Centrifugal Pump Head Curves

1. Purpose

This procedure provides characteristic pump differential head versus capacity curves which predict the developed head at various flows for all types of centrifugal pumps.

2. How to Use the Curves

Use Figure 1 to estimate the change in developed head at various flow rates and to estimate the hydraulics of a circuit. Consult a Rotating Equipment Specialist for any special applications such as a Parex Chamber Circulation pumping service.

These curves alone shall not be used for setting equipment design pressure. Refer to Inflection Point Engineering 501 Guidelines Section 4.3.b for method of estimating pump shutoff pressure and refer to Procedure for guidance on selecting equipment design pressures.

Pumps Operating in Parallel

API Std 610 11th Edition Par. 6.1.11 prefers pumps have stable head/capacity curves (i.e. continuous head rise to shutoff) for all applications and it requires this for parallel operation applications. Continuously rising pump curves are required for pumps operating in parallel to eliminate the possibility of flow imbalances between the two operating pumps. This may result in serious damage to one of the pumps if it were forced to run below minimum continuous flow or, worst case, at shutoff for an extended period of time. Also, the spare pump(s) may be incapable of getting on-line when pumps operating in parallel are not provided with flow bypasses AND the pump head-capacity curves are not continuously rising (stable).

4. Description of Curves

Pump curves were constructed based on a global cross-section of pump supplier’s published performance curves.

a. The upper curve, or “All Other” curve is typical of the majority of pumping applications and it has a relatively steep differential head rise to shutoff. For some high differential pressure pumps where the downstream equipment design pressure is at the border of a flange class break, the design engineer shall consult a rotating equipment specialist to explore whether a specific pump with a less conservative differential head rise to shutoff may be applied.

b. Larger capacity (> 1000 gpm), half speed, low head pumps typically have slightly flatter head curves due to impeller geometry reflected in the higher pump impeller specific speed.

c. For full speed, single, and two stage pumps with relatively low capacities (<125 gpm) and a low flow divided by head ratio (Q/H < 0.1), the head curve tends to be quite flat. This is primarily due to the likelihood of a poor hydraulic fit and is modeled by the bottom curve.

d. The Sundyne pump design utilizes a low specific speed Francis vane-type impeller and this generally results in an unstable, or drooping, head-flow curve. This is the reason Inflection Point Engineering mandates a minimum continuous flow bypass anytime a Sundyne pump is expected to operate below 60% of the rated capacity. Reference Procedure for additional background on Sundyne pumps.

The four vertical lines indicate the normal capacity points corresponding to rated overages of 110, 115, 120, and 125% respectively.

IPE-TM-500-18