Spillback Design

Chapter from the Pumps & Hydraulics Design Guide.

PUMP SPILLBACK & MINIMUM FLOW DESIGN

Minimum Flow Requirements

Limit Type	Basis	Typical Value	How to Determine	Consequence of Violation
Thermal minimum flow	Temperature rise in pump	Varies (5–30% BEP)	ΔT = BHP × 2545 / (500 × Q × Cp × SG)	Vaporization, seal damage, seizure
Hydraulic minimum flow	Suction recirculation onset	50–70% BEP typical	Manufacturer curve / HI guidelines	Vibration, noise, impeller damage
Continuous stable flow	Pump curve stability	Per manufacturer	Minimum flow on pump curve	Surging, unstable operation
Minimum continuous thermal	API 610 thermal limit	Calculate per formula	Energy balance on pump casing	Fluid flashing at wear rings

Spillback Configuration Options

Configuration	Description	Advantages	Disadvantages	Typical Applications
Continuous spillback	Fixed orifice in return line	Simple, reliable	Wastes energy continuously	Small pumps, non-critical
Automatic spillback (FCV)	Flow-controlled recirculation	Energy efficient	More complex, control system req'd	Large pumps, high energy cost
Automatic recirculation valve	Self-contained check/bypass	No control system needed	Higher valve cost, limited sizes	BFW pumps, critical service
Manual bypass	Hand valve in bypass line	Simple, low cost	Operator dependent	Intermittent low-flow operation

Spillback Line Sizing

1. Size for minimum flow requirement (thermal or hydraulic, whichever is greater)
2. Restrict flow with orifice, NOT line size — line sized for velocity ≤15 ft/s
3. Orifice ΔP = pump ΔP at minimum flow minus piping losses
4. Return to suction vessel (NOT suction piping) to avoid recirculation heating
5. Elevation: return nozzle below minimum liquid level to prevent flashing

Source: Pumps_Hydraulics_Design_Guide_v1.xlsx · sheet “Spillback Design”

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