I. Common Types and Applications of High-Pressure Pumps in Seawater Desalination
High-pressure pumps commonly used in membrane seawater desalination systems fall into two main categories: multistage centrifugal pumps among dynamic pumps and plunger pumps among positive displacement pumps. Generally, multistage centrifugal pumps are primarily suited for large and medium-sized seawater desalination plants, while plunger pumps are mainly used in small and medium-sized desalination plants. There are no specific data defining the capacity boundaries between large, medium, and small desalination plants. This ambiguity stems from the high efficiency of plunger pumps and the shifting investment trends enabled by parallel operation of certain plunger pump configurations. Particularly in BOT or BOT projects, total lifecycle operational energy consumption takes precedence.
Common multistage centrifugal pumps are classified by structure into horizontal split-case type and multistage series type (as shown below).
- The structural characteristics of the horizontal split-type design are:
The upper and lower pump casings are joined together, with the inlet and outlet pipes, part of the volute, and flow passages cast into the lower casing. The impellers are symmetrically mounted on a hot-fitted shaft, minimizing axial thrust. The thrust bearings experience reduced axial force, resulting in lower vibration levels. Maintenance is simplified as the pump’s piping requires no disassembly during servicing; the upper casing can be directly removed.
- The structural features of multi-stage series pumps are:
Each stage consists of an impeller housed within a diffuser casing, with the diffusers connected by bolts and connecting rods. All stages are fixed together in series by a fixed rod; The impellers are assembled in a single direction and secured to the pump shaft with keys, resembling a string of candied haws. They exhibit high efficiency at the optimum efficiency point but experience reduced efficiency and increased vibration when operating off-center. Disassembly and reassembly present significant challenges.
Structurally, the horizontal split-case design holds a significant advantage over its counterpart. However, due to the high cost of horizontal split-case equipment, both types find widespread application. Plunger pumps belong to the category of positive displacement pumps and are extensively used in scenarios requiring high head and low flow rates. Examples include the cleaning industry and the petroleum sector.
Compared to plunger pumps, multistage centrifugal pumps exhibit significant differences in practical applications due to their distinct operating principles. The following table provides a simple comparison between centrifugal pumps and plunger pumps:
2. Key Considerations When Selecting High-Pressure Pumps for Seawater Desalination
Selecting the correct high-pressure pump is critical for system safety and economical operation. For a high-pressure pump in a membrane seawater desalination system, whether it is a plunger pump or a centrifugal pump, the first consideration is the material of the wetted parts—that is, addressing the issue of pump corrosion in seawater. Currently, there are numerous material options available. Based on economic considerations, 2507 (UNS: S32750) duplex stainless steel is generally selected, with its primary chemical composition listed in the table below.
It can be seen that the trademark 2507 incorporates N to ensure the formation of a duplex structure (austenite-ferrite). The primary reason for recommending 2507 is to prevent chloride ion corrosion under stress.
Since large-scale seawater desalination plants can only utilize multistage centrifugal pumps, the following issues generally require careful consideration for such pumps:
① Select high-efficiency pumps, as efficiency directly impacts the system’s economic performance;
② For the range of high-efficiency pump coverage, a wider selection is preferable because the flux of SWRO will decay, causing the pump’s operating point to drift.
③Select the option where flow rate changes minimally with head variation, as multiple factors influence SWRO osmotic pressure. While the pump must operate at different working points, SWRO production capacity and recovery rate remain relatively stable.
④ Select pumps that are easy to maintain.
Practical Application of High-Pressure Pumps in Seawater Desalination
SDPPump Multistage High-Pressure Pump
The 100,000-ton seawater desalination project on a certain island is a key provincial project in Zhejiang Province. It serves as the platform project for the national “11th Five-Year Plan” Science and Technology Support Program initiative titled “Development and Demonstration of Key Technologies for the Domestic Production of 100,000-ton-per-day Reverse Osmosis Membrane Seawater Desalination Systems.” Upon full completion, it will become China’s largest seawater desalination project. The high-pressure pumps for seawater desalination provide sufficient feed pressure to the reverse osmosis membrane modules while integrating energy recovery to deliver the necessary pressure and flow for normal operation of the reverse osmosis membrane assembly.
Since water temperature significantly affects reverse osmosis membrane flux, to conserve energy and reduce operating costs, the high-pressure pump can be regulated using either hydraulic coupling or variable frequency drive (VFD) load control. After comparing hydraulic coupling and variable frequency control speed regulation schemes, the VFD control scheme was selected for speed adjustment.
① The materials for reverse osmosis high-pressure pumps, pipelines, and accessories are selected for corrosion resistance. Sealing methods utilize corrosion-resistant mechanical seals.
Pressure switches are installed at the inlet and outlet of the reverse osmosis high-pressure pump. An alarm will sound and the pump will stop when inlet pressure is low. An alarm will sound and the pump will stop when outlet pressure is high.
After comparing centrifugal pumps and plunger pumps, this project employs multistage centrifugal pumps (made of 2205SS duplex stainless steel) for its high-pressure pumping system. Each pump delivers a flow rate of 10,000 tons per day, with a head of 600 meters. The motor power is 1040 kW, operating at a voltage of 690 kV.
Development Trends in Seawater Desalination Pumps
Currently, there are few domestic manufacturers of high-pressure centrifugal pumps, and even fewer that can be applied to SWRO systems. High-pressure pumps for seawater desalination require higher efficiency than ordinary multistage centrifugal pumps. In response to market demand, the high-efficiency self-balancing multistage pump is a high-speed, high-pressure pump product suitable for SWRO systems. Based on a segmented multistage centrifugal pump structure, this self-balancing multistage pump employs a balanced design and an efficient hydraulic model, achieving continuous, trouble-free operation for 8,000 hours. The absence of balancing devices eliminates both disc friction losses and balancing recirculation losses, boosting pump efficiency by 3%–14% compared to conventional multistage pumps. This design completely resolves failures caused by balancing device malfunctions—such as wear on balancing discs and seats or rotor seizure—significantly extending pump service life.
Unique structural strength design offers advantages including robust mechanical sealing, corrosion resistance, low noise, compact size, and easy maintenance. It employs a combined water-lubricated bearing and rolling bearing structure to replace the traditional dual rolling bearing configuration in multistage pumps. An innovative support structure utilizes the front seal ring of the first-stage impeller as a sliding bearing, while optimizing the duplex stainless steel flow-through components. A single unit meets the 5000 t/d reverse osmosis seawater desalination requirement, achieving an efficiency exceeding 80%, significantly surpassing comparable domestic and international products.
highly efficient
The design of this pump series is based on practical safety, economic efficiency, trouble-free operation throughout the entire pump set lifecycle, and low maintenance costs. The main design features of this pump are briefly described as follows:
●Advanced modular design incorporates numerous locally sourced components proven through long-term operation, ensuring high interchangeability of parts.
●High-strength, high-quality materials and multiple options for inlet and outlet flanges and pipe connection designs enhance the pump’s permissible pipe connection load capacity. The high-strength, high-quality pump casing material features a compact structure, extending its service life to five times that of ordinary castings. The impeller and guide vanes in the flow path are precision-cast, achieving a smooth and flat surface that improves efficiency by over 2% compared to ordinary castings.