ARO PD20P Flooded Suction: Check Liquid Inlet Pressure

For an ARO PD20P flooded-suction installation, check liquid inlet pressure separately from compressed-air pressure. ARO’s PX20P operator’s manual lists a maximum material inlet pressure of 10 psig (0.69 bar), compared with 120 psig (8.3 bar) maximum air inlet pressure and maximum liquid outlet pressure. The air-side rating does not permit a pressurized supply tank to feed the liquid inlet at the same pressure.

This guide explains how to screen a proposed installation before ordering a pump or changing the supply arrangement. It applies to the PX20P family covered by the cited manual. Confirm the full nameplate code, revision and applicable instructions for your unit, including any PD20P-FPS-PTT enquiry.

Start with the highest liquid level and tank pressure

A flooded inlet can receive pressure from both the liquid column above the pump and gas pressure above the liquid. An open tank may impose only static liquid head. A blanketed or pressurized tank adds another contribution. Record the maximum permitted tank pressure, not just the normal operating reading.

For a preliminary static check, calculate:

Inlet gauge pressure (bar) = tank gauge pressure (bar) + density (kg/m³) × 9.81 × vertical liquid height (m) ÷ 100,000.

Measure height from the pump’s liquid inlet to the highest liquid surface. Use fluid density at the relevant temperature and concentration. Gauge pressure is relative to the surrounding atmosphere; do not mix it with absolute pressure. This calculation is a screening tool, not a complete piping or pressure-protection design.

A worked example: a modest tank pressure can matter

Consider an illustrative fluid density of 1,200 kg/m³ and a maximum liquid surface 4 m above the inlet:

• The liquid column contributes approximately 0.471 bar.
• With an open tank, the preliminary static inlet pressure is approximately 0.471 bar.
• Adding 0.30 bar tank gauge pressure raises the result to approximately 0.771 bar.

The pressurized example exceeds the manual’s 0.69 bar material-inlet limit. Do not release that arrangement for installation without an approved redesign. The open-tank example passes only this static comparison; it does not establish overall suitability or an adequate operating margin.

Check stopped conditions as well as running conditions

Do not subtract running suction-line friction from the static calculation to make an arrangement appear acceptable. When flow stops, that friction loss disappears. An upstream pump, changes in valve position or pressure transients can create additional conditions requiring review.

Ask the responsible engineer to assess normal operation, startup, shutdown and credible abnormal conditions. Provide both maximum and minimum tank levels: the high level matters for inlet pressure, while low-level operation may reveal a different suction problem. For the latter, see our suction-line design guide for viscous liquids.

Keep the air-side check valve in its proper role

For forced-feed or flooded-inlet service, ARO recommends a check valve at the air inlet. Its installation instructions also repeat the 10 psig flooded-suction limit. An air-side check valve does not increase the allowable liquid inlet pressure, and an air regulator does not regulate supply-tank pressure.

Have qualified personnel specify the protection, isolation and instruments required by the actual process. Follow the manufacturer’s safety instructions and site procedures before any inspection or service. Turning off compressed air alone does not remove pressure supplied by a tank or upstream equipment.

Put these conditions in the purchase enquiry

1. Full pump code and nameplate photograph, with the manual revision used for review.
2. Liquid identity, concentration, density, viscosity and operating temperature.
3. Maximum and minimum liquid levels relative to the pump inlet.
4. Tank pressure range, any upstream pump and known transient conditions.
5. Required flow, discharge pressure and available air supply at the pump.
6. Piping sketch showing isolation, instruments and the proposed air-inlet check valve.

Request a written review of the inlet-pressure conditions alongside the normal duty-point and material checks. Include the agreed limits in the site acceptance test. A matching port size alone does not resolve this installation question.

Source and enquiry support

Technical reference: ARO PX20P-XXX-XXX-AXXX Operator’s Manual, CCN 20083440, revision AA, March 31, 2023, pages 1–4, available from the official ARO 2-inch non-metallic EXP product page. The calculation above is an illustrative engineering example, not a field case or a manufacturer-approved installation.

WILLEE supplies diaphragm pumps and replacement parts and supports model identification. Send the operating conditions with your pump enquiry. Configuration, suitability, documentation and availability require confirmation for the specific application.