8.7 Special Pumps
311
8.6.7 Wear Rings
Clearance seals are used in pumps for separating the outlet area of the rotor from the
suction eye and for forming chambers for balancing the axial force. The clearances
are very small, typically of the order of 1.5–2 ‰ of the diameter in order to keep the
leakage flow small. Although there is theoretically no metal contact, the surfaces of
the clearances wear due to the high shear stress in the leakage flow. Therefore, often
on both sides of the gap, rings in hard metal are placed. These rings are called wear
rings. When the leakage flow becomes too large (two to three times the design value), these rings are replaced. In many pumps, there are no wear rings on the impeller
side (Figs. 8.21 and 8.22), which makes the pump more reliable. The casing wear
rings may then be replaced together with the impeller. Often, this is feasible since
the time between necessary replacements may be very long, of the order of several
years, and the impeller blades also wear over such a long time. It is also possible to
replace the casing wear ring by a slightly thicker version and reduce the diameter of
the corresponding collar on the impeller by turning on a lathe.
8.7 Special Pumps
Pumps are manufactured in many variants. It is not the objective of this book to discuss the different builds. Practical information may be obtained from the websites
of manufacturers or from books on practical aspects like the ones of Karassik and
McGuire [8] and Girdhar and Moniz [5] . In this section, a limited choice is made
of some special cases.
Fig. 8.22 Balance disc for a
multistage pump
311
8.6.7 Wear Rings
Clearance seals are used in pumps for separating the outlet area of the rotor from the
suction eye and for forming chambers for balancing the axial force. The clearances
are very small, typically of the order of 1.5–2 ‰ of the diameter in order to keep the
leakage flow small. Although there is theoretically no metal contact, the surfaces of
the clearances wear due to the high shear stress in the leakage flow. Therefore, often
on both sides of the gap, rings in hard metal are placed. These rings are called wear
rings. When the leakage flow becomes too large (two to three times the design value), these rings are replaced. In many pumps, there are no wear rings on the impeller
side (Figs. 8.21 and 8.22), which makes the pump more reliable. The casing wear
rings may then be replaced together with the impeller. Often, this is feasible since
the time between necessary replacements may be very long, of the order of several
years, and the impeller blades also wear over such a long time. It is also possible to
replace the casing wear ring by a slightly thicker version and reduce the diameter of
the corresponding collar on the impeller by turning on a lathe.
8.7 Special Pumps
Pumps are manufactured in many variants. It is not the objective of this book to discuss the different builds. Practical information may be obtained from the websites
of manufacturers or from books on practical aspects like the ones of Karassik and
McGuire [8] and Girdhar and Moniz [5] . In this section, a limited choice is made
of some special cases.
Fig. 8.22 Balance disc for a
multistage pump