FLEX User Instruction – 21TEM14546 EN
Page 25 of 43
6.3 Normal operation
The FLEX is a completely passive device with no controls, speed, or position settings of its own.
The device is controlled by the amount of flow and pressure put into it by other pumps and
valves in the system.
When operating a system with a FLEX installed in it, there are a few key things to control in the
system in order to obtain optimal performance and life of the FLEX device and overall system.
6.3.1 Flow control
High-pressure flow through the FLEX is controlled by the high-pressure booster pump.
Low-pressure flow through the FLEX is controlled by the low-pressure feed pump.
Both pumps can be controlled with either a variable speed drive or a throttling valve at the
discharge of the pump. If a variable speed drive is used, pump speed will control the flow rate
through the FLEX. If a fixed speed pump and throttling valve is used, the position of the valve will
control the flow rate through the FLEX.
6.3.2 Backpressure control
Backpressure on the FLEX is controlled by a throttling valve at the low-pressure outlet. The
position of this valve is adjusted to ensure that the pressure at the low-pressure outlet of the FLEX
is at or above the minimum required level. Operating the system at a backpressure lower than
the minimum required level will result in cavitation that can damage the FLEX and shorten its
lifespan, as well as increase the noise and vibration produced by the unit.
NEVER FULLY CLOSE BACKPRESSURE VALVE WHEN FLEX IS OPERATING OR PRESSURIZED
6.3.3 Leakage
The rotor inside the FLEX is supported on bearing surfaces that use the process fluid for lubrication
and cooling. During normal operation, a small amount of fluid will be leaking from the high-
pressure inlet side of the device across the precise internal clearances between the rotor and
stator, and out the low-pressure outlet of the device.
6.3.4 Rotor speed
The speed of the rotor in the FLEX is not directly controlled but it indicates whether the device is
operating properly for the given high-pressure and low-pressure flow rates through the device.
Rotor speed is driven by the low-pressure flow path, so if there is no low-pressure flow, or if the
low-pressure flow is below the rated operating range of the device, the rotor will not spin.
Increasing low-pressure flow through the device will increase rotor speed, while increasing high-
pressure flow through the device will decrease rotor speed.
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