Volute lip (tongue) cracking
Q - We have four, single stage, double suction, double volute, horizontal pumps
directly coupled to steam turbines. Each pump has rated flow, head, and speed of
9250 GPM, 1850 FT, and 4500 RPM respectively. They have been in service for 10
years pumping water.
Recently we found that their volute lips are cracked. The cracks are nearly straight
about 3/16" to 1-3/4" long. The damages are similar on all units except for one
Y-shaped crack, 2-1/8" long, that appears to be from initial casting defect and weld
repair done by the vendor. The volute lips are 5/16" thick and 3-1/4" wide. There is
no sign of severe cavitation or erosion damage on the volutes, or on the impellers.
The margin between NPSHA and NPSHR is ample.
The case material is A487-CA6NM. We tried to weld repair one unit but the result
is unsatisfactory. What are the possible causes of the cracks? Any suggestion on
repairing the cracked volute lips?
A - A487-CA6NM is readily weldable but successful result depends on qualified
welding procedure and the qualification of the welder. Post-weld heat treatment is
recommended to avoid imbrittlement of the welded part.
Investigate the possible presence of H2S in the process water. Hydrogen stress
cracking is characterized by sharp singular cracks, mentioned above, as opposed
to cracks that branch out. If H2S were present the preferred material is A487
-CA6NM CLB with typical HRC of 23 (recommended by API) compared to CLA with
typical HRC of 25. The localized cracking indicates that hydrogen stress cracking
was accelerated by very high liquid velocity at the entrance of the volute lips (the
point in a casing where liquid velocity is highest.)
In the absence of H2S, investigate the possibility of fatigue failure of the volute lips
due to high cyclic radial loads imposed on the volute lips by the impeller at
vane-pass frequency. Fatigue failure can also be accelerated by very high liquid
velocity at the entrance of the volute tongues.
Contact the original pump vendor for their recommendations.
C: design, trouble-shooting
F: volute cracking
"Make it simple"
Severe volute lip cracking can be
seen on the bottom left side of this
photo. Erosion damage can also be
seen in the volute.