Selecting a Small ATEX Pump for a Zone 1 Condensate Shaft

The workable design comes from matching the hazardous-area classification, corrosion exposure, thermal path, and actual duty cycle—not from finding a miniature version of a conventional sewage pump. At an inflow of 10 L/h, a pump rated 12 m³/h at 6 m can deliver 1,200 times the incoming volume at its stated duty point. That mismatch drives short cycles, a larger wet well, and difficult level control long before motor power becomes the main issue.

Wrong fixes and their failure modes

The number that matters is not the pump's smallest advertised motor rating. It is the minimum stable volume that the complete installation can collect, start, discharge, and stop without overheating the pump or pulling hazardous gas into an unsuitable flow path.

Attempted fix	Why it fails	Decision quantity
Select the smallest available submersible motor	A smaller motor does not automatically provide a lower hydraulic flow, a suitable thermal path, or the required hazardous-area approval.	Certified equipment marking, approved cooling condition, minimum flow, and start frequency
Use the 12 m³/h at 6 m pump and throttle it heavily	Throttling can move the operating point, but it does not prove acceptable motor cooling, internal recirculation, solids passage, or minimum stable duty. At 10 L/h, the pump would run for only a small fraction of each fill cycle.	Manufacturer's permitted operating range and measured start-to-stop volume
Reduce the shaft size without calculating cycle volume	A narrow level band can cause rapid starts, while a wide band may require a deeper shaft than the civil design permits.	Usable volume between start and stop levels
Declare the shaft nonhazardous because the condensate is not flammable	The stated gas path is through a siphon that is not treated as permanently gas-tight. Sewage gas can therefore occupy the space above the liquid even when the liquid itself is not flammable.	Area-classification drawing and gas-release assessment
Mount a suction pump outside the shaft	This removes the submerged motor but can bring gas into the pump if the suction loses liquid or draws from above the level. The installation also has about 3 m of suction head to overcome.	Net suction conditions, priming method, dry-run behavior, and classified envelope
Keep a nonapproved pump permanently submerged	Submergence helps cooling only while the stated liquid level and duty conditions remain intact. It does not by itself change the area's classification or certify the equipment.	Certified installation conditions and independent low-level protection

Current, heat, and cycle volume

This is heat, not logic. Motor losses become heat whenever the pump runs, while starts add electrical and mechanical stress. A submersible design may reject that heat through the pumped liquid, the surrounding liquid, a cast housing, or another manufacturer-defined path. Stainless sheet construction, cast construction, and plastic construction have different thermal behavior, but material alone neither grants nor prevents hazardous-area approval. The certified assembly and its permitted operating conditions decide suitability.

The hydraulic mismatch is immediate:

• Inflow: 10 L/h = 0.010 m³/h.
• Candidate duty point: 12 m³/h at 6 m.
• Flow ratio: 12 / 0.010 = 1,200.

If the pump actually discharged at 12 m³/h, it would remove 3.33 L/s. Net drawdown would be almost the same because the incoming flow is only about 0.00278 L/s. A 10 L usable level volume would therefore produce roughly three seconds of pumping before accounting for pipe filling, acceleration, check-valve response, and the pump curve. That calculation is illustrative of the stated flow mismatch; the real run time must use the pump flow at the system operating point.

Calculate the usable wet-well volume from geometry and the vertical distance between the start and stop levels. Then calculate:

t_fill = V_use / Q_in

t_run = V_use / (Q_pump - Q_in)

starts per hour = 1 / (t_fill + t_run), with time units kept consistent.

Read the permitted starts per hour, minimum run time, cooling requirement, and minimum liquid level from the selected pump's certified documentation. Those values cannot be inferred from the 0.7 kW motor rating.

Hazardous volume and equipment boundary

The installation identifies Zone 1 above the liquid level because gas from the sewage-gas pipeline can pass the siphon into the shaft. Selection must start from that classified volume and then trace every possible gas path. The liquid's nonflammability does not remove a gas hazard in the headspace.

Location or flow path	Hazard question	Where to read or measure
Space above the shaft liquid	Can gas be present during normal operation?	Area-classification drawing and process gas assessment
Submerged pump exterior	Can the pump become exposed as the level falls?	Stop level, low-low trip, pump geometry, and installation certificate
Pump interior	Can gas enter during dry running, loss of prime, or vortexing?	Suction arrangement, operating sequence, and manufacturer restrictions
External suction pump	Can a gas-liquid mixture reach the pump, seal, discharge, or vent?	Pipe profile, priming method, leak test, and classified-area boundary
Electrical connection and level instruments	Are all components and installation methods suitable for their locations?	Equipment markings, certificates, cable-entry details, and loop drawings

Resolve the required equipment category, gas group, temperature class, ambient range, and installation conditions through the project's ATEX classification and the product certificate. A generic statement that a pump is “ATEX approved” is incomplete; the marking must match the classified location and the way the equipment will operate.

