Flowserve QLQ Vertical Worthington - Manual (Page 41)

QLQ, QLQC USER INSTRUCTIONS ENGLISH 87900027 – 06/14
Page 41 of 61
5.11.1 Changing the pump speed
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
ensure the maximum pump working pressure is not
exceeded, the driver is not overloaded,
NPSH
A
>NPSH
R
, and that noise and vibration are
within local requirements and regulations.
5.11.2 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.
5.11.3 Minimum Continuous Stable Flow
The Minimum Continuous Stable Flow for the pump is
stated on the Data sheet.
5.11.4 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.
5.11.5 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).
5.11.6 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.
5.11.7 Operating at Reduced Capacity
Damage to pump may result from
prolonged operations at capacities less than MCSF
as stated in Data Sheet.
5.11.8 Minimum Submergence (Wet pit
applications)
In wet pit (sump) applications, the minimum
submergence is the minimum liquid level that shall be
granted above the upper suction bell to prevent
vortex formation.
The value depends on the pump sizes and the flow,
and it's indicated in the General Arrangement
drawing provided in the IOM book.
Operating the pump with a liquid level upon the upper
suction bell lower than the requirements may cause
vortex formation inside the pump, with high vibration,
noise, bad performances, mechanical damages to the
pump, and in certain cases actual pump failure.