API 675 Hydraulic Diaphragm Metering Pump Standard & Sizing

In petroleum refineries, natural gas dehydration plants, petrochemical synthesis units, and offshore injection skids, project specifications routinely mandate an API 675 hydraulic diaphragm metering pump. Standard water-treatment mechanical diaphragm pumps cannot withstand discharge pressures above 10–12 bar because the diaphragm bears the full differential pressure unsupported. By contrast, an API 675 hydraulic diaphragm metering pump uses a reciprocating plunger to pressurize hydraulic oil behind a balanced PTFE or metallic diaphragm, delivering hermetic zero-leakage containment at pressures up to 250 bar while holding ±1% steady-state accuracy across a 10:1 turndown range.

When reviewing EPC datasheets or preparing technical bids, process engineers need to verify four core details: the accuracy and turndown tolerances defined by the American Petroleum Institute, the internal three-valve hydraulic protection system, double-diaphragm rupture monitoring, and high-pressure capacity derating. Below we break down how an API 675 hydraulic diaphragm metering pump is engineered and specified.

1. Core performance requirements of the API 675 standard

API Standard 675 (Controlled-Volume Positive Displacement Pumps for Petroleum, Chemical, and Gas Industry Services) establishes strict mechanical and hydraulic benchmarks for continuous refinery duty. Every compliant API 675 hydraulic diaphragm metering pump must meet three volumetric performance tolerances across at least a 10:1 turndown ratio (from 10% to 100% of rated capacity):

1. Steady-state accuracy within ±1%: Under constant discharge pressure, suction head, and fluid temperature, flow variation at a fixed stroke setting cannot exceed ±1%.
2. Linearity within ±3%: Across the calibrated stroke length range, the actual flow curve must stay within ±3% of an ideal straight line, ensuring predictable proportional response to 4–20 mA DCS commands.
3. Repeatability within ±3%: When approaching a given stroke setting from either higher or lower capacity, the pump must reproduce its flow rate within ±3%.

Beyond volumetric precision, an API 675 hydraulic diaphragm metering pump must allow capacity adjustment while running under full load, use a positive-return drive mechanism (such as a variable eccentric or crank-and-crosshead drive rather than a light return spring), and be designed for a 20-year mechanical service life with at least 3 years of uninterrupted operation between major overhauls.

2. How the internal three-valve hydraulic system protects the diaphragm

Why can a thin PTFE diaphragm survive 100 to 250 bar inside an API 675 hydraulic diaphragm metering pump without rupturing? Because the diaphragm is hydraulically balanced: hydraulic oil fills the chamber behind the diaphragm at the exact same pressure as the process chemical in front of it. You can see a full diagram of this balanced chamber in our guide to the hydraulic diaphragm pump working principle.

To keep the oil volume inside the chamber constant on every stroke, the hydraulic end integrates three automatic control valves:

• Internal hydraulic pressure relief valve: If a downstream isolation valve is accidentally closed or an injection nozzle plugs, oil pressure rises to the relief setpoint and vents internally back to the oil reservoir, protecting the diaphragm and gearbox from overpressure damage.
• Vacuum oil refill (make-up) valve: During the suction stroke, if a partially clogged suction strainer creates excessive vacuum or if minor oil volume was vented, the refill valve opens at the back dead center to replenish hydraulic oil and prevent diaphragm over-stretching.
• Automatic air bleed (degassing) valve: Continuously purges micro-bubbles of dissolved air released from the hydraulic oil on each compression stroke so compressible gas cannot degrade volumetric accuracy.

3. Double-diaphragm rupture detection and capacity derating rules

When injecting hazardous, toxic, corrosive, or flammable media—such as hydrofluoric acid, methanol, corrosion inhibitors, or high-pressure amine solutions—operators cannot risk process fluid contaminating the gearbox oil if a diaphragm fatigues after 20,000+ hours of service. For these services, an API 675 hydraulic diaphragm metering pump is fitted with a double-diaphragm leak detection assembly.

Two PTFE diaphragms are clamped in series with a thin intermediate chamber filled with a neutral buffer fluid (such as white mineral oil or glycerin). A top port connects this interstitial space to a local pressure gauge or an explosion-proof pressure switch wired to the plant SIS/DCS. If the primary process-side diaphragm develops a pinhole, process pressure enters the intermediate chamber and trips the alarm immediately while the secondary diaphragm contains the chemical safely until a scheduled shutdown.

When sizing an API 675 hydraulic diaphragm metering pump from manufacturer catalogs, watch out for two mandatory derating factors:

• High-pressure compressibility derating: As discharge pressure increases, slight compression of the hydraulic oil and elastic expansion of the chamber reduce delivered volume. On the Milton Roy Milroyal B series (which reaches up to 2,616 L/h and 250 bar), nameplate flow derates proportionally as operating pressure climbs above 14 bar.
• Double-diaphragm derating (-5%): Adding a double-diaphragm rupture detection assembly reduces rated pump output by approximately 5%. Always add this 5% margin on top of your 1.2x process safety factor. For model-specific tables, see our article on the Milton Roy Milroyal B API 675 hydraulic diaphragm pump and our comparison of Doseuro plunger vs hydraulic diaphragm pumps.

Conclusion

Specifying an API 675 hydraulic diaphragm metering pump ensures ±1% steady-state dosing accuracy, internal three-valve overpressure protection, and zero fugitive emissions under extreme refinery pressures. When preparing your datasheet, remember to account for high-pressure compression losses and the 5% capacity deduction for double-diaphragm leak detection. Browse our hydraulic diaphragm metering pumps and Milton Roy brand catalog, or contact our engineers to request API 675 sizing sheets and compliance documentation.

FAQ

Does an API 675 hydraulic diaphragm metering pump still need an external safety valve?

Yes. Although every API 675 hydraulic diaphragm metering pump includes an internal oil-side relief valve to protect the pump mechanism, refinery piping codes still require an external process-side pressure relief valve on the discharge header to protect downstream piping and instruments.

Why does double-diaphragm leak detection reduce pump flow by 5%?

The intermediate fluid layer and sensing port between the two PTFE diaphragms add slight dead volume and hydraulic compliance on each compression stroke, reducing net volumetric efficiency by roughly 5%.

When should a metallic diaphragm be specified instead of PTFE?

Composite PTFE diaphragms handle almost all corrosive chemicals up to moderate temperatures with a service life often exceeding 20,000 hours. Specify a 316L or Hastelloy metallic diaphragm on an API 675 hydraulic diaphragm metering pump only when process temperatures, extreme pressures, or radiation/permeation limits exceed PTFE ratings.

How does Doseuro classify its API 675 pumps compared to standard models?

Doseuro places its positive-return crank-driven API 675 hydraulic diaphragm metering pump and plunger models in the SDP series (capable of pressures up to 200 bar), whereas its spring-return mechanical diaphragm models (Model D) are built for low-pressure water treatment.

Can an API 675 pump be automated with both a stroke actuator and a VFD?

Yes. An API 675 hydraulic diaphragm metering pump can be equipped with an explosion-proof 4–20 mA electric or pneumatic stroke length actuator, an inverter-duty motor driven by a VFD, or both for cascade control.