5 - Pumps

Abstract

This chapter focuses on pumps and describes affinity laws, horsepower, efficiency, suction systems and impeller among others. Dynamic type pumps obey the affinity laws: capacity varies directly with impeller diameter and speed, head varies directly with the square of impeller diameter and speed, and horsepower varies directly with the cube of impeller diameter and speed. Most pumps need minimum flow protection to protect them against shutoff. At shutoff practically all of pump's horsepower turns into heat, which can vaporize the liquid and damage the pump. Such minimum flow protection is particularly important for boiler feedwater pumps that handle water near its boiling point and are multistaged for high head. The minimum flow is a relatively constant flow going from discharge to suction. In the case of boiler feedwater pumps, the minimum flow is preferably piped back to the deaerator. General suction system is an important part of the pump system and should be thought of as a very specialized piping design. Considerable attention must be directed to the pump suction piping to ensure satisfactory pump operation.