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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 thefirst 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.
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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:
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