COVER SERIES | Pottable Water
Balancing the Pressure: How Thrust Control Shapes RO Pump Reliability
Reliable thrust balancing in high-pressure pumps is a cornerstone in sustainable, high-purity water production.
Richard Evans | ITT Goulds Pumps
With freshwater supplies under growing pressure, desalination is becoming an essential part of global water security. Only a tiny fraction of the planet’s water is naturally accessible as freshwater, and many coastal, arid and rapidly expanding cities can no longer rely on traditional sources. At the same time, industries that once played a minor role in water demand are now major drivers. Data centers need reliable cooling water year-round. Mining operations in water-scarce regions like Chile and Peru increasingly depend on seawater desalination to sustain production. The rise of green hydrogen, produced through electrolysis, will add new demand for high-purity water. For countries with long coastlines but limited freshwater resources, desalination offers a stable, climate-resilient supply that supports both communities and critical industries.
Reverse Osmosis
Reverse osmosis (RO) is the most commonly used technology in modern desalination plants. It uses pressure to push water through a semipermeable membrane, removing salts, minerals and other impurities. Despite its advantages, RO still demands significant energy, and reliability can be challenged by fouling and scaling, which require intensive pretreatment, frequent cleanings and periodic membrane replacements. These realities reduce uptime and raise operating costs, so improving energy efficiency and long-term durability remains essential.
RO is one of the most pump-intensive industrial processes, with pumps employed across every stage of the plant. In a two-pass configuration, intake pumps move seawater to pretreatment, filter feed pumps drive flow through multimedia filters and ultrafiltration, and the high-pressure feed pump supplies the first-pass membranes, often paired with an energy recovery device to improve energy efficiency. Clean-in-place pumps circulate chemicals during membrane cleaning without burdening the feed pump. Finally, post-treatment and distribution pumps handle remineralization, pH correction and delivery.
| IMAGE 1: Cross section of a multistage BB4 high-pressure pump showing stacked impellers and ring-section casing. Used to generate the high pressures required for RO desalination. (Images courtesy of ITT Goulds)
The High-Pressure Pump
The high-pressure pump used to push high-pressure water through the membranes is the workhorse of the plant. Since pressure is the driving force behind RO, the performance of the high-pressure pump plays a major role in both plant efficiency and operating cost. A popular pump for this application is a multistage BB4-style pump, which is particularly suited to high-pressure, high-efficiency services like desalination. The BB4 uses a ring section casing with multiple stages stacked in series to build pressure, with each stage adding incremental head as the water moves through the pump. Losses remain low and clearances stay tight, allowing the pump to maintain excellent hydraulic efficiency over long periods of continuous operation. In seawater service, duplex or super duplex stainless steels are commonly specified for key wetted parts to combine chloride resistance with mechanical strength. While efficiency is critical, so too is durability. No operator wants a shutdown driven by maintenance on the high-pressure pump. To understand the features that drive uptime, it helps to look at axial thrust and how these pumps manage it.
Any multistage pump that develops high differential pressure generates a significant axial force on the rotor. As each impeller adds head, the pressure distribution across the rotor produces a net push toward the suction end. If that force is not controlled, it overloads the thrust bearings, increases vibration, upsets the internal hydraulics and shortens the life of the mechanical seals. In desalination, where RO feed pumps run at high pressure for long periods, stable thrust control is essential for reliability.
How Thrust Is Balanced
BB4 pumps counter this axial load with a dedicated balance device. The goal is to generate an opposing hydraulic force so the thrust bearing sees only a small, manageable residual load. In practice, a controlled internal leakage passes through a precision clearance, creating a pressure drop and a counter thrust. Two common approaches are the balance drum and the balance disk.
| IMAGE 2: Diagram of a balance drum showing radial clearance and controlled leakage flow used to generate hydraulic counter thrust in multistage pumps
Balance drum
A balance drum uses a long radial clearance between a rotating drum and a stationary bushing. Pressure drops progressively along that clearance, and the resulting hydraulic area produces a steady, predictable counterforce. Because the clearance is relatively generous compared with a disk, the drum is tolerant of small particles, minor misalignment and day-to-day transients during starts, stops and cleaning cycles. Leakage is modest and consistent, which helps overall efficiency and keeps axial position stable as operating conditions change. In desalination service, this stability shows up as smoother pressure control to the membranes, gentler loading on the thrust bearing and fewer surprises for the mechanical seals.
| IMAGE 3: Diagram of a balance disc showing the tight axial clearance and leakage flow path that produce a rapid hydraulic counter thrust in multistage pumps
Balance disk
A balance disk is a flat rotating surface running very close to a stationary mating surface with a small axial gap. The high-pressure fluid is applied to one side of the disk with lower pressure fluid on the other side. This pressure difference provides a counteracting force to balance the thrust. A disk’s tighter axial clearance provides a more efficient solution, and a fast axial response can deliver precise balancing in pristine services, whereas a drum’s longer radial clearance and gradual pressure drop make it more tolerant when real world conditions vary.
Thrust control is not a detail; it decides bearing life, seal life, uptime and energy cost. In continuous RO duty, a balance drum keeps axial movement smooth with predictable leakage, which calms thrust loads and helps the pump hold its efficiency over time. Disks can work in very clean, tightly controlled services, but for large BB4 feed pumps in continuous high pressure desalination, a drum is the pragmatic choice for reliability and life cycle cost.
Chris Schantz Richard Evans is the global products manager for Goulds sump, ISO, multistage and Rheinhütte products within ITT’s Flow Technologies platform. For more, visit gouldspumps.com.
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