Phosphoric Acid Storage — 85% H<sub>3</sub>PO<sub>4</sub> Tank System Selection
Phosphoric Acid Storage — 85% H3PO4 Tank System Selection
85% phosphoric acid in HDLPE polyethylene tanks: the water-treatment and food-processing workhorse with a stable MOC stack.
Overview
Phosphoric acid (H3PO4) at 75% to 85% industrial concentration is produced in enormous volume for fertilizer manufacture, metal surface treatment (phosphate conversion coatings), cola beverage acidulation, and wastewater pH control. Commercial 85% phosphoric is a viscous, oily-feeling liquid with specific gravity 1.69 and a relatively low vapor pressure — making it one of the easier acids to store in polyethylene at ambient temperature.
Why HDLPE — Straightforward
Snyder specifies HDLPE at 1.9 ASTM specific gravity for 85% phosphoric. Crosslinked polyethylene (XLPE) is not approved. The 1.9 ASTM rating exceeds the mechanical requirement (phosphoric SG is 1.69) but is specified because the commercial tank is pre-engineered for oxidizing-acid service at that wall thickness.
Gasket: Viton — Not EPDM
EPDM fails in concentrated phosphoric — it swells and loses rebound within months. Viton (FKM) is the correct choice for all wetted gaskets: manway, bulkhead, and flanged connections. Viton in phosphoric service typically lasts 4–7 years between replacements, significantly longer than in sulfuric or hypo service.
Hardware: 316SS Is Adequate
Unlike sulfuric or hydrochloric, 316 stainless steel is adequate for phosphoric-acid bolting. 316SS does not pit measurably in phosphoric service at ambient temperature — the acid is passivating rather than chloride-attacking. Hastelloy or Titanium are not required and are economically unwarranted unless the service also includes chloride impurities (trace sea-water in feed stock, for instance).
Fittings: PVC Standard
PVC bulkhead fittings and piping are the standard for ambient phosphoric service. For heated service above 130°F, step up to CPVC or polypropylene. Glass-filled nylon threaded fittings are NOT approved — phosphoric attacks the glass-resin bond.
Dilution Heat
Diluting 85% phosphoric with water is mildly exothermic — nothing like sulfuric or caustic, but enough to warm the tank measurably if done in bulk. Add acid to water, not the reverse, and plan dilution volumes to keep the tank below 100°F during any single dilution event.
System-of-Construction Table (Snyder Industries)
This is the exact specification Snyder Industries publishes for this chemistry. Every column is required — changing any of them voids the service rating.
| Concentration | Resin | Specific Gravity | Fitting | Gasket | Bolt |
|---|---|---|---|---|---|
| 85 | HDLPE | 1.9/ASTM | PVC | Viton | 316SS |
Concentration-Band Compatibility (Enduraplas / Equistar Data)
Polyethylene chemical resistance by concentration and service temperature. Satisfactory (S) = long-term service. Limited (O) = occasional only. Unsatisfactory (U) = do not use.
| Concentration | LDPE/MDPE @ 70°F | LDPE/MDPE @ 140°F | HDPE @ 70°F | HDPE @ 140°F |
|---|---|---|---|---|
| 0-30% | Satisfactory | Satisfactory | Satisfactory | Satisfactory |
| 90% | Satisfactory | Unsatisfactory | Satisfactory | Satisfactory |
Frequently Asked Questions
- Is phosphoric acid safe around polyethylene long-term?
- Yes. 85% phosphoric at ambient is one of the more benign strong-acid chemistries for polyethylene. Tanks routinely last 15-25 years in phosphoric-only service.
- Can I use the same tank for phosphoric and sulfuric?
- Technically the MOC stack is similar (HDLPE, Viton, PVC) but the bolt spec differs — sulfuric needs Hastelloy while phosphoric only needs 316SS. If the tank is spec'd for sulfuric, it is also adequate for phosphoric but not vice versa. Never store mixed acids in the same tank without an engineer review of the actual product chemistry.
- Do I need secondary containment?
- Most states require secondary containment for phosphoric acid in storage volumes above 55 gallons, though the specific trigger varies. Double-wall tanks (DW series) satisfy code; lined concrete berms at 110% of tank volume also work.
- What about vent sizing?
- Standard atmospheric vent sized for fill/draw rate. Phosphoric has low vapor pressure and is not a venting-intensive service.
Source Citations
- Snyder Industries — Chemical Resistance Recommendations (current edition)
- Enduraplas / Equistar Technical Tip — Chemical Resistance of Polyethylene (12-page reference)
Chemical-Service Tanks
These HDPE vertical chemical-storage tanks from Snyder Industries ship pre-engineered for industrial chemistry service at 1.9 ASTM design specific gravity. When you order for phosphoric acid service, our team verifies the full materials-of-construction stack (resin grade, fittings, gaskets, bolts) against the OEM recommendations above before shipment — no surprises at commissioning.
Need your state's septic or tank regulations?
Chemical service tanks are spec'd at the manufacturer level, but the installation still has to comply with your state and county rules — setbacks, containment, permitting, and in some states, construction-authorization review. Our State Regulation Guides cite actual statutes, not generic lore.