Material Insights · Understanding the Material World

Pressure and flow recovery: Engineering and integration

Fitted beside the old valve, so failure does no harm

What this tab covers Engineering and operating integration: how tightly it is coupled to the host process, how steady the waste stream is, and how hard it is to retrofit.

Most variants sit in parallel with an existing valve, so a failure simply falls back to the old way of dropping pressure. Output follows the host's flow, which varies a lot in gas and water networks.

Process interdependence (coupling risk)

Favourable

A turbine at a water main is usually fitted in a parallel loop with the pressure-reducing valve, as at Manitou Springs, so the valve takes over if the turbine trips. Gas expanders likewise sit beside the regulators, and the station keeps working if the expander stops. RO pressure exchangers are different. They are part of the plant's core design, and the plant cannot run economically without them.16

Load factor and intermittency

Mixed

Gas and water flows swing with time of day and season. In a 2010 Canadian study, a single expander averaged only about 58 to 134 kW against a peak of about 658 kW. A 30 kW InPipe Energy unit at EBMUD in California expects 150,000 kWh a year, a capacity factor of about 57% (calculated). RO plants run close to continuously. Compressor heat is available only while compressors run. A two-shift plant gives about 4,160 hours a year.245

Expander average vs peak output about 58-134 kW vs about 658 kW4 EBMUD in-pipe hydro capacity factor about 57% (calculated)5 Two-shift compressor hours 4,160 h/y2

Footprint and retrofit complexity

Favourable

In-pipe hydro and compressor heat recovery are the easiest brownfield fits. They need a bypass loop, a grid tie or some ducting, a fan and a thermostatic damper. A US conduit site also needs three-phase power within about a quarter mile. Gas expanders need a heater, a skid and grid connection inside a hazardous-area station. PX units are compact and modular, but adding them to an old RO train means plant downtime.25

US conduit grid-access rule of thumb three-phase within about a quarter mile5

Sources

  1. Energy Recovery, Inc. Form 10-K for fiscal year 2025 (company filing) Energy Recovery, Inc. (US SEC filing), 2026
  2. Fact Sheet 10: Heat recovery with compressed air systems (updated 2026) Compressed Air Challenge (sourcebook developed with US DOE), 2026
  3. Hybrid turboexpander and fuel cell system for power recovery at natural gas pressure reduction stations (Howard, Oosthuizen and Peppley), Chemical Engineering Transactions 21 AIDIC, 2010
  4. Water and wastewater systems hydropower guide (document 24-50213) Pacific Northwest National Laboratory and Idaho National Laboratory for US DOE Water Power Technologies Office, 2024
  5. Manitou Springs water treatment plant case study (vendor case study) Rentricity, 2022