Reverse osmosis membrane operating pressure and commissioning

What is the impact of operating pressure on reverse osmosis membranes?

1 Impact of excessively low operating pressure on reverse osmosis membranes:

If the operating pressure of the reverse osmosis membrane is lower than the manufacturer's requirements, the water yield will decrease, and the desalination rate will also be affected.

2 Impact of excessively high operating pressure on reverse osmosis membranes:

Excessively high operating pressure on the reverse osmosis membrane may damage the membrane. In theory, increasing the operating pressure can increase water yield and desalination rate. However, each membrane has an upper limit for water yield and desalination rate. When this upper limit is reached, increasing the operating pressure will not increase the water yield and may damage the membrane.

3 Impact of feed water pressure on reverse osmosis membranes

The feed water pressure of the reverse osmosis membrane does not directly affect the salt permeation rate, but increasing the feed water pressure will increase the static pressure of the reverse osmosis, because the water yield is increased, the change in salt permeation rate will not be too large at this time, the increased water yield dilutes the salt that permeates the membrane, reducing the salt permeation rate, which is equivalent to increasing the desalination rate. When the feed water pressure exceeds a certain value, due to the excessively high recovery rate, the concentration polarization is increased, which will lead to an increase in salt permeation rate, offsetting the increased water yield, so that the desalination rate will no longer increase.

How to adjust the pressure switch of the reverse osmosis equipment?

The high/low pressure protection of the water treatment reverse osmosis equipment is to prevent the RO high-pressure pump from idling or working under excessive high/low pressure.

The high-pressure switch is used for high-pressure protection. As the equipment is used for a longer time, the membrane will be fouled, and the membrane inlet pressure will naturally increase. When the pressure increases to a certain extent, in order to protect the equipment and pipes from damage, a high-pressure switch is needed for protection. When the pressure increases to the preset value of the pressure switch, the equipment will be forced to shut down.

The low-pressure switch, when the pressure in the feed water pipe is low, the reverse osmosis membrane cannot work normally, and will continuously discharge wastewater, so when the feed water pressure does not meet the working requirements, it will cut off the circuit and stop the machine. When the pressure in the feed water pipe reaches the working requirements, it will start working again.

How to adjust the water treatment reverse osmosis pressure switch? ?

The low-pressure switch has two adjustable screws, one is to adjust the base pressure, generally 0-6bar, and the other is to adjust the differential pressure, generally 0-2bar, which can be adjusted with a screwdriver.

There are two possibilities for the common low-pressure switch always tripping: one is that the fluctuation of the raw water pressure is large, and the other is that the base pressure is near the raw water pressure value and the differential pressure is small.

The first one is generally easy to solve, such as venting the pretreatment equipment and increasing the opening of the outlet valve of the raw water pump, so that the raw water pressure is relatively larger, it is not easy to trip.

For the second one, the following methods can be used for adjustment:

　　1. First observe the pressure during normal operation of the equipment, and adjust the base pressure slightly lower than it. For example, if the pretreatment water pressure is 2.5bar, the base pressure can be adjusted to 2bar;

　　2. Set the pressure value for low-pressure tripping. For example, when the raw water pressure is lower than 1.5bar, the pressure switch starts to trip, so the differential pressure is adjusted to 0.5bar;

　　3. Although the adjustment is made, there is also a difference in the adjustment accuracy, so adjust according to the actual pressure situation until the requirements are met, for example, it is engaged at 2bar pressure and disengaged at 1.5bar pressure (at this time, the pressure can be adjusted by adjusting the high-pressure pump valve or the raw water pump outlet valve).

　　4. If you want to increase the pressure differential range, you can also adjust the differential pressure to 0.8-1.0bar, so that it is engaged at 2bar pressure and disengaged at less than 1bar pressure.