High Pressure Pump Selection
(1) Forms and characteristics of high pressure pump
There are two main types of high pressure pumps commonly used in membrane desalination systems: multistage centrifugal pump in powerful pumps and piston pump in positive displacement pumps.
In general, multistage centrifugal pump is mainly suitable for large and medium-sized desalination plants. Piston pump is mainly suitable for small and medium-sized desalination plants.
We will introduce multistage centrifugal pump as the key product. The common multistage centrifugal pump is single suction multistage centrifugal pump (conventional structure) or self-balancing multistage centrifugal pump (innovative structure).
The features of conventional structure:
The pump casing of the suction stage, middle stage, and discharge stage are integrally connected by tension bolts.
The bearing parts are arranged t both ends.
The rotor part is composed of the impellers, shaft sleeves and balance disc which all mounted on the shaft. The bearings are different according to the model, they are not subjected to axial force, and the axial force is balanced by the balance disc.
The disadvantage is that the balance plate is easy to wear, resulting in a decrease in pump efficiency and a reduction in service life.
The features of self-balancing structure: Although it is a segmented centrifugal pump like the conventional pump, there is no balancing device, the impeller is symmetrically arranged, so that the axial thrust generated by the positive and negative of impellers during operation cancels each other out, and there is no axial movement, efficiency is obviously enhanced and stable. (If you want to learn more about the differences between the above two, please click here).
Self-balancing multistage pump has a large advantage compared to the conventional one. As the price is more expensive, both of the pump are widely used.
(2) Considerations of selecting high pressure pump
The correct choice of high pressure pump is the key to safe and economic operation of the system. For a high-pressure pump of a membrane desalination system, whether it is a piston pump or a centrifugal pump, the first thing to determine is the material of the overflow part, that is, to solve the corrosion problem of the water pump in seawater.
At present, there are many materials available. Based on economic considerations, 2507 (UNS: S32750) duplex stainless steel is generally used. The main chemical composition is as follows.
It can be seen that N is added to 2507 to ensure the formation of two-phase (austenite-ferrite). The main reason for recommending 2507 is to avoid chloride ion corrosion under stress.
Since large-scale desalination plants can only use multi-stage centrifugal pumps.
For multistage centrifugal pump, it is generally necessary to consider the following issues:
1. Select high-efficiency pump because the efficiency directly affects the cost. In terms of the range of high-efficiency coverage of pumps.
2. Select the wide range pump because the flux of SWRO will decay and the operating point of the pump will drift.
3. Select the pump with less change in flow when the head changes, because there are many factors affecting the osmotic pressure of SWRO, the pump should work at different working points, but the SWRO capacity and recovery rate change little.
4. Select a pump with easy maintenance.
Project Application Case
The Liuheng 100,000-ton seawater desalination project is a national reverse osmosis membrane seawater desalination demonstration project.
The high pressure pump provides sufficient inlet pressure for the reverse osmosis desalination membrane module, while combined with energy recovery to provide the reverse osmosis membrane group with the pressure and flow required for normal operation.
Since the water temperature has a great influence on the flux of the reverse osmosis membrane, in order to save energy and reduce operating cost, the high pressure pump can adjust the load control by liquid force coupling or frequency conversion. After comparing the two speed control schemes of liquid force coupling or variable frequency control, the frequency conversion control scheme is adopted to adjust the speed.
1. Choosing corrosion resistant when it comes to materials for reverse osmosis high pressure pumps, pipes and accessories.
2. Choosing mechanical seal for sealing. The pressure switch is installed at the inlet and outlet of the reverse osmosis high pressure pump. When the inlet pressure is low, the alarm ring and pump stop, and when the outlet pressure is high, the alarm ring and pump stop.
The high pressure pump of this project uses a multistage centrifugal pump (material: 2205SS duplex stainless steel). Flow rate of each pump is 10,000 tons / day, head is 600m, motor power is 1040kW, voltage is 690KV.