1
Function in Dialysis Water Systems
ℹ A check valve (non-return valve / NRV) allows flow in one direction only and closes automatically when flow stops or reverses. In dialysis water treatment, it is a critical contamination barrier — preventing backflow of contaminated water into purified sections of the system.
| Location in System | Purpose | Consequence if Missing |
|---|
| After feed pump outlet |
Prevents backflow when pump stops — protects pump from water hammer |
Pump seal damage, water hammer, reverse flow contamination |
| After RO membrane outlet |
Prevents permeate backflow into membrane when system is off |
Membrane fouling, bacteria ingress into permeate side |
| Before distribution loop entry |
Prevents loop water from flowing back toward RO unit |
Biofilm from loop contaminates RO permeate |
| Chemical dosing injection points |
Prevents main line water from siphoning back into chemical tank |
Chemical tank contamination, incorrect dosing concentration |
| Storage tank inlet |
Prevents stored water from back-siphoning to RO unit |
Cross-contamination between storage and treatment sections |
| Parallel filter bypass connections |
Ensures flow direction through intended filter path |
Unfiltered water bypasses treatment stage undetected |
2
Check Valve Types — Selection for Dialysis
Swing Check Valve Pretreatment ✓
| Mechanism | Hinged disc swings open on forward flow, falls closed on reverse |
| Cracking pressure | 0.05 – 0.15 bar |
| Orientation | Horizontal pipe only (disc must swing freely) |
| Best for | Feed water lines, sediment filter inlet/outlet |
| Avoid when | Vertical pipework or pulsating flow |
| Dead space | Moderate — adequate for pretreatment |
Spring-Loaded Piston Post-RO ✓✓
| Mechanism | Spring-loaded disc or piston closes instantly on zero/reverse flow |
| Cracking pressure | 0.07 – 0.35 bar |
| Orientation | Any — horizontal or vertical |
| Best for | RO outlet, loop entry, pump discharge — all critical points |
| Advantage | Fast closing — minimal backflow, low dead volume |
| Dead space | Low — preferred for sanitary applications |
Dual Plate Wafer High Flow ✓
| Mechanism | Two spring-loaded half-discs fold open on flow |
| Cracking pressure | 0.03 – 0.10 bar |
| Orientation | Any orientation — very versatile |
| Best for | Main feed headers, high flow rate lines |
| Advantage | Compact, low pressure drop, full bore available |
| Dead space | Very low |
Lift Check Valve Conditional
| Mechanism | Disc lifts vertically off seat on forward flow |
| Cracking pressure | 0.10 – 0.50 bar |
| Orientation | Vertical pipe only (upward flow) |
| Best for | Vertical pump discharge lines only |
| Limitation | Higher pressure drop, limited orientation |
| Dead space | High — avoid for post-RO loop |
Diaphragm Check Low Pressure
| Mechanism | Flexible diaphragm flexes open on forward differential pressure |
| Cracking pressure | 0.02 – 0.07 bar |
| Orientation | Any |
| Best for | Chemical dosing injection points, low pressure lines |
| Diaphragm material | PTFE or EPDM — specify PTFE for RO water |
| Dead space | Very low |
Ball Check Valve Avoid in Dialysis
| Mechanism | Ball rolls off seat on forward flow, gravity closes on reverse |
| Cracking pressure | 0.10 – 0.30 bar |
| Orientation | Vertical only — gravity dependent |
| Problem | Large dead volume behind ball — biofilm trap |
| Problem | Ball can stick open or closed over time |
| Recommendation | Do not use in dialysis systems |
3
Materials of Construction
| Component | Required Material | Acceptable Alternative | Avoid | Reason |
|---|
| Body |
SS 316L (EN 1.4404) |
SS 316 (EN 1.4401) |
SS 304, Brass, Bronze |
Mo content resists RO water corrosion. Brass/Bronze leach copper and zinc ions. |
| Disc / Piston |
SS 316L Electropolished |
SS 316 + PTFE coated |
Zinc alloy, Carbon steel |
Disc contacts the water stream directly — must be corrosion-proof and low biofilm adhesion |
| Spring |
SS 316 spring wire |
Hastelloy C276 (aggressive media) |
Carbon spring steel, SS 302 |
Chlorinated water and H₂O₂ sanitization corrode carbon steel springs — debris enters system |
| Seat / Seal |
PTFE (Teflon) |
PEEK |
NBR, EPDM, Natural rubber |
PTFE: chemically inert, zero TOC contribution. Elastomers leach organics into RO permeate |
| Body O-ring |
PTFE encapsulated FKM |
Pure PTFE |
Bare EPDM, NBR |
Encapsulated FKM combines chemical resistance with sealing performance without direct water contact |
| Internal surface Ra |
≤ 0.4 µm Electropolished |
0.4 – 0.8 µm (pretreatment only) |
> 0.8 µm — rough cast finish |
Rough surfaces are biofilm nucleation sites — impossible to sanitize chemically once colonized |
✗ Never use brass or bronze check valves anywhere in a dialysis water system — they leach copper, zinc, and lead ions that are toxic at dialysis water concentrations and will fail AAMI chemical purity testing.
