When we speak of explosions, we usually think of dynamite or C4, but there are actually many types of explosions. The most common man-made type of explosion is chemical, to which dynamite and C4 as well as other flammable compounds belong. There are also electrical, nuclear and mechanical explosions. No matter what kind of explosion, there will always be heat, destruction, noise and pressure as by-products.
In the case of pressure, however, it can also be the cause of an explosion. In a chemical, nuclear or electrical explosion, pressure comes after the explosion, and the amount of pressure will depend on the magnitude of the explosion. In some instances, the pressure is so great that it creates a shock wave with rapidly expands from the epicenter of the explosion, usually causing massive destruction over a large area in a very short time.
In a mechanical explosion, however, pressure is necessary for the explosion to take place. Mechanical explosions are mainly due to the buildup of pressure within a confined space through the transformation of a liquid substance to gas through heating.
A good example would be throwing a tin can in the fire. If the can is not vented, the contents of the can will expand as it heats up because the liquid is being turned into gas, which has a higher volume, and it has nowhere to go. This increase in volume may happen slowly or fast, but unless there is a reduction in the heat source, the integrity of the tin can will eventually fail, resulting in a rapid release of pressure localized at the point of rupture or failure. This high pressure release can cause the container to fragment, sending projectiles in every direction.
This type of explosion is duplicated, albeit in a larger scale, in boiler room explosions. The Boston Marathon explosion was also a mechanical explosion. In some instances, there may also be a chemical explosion resulting from a mechanical one, such as the heating of pressurized aerosol cans of hair spray, or propane tanks. Heating these containers may cause the liquid inside to expand, but the explosion may set off a chemical reaction i.e. ignite the flammable gas thus increasing the resulting heat and pressure which would make it much more devastating.
High pressure is a destructive force when it comes to explosions. Under controlled, circumstances, however, it can be used for good. An example would be a combustion engine, in which controlled chemical explosions turn fuel into mechanical energy. Without high pressure, many of the processes we take for granted would not be possible.