Regarding the recommendation not to use diaphragm structures for the seals of light hydrocarbon (flash) pumps

Thread Content

Some may wonder why bellows designs are not recommended for sealing light hydrocarbon (flash hydrocarbon) pumps. Looking at history, metal bellows seals became very popular in the late 1980s and early 1990s; the reason is that, compared to spring-based designs, bellows structures lack a dynamic O-ring, which reduces the number of potential leakage points. In spring-based designs, the dynamic O-ring is constantly in a state of movement, leading to micro-wear and subsequent leakage. Therefore, metal bellows designs are specified for use in most applications. However, in practice, many light hydrocarbon fluids in the field have experienced failure due to bellows rupture. Subsequently, a change was made to avoid using metal bellows seals in fluids that are light hydrocarbons; this is reflected in the description in API682, which states that \"under flashing conditions, viscous sliding vibrations occur due to steam suppression, which in turn leads to fatigue failure of the metal bellows seals.\" ” Simply put, the stick-slip phenomenon involves the dynamic interaction between two surfaces. Generally, when two surfaces are at rest, the force required to initiate motion is greater than the force needed to maintain that motion. Once the applied force exceeds the maximum value of static friction, the friction force decreases to a lower level, namely kinetic friction, which results in a sudden increase in the speed of motion. This behavior stems from the difference between the static friction coefficient and the kinetic friction coefficient, and it can lead to interesting and sometimes unpredictable movements in various systems. For example, when trying to slide a fully loaded cardboard box across hardwood floors, one must push hard on the box in order to overcome the friction that causes it to stick to the floor. But once it starts to slide, movement becomes much smoother; if the speed is reduced, the box will move in a discontinuous manner whenever it is tried to set in motion again. For example, the common stickiness and slippiness phenomenon: the squeaking sound of chalk on a blackboard ； While watching basketball games, one often hears the crisp squeaking sound of basketball shoes on the floor ； The sounds a car makes when braking, and so on. Therefore, the stick-slip phenomenon is a recurring motion that often confuses people; it is a frequent, intermittent movement that alternates between movement and stoppage. Understanding this mechanism is crucial for industries and applications that require controlled and smooth motion. Turning now to the properties of the fluid, light hydrocarbons (vaporizing hydrocarbons): vaporization is the phenomenon by which a fluid rapidly changes from a liquid state to a gas state. In dynamic sealing devices, vaporization occurs when the fluid flows over the sealing surface; friction can increase the energy of the fluid, or the pressure of the fluid may drop below its vapor pressure as it passes over the sealing surface. Light hydrocarbons are non-lubricating fluids with low viscosity; they tend to flash and turn into gas. The lack of lubrication between sealing surfaces can lead to destructive vibrations, namely slippage. Without proper lubrication at the end faces, the ground seal surfaces tend to \"stick\" together; however, when the driving mechanism of the seal rotates and causes the seal surfaces to rotate as well, \"sliding\" occurs” ； Subsequently, due to poor lubrication, the sealing surface will slow down again ； This alternating \"sliding\" and \"sticking\" leads to severe vibrations, and the resulting stresses are transmitted to the bellows welds near the sealing surface, thereby causing failure through fracture.