SERIES AND PARALLEL OPERATION OF CENTRIFUGAL PUMPS 18-1

A centrifugal pump will pump fluid at the point where the system curve intersects the pump curve.

If you need more flexibility you can install another pump and operate it in either series or parallel with the first pump.

SERIES OPERATION

Centrifugal pumps are connected in series if the discharge of one pump is connected to the suction side of a second pump. Two similar pumps, in series, operate in the same manner as a two-stage centrifugal pump.

Each of the pumps is putting energy into the pumping fluid, so the resultant head is the sum of the individual heads.

Some things to consider when you connect pumps in series:

• Both pumps must have the same width impeller or the difference in capacities (GPM or Cubic meters/hour.) could cause a cavitation problem if the first pump cannot supply enough liquid to the second pump.
• Both pumps must run at the same speed (same reason).
• Be sure the casing of the second pump is strong enough to resist the higher pressure. Higher strength material, ribbing, or extra bolting may be required.
• The stuffing box of the second pump will see the discharge pressure of the first pump. You may need a high-pressure mechanical seal.
• Be sure both pumps are filled with liquid during start-up and operation.
• Start the second pump after the first pump is running.

PARALLEL OPERATION

Pumps are operated in parallel when two or more pumps are connected to a common discharge line, and share the same suction conditions.

Some things to consider when pumps are operated in parallel:

• Both pumps must produce the same head this usually means they must be running at the same speed, with the same diameter impeller.
• API 610, states that when pumps are run in parallel, "the head shall rise at least 10% of the head at rated capacity."(this is called a "stable curve because there is a continious rise to shutoff.)
• Two pumps in parallel will deliver less than twice the flow rate of a single pump in the system because of the increased friction in the piping.
• The shape of the system curve determines the actual increase in capacity. If there is additional friction in the system from throttling (see dotted line in the following diagram), two pumps in parallel may deliver only slightly more than a single pump operating by its self.
• If you run a single pump only, it will operate at a higher flow rate (A) than if it were working in parallel with another pump (B) because it will be operating further out on the curve requiring increased power. The rule is that if a pump is selected to run in parallel, be sure it has a driver rated for single operation.

Remember the following:

• Most plants read only total flow and cannot see the differences in individual pump performance
• Parallel pumps are notorious for operating at different flows. Often a weaker pump is operating close to its shut off point while a stronger pump is operating to the far right of its curve and running out of NPSHA.

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