Summary
Refurbishing large non-return valves should reduce risk, not create it. But change the closing time on a 72-inch valve and you can aggravate reverse pump rotation, raising the risk of excessive shaft torque, valve torque, and motor overcurrent, potentially taking out every pump and forcing a multi-week plant shutdown.
That is exactly what Sasol faced during a valve refurbishment project on a petrochemical cooling water system: five pumps, 100,000 m3/h (440,000 gpm) of flow, and a narrow window between too fast and too slow.
On September 8, Trey Walters, Principal, R&D at Datacor, and Nelius Joubert, Principal Process Engineer at Sasol, walked through the full analysis live. Access to the full recording is above.
The Challenge
Sasol's cooling water system supplies 100,000 m3/h (440,000 gpm) at 450 kPa through five pumps and 72-inch hydraulically actuated non-return valves, with surge tower protection in place. A valve refurbishment project raised a critical question: would a new valve closure time protect the system, or introduce new risk?
The Approach
Using Datacor Impulse (formerly AFT Impulse), the team modeled valve closure times from 4 to 40 seconds, calibrated against real field pressure data, motor current readings, and pump stopping times. Four-quadrant pump modeling captured what happens during a pump trip, including reverse flow and reverse rotation, rather than assuming the pump simply coasts to a stop.
What the Data Showed
- A 40-second valve closure allowed a pump to reach 286 rpm in reverse, 87% of synchronous speed, and pushed motor current on the remaining operating pumps past a 350 A trip threshold.
- A 6-second closure generated valve differential pressure and torque high enough to raise its own concerns.
- The safe range for this system: 6 to 12 seconds.
- The surge tower eliminated water hammer across every scenario tested.
- Model the full transient, not just steady-state flow. Steady-state analysis will not surface reverse rotation, valve torque spikes, or overcurrent risk.
- Use four-quadrant pump modeling to capture pump behavior across normal, zero-flow, and reverse-flow conditions.
- Calibrate the model against field data (pressure, motor current, pump stopping time) rather than assumptions alone.
- Test a range of valve closure times to find the safe window, not just the current or proposed setting.
Best Practices for Modeling Pump Transients With Reverse Flow
- Model the full transient, not just steady-state flow. Steady-state analysis will not surface reverse rotation, valve torque spikes, or overcurrent risk.
- Use four-quadrant pump modeling to capture pump behavior across normal, zero-flow, and reverse-flow conditions.
- Calibrate the model against field data (pressure, motor current, pump stopping time) rather than assumptions alone.
- Test a range of valve closure times to find the safe window, not just the current or proposed setting.
See This in Your Own System
If your plant has pumps that trip, valves that close, or a shutdown scenario you have never modeled, the questions Sasol answered apply to you too. Datacor Impulse is built to find the answers before they show up in the field.
