Most process pumps in a plant are centrifugal, built to API 610. But for high-viscosity fluids, high-pressure/low-flow duties, or services that need flow largely independent of discharge pressure, positive displacement pumps take over — and API 610 does not apply to them. Two API standards cover the two mechanical families: API 674 for reciprocating pumps and API 676 for rotary pumps.
Why positive displacement at all
A centrifugal pump adds velocity to a fluid with a spinning impeller; how much flow it produces depends on where the system resistance curve intersects the pump’s own performance curve, and flow falls as discharge pressure rises. Efficiency also falls away quickly once a centrifugal pump is run far from its best efficiency point, whereas a well-selected PD pump holds efficiency across a much wider turndown range, which matters for services that spend significant time away from a single fixed operating point. A positive displacement (PD) pump instead traps a fixed volume of fluid and physically moves it from suction to discharge each stroke or revolution, so flow stays essentially constant regardless of discharge pressure (within the pump’s pressure rating) and depends mainly on speed. That property makes PD pumps the standard choice for fluids too viscous for an efficient centrifugal design, for high-pressure, low-flow duties such as chemical injection or high-pressure cleaning, and for services where accurate, pressure-independent flow matters more than raw capacity. The trade-off is that a PD pump cannot simply be throttled with a discharge valve the way a centrifugal pump can — closing a valve downstream of a running PD pump does not reduce its flow, it raises pressure until something gives, which is why relief protection sized for the pump itself, not just the system, is a fixed requirement rather than a site-specific option on both API 674 and API 676 packages.
API 674: reciprocating pumps
API 674 (Positive Displacement Pumps — Reciprocating, 3rd Edition, December 2010) covers the minimum requirements for reciprocating PD pumps and pump units in petroleum, petrochemical and gas industry service. A plunger or piston moves back and forth in a cylinder, displacing fluid on each stroke through suction and discharge check valves. The standard splits the family into two drive types:
Direct-acting
The pumped-side plunger is driven straight by steam or pneumatic pressure acting on the opposite side of the piston — no crankshaft or rotating drive train. Common where a site already has a steam or compressed-air utility and wants a simple, self-regulating stroke rate.
Power-frame
A motor drives a crankshaft and crosshead, converting rotary motion into the plunger's linear stroke — mechanically similar to a reciprocating compressor's drive end. This is the more common arrangement for continuous, motor-driven process duties and is what most API 674 pump packages in a refinery or petrochemical plant use.
Because a reciprocating pump delivers flow in discrete pulses rather than a steady stream, API 674 places heavy emphasis on pulsation and vibration control — acceptance criteria for pulsation dampeners, piping-induced vibration limits, and torsional analysis of the drive train — alongside the more familiar requirements for materials, bearing life and testing. Controlled-volume metering pumps and hydraulically driven pumps are explicitly excluded from API 674’s scope; they fall under API 675 instead.
API 676: rotary pumps
API 676 (Positive Displacement Pumps — Rotary, 4th Edition, February 2022) covers rotary PD pumps — gear, screw, lobe and vane types — that displace fluid through continuously rotating elements rather than a reciprocating stroke. Because the elements rotate rather than reverse direction, rotary pumps produce a much smoother, less pulsating flow than reciprocating pumps, while keeping the same essential advantage of flow that is largely independent of discharge pressure. Twin-screw pumps are common enough in oil and gas service that API 676 includes a dedicated annex on the factors affecting twin-screw efficiency, alongside general datasheets and an inspector’s checklist that apply across all the rotary types it covers. As with API 674, controlled-volume and reciprocating pumps are outside its scope, and rotary pumps in purely auxiliary services (such as a lube-oil system) can often be sourced to a manufacturer’s standard design rather than a full API 676 build, where the purchaser accepts that lower level of customization.
API 674 vs API 676 at a glance
| API 674 | API 676 | |
|---|---|---|
| Pump motion | Reciprocating plunger/piston | Continuously rotating elements (gear, screw, lobe, vane) |
| Flow character | Pulsating — needs dampener/pulsation control | Smooth, near-continuous |
| Typical drive | Direct-acting (steam/pneumatic) or power-frame (motor + crankshaft) | Motor through a gearbox or direct coupling |
| Typical duty | High-pressure, low-flow — injection, high-pressure cleaning, well service | High-viscosity transfer — heavy crude, asphalt, lube oil, polymer |
| Excludes | Controlled-volume pumps (API 675), rotary pumps (API 676) | Controlled-volume pumps (API 675), reciprocating pumps (API 674) |
Where each type shows up in the field
Reciprocating pumps built to API 674 tend to cluster around high-pressure, comparatively low-flow duties: well-service and hydraulic fracturing pumps, hydrotest pump skids, high-pressure water-jetting units, and chemical injection at pressures beyond what a metering pump under API 675 is built for. Their triplex or quintuplex plunger arrangements (three or five plungers, phased to smooth out the combined pulsation) are chosen specifically to keep discharge pulsation manageable at those pressures. Rotary pumps built to API 676 cluster instead around viscous, continuous transfer duties: gear pumps for lube oil, fuel oil and other clean, moderate-viscosity fluids; twin-screw pumps for heavy crude, bitumen and multiphase (liquid-and-gas) transfer, where their tolerance for entrained gas is a genuine advantage over both centrifugal and gear designs; and lobe or vane pumps where gentle, low-shear handling matters, such as with fluids prone to emulsifying or shearing apart under a spinning gear mesh. Knowing which family a service falls into before a request for quotation goes out saves a round of vendor questions before a quote can even be prepared.
