Head Curves

Chapter from the Pumps & Hydraulics Design Guide.

PUMP CURVE INTERPRETATION & SYSTEM MATCHING

Key Pump Curve Parameters

Parameter	Definition	Typical Range	Impact of Operating Off-Design	How to Verify in Field
Best Efficiency Point (BEP)	Flow at maximum pump efficiency	Peak of efficiency curve	Reduced bearing/seal life, vibration	Measure flow, compare to curve
Preferred Operating Region	80–110% of BEP flow	Per API 610	Acceptable continuous operation	Verify flow is in range
Allowable Operating Region	70–120% of BEP flow	Per manufacturer	Acceptable for limited periods	Monitor vibration, bearing temp
Shutoff Head	Head at zero flow	110–140% of rated head	Maximum pressure on system	Block discharge, read pressure
Minimum Continuous Flow	Lowest stable flow	Varies, 30–60% BEP	Recirculation, vibration, heating	Never operate below
Runout Flow	Flow at zero head (end of curve)	150–200% BEP	Maximum flow, motor overload risk	Check motor amps at high flow

System Curve Matching

System Head = Static Head + Friction Head
• Static Head: fixed (elevation difference + pressure difference)
• Friction Head: varies with Q² (H_f = K × Q²)

Operating Point: intersection of pump curve and system curve

Common Issues:
• Pump curve too flat + system mostly static head → unstable operation, flow hunting
• Pump curve too steep + system mostly friction → pump sensitive to downstream changes
• Operating far right of BEP → cavitation risk, motor overload
• Operating far left of BEP → recirculation, seal/bearing damage

Parallel & Series Operation

Parallel Pumps (same discharge header):
• Combined curve: add flows at same head
• Both pumps must have stable, rising curves
• Flat curves can cause one pump to "dead-head" the other
• Check valve required on each pump discharge
• Each pump must be capable of operating alone (if one trips)

Series Pumps (booster + main):
• Combined curve: add heads at same flow
• Intermediate pressure must be within booster casing rating
• Common for high-head applications (BFW, injection)
• First pump NPSH sets the system

Source: Pumps_Hydraulics_Design_Guide_v1.xlsx · sheet “Head Curves”

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