Radially Split Double-Casing Pumps
The oldest form of radially split casing multistage pump is commonly called the ring-section, ring-casing, or the doughnut type. When more than one stage was found necessary to generate higher pressures, two or more single-stage units of the prevalent radially split casing type were assembled and bolted together.
In later designs, the individual stage sections and separate suction and discharge heads were held together with large throughbolts. These pumps, still an assembly of bolted-up sections, can present serious dismantling and reassembly problems because suction and discharge connections have to be broken each time the pump is serviced. The double-casing pump retains the advantages of the radially split casing design and minimizes the dismantling problem.
The basic principle consists of enclosing the working parts of a multistage centrifugal pump in an inner casing and building a second casing around this inner casing. The space between the two casings is maintained at the discharge pressure of the last pump stage. The construction of the inner casing follows one of two basic principles: (1) axial splitting (see Figure 21) or (2) radial splitting (see Figure 22).
The double-casing pump with radially split inner casing is an evolution of the ring casing pump with added provisions to ease dismantling. The inner unit is generally constructed exactly as a ring-casing pump. After assembly, it is inserted and bolted inside a cylindrical casing that supports it and leaves it free to expand under temperature changes. In Figure 23, the inner assembly of such a pump is being inserted into the outer casing. Figure 24 shows the external appearance of this type of pump. The suction and discharge nozzles form an integral part of the outer casing, and the internal assembly of the pump can be withdrawn without disturbing the piping connections.