Parallel installation of ISG vertical pipeline centrifugal pumps is a common configuration for systems that need flexible flow control, redundant operation, or staged capacity expansion without replacing existing equipment. When executed correctly, this setup allows operators to activate additional units during peak demand and shut down idle pumps during low-load periods, reducing unnecessary energy consumption and extending the overall service life of every unit in the system. Poorly planned parallel installation, however, often leads to uneven load distribution, unstable operating points, unexpected cavitation, and frequent premature shutdowns that defeat the original purpose of the configuration.

Hydraulic Balance for Shared Suction and Discharge Manifolds

The foundation of reliable parallel operation lies in eliminating uneven flow resistance across every pump’s suction and discharge lines. Even minor differences in pipe length, elbow count, or local resistance can create significant performance gaps between units, forcing one pump to carry far more load than the others or even operate completely outside its stable performance curve. The first rule for manifold layout is to design all suction branches with equal flow path length, equal number of bends, and consistent pipe diameter from the main suction header to each pump inlet. This ensures that every unit receives nearly identical inlet flow conditions, eliminating the uneven pressure drop that causes one pump to starve for fluid while another receives excess flow.

Avoid sharp 90-degree elbows directly at the pump suction flange, as these create uneven velocity profiles that disrupt uniform flow into the impeller and increase the risk of cavitation during parallel startup. If space constraints make a nearby elbow unavoidable, install a straight pipe section with a length of no less than five times the suction pipe diameter between the elbow and the pump inlet to allow the flow pattern to fully stabilize. On the discharge side, arrange all branches to merge into the common discharge manifold at equal distances, and use symmetric tee connections instead of sharp angled junctions to prevent high-velocity flow from one pump from creating backpressure that disturbs the discharge of adjacent units.

Each individual pump must have its own dedicated isolation valve on both the suction and discharge sides, placed at a reasonable distance from the pump flanges to avoid introducing additional turbulence near the impeller or seal assembly. A separate check valve installed between the pump discharge flange and the discharge isolation valve prevents reverse flow from the common manifold from spinning the impeller backward when an idle unit is offline. This prevents unexpected water hammer events that can damage the impeller, shaft, and mechanical seal when the system switches operating modes.

Startup Sequence and Load Sharing Calibration

Improper startup procedure is one of the most common causes of immediate instability in parallel pump systems. Many operators attempt to start all units at the same time, which creates conflicting pressure surges in the shared manifold that push every pump far outside its designed stable operating range. The correct process begins with opening the suction isolation valves on all standby pumps fully, ensuring every casing is properly primed and all trapped air is completely vented before any unit is energized. Never leave suction valves partially closed during standby, as this creates uneven inlet pressure that can cause severe cavitation the moment the pump starts.

Start the first pump with its discharge isolation valve partially closed, then slowly open the valve until the system pressure and flow stabilize at a steady operating point that sits well within the pump’s recommended efficiency band. Once the first unit is running smoothly and the system pressure holds