Engineering Problems

The pumps described in the preceding pages, although widely diverse in configuration, still have a number of common problems. First, there is the extreme, paralyzing cold. It is small wonder that the early pumps were considered successful when they did not freeze into immobility. The cold penetrated the motor, causing grease-lubricated bearings to seize and filling the inside of the motor with ice that locked the rotor fan blades and kept them from turning. The piping attached to the pumps shrank, distorting the pump casings into heavy rubbing contact with the impeller. Frost and ice entered the external side of the seal, immobilizing it so that leakage was uncontrollable. It was difficult to find materials for construction that did not become unusually brittle, and there were almost no data available on the total shrinkage of common materials at cryogenic temperatures.

Sources of data on the properties of materials at low temperatures are listed at the end of this section.

Other problems resulting from low-temperature operation are mainly mechanical and are surmounted by using thermal barriers, flexible sections of piping, and dry purge gas to prevent the ingress of moist air. Gasketing and lubrication problems are both currently being solved by materials that did not exist when the problems were originally encountered.

One common problem has to do with the characteristics of the fluids being pumped. Cryogenic fluids are stored in insulated tanks maintained at pressures only slightly above atmospheric, and the fluids become saturated at the storage pressure. Because it is not possible to obtain any pump elevation relative to the tank bottom, net positive suction head becomes a real problem. Various style of inducers have been used to improve the suction performance of the pumps, but in nearly all cases some degradation of performance must be anticipated as the tank is drawn down to low levels and cavitation operation is inevitable. Fortunately, operation in cavitation is not as severe a problem in cryogenic fluids as it is in water, because of the much lower energy of the cryogenic fluid bubble, and so severe cavitation damage to pump parts is seldom encountered.

Testing of pumps for cryogenic service is difficult and requires a heavy investment in test loop equipment. It is also fraught with problems that are peculiar to testing and will not be present in actual operation. One such problem has to do with the recirculation operation in a test loop. The work put into a saturated fluid is usually released by allowing some of the fluid to boil away. Because it proves nearly impossible to extract all the vapor from the liquid, the test fluid specific gravity effectively decreases by an unknown amount. This makes it difficult to establish the exact performance that has been obtained because the data will be lower than true values based on the specific gravity of the unadulterated fluid. It has been found with careful testing that exact duplication of water head-capacity performance will be obtained on any cryogenic fluid, subject only to adjustment for the shrinkage in impeller diameter.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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