Not every dosing pump in a plant needs a standard behind it — but for services where an under-dose or over-dose of a chemical has safety, environmental, or production consequences, a general-purpose metering pump is not enough. API 675 (Positive Displacement Pumps — Controlled Volume for Petroleum, Chemical, and Gas Industry Services, 3rd Edition, November 2012) is the API standard that defines what a controlled-volume metering pump must actually deliver — tested accuracy, mandatory design features, and a documentation package that ties the pump’s performance to the contract rather than a catalogue brochure.
What API 675 covers
API 675 covers the minimum requirements for reciprocating, controlled-volume pumps and pump units for use in the petroleum, petrochemical, and gas industry services. The two pump constructions within its scope are hydraulic diaphragm and packed plunger designs. Rotary positive displacement pumps are not included, and diaphragm pumps that use direct mechanical actuation (e.g. solenoid-driven or direct-drive diaphragm pumps) are also excluded — those fall under API 674 or no specific API standard at all.
The standard covers the complete pump unit: liquid end, drive, gearbox, capacity adjustment mechanism, baseplate, relief arrangement, and instrumentation. It extends to multiplex arrangements where several liquid ends share one common drive. The standard requires the purchaser to specify certain details and features — it is a purchase specification, not a design manual, and the accuracy figures it specifies are contractual claims that the manufacturer must demonstrate through testing.
Hydraulic diaphragm vs packed plunger
Hydraulic diaphragm
A piston moves hydraulic oil (typically silicone oil) in a cavity behind a flexible PTFE or elastomer diaphragm. The hydraulic oil transmits the piston’s displacement evenly across the diaphragm surface, which flexes to draw in and discharge the process fluid through check valves. This design isolates the process fluid entirely from the drive mechanism and is the preferred choice where the fluid is toxic, corrosive, or where leakage past a plunger packing is unacceptable. API 675 requires diaphragm rupture detection on this type — typically a pressure sensor or switch in the hydraulic circuit that alarms if the secondary diaphragm is breached. Double-diaphragm designs with a fluid-filled intermediate chamber are also covered.
Packed plunger
A reciprocating plunger slides through a packed stuffing box, displacing fluid directly in the cylinder on each stroke through suction and discharge check valves. This is a simpler, more robust design with fewer failure modes than a diaphragm, and is suitable for fluids that are compatible with the plunger and packing materials and where minor leakage past the packing is tolerable. API 675 requires a lantern ring in the stuffing box for flushing or adding lubricant to the packing, with drilled and tapped holes on the top and bottom of the pump body for inlet and outlet. Packed-plunger pumps require an external relief valve mounted in the purchaser’s piping, unlike diaphragm pumps which use an integral hydraulic relief valve.
The three numbers that define API 675 performance
Three measured properties define what API 675 actually requires of a metering pump, and a specification must cite all three to be meaningful:
| Property | Requirement | What it means |
|---|---|---|
| Steady-state accuracy | Within ±1% of rated flow | At a fixed setting, assess the deviation using the stated rated-flow basis and agreed test conditions; this is not necessarily 1% of the selected set point |
| Linearity | Within ±3% of rated flow | As stroke length is adjusted from 10% to 100% of rated capacity, the actual delivered flow follows a straight line against the dial setting within ±3% |
| Repeatability | Within ±3% of rated flow | If the pump is set to the same stroke length and run repeatedly, each run delivers the same flow within ±3% of the previous runs |
The guaranteed operating range and accuracy basis must be stated in the offer. A turndown ratio describes the span between maximum and minimum flow within the specified performance envelope; it does not make percentage-of-rated-flow accuracy the same as percentage-of-set-point accuracy. Milton Roy provides manufacturer examples of API 675 performance criteria in the reference linked above. Confirm the applicable standard edition, test conditions and any extended range for the actual pump.
Turndown ratio — what 10:1 actually means
The turndown ratio is the ratio between the pump’s maximum rated flow and the minimum flow at which accuracy, linearity, and repeatability are still guaranteed. API 675 requires a minimum turndown ratio of 10:1 — so a pump rated at 100 litres per hour must maintain ±1% accuracy at flows as low as 10 litres per hour. Below 10% of rated capacity, the manufacturer does not guarantee the standard’s accuracy figures. Stroke length adjustment is performed manually via a scaled rotary dial or remotely via an Actuated Capacity Controller (ACC) that receives signals from a PLC or control system, and adjustment must be possible while the pump is running — stopping the pump to change the dose is not acceptable under the standard.
Relief valve and safety requirements
Like all positive displacement pumps, a controlled-volume pump will keep building pressure against a closed or blocked downstream valve until something fails. API 675 addresses this with two different relief valve arrangements depending on pump type:
- Diaphragm pumps must have an integral, adjustable hydraulic relief valve to protect the drive mechanism from excessive discharge pressure; the setting must follow the approved design and the instructions for the actual pump
- Packed-plunger pumps require an external relief valve in the purchaser’s piping, sized for the pump’s full rated capacity
- Relief valves must be self-seating and easily accessible for adjustment, repair, or replacement
- Gears in the pump drive assembly must be all-metal, designed with AGMA criteria, and carry a minimum service factor of 1.5
- Bearings must be selected for a defined minimum rated life
- Capacity adjustment must include a locking arrangement to prevent accidental drift from the set point
Diaphragm rupture detection
A double-diaphragm arrangement can provide a monitored space between the process and hydraulic sides. The manufacturer should identify how rupture detection works on the offered liquid end, the detection device and the associated alarm or control interface. Do not assume that every model detects a breach through contamination of the hydraulic oil: intermediate-space monitoring arrangements differ. Specify the required detection function and obtain the appropriate manufacturer drawing and project compliance statement.
