Danfoss Pumps Buyer's Guide: APP Axial Piston Pumps for SWRO and High-Pressure Applications
Danfoss APP axial piston pumps are the industry standard for seawater reverse osmosis (SWRO) from 200 GPD portable watermakers to 1 million GPD industrial plants. This guide covers every APP model (APP 0.6 through APP 86), specifications, part numbers, pricing, spare parts, and how to choose the right Danfoss pump for your desalination or high-pressure water application.
ForeverPure is an authorized Danfoss High Pressure Pumps distributor. We stock the full Danfoss APP line and provide technical sizing, spare parts, and rebuild support to commercial and industrial customers worldwide.
The Danfoss APP (Axial Piston Pump) series is a family of positive-displacement high-pressure pumps engineered specifically for seawater reverse osmosis and technical water applications. Built from seawater-resistant stainless steel and lubricated by the seawater itself (no external oil lubrication), APP pumps deliver the 800–1,000 psi (55–70 bar) required to push seawater through RO membranes, at overall efficiencies of 90%+.
Danfoss manufactures the APP line in Nordborg, Denmark. The pumps are available in nine standard flow capacities (APP 0.6 through APP 86) covering water output from approximately 0.6 to 86 m³/hour at 800 psi operating pressure. Each size is available in multiple variants (standard APP, APP S for stainless steel manifold, APP T/W for high-temperature or solids-tolerant service).
Why Danfoss Axial Piston Pumps Dominate SWRO
Seawater-Lubricated Design
Traditional triplex plunger pumps require a separate oil-lubricated drive-end crankcase and use packing or seals to isolate the pumped fluid from the lubricant. Danfoss APP pumps are radically different: the seawater being pumped also lubricates the swash-plate bearings and piston shoes. No oil. No packing. No contamination risk. The result: simpler installation, lower maintenance, and no lubricant spills into product water.
Super-Duplex Stainless Steel Construction
All wetted parts are 2507 super-duplex stainless (pitting resistance equivalent number >40), which is essentially immune to chloride corrosion in full-strength seawater up to 40,000 ppm TDS. This is critical — conventional 316L stainless fails in high-chloride service within 2–5 years.
Long Service Life at Low Noise
A Danfoss APP 16 running 16 hours/day in standard SWRO duty typically reaches 50,000–80,000 operating hours before a major overhaul. That's 8–15 years of continuous service. Noise levels are 65–75 dB(A), quiet enough for shipboard and residential adjacent installations.
Low Minimum-Flow Capability
APP pumps operate reliably down to 30% of rated capacity, allowing VFD-based capacity control without cavitation. This is particularly valuable for solar-powered or variable-demand installations.
Danfoss APP Model Range: Complete Reference
The table below covers every standard Danfoss APP model with part numbers, flow rates, and typical applications.
All flow rates assume 40% system recovery. Actual output varies with feed water salinity, temperature, membrane selection, and system design.
Each model is available as standard APP, APP S (stainless steel valve block), APP T (high-temperature), and APP W (high-solids tolerance). Part numbers for S/T/W variants use the same prefix with different suffix digits.
How to Size a Danfoss APP Pump
Sizing a Danfoss APP pump requires three inputs: required feed flow to the membrane system, operating pressure, and recovery rate.
Step 1: Determine membrane feed flow
For a target permeate output P (gallons per day) at recovery rate R (typically 35–45% for SWRO), required feed flow F is:
If using an ERI pressure exchanger or FEDCO HPB, the high-pressure pump only needs to supply the permeate portion of feed (40% of total), because the ERD recirculates the 60% brine to feed pressure. This dramatically reduces required pump size.
With ERD: High-pressure pump flow = Permeate flow ≈ 2,500 GPD = 0.41 m³/h → select APP 0.6 or APP 0.8 (significantly smaller than without ERD).
Without ERD: High-pressure pump must supply full feed flow = 3.54 m³/h → select APP 3.5 or APP 5.1.
Step 3: Verify pressure rating
Standard seawater RO operates at 800–1,000 psi. All APP models are rated for 80 bar (1,160 psi) continuous. No derating required for standard SWRO duty.
Step 4: Add 10% design margin
Membrane fouling increases pressure requirement over time. Size the pump at 10% above calculated duty to ensure adequate capacity at end of membrane life.
Worked example: A coastal hotel needs 30,000 GPD SWRO with ERI PX-Q300 energy recovery. Permeate flow = 30,000 GPD = 20.8 GPM = 4.73 m³/h. With 10% margin: 5.2 m³/h. Select APP 5.1 (180B3005).
Send us your feed-water analysis and target permeate output — we'll confirm pump selection and quote within 24 hours. Request engineering support.
Spare Parts, Seal Kits, and Rebuild Components
Danfoss publishes standardized spare parts kits for each APP size. The most common service items are:
O-ring kit — annual service on heavy-duty installations. Part numbers follow the pattern 180BXXXX.
Seal kit (high-pressure seals + wear rings) — typically replaced every 8,000–12,000 operating hours.
Valve kit (inlet/outlet plate assemblies) — inspected annually, replaced every 20,000–40,000 hours depending on water quality.
Swash plate assembly — rare replacement, typically at major overhaul (>50,000 hours).
Cylinder barrel set (Danfoss 180B4140 is one common example) — inspected at major overhaul.
Piston/shoe assembly — replaced at major overhaul.
ForeverPure stocks all standard service parts. Overnight shipping is available for production-stopping failures.
