Some safety valves are unable to re-close. This was the observation of a prominent executive during a meeting in 1990 about the various problems related to valve instability and valve closure as well as sealing failures in these important mechanisms.
According to industry standards, the definition of a safety valve is : A safety mechanism that opens automatically to relieve excessive pressure – and which evacuates a sufficient flow of fluid – it closes again automatically by stopping the flow of fluid.
Yet, the fluid flowing past the open valve carries with it kinetic energy. The motive power of that kinetic energy—variable, of course, relative to the conditions—often reaches a megawatt, to give an idea of the order of magnitude. The fluid, ridding itself of such motive power, is capable of causing serious mechanical problems.
Degrading the kinetic energy contained in the evacuating fluid therefore constitutes an urgent priority that is not, to my knowledge, acknowledged in the standards.
In order to improve the safety of installations and prevent harm to people and the environment, this second function of safety valves—that is, degrading the kinetic energy of flows in a controlled manner—should be, in my opinion, included in the standards. This would improve closure in valves.
As soon as the valve is open, the resulting phenomena becomes very complicated to analyze since the flows deeply penetrate the chaos. In applications in power plants, the motive power contained in the fluid can reach extremely high levels; the corresponding kinetic energy generally dissipates in whatever manner possible by transforming into other—often harmful—forms of energy (e.g. sound energy, vibration energy) that may disrupt smooth operation of the installation and weaken the structures.
Malfunctioning valves are a relatively common occurrence in thermal power stations, as well as in chemical and petroleum installations. If valves sometimes frustrate the smooth operation of these industrial installations, it is because their flows are insufficiently controlled.
For example, what initially triggered the chain of events that lead to the partial nuclear meltdown at Three Mile Island was the failure of a relief valve. In response to an open letter, former President Jimmy Carter, a specialist in nuclear engineering, expressed his hope that this opinion would be shared more widely.
The massive and rapid degradation of kinetic energy—a common occurrence in valves and pressure regulators—involves a poorly understood field of thermodynamics. I thought it might be useful to explore the subject in a book of applied physics. This book describes how it is possible, through an unusual use of supersonic flows, to introduce profound disorder at the molecular level in order to escape chaos at the macroscopic level.