Flowserve ESP2 - Manual (Page 34)

ESP2 USER INSTRUCTIONS ENGLISH PCN-(71569292) 4-12
Page 34 of 64
5.8.7 Normal vibration levels, alarm and trip
Alarm and trip values for installed pumps should be
based on the actual measurements (N) taken on site
on the motors of vertical pumps in fully commissioned
as new condition. The example (N) value is given for
the preferred operating flow region (typically this may
extend to 70 to 120% of the pump best efficiency
point); outside the preferred flow region the actual
vibration experienced may be multiplied by up to two.
These standard values can vary with the rotational
speed and the power absorbed by the pump. For any
special case, contact your nearest Flowserve office.
Measuring vibration at regular intervals will show any
deterioration in pump or system operating conditions.
Vibration velocity –
unfiltered mm/s (in./s)
r.m.s. mm/s (in./s)
Peak value
Normal N 5.6 (0.22) 8.0 (0.31)
Alarm N x 1.25 9.0 (0.35) 12.7 (0.50)
Shutdown N x 2.0 11.3 (0.45) 16. (0.63)
5.9 Stopping and shutdown
The pump should be shut down
rapidly, especially on pumps equipped with product-
lubricated bearings.
Pumps driven by electric motors do not require any
special shut-down procedure. If turbine drive is used,
the operator must manually trip the over speed trip to
obtain rapid shut-down. Close the gate valve in the
discharge line if maintenance work is to be done on
pump.
5.10 Hydraulic, mechanical and electrical
duty
5.10.1 Net positive suction head (NPSH)
Net positive suction head - available (NPSH
A
) is the
measure of the energy in a liquid above the vapor
pressure. It is used to determine the likelihood that a
fluid will vaporize in the pump. It is critical because a
centrifugal pump is designed to pump a liquid, not a
vapor. Vaporization in a pump will result in damage to
the pump, deterioration of the Total differential head
(TDH), and possibly a complete stopping of pumping.
Net positive suction head - required (NPSH
R
) is the
decrease of fluid energy between the inlet of the
pump, and the point of lowest pressure in the pump.
This decrease occurs because of friction losses and
fluid accelerations in the inlet region of the pump and
particularly accelerations as the fluid enters the
impeller vanes. The value for NPSH
R
for the specific
pump purchased is given in the pump data sheet, and
on the pump performance curve.
For a pump to operate properly the NPSH
A
must be
greater than the NPSH
R
. Good practice dictates that
this margin should be at least 1.5 m (5 ft) or 20%,
whichever is greater.
Ensuring that NPSH
A
is larger than
NPSH
R
by the suggested margin will greatly enhance
pump performance and reliability. It will also reduce
the likelihood of cavitation, which can severely
damage the pump.
5.10.2 Specific gravity (SG)
Pump capacity and total head in meters (feet) of
liquid do not change with SG, however pressure
displayed on a pressure gauge is directly proportional
to SG. Power absorbed is also directly proportional
to SG. It is therefore important to check that any
change in SG will not overload the pump driver or
over pressurize the pump.
5.10.3 Viscosity
For a given flow rate the total head reduces with
increased viscosity and increases with reduced
viscosity. Also for a given flow rate the power
absorbed increases with the increased viscosity, and
reduces with reduced viscosity. It is important that
checks are made with your nearest Flowserve office if
changes in viscosity are planned.
5.10.4 Pump speed
Changing the pump speed affects flow, total head,
power absorbed, NPSH
R
, noise and vibration levels.
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 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.
5.10.5 Minimum submergence
Each ESP pump has a minimum submergence
depending on the design conditions of that pump.
See Figure 3-3 in section 3.4 to determine the
minimum submergence based upon pump flow and
suction size. All minimum submergence values are
taken from the bottom of the strainer.
If a tailpipe (1428) is used, the sump
(pit) liquid level must engulf the casing and adapter
wcompletely during start-up.