Grundfos BM Booster Modules for SWRO Desalination
The complete buyer's guide to Grundfos BM and BM hp horizontal booster modules — the standard high-pressure pump in seawater reverse osmosis (SWRO) under 500 m³/day.
The Grundfos BM is the workhorse high-pressure pump in small- and mid-scale seawater reverse osmosis (SWRO) desalination. It packs a CRN-style multistage hydraulic stack inside a cylindrical stainless sleeve, driven by an integral submersible motor — no shaft seal, no leakage, and a footprint that fits inside a 20-foot ISO container. This guide walks through sizing, materials, motor selection and integration with energy-recovery devices.
Full BM Range — Flow, Head and Frame Size
| Model | Max flow (GPM) | Max head (ft) | Max pressure | Frame | Typical use |
|---|---|---|---|---|---|
| BM 3 | 18 | 1000 | 435 psi | 4" | SWRO yacht / villa (5–15 m³/day) |
| BM 5 | 32 | 1000 | 435 psi | 4" | Small SWRO (20–40 m³/day) |
| BM 6 | 42 | 1000 | 435 psi | 4" | Resort / boat SWRO |
| BM 9 | 60 | 1000 | 435 psi | 4" | Resort / commercial SWRO |
| BM 17 | 110 | 1000 | 435 psi | 6" | Commercial / hotel SWRO |
| BM 30 | 190 | 1000 | 435 psi | 6" | Municipal SWRO containerized |
| BM 46 | 290 | 900 | 435 psi | 6" | Municipal SWRO |
| BM 60 | 380 | 900 | 435 psi | 6" | Municipal SWRO |
| BM 77 | 480 | 900 | 435 psi | 8" | Large SWRO |
| BM 95 | 600 | 750 | 435 psi | 8" | Large SWRO / industrial |
| BM 125 | 790 | 700 | 435 psi | 8" | Large municipal |
| BM 160 | 1000 | 600 | 435 psi | 8" | Largest BM range |
| BM 215 | 1350 | 500 | 435 psi | 8" | Largest BM range |
Stainless Steel Options
- AISI 316L (standard) — covers normal SWRO operating temperatures up to ~30 °C and chlorides up to ~45,000 ppm.
- 904L — for warmer SWRO (Arabian Gulf, Red Sea, Caribbean) or higher recovery where chloride concentration rises in the pump.
- Super-duplex (UNS S32750) — for the most aggressive SWRO concentrate streams and high-temperature service.
Motor Selection
BM modules use a Grundfos MS or MMS submersible motor (4", 6" or 8" depending on frame) sealed inside the same cylindrical sleeve as the pump end. Motor power ranges from 4 kW (BM 3) to 250 kW (BM 215). All BM motors can be operated at fixed speed across the line, or with a Grundfos CUE drive for soft start and capacity control.
How a BM Sits Inside a SWRO Train
- Seawater intake through a beach well or open-water structure.
- Pretreatment — coagulation, filtration, antiscalant, possibly UF membranes.
- Low-pressure booster — small CRN or BM brings pretreated seawater up to 30–80 psi.
- High-pressure pump (BM hp or BM) — raises pressure to RO operating pressure (60–82 bar for seawater).
- Energy Recovery Device — iSave, APM, isobaric chamber recovers pressure from concentrate.
- RO membranes produce permeate (fresh water) and concentrate (brine).
- Post-treatment — remineralization, disinfection, distribution.
BM vs Vertical CRN — When Is Each Right?
| Criterion | BM / BM hp | Vertical CRN |
|---|---|---|
| Footprint | Compact horizontal | Tall vertical |
| Max pressure | Up to 1,450 psi | Up to 435 psi |
| Shaft seal | No (submersible) | Standard mech seal |
| Maintenance access | Pull cartridge from sleeve | Drop top motor + stack |
| SWRO suitability | Designed for it | Brackish only |
| Service life expectation | 40,000+ h with proper pretreatment | 50,000+ h on clean water |
Energy Recovery Integration
Pairing the BM with an energy recovery device (ERD) is what makes SWRO economically viable. Common pairings:
- BM + Danfoss iSave — preferred under ~500 m³/day. Compact, no separate motor.
- BM hp + Grundfos APM — Grundfos energy recovery for medium plants.
- BM hp + isobaric exchanger (PX / ERD) — large plants over 500 m³/day, lowest specific energy consumption.
Sizing in Practice — A Worked Example
Target: 250 m³/day SWRO at 45 % recovery, 35,000 ppm seawater, 25 °C.
- Feed flow = 250 / 0.45 ÷ 24 = 23.1 m³/h ≈ 102 GPM
- Operating pressure (end of life): ~62 bar (900 psi)
- Pump choice: BM 17 hp (handles 110 GPM at 1000 ft / ~430 psi) — combined with an iSave 21 ERD so the BM only provides the differential pressure across the membrane (~10–15 bar / 150–220 psi), not the full 62 bar.
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