Flashing hydrocarbon and I.
Here at KSTC, we work on a wide range of turbomachinery components designed for fluids ranging from gases to liquids, including flashing hydrocarbons like propylene which is a flammable fluid with a very high vapour pressure at ambient temperature. We do a lot of pump design, consulting work and problem solving. Many engineers in the gas compressor industries assume pump design simple vanilla, without the added complexities of shock and Mach effects, but pump design can be very problematic.
Maybe the most crucial factor in pump design, relative to other turbomachinery, is the risk of cavitation where the NPSHa is low specially in case of flashing hydrocarbon. Usually, in hydrocarbon services, the suction pressure of the pump is equal to the saturation pressure of the fluid, and the only factor that provides NPSHa is the height of the liquid column in the suction reservoir. The design changes required to minimize cavitation risk range from the correct sizing of the pump inlet to the design of the impeller blades itself at the impeller eye, sometimes referred to the inducer section. Indeed, some pumps require a separate inducer for the sole purpose of minimizing cavitation risk while slowing increasing the pressure in the pump so that local static pressure rises sufficiently above the fluid vapor pressure, allowing downstream components, to be less prone to cavitation.
Another key parameter in hydrocarbon pump design is the high axial thrusts associated with the pressure distribution around the impeller. Handling the high axial thrust loads can be problematic due to restrictions on size and capacity of thrust bearings. It is required to decrease axial thrust load, leading to a whole series of techniques the designer can utilize to decrease the load. These changes usually come at the expense of pump performance, so the designer must carefully balance performance needs with mechanical and reliability requirements when axial thrust load is a problem. Features that have arisen due to the need to control thrust include holes through the impeller itself connecting the pump inlet to the impeller back-face to allow for better pressure distribution and hence thrust balance. Sometimes the attempt to control the thrust load may lead to a decrease in seal stuffing box pressure, and this issue is extremely critical for flashing hydrocarbons. Because it is possible that the reduction of pressure and the increase in fluid temperature caused by seal face loses may lead to the flashing of hydrocarbons and make it difficult to seal. In such cases the designer must use non-contact dry gas seal instead of mechanical seal. It may seem strange to use dry gas seals for the pump, but in some cases, it may be the only and best solution.