Abstract
Some fundamental and specific hydraulic considerations associated with the operation of centrifugal pumps on condenser cooling-water circulation, condensate extraction and boiler feeding services are discussed and illustrated by simple empirical formulae and practical diagrams.
Attention is drawn to the importance of selecting the correct speed for reliable operation. The hydraulic gradient through a pump is represented diagrammatically to illustrate the meaning of the term “depression head” and to show how this is related to the throttling condition. It is claimed that in certain circumstances a pump can operate advantageously and safely on the throttling point, but otherwise it is essential to avoid what is described as the “cavitation area”.
A relation is established between pump output, “depression head”, and maximum permissible speed. The “speed coefficient” used in the formula is shown to depend on several factors.
For circulating water pumps the relation between “depression head” and suction lift is illustrated. Maximum speed in relation to output and variation of pump type with specific speed and net head are presented graphically.
Diagrams and curves show the fundamental difference in operating methods of extraction pumps on the two main types of surface condenser and on de-aerators. Factors influencing the choice of speed are outlined.
A typical build-up of the pressure characteristics of a boiler feed system is illustrated and the requirements of the system are compared with the characteristics provided by the feed-pumps, both constant and variable speed. Reference is made to the importance of providing suitable pressure-characteristics to ensure satisfactory parallel operation, load sharing and freedom from pressure surge. A formula is developed for calculation of the temperature rise across a feed-pump at any load, and proposals are outlined for keeping this rise within safe limits. Special reference is made to the feed-pump drawing direct from a de-aerator and the problem of avoiding vapour-locking with sudden fall in pressure. The main factors which determine the operating speed of a feed-pump are enumerated. Attention is drawn to the importance of, and means of constructing, the pump to withstand a sudden reduction in temperature without incurring casing joint leakages. The relative merits of placing the feed-pumps before or after the heaters are discussed, and it is claimed that considerable thermal economy is achieved by using high-pressure heaters.
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