Setting HEDP Feed Rates Using Makeup Water Quality and Blowdown Data

A practical HEDP scale inhibitor feed rate starts with two decisions: the active residual needed under the actual cooling-water conditions, and the amount of treated water leaving the system. Makeup water analysis helps establish the first; blowdown and other liquid losses establish the second. Together, they provide a defensible starting point for pump settings.

Hardness and alkalinity alone cannot determine a reliable dosage. The same makeup water can produce different scaling conditions at different cycles of concentration, pH values, and heat-transfer surface temperatures. Operators therefore need a water balance, a treatment target validated for those conditions, and a residual test that measures the intended chemical basis.

Use makeup quality to establish the operating conditions

Start with representative makeup and circulating-water samples taken during stable operation. Record calcium hardness, total alkalinity, pH, conductivity, and temperature. Include silica, sulfate, and other constituents where they could constrain concentration or treatment performance. Keep hardness and alkalinity units consistent: results expressed as calcium carbonate cannot be compared directly with concentrations expressed as calcium ions or bicarbonate.

Estimate cycles of concentration using a dissolved constituent that remains substantially conservative in the system:

Cycles of concentration = circulating-water tracer concentration ÷ makeup-water tracer concentration

Chloride can be useful when chemical additions and process contamination do not materially change its concentration. Conductivity offers a convenient operating indicator, but acid addition, treatment chemicals, and changing makeup composition can shift its relationship with cycles. Compare it periodically with a suitable analytical tracer.

Multiplying makeup calcium hardness by cycles gives an initial estimate of circulating calcium hardness before precipitation or other removal occurs. Alkalinity requires more care because acid dosing and carbonate precipitation can alter it. Use measured circulating-water chemistry when assessing scale risk, and include the temperature at the most demanding heat-transfer location. Basin temperature alone may understate the conditions where deposits form.

These measurements help define an acceptable operating window. They do not produce a universal HEDP dosage. Select the initial active residual through a treatment program supported by relevant water chemistry calculations, compatibility checks, and performance evidence. Increasing HEDP feed cannot indefinitely compensate for excessive concentration, unsuitable pH, or a limiting scale species.

Calculate replacement demand from liquid losses

For an evaporative cooling system at approximately steady inventory, use the following water balance:

M = E + B + D + L

Here, M is makeup flow, E is evaporation, B is blowdown, D is drift, and L represents leaks and other liquid withdrawals. Use the same flow units throughout. Evaporation removes water while leaving nonvolatile dissolved treatment chemicals behind; blowdown, drift, and leaks carry circulating-water treatment out of the system.

For a conservative dissolved tracer under steady conditions:

Cycles ≈ M ÷ (B + D + L)

This provides a useful cross-check between laboratory results and flow records. If the measured cycles and water balance disagree substantially, investigate meter accuracy, unrecorded drains, intermittent blowdown, changing basin inventory, or a tracer affected by chemical addition. Calculate balances over a representative operating period rather than pairing an instantaneous flow reading with an unrelated sample.

Once a target circulating residual has been selected, the theoretical replacement rate is:

Active HEDP feed, kg/h = target active residual, mg/L × total liquid loss, m³/h ÷ 1,000

This equation assumes good mixing and negligible additional chemical demand. Adsorption onto solids, interaction with deposits, and degradation can increase the required feed. Treat the result as a mass-balance starting point, then adjust against verified residuals and system performance.

If blowdown is the only measured loss, identify whether drift and other withdrawals are small enough to neglect. A persistent leak or routine filter backwash can make a blowdown-only calculation understate chemical replacement. Conversely, using total makeup flow directly in the replacement equation can overstate feed unless the selected dosage is explicitly defined on a makeup-water basis.

Convert active demand into a pump setting

Consider a hypothetical system with makeup flow of 100 m³/h and verified operation at five cycles. Its calculated total liquid loss is 20 m³/h. Suppose the treatment program has established a target of 4 mg/L active HEDP for its operating conditions. This concentration illustrates the calculation; it is not a recommended dosage for other systems.

Active replacement = 4 × 20 ÷ 1,000 = 0.080 kg/h

If the dosing product contains 60% active HEDP by mass, its required feed is:

Product feed = 0.080 ÷ 0.60 = 0.133 kg/h

With an illustrative product density of 1.40 kg/L, this corresponds to approximately 0.095 L/h. Use the actual product assay and density for the delivered material. If the product is diluted in a day tank, calculate the active mass fraction and density of that dosing solution before converting to pump volume.

Check what “active” means on both the product specification and the analytical report. HEDP acid equivalent, a sodium-salt basis, and phosphorus expressed as P are different reporting bases. A blended inhibitor may also contain several active ingredients, so its total solids content does not establish its HEDP content.

The same distinction matters when evaluating another phosphonate, such as Diethylene Triamine Penta (Methylene Phosphonic Acid) Pentasodium Salt (DTPMP·Na5). Its specification lists 24.0–26.0% active component as DTPMPA and 28.5–31.0% as the pentasodium salt. Those figures describe different mass bases for the same product. It is a different phosphonate from HEDP, and any substitution requires reassessing performance, compatibility, and residual measurement rather than transferring the HEDP setpoint.

Confirm actual pump delivery by a suitable calibration method at representative discharge pressure. A calculated requirement of 0.095 L/h is useful only if the pump can deliver that flow consistently. Where the required output falls below its reliable operating range, review pump sizing or controlled dilution.

Separate initial charging from continuous replacement

A system with little or no existing residual needs an inventory charge before the maintenance feed can hold the target:

Initial active charge, kg = required residual increase, mg/L × system volume, m³ ÷ 1,000

Subtract any verified existing residual when calculating the required increase. This charge establishes the chemical inventory; continuous feed replaces subsequent losses and demand. A maintenance setting alone may take considerable time to bring a large system to target.

Introduce treatment through a well-mixed location and follow product handling and compatibility requirements. Avoid concentrated contact with incompatible chemicals. Account for the system’s mixing and turnover time before interpreting a sample taken after a feed adjustment.

Adjust feed using residuals and operating evidence

A useful control arrangement ties feed to measured losses or to makeup flow with a verified cycles relationship. Where feed follows makeup, the theoretical active dose per unit of makeup equals the target circulating residual divided by cycles, before allowing for additional demand. Review that relationship whenever the cycles setpoint changes.

Use a residual method suitable for the treatment formulation. Total phosphorus can include orthophosphate, other phosphonates, and degradation products; it does not by itself establish the concentration of intact HEDP. Record the analytical basis alongside the control limit so that operators compare equivalent results.

When residuals fall, first check pump delivery, tank concentration, blowdown activity, leaks, and sampling consistency. Then investigate chemical demand, including solids, iron contamination, and the oxidizing biocide program. Raising feed immediately can conceal a mechanical fault or an analytical mismatch.

If residual remains within range while deposits increase, revisit circulating chemistry, surface temperature, flow distribution, and deposit composition. HEDP residual alone cannot demonstrate scale control. Track it alongside heat-transfer performance, pressure drop where relevant, and inspection findings.

After a makeup-source change or blowdown adjustment, recalculate the water balance and reassess the treatment target. Higher cycles reduce liquid losses but increase dissolved constituent concentrations. The resulting feed requirement depends on both effects: how much chemical leaves the system and what residual the more concentrated water actually requires.