Testing and materials
Both standards require a hydrostatic test of the pressure-containing parts and a witnessed (or at minimum documented) performance test before shipment, the same principle familiar from API 610 centrifugal pumps. Where PD pumps diverge is in what the performance test actually has to demonstrate: rather than a full head-versus-flow curve, the test confirms volumetric efficiency (how close actual delivered flow comes to the theoretical displacement at rated speed) and, for reciprocating pumps, that pulsation levels in the discharge piping stay within the levels assumed in the pulsation study. Sealing method is another point that needs to be explicit on the datasheet rather than left to the vendor’s standard offering — many rotary and reciprocating PD pumps in less demanding services still use a packed gland rather than a mechanical seal, which is perfectly acceptable where minor leakage past the packing is tolerable, but is a materially different maintenance and emissions proposition from a seal built to API 682. Materials selection follows the fluid rather than a fixed table: abrasive or high-solids service pushes toward hardened wear rings and clearances on the generous side, while highly viscous but clean fluids (heavy crude, bitumen, polymer melts) push toward tighter clearances and, on rotary pumps, larger rotor diameters running at lower speed to keep shear and internal heating manageable.
A third standard sits alongside these two: API 675 covers controlled-volume (metering) pumps — reciprocating or diaphragm pumps built specifically for accurate, adjustable dosing rather than bulk transfer, such as corrosion inhibitor or methanol injection skids. A pump that needs to deliver a precisely adjustable, repeatable flow rate at low volume belongs under API 675, not API 674, even though the underlying reciprocating mechanism looks similar at a glance.
What to check on a PD pump datasheet
Positive displacement pumps raise a few datasheet questions that rarely come up with a centrifugal pump, precisely because their failure modes are different: a centrifugal pump running against a closed valve simply recirculates and heats up, while a PD pump keeps displacing fluid regardless, turning a closed valve into an overpressure event within seconds rather than minutes.
- A relief valve dedicated to the pump and sized for its full rated capacity is included — a PD pump will keep building pressure against a blocked line rather than stalling
- Pulsation dampener sizing and acoustic/vibration analysis are included for reciprocating (API 674) packages operating near piping natural frequencies
- NPSH margin is checked at the pump's actual acceleration head, not just the steady-state figure used for centrifugal pumps — reciprocating pumps are far more sensitive to suction piping layout
- Materials and clearances are matched to the fluid's viscosity, abrasiveness and temperature range across the full operating envelope, not just the design point
- Seal or packing type (mechanical seal vs packed gland) is specified explicitly, since many rotary and reciprocating PD pumps still use packing in less critical services
- The edition of API 674 or API 676 the pump and its testing comply with is stated on the datasheet
Sourcing a reciprocating or rotary PD pump package? Browse our pumps & rotating equipment category, or send us your pump datasheet and we will circulate it to potential manufacturers and check every returned offer against the standard, materials and testing you specified, including the relief valve and pulsation control items that are easy for a rushed quotation to leave out. If the pump also needs a mechanical seal and support system, see our guide to API 682 seal piping plans.
Frequently asked questions
What is the actual difference between API 674 and API 676?
API 674 covers reciprocating positive displacement pumps — direct-acting and power-frame (crankshaft) types that move fluid with a plunger or piston stroking back and forth in a cylinder. API 676 covers rotary positive displacement pumps — gear, screw, lobe and vane types that move fluid through continuously rotating elements. Both exclude controlled-volume metering pumps, which fall under API 675, and both exclude hydraulically driven pumps.
Why choose a positive displacement pump instead of a centrifugal pump built to API 610?
A centrifugal pump's flow rate falls as system pressure rises, following its performance curve, and it struggles with high-viscosity or shear-sensitive fluids. A positive displacement pump delivers essentially the same flow per revolution or stroke regardless of discharge pressure, within its rated limits, which makes it the standard choice for high-viscosity fluids (heavy crude, asphalt, lube oils), high-pressure/low-flow duties, and services needing accurate, pressure-independent flow.
Does a positive displacement pump need a relief valve even if the pipeline already has one?
Yes. A positive displacement pump will keep building pressure against a closed or blocked downstream valve until something fails, since it does not stall the way a centrifugal pump does against shutoff head. API 674 and API 676 both require a relief valve dedicated to protecting the pump and its immediate piping, sized for the pump's full rated capacity, independent of any relief protection elsewhere in the system.
What is the difference between direct-acting and power-frame reciprocating pumps under API 674?
A direct-acting pump is driven straight by steam or pneumatic pressure on one side of a piston, with no crankshaft — the pumped-side plunger moves in direct response to the driving fluid. A power-frame pump is driven by a motor through a crankshaft and crosshead, converting rotary motion into the plunger's reciprocating stroke, and is the more common arrangement for continuous, motor-driven process duties.
What wears out first on a positive displacement pump, and does that affect spare parts planning?
On reciprocating pumps, the packing or seal around the plunger and the suction/discharge check valves are usually the first wear items, since they see continuous cyclic loading every stroke; plungers and cylinder liners follow over a longer horizon, especially in abrasive service. On rotary pumps, wear concentrates on the close clearances between the rotating elements themselves — gear teeth, screw rotors, or lobes — and on the bushings or bearings supporting them. Both point to the same procurement habit: order a starter set of the specific wear items (packing, valves, or rotor sets) alongside the pump itself, rather than waiting for the first overhaul to find out lead times on parts that are effectively consumables.
