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Engineering A nswers 1. A. Suction pressure hold back valve Correct answer. This valve is situated in the compressor suction line and senses the pressure downstream at the compressor inlet. When box temperatures are in range, the compressor suction pressure is in the normal range and below the set point of the suction pressure hold back valve; and, consequently, the valve is wide open and the compressor capacity is not limited. With higher box temperatures, however, the suction pressure will rise to the set point of the valve, and the valve will then throttle the Áow of suction gas to the compressor, reducing the compressor volumetric displacement during the initial pull-down period. The suction pressure hold back valve is also known as a crankcase pressure regulator. B. Incorrect answer. This is a safety switching device, which shuts down the compressor in the event of unusually high discharge pressure, regardless of the cause. It does not directly sense the compressor suction pressure and does not function to prevent excessive gas pressure at the compressor suction. High pressure cutout C. Solenoid valve D. Low pressure cutout Incorrect answer. This is an operating control switching device, which normally starts and stops the compressor and is part of a pump down circuit, which also includes thermostatically controlled box solenoid valves. Even though it does directly sense suction pressure, it does not function to prevent excessive gas pressure at the compressor suction. In fact, it closes (to start the compressor) on a rise in suction pressure and opens (to stop the compressor) on a fall in suction pressure. A. reduces radial thrust on the impeller Correct answer. A centrifugal pump double volute casing has a dividing wall existing in the volute through 180 degrees. This dividing wall is designed to neutralize the radial reaction forces when at less than design capacity. As such, the radial loading on the pump shaft bearings is minimized throughout the entire range of pump capacities. B. 2. Incorrect answer. This is a generic term referring to any number of electro-magnetically operated valves in the system, which depending on the application may vary widely in terms of function. Incorrect answer. The liquid velocity through the pump is a function of impeller speed and diameter. All other factors being equal, there would be no essential difference between liquid velocity through a single volute pump as compared to a double volute pump. double the liquid velocity through the pump when compared to a single volute C. reduce the hydraulic end thrust D. provide the effect of multistaging 3. Incorrect answer. A centrifugal pump double volute casing is designed to handle issues related to radial thrust, and is not designed to handle any issues related to end (axial) thrust. Centrifugal pumps are sometimes designed with an impeller with a double suction, which is used to neutralize axial (end) thrust. This is not to be confused with a double volute casing. Incorrect answer. Multi-staging is used to increase the pressure output of a centrifugal pump beyond what one pump impeller can deliver. This increase in pressure output is accomplished by the use of multiple impellers, with each impeller discharging into the suction of the impeller of the following stage. The centrifugal pump double volute casing does not increase pressure over that of a single volute casing. A. valve spring Incorrect answer. The valve spring is the closing force for the valve itself. Its function is to keep the valve seated when the valve lifter or cam follower is on the base circle of the cam. It has no direct impact on valve lash. B. Correct answer. The plunger spring acts so that the pushrod seat remains in contact with the pushrod at all times. As such, the hydraulic valve lifter maintains "zero" valve lash. plunger spring C. oil pressure Incorrect answer. Whether valve lifters or cam followers are mechanical or hydraulic in nature, oil is required as a lubricant, as the device reciprocates within its bore. With hydraulic valve lifters, oil is also used as a hydraulic medium to make up for changes in overall lifter length; however, it is the plunger spring itself which maintains "zero" lash. D. rocker arm Incorrect answer. The rocker arm is designed to translate the upward motion of the valve lifter or cam follower to the downward motion of the diesel engine valve and vice versa. The rocker arm is used to set valve lash on mechanical lifters. The rocker arm does not maintain 'zero' lash. Only hydraulic valve lifters do, by the action of the plunger spring. 68 Proceedings Winter 2012 | Spring 2013 www.uscg.mil/proceedings