4
Cracking Pressure Selection
0.03 – 0.07 bar
Ultra Low
Chemical dosing injection points. Low-pressure gravity feed lines. Prevents siphon backflow without significant pressure drop.
0.07 – 0.15 bar
Standard ✓ Most Common
RO outlet, pump discharge, filter inlet/outlet, distribution loop entry. Correct for most dialysis system locations. Specify this as default.
0.15 – 0.50 bar
High — Use with Caution
Only where backpressure risk is high and system pressure is sufficient to open. Risk: valve stays closed if system pressure drops — no flow. Verify system pressure first.
⚠ Cracking pressure must be significantly lower than system operating pressure. Rule of thumb: cracking pressure ≤ 20% of minimum expected operating pressure at that point. A 0.15 bar cracking pressure on a 0.3 bar line will cause intermittent no-flow conditions.
5
Pressure Rating & Pipe Sizing
| Parameter | Pretreatment Section | Post-RO / Distribution Loop | Minimum Valve Rating | Standard |
|---|
| Working pressure |
3 – 6 bar |
2 – 4 bar |
≥ 10 bar WP |
ASME B16.34 |
| Test pressure |
1.5 × WP |
1.5 × WP |
≥ 15 bar TP |
ASME B16.34 |
| Temperature range |
5 – 40°C (ambient) |
5 – 80°C |
≥ 100°C rated |
For hot sanitization cycles |
| Pipe size (DN) |
DN15 – DN32 |
DN15 – DN25 |
Match pipe diameter exactly |
ISO 6708 |
| End connection |
NPT / BSP thread or compression |
Tri-Clamp (sanitary) preferred |
Tri-Clamp = ISO 2852 |
Allows disassembly for inspection |
| Pressure drop (fully open) |
≤ 0.3 bar |
≤ 0.2 bar |
Per system hydraulic calc |
Minimize pump head losses |
| Flow characteristic |
Full bore strongly preferred |
Full bore mandatory |
No reduced port valves |
Prevents flow stagnation zones |
6
Recommended Placement Locations
| Location | Valve Type | Cracking Pressure | Orientation | Priority |
|---|
| Feed pump outlet (each pump) |
Spring-loaded piston or swing |
0.07 – 0.15 bar |
Horizontal |
Mandatory |
| After sand / multimedia filter |
Swing check or spring piston |
0.05 – 0.15 bar |
Horizontal |
Recommended |
| After carbon filter outlet |
Spring-loaded piston |
0.07 – 0.15 bar |
Any |
Mandatory |
| After water softener outlet |
Spring-loaded piston |
0.07 – 0.15 bar |
Any |
Mandatory |
| RO high-pressure pump outlet |
Spring-loaded piston (heavy duty) |
0.10 – 0.20 bar |
Horizontal preferred |
Mandatory |
| RO permeate outlet |
Spring-loaded piston (SS316L + PTFE) |
0.07 – 0.10 bar |
Any |
Mandatory |
| Distribution loop return line |
Spring-loaded piston or dual plate |
0.05 – 0.10 bar |
Any |
Mandatory |
| Chemical dosing injection points |
Diaphragm check (PTFE) |
0.02 – 0.07 bar |
Any |
Mandatory |
| Storage tank inlet |
Spring-loaded piston or swing |
0.05 – 0.15 bar |
As designed |
Recommended |
| UV unit inlet/outlet |
Spring-loaded piston |
0.07 – 0.15 bar |
Per UV orientation |
Optional |
7
Common Failure Modes & Prevention
Valve Stuck Open — Fails to Close
Cause: Worn PTFE seat, debris lodged under disc, spring fatigue, scale buildup on disc/seat.