Testing and documentation
API 675 requires a hydrostatic test of the pressure-containing parts and performance testing to the agreed requirements before shipment; witnessing must be defined in the inspection plan. The performance test confirms that the pump meets the accuracy, linearity, and repeatability figures at the specified conditions. If specified by the purchaser, a mechanical running test of the agreed duration is also performed. Documentation includes Certified Material Test Reports (CMTRs) for wetted parts, the manufacturer’s data and drawing requirements (often called VDDRs), an inspector’s checklist, and the test results. The manufacturer’s data sheets — which define site details, performance criteria, and equipment furnished — are the contractual link between the purchaser’s process requirements and the pump that gets built. An incomplete or generic datasheet is the most common specification error: it leaves the manufacturer to guess at the test scope and produces quotations that cannot be compared.
Where API 675 metering pumps show up in the field
API 675 pumps appear in services where the dose matters and where an under-dose or over-dose has consequences beyond a minor process upset. Common applications include corrosion inhibitor injection (film-forming amines, oxygen scavengers) on offshore platforms and in refinery crude units, methanol injection for gas hydrate prevention in subsea and pipeline systems, polymer and flocculant dosing in produced water treatment, scale inhibitor injection in water injection systems, and chemical injection skids for pH control and neutralisation. The standard is less common in municipal water treatment and general industrial dosing, where the cost premium of an API 675 package — with its tested accuracy, documentation, and mandatory design features — is not justified by the consequences of a dosing error.
What to check on an API 675 datasheet or RFQ
Citing API 675 on a datasheet without selecting any of the optional requirements leaves the manufacturer to guess at the test and certification scope, and produces quotations that cannot be compared. Specifying a duty that no controlled-volume pump can hold — such as a turndown far beyond 10:1 — means the accuracy figures are not contractually enforceable at the actual operating point. The datasheet must be complete and explicit.
- State the API 675 edition (3rd Edition, November 2012) and select which optional requirements apply — do not just cite the standard without specifying the test scope
- Specify the turndown ratio as 10:1 or state the actual turndown needed; if more than 10:1 is required, explain how variable speed or multiple pumps will cover the range
- Specify whether the pump is hydraulic diaphragm or packed plunger — this determines the relief valve arrangement and whether diaphragm rupture detection is required
- State the process fluid, temperature, viscosity, specific gravity, and whether the fluid is abrasive, corrosive, or shear-sensitive — diaphragm and packing material selection depends on this
- Specify the required suction conditions: NPSH available and NPIP (Net Positive Inlet Pressure) — controlled-volume pumps are more sensitive to suction-side conditions than centrifugal pumps because of the acceleration head
- Include the discharge pressure, required flow rate, and the number of pump heads in any multiplex arrangement
- Specify the electrical area classification (IEC 60079 or NFPA 70 / API 500) and whether the pump requires an Actuated Capacity Controller (ACC) for remote stroke adjustment
Sourcing API 675 metering pumps or chemical injection skids? Browse our pumps & rotating equipment category, or send us your metering pump datasheet and Oillinko will circulate it to potential manufacturers, checking every returned offer against the API 675 edition, accuracy requirements, and testing scope you need confirmed before the order is placed. If the pump also needs a mechanical seal or support system, see our guide to API 682 seal piping plans.
Frequently asked questions
What does API 675 actually require of a metering pump?
API 675 requires steady-state accuracy within ±1%, linearity within ±3%, and repeatability within ±3% of rated flow, each measured across the turndown ratio from 10% to 100% of rated capacity. The pump must be adjustable over this range while running, and rated capacity must be at least 110% of the maximum capacity specified.
What is the difference between a hydraulic diaphragm and a packed plunger under API 675?
A hydraulic diaphragm pump uses a flexible PTFE or elastomer diaphragm that is actuated by hydraulic oil from a piston, isolating the process fluid from the drive mechanism. A packed plunger pump displaces fluid directly using a reciprocating plunger sliding through a packed stuffing box. Both designs must meet the same accuracy and testing requirements under API 675, but the diaphragm type is preferred where the process fluid is toxic, corrosive, or where zero leakage from the liquid end is required.
Is API 675 required for every chemical injection pump in a refinery?
Not every dosing service justifies API 675. The standard is intended for hydrocarbon and high-consequence services — refinery chemical injection, gas processing, offshore platforms, and pipeline additive injection — where a dosing failure creates a safety or environmental event. Municipal water treatment, effluent plants, and general process dosing rarely justify the additional cost.
What does API 675 require for diaphragm pumps regarding rupture detection?
Specify diaphragm rupture detection and ask the manufacturer to identify the arrangement provided. Double-diaphragm designs can monitor the space between diaphragms to detect a failure. The exact design, alarm function and applicable requirements must be confirmed for the offered pump and the project specification.
