Page 96 - Root Cause Failure Analysis

Pumps 87

With two exceptions, the certified hydraulic curve for any centrifugal pump provides
the data required to calculate the actual brake horsepower. Those exceptions are spe-
cific gravity and TDH.

Specific gravity must be determined for the particular liquid being pumped. For
example, water has a specific gravity of 1.0. Most other clear liquids have a specific
gravity of less than 1.0. Slurries and other liquids that contain solids or are highly vis-
cous materials generally have a higher specific gravity. Reference books, like Inger-
sol1 Rand’s Cameron Hydraulic Databook, provide these values for many liquids.

The TDH can be measured directly for any application using two calibrated pressure
gauges. Install one gauge in the suction inlet of the pump and the another on the dis-
charge. The difference between these two readings is the TDH.

With the actual TDH, flow can be determined directly from the hydraulic curve. Sim-
ply locate the measured pressure on the hydraulic curve by drawing a horizontal line
from the vertical axis (i.e., TDH) to a point where it intersects the curve. From the
intersection point, draw a vertical line downward to the horizontal axis (i.e., flow).
This provides an accurate flow rate for the pump.

The intersection point also provides the pump’s efficiency for that specific point.
Since the intersection may not fall exactly on one of the efficiency curves, some
approximation may be required.

lnstallation

Centrifugal pump installation should follow the Hydraulic Institute standards, which
provide specific guidelines to prevent distortion of the pump and its baseplate. Distor-
tions can result in premature wear, loss of performance, or catastrophic failure. The
following should be evaluated as part of a root cause failure analysis: foundation, pip-
ing support, and inlet and discharge piping configurations.

Foundation
Centrifugal pumps require a rigid foundation that prevents torsional or linear movement
of the pump and its baseplate. In most cases, this type of pump is mounted on a concrete
pad having enough mass to securely support the baseplate, which has a series of mount-
ing holes. Depending on size, there may be three to six mounting points on each side.

The baseplate must be securely bolted to the concrete foundation at all these points.
One common installation error is to leave out the center baseplate lag bolts. This per-
mits the baseplate to flex with the torsional load generated by the pump.

Piping Support

Pipe strain causes the pump casing to deform and results in premature wear or failure.
Therefore, both suction and discharge piping must be adequately supported to prevent