Pump architecture decision

A certified stainless submersible pump remains the direct arrangement when frost protection, compact piping, and flooded suction dominate. Its drawback at 10 L/h is usually hydraulic scale: the available pump may require more cycle volume than the shaft can economically provide.

An external suction pump changes the problem rather than eliminating it. A peristaltic pump isolates the process fluid inside a hose and tolerates metering-scale flow well, but hose life, suction lift, gas handling, and the hazardous classification of the complete assembly require review. A progressive-cavity pump can also serve low-flow transfer, with stator and seal wear, dry-running limits, and priming as key checks. Both options must be evaluated against the stated suction head of about 3 m, including pipe losses and vapor conditions.

An air-operated double-diaphragm pump could place the compressor outside the shaft, but no submersible example is established for this installation. Treat it as a new architecture requiring confirmation of immersion suitability, material compatibility, exhaust routing, static control, starting at the available head, and hazardous-area marking. Pneumatic drive does not make every component automatically suitable for a classified zone.

Selection procedure

1. Freeze the area classification. Mark the Zone 1 boundary, identify the gas entering from the pipeline, and define whether the interior of any suction line can receive gas during normal operation or an expected fault.
2. Build the hydraulic profile. Record the 10 L/h minimum inflow, maximum inflow, static discharge head, pipe losses, discharge pressure, check-valve behavior, and the stated 3 m suction head if an external pump is considered.
3. Calculate cycle volume. Use the pump curve at the calculated system head, not the nominal 12 m³/h at 6 m point unless that is the actual intersection. Calculate fill time, run time, and starts per hour for the proposed start and stop elevations.
4. Screen certified submersible pumps. Require matching hazardous-area markings, acceptable stainless grade such as 1.4436 or a documented alternative, permitted immersion conditions, and a duty point inside the manufacturer's operating range.
5. Screen low-flow suction alternatives. For peristaltic and progressive-cavity pumps, obtain the allowable suction lift, dry-run limit, wetted-material list, wear-part interval, priming behavior, and certification for every classified location.
6. Control gas ingestion. Place the suction pickup and stop level to prevent vortexing and loss of liquid seal. Add independent low-level shutdown when the selected pump relies on liquid for cooling or exclusion of gas.
7. Check thermal conditions. Compare expected run time, starts per hour, surrounding medium, ambient temperature, and minimum submergence with the certified instructions. Confirm protection settings from the motor data and approved protection concept.
8. Obtain written configuration acceptance. Give the pump manufacturer the liquid chemistry, temperature, classification, duty curve, cycle calculation, shaft geometry, and possible gas exposure. Request confirmation for the complete operating mode, not just a material quotation.

Materials and corrosion control

1.4436 stainless steel is the stated preference, but “or similar” needs a corrosion basis. Record condensate pH, chlorides, sulfides, cleaning chemicals, temperature, and stagnant exposure. Review the housing, shaft, fasteners, elastomers, cable sheath, mechanical seal, hose, and check valve separately; a stainless casing does not protect incompatible internal or auxiliary parts.

Plastic may provide suitable chemical resistance, yet its lower thermal conductivity changes heat rejection and static behavior. Those issues are design inputs, not an automatic rejection. Select plastic only where the manufacturer certifies the assembly for the classified location and documents its temperature, immersion, electrostatic, and mechanical limits.

Cast housings can provide thermal mass and a useful heat path, but cast construction is not a substitute for certification. Likewise, sheet-metal stainless construction is not inherently disqualified. Read the exact equipment marking and certificate conditions for the proposed pump.

Commissioning and proof of operation

1. Measure the actual start and stop elevations and calculate the corresponding usable volume.
2. Time several fill and pump-down cycles. Compare measured run time and starts per hour with the selected manufacturer's limits.
3. Measure discharge pressure or head during operation and plot the observed duty point against the pump and system curves.
4. Test normal stop, independent low-level shutdown, level-sensor failure response, blocked discharge response, and restoration after power loss.
5. Inspect for vortexing, gas ingestion, loss of prime, unstable check-valve action, leakage, abnormal vibration, and excessive temperature.
6. Verify that equipment markings, cable entries, protection devices, bonding, installation orientation, and minimum submergence match the approved documents.

A successful test has stable priming, no gas draw at the stop level, a repeatable operating point, and cycle values inside the documented limits. Trend cycle count and run time after commissioning: rising run time can indicate restriction or wear, while falling run time or rapid starts can indicate reduced level volume, a control fault, or check-valve leakage.

FAQ

Stop selection when the classification, gas group, temperature class, certified cooling condition, or actual duty point remains unresolved. Escalate the completed classification drawing, hydraulic calculation, liquid analysis, shaft geometry, and operating sequence to the pump manufacturer's official technical support or certification department for written confirmation.