English (GB)
9
3.4 Pipes
Inlet and outlet pipe
In order to minimise friction losses and hydraulic
noise in the pipes, choose a pipe that is one or two
sizes larger than the pump inlet and outlet ports.
Typically, flow velocities must not exceed 2 m/s (6
ft/sec) for the inlet pipe (port) and 3 m/s (9 ft/sec) for
the outlet pipe (port).
Make sure that the NPSH available (NPSHA) is
higher than the NPSH required (NPSHR). NPSH =
Net Positive Suction Head.
3.4.1 General precautions
When installing the pipes, observe these
precautions:
1. Always run the pipes directly to the pump.
2. Do not move the pump to the pipes. This could
make the final alignment impossible and cause
stress to the pump flanges and pipes.
3. When you use expansion joints in the pipe
system, fit the joints at a minimum distance of 2
pipe diameters away from the pump on the inlet
side. This prevents turbulence in the joints, thus
ensuring optimum inlet conditions.
4. Install the pipes as straight as possible and avoid
unnecessary bends. Where necessary, use 45 °
or long-sweep 90 ° fittings to reduce friction loss.
5. Make sure that all pipe joints are tight.
6. Where you use flanged joints, ensure that the
inside diameters match properly.
7. Remove burrs and sharp edges when making up
joints.
8. Make sure that the pipes do not cause stress or
strain in the pump.
9. Provide for expansion of pipe material by means
of expansion joints on both sides of the pump.
10. Always allow sufficient space and accessibility for
maintenance and inspection.
3.4.2 Inlet pipes
Place the pump below system liquid level whenever
possible. This will facilitate priming, assure a steady
liquid flow and provide a positive suction head.
You can avoid many NPSH problems if you install the
inlet pipe properly. See section
In section
, common
inlet pipe installations are illustrated for two
situations:
•
flooded systems
Closed systems and open systems where the
liquid level is above the pump inlet.
•
suction lift systems
Closed systems and open systems where the
liquid level is below the pump inlet.
3.4.3 Valves in the inlet pipe
If the pump is operating under permanent suction lift
conditions, install a non-return valve in the inlet pipe
to avoid having to prime the pump each time it is
started. The non-return valve must be of the flap type
or a foot-valve with a minimum of pressure loss.
3.4.4 Outlet pipe
The outlet pipe is usually preceded by a non-return
valve and an isolating or throttle valve. The
non-return valve protects the pump against
excessive counterpressure and reverse rotation of
the pump and prevents backflow through the pump in
case of operational stop or failure of the motor.
In order to minimise friction losses and hydraulic
noise in the pipes, flow velocities must not exceed 3
m/s (9 ft/sec) in the outlet pipe (port).
On long horizontal runs, keep the pipe as level as
possible.
Avoid high spots, such as loops, as they will collect
air and throttle the system or lead to uneven
pumping.
Protective covers are fitted to the inlet and
outlet ports to prevent foreign bodies from
entering the pump during transportation
and installation. Remove these covers
from the pump before connecting any
pipes.
Make sure that both the inlet and outlet
pipes are independently supported near
the pump so that no strain is transmitted to
the pump when you tighten the flange
bolts. Use pipe hangers or other supports
with necessary spacing to provide support.
The sizing and installation of the inlet pipe
is extremely important.
