In the field of reverse osmosis water treatment, back pressure refers to the situation where the pressure on the water side of the product is greater than the pressure on the feed water side. The roll-type membrane element resembles a long envelope membrane pocket with the open side glued to a product water center tube containing an opening. Multiple membrane pockets are wound to the same product center, so that the water flow from the outside of the membrane flow, under the water supply pressure, the fresh water through the membrane into the membrane pocket after the confluence of the product water center pipe.
In order to facilitate the flow of product water in the membrane bag, a fabric supporting layer of product water diversion is sandwiched in the envelope membrane bag. In order to make the water flow evenly through the surface of the membrane bag and to disturb the water flow, a screen layer is sandwiched in the water supply channel between the membrane bag and the membrane bag.
The three sides of the membrane pocket are bonded together with a binder. If the pressure on the water side of the product is greater than the pressure on the water supply side, then these adhesive wires will break and lead to the loss or significant reduction of the desalination rate of the membrane element. Therefore, from the perspective of safety, the reverse osmosis system can not exist back pressure.
General technical manuals related to reverse osmosis membranes have back pressure limit parameters. It is assumed that the maximum back pressure that reverse osmosis membranes can bear is 0.1MPa, that is, about 1bar pressure, about equal to the pressure generated by 10 meters of water column. When reverse osmosis is shut down, the water in the production pipe will inevitably produce a certain pressure on the water-producing side of the reverse osmosis membrane, and the higher the water pipe climbs, the greater the pressure will be. If the water pipe climbs more than 10 meters, the back pressure on the water-producing side will be enough to cause damage to the membrane. Therefore, in the process of reverse osmosis design, the maximum climbing height of the production pipe should be specified as 8 meters, and the production pipe should be equipped with a check valve to prevent the water in the production pipe from producing pressure. Sometimes the check valve may be lax or the production pipe must climb higher. In this case, the drain pipe can be installed behind the production pipe check valve to prevent back pressure.
Another type of back pressure is caused by operational error. In the process of reverse osmosis, if the operation is wrong, when the water production valve and the water production and discharge valve are not opened, the pump will cause the continuous increase of system pressure and water production side pressure, but no water can be produced. The water production pipe is usually made of UPVC material. If the pressure is too high, the pipe may burst and cause safety accidents. At this time, if the operator urgently opens the production valve to relieve pressure, the accident can be avoided. And if the operator takes the way is to stop the pump, then the inlet side pressure suddenly decreases, the water side pressure is very high, so high back pressure will immediately cause the damage of the film. Therefore, designers should also take this into account in the design process. Bypass can be installed at the side of the production and discharge valve, and blasting film can be installed on the bypass. Assuming that the bursting pressure of the blasting film is 0.3MPa, once the system holds pressure and the water side pressure exceeds 0.3MPa, it will automatically burst pressure relief to prevent greater losses.
There is also the bursting film due to non-standard, not to burst pressure explosion, or to the bursting pressure does not explode, sometimes, the site personnel in order to save trouble, directly take iron plate or other materials to replace the bursting film, so that once the pressure occurs, it is really irreversible. So we must follow the specification and do a good job of bursting film experiment.
