in the tank becomes too low during suctioning, a spring-loaded release valve will be drawn open by the low pressure within the tank, allowing air into the tank.  This would stop the tank pressure from falling too low, and protect it from the risk of an implosion.  However, if the valve is seized in the shut position, this protection would be lost.  A seized valve in itself may not be enough to allow the collapse of a tanker, as tanks are usually fitted internally with baffles and reinforcing rings, giving them structural support across their diameters.  However, these components are usually manufactured from a low grade carbon steel, and they would be susceptible to corrosion caused by the damp environment, with some of the chemicals in slurry such as uric acid, ammonia and sodium chloride (salt) enhancing the rate of corrosion. 

The wall thickness of the tanker also plays a critical part in the tanker’s strength and resistance to implosion.  The tanker wall can corrode from the inside out, varying the wall thickness, and leaving the tanker vulnerable to collapse under a vacuum.  Hawkins utilises ultrasonic wall thickness testers to determine, in a non-destructive way, the thickness and condition of the tanker walls.  If these integral, structural supports within the tanker are lost, or the walls become too thin, when the pressure in the tanker falls below that atmospheric pressure outside of the tanker, an implosion is highly likely. 

To avoid an implosion, regular service and maintenance of a slurry tanker is imperative.  Internal surfaces need to be cleaned and examined on a regular basis with repairs being completed as necessary.  Furthermore, the vent valve functionality needs to be routinely checked.  By doing this, the risk of an implosion is significantly reduced.

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