Service manual

10
D.7 PIPING AND VALVES
D7.1 General
Piping connected to pump MUST be independently supported and not allowed to
impose strains on pump casing including allowing for expansion and contraction due to
pressure and temperature changes.
To prevent foaming and air entrainment, all return lines in recirculating systems should
end well below liquid surface in reservoir. Bypass liquid from relief pressure and flow
control valves should be returned to source (tank, reservoir, etc.), NOT pump inlet line.
Shut-off valves should be installed in both suction and discharge lines so pump can be
hydraulically isolated for service or removal. All new piping should be flushed clean
before connecting to pump.
CAUTION ATTENTION
• Pipe strain will distort a pump. This could lead to pump and piping malfunction or failure.
• Return lines piped back to pump can cause excessive temperature rise at pump which
could result in catastrophic pump failure.
D.7.2 Variable Frequency Drives (VFD)
When utilizing VFD’s with positive displacement pumps, consideration must be given to
the minimum and maximum speed range intended for the pumping application.
Maximum speed must consider the inlet pressure available for the fluid viscosity range.
The minimum operating speed is dependent on the minimum operating viscosity and
associated discharge pressure.
When starting positive displacement pumps with VFD’s, there will be a delay from the
initial start until the motor reaches the selected minimum operating speed. In some
applications, such as crude oil pipeline transfer, the pump needs to be operated in full
fluid bypass mode through a control valve loop during the VFD speed ramp-uptime.
Once the minimum operating speed is attained, the control valve should completely
close and the system operated as intended. The time delay should be relatively short,
approximately 30 seconds to two minutes. For system shut down, it is recommended
this procedure be followed in reverse order. If a pump is to be driven by a VFD, this
detail should be addressed during the pump selection process. Contact Imo Pump if
there are any questions concerning VFD operation with positive displacement
pumps.
D.7.3 Relief Valve
Use relief valves to protect pumps from overpressure. They need to be connected to
pump discharge lines as close to pumps as possible and with no other valving between
pumps and relief valves. Relief valve settings should be set as low as practical.
DO NOT set relief valve higher than maximum pressure rating of pump, including
pressure accumulation at 100% bypass. Relief valve return lines should NOT be piped
into pump inlet lines because they can produce a loop that will overheat pump. See
Figure 7.