Installation and Commissioning Guidelines
Suction-side requirements
APP pumps require positive inlet pressure (NPSH required is approximately 2 bar / 30 psi above vapor pressure). A booster pump is always required — these are not self-priming. The booster must supply clean, pre-filtered feed water (recommend 5 μm cartridge filter minimum).
Mounting and alignment
Direct-couple to motor using a flexible coupling. Laser-align shaft to within 0.002" TIR. Misalignment is the #1 cause of premature bearing failure.
Pre-commissioning flushing
Flush the feed line with 5 μm-filtered fresh water for minimum 30 minutes before first startup. Residual construction debris in new pipework will destroy the piston/shoe interface within hours.
First startup
Always start with the discharge valve cracked open (no full pressure build-up). Bleed air from the pump head. Ramp pressure gradually over 2–3 minutes. Verify rotation direction before engaging load.
Maintenance Schedule & Service Life
Interval
Task
Daily
Verify discharge pressure within ±5% of setpoint. Log RPM.
Weekly
Inspect for leaks at manifold and shaft seal.
Monthly
Check coupling alignment. Monitor current draw — rising current indicates bearing or valve wear.
Replace O-ring kit. Verify torque on mounting bolts.
8,000–12,000 hrs
Replace seal kit and wear rings.
20,000–40,000 hrs
Replace valve kit. Inspect swash plate.
50,000+ hrs
Major overhaul. Replace piston/shoe assemblies, cylinder barrel. Return to factory-authorized service center.
Common Problems and Troubleshooting
Pump fails to build pressure
Air in suction line — bleed and verify booster pump operation
Fouled suction filter — replace and check inlet pressure
Worn inlet/outlet valves — replace valve kit
Discharge relief valve stuck open — inspect and test
Excessive vibration
Coupling misalignment — laser-align within 0.002" TIR
Bent shaft or worn bearings — overhaul required
Cavitation from insufficient suction pressure — verify booster NPSH
Premature seal failure
Abrasive particles in feed — upgrade pretreatment to 1 μm
Dry-running — ensure booster never loses prime
Chemical incompatibility — verify feed water chemistry within spec
Rising current draw
Bearing wear — measure shaft axial/radial play
Valve seat wear — replace valve kit
Viscosity change in feed (temperature drop) — verify feed is within operating temperature range
Pricing and Lead Times
Danfoss APP pumps are priced per model, typically on a request-for-quote basis for commercial and industrial customers. Indicative ballparks (USD, Danfoss factory price before freight/spares):
Bottom line: For commercial SWRO from 2,000 to 35,000 GPD, Danfoss APP wins decisively on efficiency, materials, and TCO. For tiny portable systems under 2,000 GPD, CAT Pumps is often cheaper. For very large municipal systems (250,000+ GPD), Grundfos BME or dedicated high-pressure multi-stage centrifugals with ERD become more economical.
Frequently Asked Questions
How long do Danfoss APP pumps last?
A properly sized and maintained Danfoss APP pump runs 50,000 to 80,000 operating hours before major overhaul — typically 8 to 15 years of continuous SWRO duty. Total pump service life including overhauls often exceeds 25 years. The most common failure mode is poor pretreatment letting abrasive particles reach the piston/shoe interface.
What's the difference between Danfoss APP and APP S?
Standard APP has a cast bronze manifold block. APP S uses a 316L or super-duplex stainless steel manifold block for installations with very high chloride or trace contaminant concerns. APP S costs approximately 20-25% more than standard APP but is recommended for high-salinity Gulf or Red Sea waters.
Can Danfoss APP pumps run dry?
No. Danfoss APP pumps are seawater-lubricated — running dry even briefly will cause severe damage to the piston/shoe interface within seconds. Always verify the booster pump has prime and positive inlet pressure before starting the APP. Install low-pressure switches that interlock shutdown.
What pressure can a Danfoss APP pump generate?
All standard Danfoss APP models are rated for 80 bar (1,160 psi) continuous operating pressure with brief peaks to 85 bar. Standard SWRO operates at 55–70 bar (800–1,000 psi), leaving generous design margin.
How much does a Danfoss APP pump cost?
Danfoss APP pumps range from approximately $2,500 for an APP 0.6 (smallest portable watermaker size) to $85,000+ for an APP 86 (industrial-scale). Commercial sizes (APP 5.1 through APP 22) typically fall between $6,500 and $22,000 depending on model and configuration. Prices vary by variant (standard vs S/T/W) and current exchange rates.
Are Danfoss APP pumps axial piston or swash plate pumps?
Both descriptions apply — Danfoss APP pumps use a swash-plate mechanism to drive axial pistons. The swash plate sits at a fixed angle to the drive shaft; as the shaft rotates, the swash plate pushes each piston through a stroke, pressurizing and discharging seawater through inlet/outlet valves.
Do Danfoss APP pumps need oil?
No. Danfoss APP pumps are uniquely seawater-lubricated. The pumped seawater itself lubricates the internal bearings and piston shoes, eliminating the need for a separate crankcase oil system. This is a major advantage over traditional triplex plunger pumps for marine and remote installations.
Where can I buy Danfoss APP pump spare parts?
ForeverPure is an authorized Danfoss distributor stocking the complete APP spare parts range including O-ring kits, seal kits, valve kits, cylinder barrel sets, and swash plate assemblies. Overnight shipping is available from our Denmark and California warehouses. Contact us with your APP model and part number for immediate quote.
Request a Danfoss APP Pump Quote
Tell us your target permeate output, feed water source, available power, and site location. Our engineers will recommend the correct APP model, motor, baseplate, and spare parts package, and provide a formal quote within 24 hours.
