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3.6
Minimum Continuous Stable Flow
The Minimum Continuous Stable Flow for the pump is stated on the Data sheet.
3.7
Minimum Flow Control
In all cases, it is the customer's responsibility to supply a system and/or control which assures that
any pump within a system is not operated below its minimum flow condition.
In many cases, this is not a problem because the system is operating within its own flow range to
assure product delivery. A simple high pressure alarm, shut down and/or bypass control can be
used. However, in systems where product demand has high swings or where more than 100%
capacity units are desired to support a product system, additional care must be taken.
3.8
Control Notes
3.9
Thermal Control
The actual protection of the given unit is best provided by thermal sensors which read direct or
"related to" fluid temperatures and respond accordingly by opening additional flow paths until the
given unit re-establishes the acceptable temperature rise, and sets off alarms if not achieved
within reasonable/normal time periods. (High limit could actually shut down unit).
3.10
Pressure and/or Flow Control
Pressure and/or flow sensors can be used to hold the unit at higher flows by opening additional
flow paths once a "high pressure limit" or " low pressure limit" was indicated. Upon system
reaching increased flow a "low pressure limit" or "high pressure limit" setting would close the
bypass flow path. Care must be taken to allow for signal spread to avoid cyclic conditions.
3.11
Continuous Bypass Control
This is not a desired system, as it requires an oversize bypass system as dictated below with
excessive power losses and oversize pumping units, however, if used the following must be
considered.
Continuous Bypass Sizing for Parallel Units:
The limits and/or protection requirements of two or more units in parallel operation is dictated by
the minimum flow of each particular unit and the performance tolerances of the pump and system.
The following is required to provide unit protection.
1.
Determine the minimum flow of one of the units. This will be the smallest flow allowed for
any of the units in the parallel system.
2.
By general specification, while the rated head condition has a tolerance of +2% to -2% at
the guarantee point, the tolerance at "shutoff" (low flow = 0) is +5% to -5%.
Therefore: draw two head capacity curves; one at -2% head at rated capacity and fared to -
5% at shutoff, the other at 2% head at rated capacity fared to +5% at shutoff.
The lower curve represents the "weakest" pump in the system and the upper represents
the "strongest" pump or pumps in the system.
At the minimum flow from step 1 above, find the Total Developed Head (TDH) of the
Weakest pump. Draw a straight line at this TDH until it intersects the head-capacity curve
of the strongest pump. Read the corresponding "Strong Pump Flow".
