Field-style article image prepared for pump shut off head.
Pump shut-off head is the maximum head a centrifugal pump can develop when the discharge valve is fully closed and flow drops to zero. It appears as the leftmost point on the pump performance curve where the head-capacity line intersects the vertical axis. Engineers use this value to size pressure instruments, check system compatibility, and verify that downstream components can withstand the highest possible discharge pressure the pump will generate.
Shut-off head differs from the pump duty point (https://industrialmonitordirect.com/blogs/knowledgebase/centrifugal-pump-shut-off-head-definition-testing-safety), which is the operating point where the system curve meets the pump curve during normal flow. Running a pump continuously at shut-off is called deadheading and typically damages the pump through overheating and mechanical stress.
Key Takeaways
- Shut-off head is always higher than the rated head at the pump’s best efficiency point
- Pressure gauges and transmitters must be rated above shut-off head converted to pressure units
- The shut-off head rise—difference between shut-off and duty head—varies by impeller design and specific speed
- Pumps with flat curves show minimal head rise; steep curves show significant rise that requires wider instrument ranges
- System design must account for shut-off pressure in pipe class ratings, valve body ratings, and relief valve settings
Reading Shut-Off Head From Pump Curves
The pump manufacturer’s curve plots head (vertical axis) against flow rate (horizontal axis) for a given impeller diameter and speed. Shut-off head is the head value where the curve intersects the vertical axis at zero flow.
For a pump rated at 100 feet of head at 500 GPM, the shut-off head might read 115 feet. That 15-foot difference is the head rise, expressed as a percentage: (115 – 100) / 100 = 15% rise.
Manufacturers test shut-off head by closing the discharge valve during a shop test and recording the pressure at the discharge flange. The head value H (in feet) relates to pressure P (in psi) through the fluid specific gravity SG:
H = (P × 2.31) / SG
For water (SG = 1.0), a shut-off pressure of 50 psi converts to 115.5 feet of head. For fluids with different densities, adjust the specific gravity accordingly.
Why Shut-Off Head Controls Instrument Sizing
Pressure instruments must be sized for shut-off conditions (https://industrialmonitordirect.com/blogs/knowledgebase/pump-shut-off-head-pressure-instrument-sizing-calculations), not normal operating pressure. If the pump duty point is 100 feet but shut-off is 115 feet, a gauge sized only for 100 feet will peg and potentially fail when someone closes a valve during maintenance or troubleshooting.
Standard practice adds a safety margin above shut-off pressure. For a pump with 115 feet shut-off head, convert to pressure (approximately 50 psi for water) and select an instrument with a range of 0-100 psi, giving roughly 2:1 margin.
Process transmitters follow the same logic. A 4-20 mA transmitter calibrated to span the normal operating range will saturate at 20 mA if pressure reaches shut-off during a transient. Spanning the transmitter from zero to 1.5× shut-off pressure maintains measurement capability during valve closures and startup sequences.
Pump Curve Shape and Head Rise Behavior
The magnitude of head rise depends on the pump’s specific speed and impeller geometry. Radial-flow impellers (low specific speed, high head per stage) typically show 10-25% head rise from duty point to shut-off. Mixed-flow and axial-flow designs (high specific speed, low head per stage) may show 5-10% rise or even a drooping curve where shut-off head is lower than rated head.
A steep pump curve (https://www.eng-tips.com/threads/pump-shut-off-head.110888/) means head changes significantly with small flow changes. These pumps show larger shut-off head rises and require more careful pressure rating of downstream components. Flat curves provide more stable pressure across the operating range but may have broad shut-off zones where the pump can operate at multiple flow points for the same head.
Engineers check the curve shape when selecting pumps for systems with variable demand. A flat curve tolerates flow variation without large pressure swings, while a steep curve provides better flow control but needs tighter system design margins.
System Design Checks Against Shut-Off Pressure
Every component downstream of the pump discharge must withstand shut-off pressure continuously, since valve closures, blockages, or control failures can expose any part of the system to maximum pump pressure.
Check these ratings against shut-off head converted to pressure:
- **Pipe pressure class**: ANSI 150, 300, etc., must exceed shut-off pressure with appropriate safety factor per B31.3 or applicable code
- **Valve body ratings**: gate, globe, check, and control valves must all be rated for shut-off pressure at operating temperature
- **Heat exchanger and vessel design pressure**: specified as maximum allowable working pressure (MAWP) on the data sheet
- **Relief valve set point**: typically set 10% above shut-off pressure to protect against pressure excursions while avoiding nuisance lifting
For a pump with 50 psi shut-off pressure in a water system, ANSI Class 150 flanges (rated approximately 285 psi at ambient temperature for most materials) provide adequate margin. In high-temperature or corrosive service, derate the flange class per ASME B16.5 temperature-pressure tables.
