FLEX User Instruction – 21TEM14546 EN
Page 26 of 43
A sight glass in the vessel allows observation and measurement of rotor speed with an optical
tachometer. Measured speed can be compared to the rated speed on the data sheet at the
conditions of operation. A speed significantly higher or lower than expected may indicate a
malfunction of the FLEX, an imbalance of high-pressure and low-pressure flow, or an imbalance
of flow between units when multiple units are installed in parallel.
6.3.5 Mixing and flow balance
During normal operation, the brine and seawater come into direct contact with each other
inside the ducts of the rotor in the FLEX. A small amount of mixing between the two fluids occurs
at this location. The result of this mixing is that the feedwater will leave the FLEX at a slightly
higher salinity than it entered, and the brine will leave with a slightly lower salinity than when it
entered.
While the FLEX has been designed to minimize mixing, the balance of brine and feedwater flow
rates can be used to further control the amount of mixing in the overall system.
There are three ways that the flow rates through the FLEX can be set to control mixing:
• Balanced flow is when brine and feedwater flow rates are equal. In this case, mixing is
determined by the design of the FLEX and the mixing region inside the rotor ducts.
• Over-flush is when feedwater flow is higher than brine flow. This causes the rotor to spin
faster which reduces the duct utilization, and the extra feedwater flow flushes the brine
out of the rotor ducts. These two effects result in reduced mixing, and a lower feedwater
salinity leaving the FLEX. The benefit of this is a lower feedwater salinity at the
membranes to reduce high-pressure requirements. The drawback is that it requires more
input power from the feedwater pump.
• Under-flush is when feedwater flow is less than brine flow. This reduces rotor speed
thereby reducing rotor utilization and allows some of the brine flow to leave the
feedwater side of the device. This results in increased mixing with higher feedwater
salinity leaving the FLEX. The benefit is that the feedwater input power is reduced, but
the drawback is that higher feedwater salinity requires higher pressures at the
membranes.
6.4 Shut down
When the unit is being shut down, the procedure should be the reverse of the start-up
procedure.
1. Shut down the high-pressure feed pump.
2. Allow the high-pressure booster pump to keep operating for a few minutes to flush brine
from the train and allow system pressure to drop to seawater osmotic pressure.
3. Shut down the high-pressure booster pump. The FLEX may speed up at this point.
4. Shut down the low-pressure feed pump. The FLEX will stop spinning at this point.
5. Leave an outlet on the low-pressure system open to allow depressurization.
If the system is to be shut down for an extended period, the following steps must be taken.
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