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.
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
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
- Be sure both pumps are filled with liquid during start-up and
- Start the second pump after the first pump is running.
Pumps are operated in parallel when two or more pumps are
connected to a common discharge line, and share the same suction
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
- 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
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.
about my CD with over 600 Seal & Pump Subjects
Link to Mc Nally home page