OTP Industrial Solutions for Pump Repair
Written by Tom Bland
September 15, 2009
We received a call from a large stone quarry requesting a budget quote to rebuild a 200 HP, 4-stage vertical turbine pump used for quarry dewatering. We visited the site to review the installation and to gather additional information as to the nature of the problem. While driving deep into the quarry, the site manager explained that these pumps were mounted ten feet above the normal high level and would draw down to 23 feet to the minimum level. The pumps are also mounted on a mezzanine-like structure. This high mounting distance, and the mezzanine mounting was necessary to prevent flooding of the motors should the quarry lose power or experience an abnormally heavy rainfall.
As we climbed the stairs to the top of the structure, the excessive vibration became very evident. In fact, the whole structure had such severe vibration that we questioned the ability of the welds and fasteners to keep it together!
Although the pumps were clearly causing excessive vibration, we decided to confirm the pumps were correctly sized for the job before we assumed that a rebuild was required.
After gathering the necessary information including the pump discharge pressure of 70 psi and the distance from the gauge to the water level, we then compared the actual operating conditions against the design conditions.
The design condition was 2200 gpm at 250 feet of head with a requirement of 34 feet of net positive suction head (NPSH).
Calculated NPSH available:
33 ft. atmospheric pressure
– 1 ft. vapor pressure at 75F
+ 3 ft. minimum submergence of bowl assembly at low level cut-off
35 ft. NPSH available
The NPSH available is barely sufficient for the design condition requirement of 34 feet, but more than ample for higher water levels. But is the pump operating at design condition?
70 psi discharge pressure x 2.31 = 162 ft. head
Distance from gauge to low water level = 23 ft.
Operating condition 185 ft. head
Referring to the pump curve, at 185 ft. hd. this pump will deliver 2600 gpm with 50 ft. of NPSH required! The NPSH required (50 ft.) exceeds the NPSH available (35 ft.) by 15 ft.
At run-out, or maximum flow, of 2860 gpm at 141 ft. head, this pump requires 72 ft. of NPSH. It was clear that cavitation was the cause of the vibration.
Returning to the pump, we throttled the discharge valve to return the pump to the original design condition and as expected, the vibration disappeared. This unit also operates in parallel with additional pumps. When all three pumps are running, the total discharge head (TDH) is sufficient to restrict the flow and keep the pump near the original design point. But operation of this pump alone or with only one additional pump will always require close monitoring and throttling to prevent excessive wear or failure due to cavitation.
By reviewing the installation prior to removal, we prevented an unnecessary rebuild on this VTP pump.
Proper selection of all design conditions is critical to correct pump selection. In this real world example, the VTP pump supplier did not consider that the pump may be operated beyond the design condition. In fact most pumps operate outside of their design condition as they are typically oversized for worst case conditions.