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What is HDEP

 

  1. HEDP stands for 1-hydroxyethylidene-1,1-diphosphonic acid, also known as etidronic acid. It is an organophosphonate used widely in industrial water treatment, including cooling towers, due to its excellent scale inhibition, moderate corrosion control, and good thermal and chemical stability.
  2. Function in Cooling Tower Water Treatment

Function

Mechanism

Scale Inhibition

Sequesters calcium, magnesium, and iron; distorts crystal growth of CaCO₃, CaSO₄, and other salts

Corrosion Inhibition

Chelates metal ions and forms thin passivation layers (mainly on steel) when combined with Zn or phosphate

Stabilizes Other Additives

Helps prevent precipitation of zinc, phosphate, and dispersants in hard/high-alkalinity water

Chlorine Resistance

More stable to oxidizers (like bleach or bromine) than other phosphonates such as ATMP or PBTC

 

  1. Typical Use in Cooling Tower Programs

Use Case

Dose Range (active HEDP)

Notes

As a scale inhibitor only

5–15 ppm

Higher end for harder water or high cycles

As part of a corrosion control program

5–10 ppm

Typically combined with Zn²⁺ and/or orthophosphate

In high-alkalinity systems

10–20 ppm

Used to prevent scaling when cycles are pushed high

  1. Limitations
    1. Less effective on silica scale compared to PBTC or specialty polymers
    2. Does not perform as well as PBTC in high-temperature or high-iron environments
    3. Can contribute to orthophosphate accumulation if dosed with phosphate
    4. Still subject to oxidative degradation over time, though slower than ATMP
  1. When to Use HEDP

System operates with moderate cycles (3–6) and moderate hardness

Chlorinated or brominated oxidizer is present

Water has medium pH (7.5–8.5)

Budget or formulation favors commodity phosphonates

 

 

 

 

 

 

 

 

 

 

 

 

  1. 📊 Comparison of Scale/Corrosion Inhibitors in Cooling Tower Water Treatment

Inhibitor                      Scale Control  Corrosion        Oxidizer Stabler          Note

HEDP                           Good               Moderate        Good                           Commodity

CaCo3              Need Zn

CaSO4             Or PO4

 

            PBTC                            Excellent         Moderate        Very Good                   Best in class

                                                CaCO3                                                                         with oxidizers

                                                Fe

                       

            AMP                            Good

                                                CaCO3             Low                 Poor                            Cheap 

                       

            Poly Maleic Acid         Excellent         Low to None   Excellent                     Non Phosphorus Polymer

                                                CaCo3

                                                Silica

 

  1. When to Prefer Each:
    1. PMA: High-silica systems, zero-P applications, or high-temperature loops
    2. PBTC: High-temp systems with chlorine/bromine and metal risk
    3. HEDP: General-purpose, cost-effective programs
    4. ATMP: Budget-constrained programs without oxidizers