Page 93 - Root Cause Failure Analysis

84 Root Cause Failure Analysis

Total System Head
Centrifugal pump performance is controlled by the total system head (TSH) require-
ment, unlike positive-displacement pumps. TSH is defined as the total pressure
required to overcome all resistance at a given flow. This value includes all vertical lift,
friction loss, and back pressure generated by the entire system. It determines the effi-
ciency, discharge volume, and stability of the pump.

Total Dynamic Head
The total dynamic head (TDH) is the difference between the discharge and suction
pressure of a centrifugal pump. This value is used by pump manufacturers to generate
hydraulic curves, such as those shown in Figures 7-5,7-6, and 7-7. These curves rep-
resent the performance that can be expected for a particular pump under specific oper-
ating conditions. For example, a pump having a discharge pressure of 100 psig
(gauged pounds per square inch) and a positive pressure of 10 psig at the suction will
have a TDH of 90 psig.

Hydraulic Curve
Most pump hydraulic curves define pressure to be TDH rather than actual discharge
pressure. This is an important consideration when evaluating pump problems. For
example, a variation in suction pressure has a measurable impact on both the dis-
charge pressure and the volume. Figure 7-5 is a simplified hydraulic curve for a sin-
gle-stage, centrifugal pump. The vertical axis is TDH and the horizontal axis is the
discharge volume or flow.

a Iu 70% 75%

Figure 7-5 Simpb hydraulic curve for centrifugal pump.