Pump Dead-Head, Dry Running and Run-Out on VFD Pumps: Causes and Protection

BEP, POR and why they matter

Every centrifugal pump has a best efficiency point (BEP): the flow at which internal flow is smoothest and radial forces on the impeller and shaft are lowest. ANSI/HI 9.6.3 defines a preferred operating region (POR), typically about 70–120% of BEP flow for most centrifugal pumps, where seal and bearing life stay close to design. The wider allowable operating region (AOR) set by the manufacturer is acceptable, but life is noticeably shorter.

• Left of BEP (low flow): higher radial load, shaft deflection, wear-ring rubbing, internal recirculation, vibration and heating of the liquid in the seal chamber.
• Right of BEP (high flow): higher NPSH required, cavitation risk, motor overload, noise and vibration.

Dead-head: running against a closed system

Dead-head means the pump runs with no discharge flow. Typical causes are a closed or stuck discharge or check valve, system pressure above the pump’s shut-off head (common when a weak pump runs in parallel with stronger ones), or a VFD turned down too far in a system with high static head. Almost all shaft power becomes heat in the small volume of liquid in the casing. As a rough example, about 30 kW into about 20 kg of water raises its temperature by around 20 °C per minute. Within minutes the liquid at the seal faces can flash to vapour, cracking seal faces, hardening O-rings and overloading bearings.

Dry running: no liquid at the seal

Dry running happens when the suction tank is empty, the suction valve is closed, gas enters the suction line or the pump is not primed. Mechanical seals depend on the pumped liquid for lubrication and cooling, so face temperature climbs quickly and failure can follow in a short time. Conventional protection uses level switches or low-current relays, but low-current relays can trip falsely on entrained gas or legitimate low-speed operation, and repeated nuisance trips often lead operators to bypass the protection altogether.

Run-out: too much flow

Run-out occurs after a pipe break, when an empty system is started with the discharge valve fully open, or when one pump carries a system designed for two. Flow moves far beyond BEP, NPSH required rises, cavitation and motor overload follow.

Why a VFD can make things worse

The affinity laws say flow scales with speed, head with speed squared and power with speed cubed. At 80% speed a pump delivers about 80% of the flow, 64% of the head and about 51% of the power. That holds only when system losses are mostly friction. In systems with large static head, such as pumping up to a high tank or into a pressurised vessel, head drops faster than the system curve as speed falls, pushing the operating point left toward dead-head while efficiency collapses. A drive lets you choose an operating point; it does not know which point is safe.

The hidden dead-head in parallel pump stations

Running all pumps in a station at the same speed is common practice, but worn pumps develop less head than new ones. At equal speed and a shared header pressure, a worn pump can deliver little or no flow and effectively dead-head while the control panel shows normal operation. The better approach is flow balancing: adjusting each pump’s speed so flow is shared evenly, and running the minimum number of pumps that keeps every one inside its POR.

Sensorless protection using drive data

A modern VFD already knows speed, current, power and torque. Combined with a model of each pump’s curves, a controller can estimate flow and head in real time and detect dead-head, dry running and run-out without adding flow meters or pressure transmitters. Accuracy depends on the pump: where the power curve is very flat, estimation is harder, so each pump should be assessed individually.

PlusSmart ReliaPump

ReliaPump applies these principles to pump stations:

• ReliaPumpIQ protects against dry running, dead-head and run-out, designed to avoid the false trips typical of simple low-current relays.
• ReliaPumpSync balances flow across multi-pump stations instead of forcing equal speeds.

PlusSmart reports a case at an iron-ore export terminal where a multi-pump dust-suppression system had repeated seal failures caused by dead-head. After the issue was addressed, the vendor reports no seal failures since 2015, maintenance cost falling from about AUD 350,000 to under AUD 5,000 per year, and unplanned stops falling from 52 to zero. These are vendor-reported figures.

A practical checklist

1. Plot each pump’s actual operating point on its curve and confirm it sits inside the POR across the speed range.
2. Design VFD control around real pump and system curves, not only pressure or level setpoints.
3. Check parallel stations for weak pumps that dead-head silently, especially with high static head.
4. Use flow balancing rather than equal speeds.
5. Use drive data for sensorless protection, and review seal selection and flush plans rather than replacing seals like-for-like.

How we help

HOANTRANTDH works with PlusSmart on pump reliability projects in Vietnam and ASEAN. Send us DCS or VFD trend data and pump curves and our engineers will identify pumps running outside their POR and propose protection or flow-balancing settings. Learn more about PlusSmart predictive maintenance, pump reliability and seal failure prevention and wireless vibration monitoring for pumps and motors.