Flowserve LPNV Worthington - Manual (Page 16)

LPNV USER INSTRUCTIONS ENGLISH 87900031 – 06/14
Page 16 of 48
3.4.3 Changing the pump speed
The pump must be operated at the rated speed
defined in the contract, corresponding to the motor
speed or to the maximum speed specified in case
of VFD (variable frequency driver).
In case that it is needed to operate the pump at
increased speed, it is mandatory to refer to the
Pump Manufacturer.
Changing pump speed effects flow, total head,
power absorbed, NPSH
R
, noise and vibration.
Flow varies in direct proportion to pump speed.
Head varies as speed ratio squared. Power varies
as speed ratio cubed.
If increasing speed, it is important therefore to
refer to the Pump Manufacturer in order to ensure
that the maximum pump working pressure is not
exceeded, that the driver is not overloaded, that
NPSH
A
>NPSH
R
, and that noise and vibration are
within local requirements and regulations.
3.4.4 Net Positive Suction Head (NPSH)
Any liquid, hot or cold, must be pushed into the
impeller of the pump by absolute pressure, such
as the atmospheric or vessel pressure from which
the pump takes its suction.
The head in feet of liquid necessary to push the
required flow into the pump is called Net Positive
Suction Head.
This value, more commonly called NPSH, is
measured above the vapor pressure of the liquid at
the pumping temperature.
There are two kinds of NPSH: the NPSH
R
is the
head required by the pump to cover the losses in
the pump suction - that is shown on the pump
characteristic curve.
The second, NPSH
A
, is the head available in the
system, taking into account friction loss in suction
piping, valves, fittings etc. In all cases the NPSH
A
,
measured above vapor pressure, must exceed the
NPSH
R
in order to push the liquid into the pump.
Failure to have this will result in both bad
performance and mechanical damage to the
pump, and in certain cases actual pump failure.
If any change in NPSH
A
is proposed, ensure its
margin over NPSH
R
is not significantly eroded.
Refer to the pump performance curve to determine
exact requirements particularly if flow has
changed. If in doubt please consult your nearest
Flowserve office for advice and details of the
minimum allowable margin for your application.
3.4.5 Minimum Continuous Stable Flow (MCSF)
The Minimum Continuous Stable Flow for the
pump is stated on the Data sheet.
3.4.6 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.
3.4.7 Thermal control
A thermal control of the unit can be 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).
3.4.8 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 flow
limit" was indicated.
Upon system reaching increased flow a "low
pressure limit" or "high flow limit" setting would
close the bypass flow path. Care must be taken to
allow for signal spread to avoid cyclic conditions
.
3.4.9 Operating at Reduced Capacity
Damage to pump may result from
prolonged operations at capacities less than
MCSF as stated in Data Sheet.