Testing and Verification During Commissioning
Field verification of shut-off head confirms that the installed pump matches the manufacturer’s curve and that no installation issues affect performance. The test requires closing the discharge valve slowly while monitoring discharge pressure with a calibrated gauge.
Start with the pump running at normal flow. Close the discharge valve in increments while watching suction pressure, discharge pressure, pump current, and bearing temperatures. The test should take 30-60 seconds total to avoid overheating.
Record discharge pressure at the instant flow stops (discharge valve fully closed). Convert pressure to head and compare against the curve value. Acceptance criteria typically allow ±5% deviation from the published shut-off head.
If measured shut-off head is significantly low, check for impeller diameter mismatch, speed error (VFD frequency setting), worn wear rings, or internal recirculation. High shut-off head may indicate incorrect impeller diameter, higher-than-rated speed, or specific gravity error in the head calculation.
Never leave a centrifugal pump running at shut-off for more than one minute. The fluid recirculates inside the pump casing, temperature rises rapidly, and mechanical seals or packing can fail. Some pump designs include minimum-flow bypass lines that open automatically to prevent deadhead damage.
Shut-Off Head vs. Duty Point vs. Deadheading
These three terms often confuse buyers because they all relate to the pump curve but describe different operating conditions.
**Duty point** is the intended operating point where the pump will run most of its life. The system curve (static head plus friction losses) intersects the pump curve at this point. You specify the pump based on duty flow and duty head.
**Shut-off head** is a specification point, not an operating point. It’s the maximum head the pump can develop, used for system design and instrument sizing. You don’t want to operate there, but you design for it.
**Deadheading** means running the pump at or near shut-off, which happens when someone closes the discharge valve. Short deadhead periods (less than one minute) occur during commissioning tests. Prolonged deadheading causes cavitation, overheating, and mechanical failure.
The relationship: duty point tells you where the pump runs; shut-off head tells you what the system must withstand; deadheading is what you avoid in operation.
Shut-Off Head in Pump Selection and Specification
When you request a pump quotation, the vendor needs duty flow, duty head, fluid properties, temperature, and site conditions. The vendor selects an impeller and provides a certified curve showing performance across the full flow range including shut-off.
Review the shut-off head value and calculate the head rise percentage. If the rise exceeds 20%, confirm that downstream pressure ratings accommodate the higher value. For systems with pressure-sensitive equipment, consider requesting a pump with flatter curve characteristics or specifying maximum allowable shut-off head in the purchase specification.
Multi-stage pumps add complexity because each stage contributes head, and shut-off head multiplies by the number of stages. A three-stage pump with 40 feet shut-off per stage reaches 120 feet total shut-off head. High-stage-count pumps require careful attention to case splitting flanges, shaft seals, and discharge piping pressure class.
Variable-speed pumps operated by VFDs change the entire curve according to affinity laws. Shut-off head varies with the square of speed ratio. A pump with 100 feet shut-off at 1750 RPM will have 64 feet shut-off at 1400 RPM (1400/1750 = 0.8; 0.8² = 0.64). Size instruments for the maximum expected speed, not reduced-speed operation.
FAQs
Can shut-off head exceed the pump’s rated head significantly?
Yes, especially in low-specific-speed radial pumps. A 15-25% rise from duty head to shut-off head is common. This is why you must check the actual curve value rather than assuming shut-off equals rated head.
Do positive displacement pumps have shut-off head?
No, positive displacement pumps (gear, lobe, screw, piston) develop pressure based on system resistance with no maximum head limit. They require relief valves to prevent over-pressurization, unlike centrifugal pumps that self-limit at shut-off head.
How do you handle shut-off head when the pump curve is not available?
Request the certified curve from the manufacturer. If unavailable, assume 20% head rise as a conservative estimate for sizing instruments and checking pressure ratings. Replace with actual values once the curve is provided.
Does shut-off head change as the impeller wears?
Yes, shut-off head decreases as wear ring clearances increase and impeller vanes erode. A pump that originally achieved 115 feet shut-off may drop to 105 feet after several years of service with abrasive fluids. Plan instrument ranges to remain useful across the wear life.
Conclusion
Shut-off head determines the maximum pressure your pump system will see and directly controls how you size pressure instruments, rate piping and valves, and set relief devices. Read it from the leftmost point of the pump curve, convert to pressure using fluid density, and apply it to every component downstream of the discharge flange. Before accepting a pump, verify shut-off head during commissioning by briefly closing the discharge valve and comparing measured pressure to the curve value. When the installed shut-off head matches the specification and your instruments span the full range including safety margin, you’ve closed the loop between pump selection and system protection.