The following are some technical notes (In case you want to bypass the equations and theory to get to the Chase)
- For a given set of hydraulic conditions, a centrifugal pump is designed to operate for one set of flow and head. Deviation from this operating point is allowed only to some degree.
- Pump selection closer to the Best Efficiency Point (BEP) will yield a more efficient pump with the least amount of vibration and radial forces acting on the shaft. Pump system resistance curve should be calculated accurately because the pump operates where the performance curve intersects the system curve.
- In the case of single volute pumps, operating away from the BEP will cause the shaft to deflect with bearings and seals rubbing against the casing components. The fluid flow angle into the impeller will also not align to match impeller speeds and vane angles causing suction recirculation, fluid to stall and cavitation.
- It is not always possible to operate the pump at the BEP for the conditions required and hence a flow variation of ±10% of BEP is allowed.
- Minimum stable continuous flow (MSCF) is the minimum flow below which the pump is not allowed to operate. Although API 610 recommends that the rated region is located between 80% to 110% of BEP, the preferred region of flow is between 70% to 120% of BEP.
- Clause 6.1.12 of API 610 11th edition states “Setting limits for preferred operating region and the location of rated flow is not intended to lead to the development of additional sizes of small pumps or preclude the use of high-specific-speed pumps. Small pumps that are known to operate satisfactorily at flows outside of the specified limits and high specific speed pumps that may have a narrower preferred operating region than specified should be offered…” Therefore the Allowable Operating Region is set by the manufacturer as the allowable region to operate with stability whilst conforming to predefined API 610 vibration limits.
- The Net Positive Suction Head Available (NPSHA) should always be higher than the (NPSHR) required.
- Pumps that are expected to operate less frequently can be chosen such that they operate at lower speeds at the cost of efficiency. Since the pump is selected to operate intermittently, a slightly lower efficiency pump is acceptable compared to a higher speed pump. This will also ensure a longer operating life cycle
- Ensure check valves are installed at the pump discharge to prevent the pump impeller from rotating in the opposite direction in the event of a pipeline/pipe surge.
- Centrifugal pumps are always to be operated with suction open with recycling present followed by the partially open discharge lines being fully opened. Failure to keep the discharge line open after a pump startup, would lead to overheating of the pump. Additionally, to avoid overheating, centrifugal pumps must always operate above the MSCF point.
Vijay Sarathy holds a Master’s Degree in Chemical Engineering from Birla Institute of Technology & Science (BITS), Pilani, India and is a Chartered Engineer from the Institution of Chemical Engineers, UK. His expertise over 16 years of professional experience covers Front End Engineering, Process Dynamic Simulation and Subsea/Onshore pipeline flow assurance in the Oil and Gas industry. Vijay has worked as an Upstream Process Engineer with major conglomerates of General Electric, ENI Saipem and Shell.