Consequence: Backflow contamination — bacteria from downstream loop enters RO permeate.
Prevention: Annual inspection and seat replacement. Use strainer upstream of valve. Avoid carbon steel springs.
Valve Stuck Closed — Fails to Open
Cause: Scale or biological fouling on disc/piston. Spring corrosion. Cracking pressure too high for system.
Consequence: No flow through that branch — undetected if no flow meter installed.
Prevention: Correct cracking pressure selection. SS316 spring. Regular sanitization cycle. Install differential pressure indicator.
Water Hammer on Valve Closure
Cause: Swing check or slow-closing check on pump discharge — disc slams shut when pump stops causing pressure surge.
Consequence: Pipe joint failure, membrane damage, fitting leaks throughout system.
Prevention: Use spring-loaded piston check on all pump discharge lines — closes fast enough to prevent hammer before reverse flow builds velocity.
Biofilm Formation Inside Dead Legs
Cause: Check valve creates dead volume (dead leg) behind closed disc where water stagnates and bacteria colonize.
Consequence: Continuous biofilm source — chemical sanitization cannot reach stagnant zone.
Prevention: Use spring-loaded piston or dual plate types (lowest dead volume). Ensure sanitization flow velocity ≥ 1.5 m/s reaches all valve internals.
8
On-Site Inspection & Testing
🔍
Acceptance Test Before Installation
- Magnet test: Hold strong magnet to valve body — SS316L is non-magnetic or very weakly magnetic. Hard stick = wrong grade — reject.
- Material certificate: Request EN 10204 Type 3.1B MTC per heat number. Verify Mo content ≥ 2% in the certificate.
- Visual internal inspection: Use penlight — internal surface must be smooth and bright (Electropolished). Dull, rough, or discolored surface — reject.
- Bench flow test: Apply forward pressure — valve must open cleanly at stated cracking pressure. Apply reverse pressure — valve must hold with zero leakage.
- Spring check: Remove disc/piston (if serviceable) — spring must be SS316, bright, no rust spots. Any reddish discoloration = carbon steel spring — reject.
- Seat inspection: PTFE seats must be white, smooth, and have clean seating surface. Yellowish or discolored PTFE may indicate degradation.
9
Required Certifications
NSF/ANSI 61
Drinking water contact materials — mandatory for all wetted components
ISO 9001
Quality management system — manufacturer certification required
MTC 3.1B
EN 10204 Material Test Certificate — mandatory per heat number
CE / FDA
CE Mark (EU) or FDA 21 CFR (USA) for medical water system compliance
10
Recommended Brands — Medical Grade
Gemü
🇩🇪 Germany
Best for pharmaceutical and dialysis water. Full MTC documentation. PTFE diaphragm and piston types available.
Watts
🇺🇸 USA
NSF/ANSI 61 certified range. Comprehensive check valve portfolio including Tri-Clamp sanitary types.
Bürkert
🇩🇪 Germany
Precision spring-loaded types. Excellent for chemical dosing injection points and low-pressure lines.
Parker
🇺🇸 USA
High-purity check valves for RO permeate lines. PTFE-lined body options available for aggressive pure water.
⚠ Critical Engineering Warning
Never use brass, bronze, or chrome-plated check valves in any part of a dialysis water system — not even in the raw water feed section.
Copper and zinc leaching from brass valves can travel through carbon filters and soften the water but will not be fully removed by the RO membrane at trace concentrations.
AAMI water quality standards set maximum limits for these heavy metals specifically because of this failure mode.
Always request EN 10204 Type 3.1B Material Test Certificate. Valves labeled "SS316" from unknown manufacturers frequently test as SS201 or SS430 — both lack Molybdenum and will corrode in RO water within one to two years, releasing particulates and metal ions directly into dialysis water.