Questions & Answers

A clear, practical guide to managing inlet (suction) pressure on air-operated double-diaphragm (AODD) pumps—based on widely accepted OEM guidance and field experience.

Key takeaways (TL;DR)

• Keep inlet pressure modest: target ≤ 0.7 bar (10 psi) at the pump inlet.

• Avoid high inlet pressure: around 1.0 bar (15 psi) can shorten diaphragm life.

• Balance the system: if you have strong flooded suction or a pressurised source, add discharge back-pressure to stabilise check-ball seating.

• Healthy differential: keep inlet < ~25% of discharge pressure to minimise check-valve chatter.

• Stay within ratings: air set-point + inlet pressure ≤ pump’s max fluid working pressure (and the lowest rated component).

What “inlet pressure” is—and why it matters

• Inlet pressure is the gauge pressure at the fluid inlet port of the pump.

• AODDs are essentially 1:1 differential-pressure machines: the pump develops discharge pressure roughly equal to air pressure + inlet pressure (minus losses).

• Too much positive inlet pressure can hold the inlet checks off their seats, causing chatter, slip, lower efficiency, and accelerated diaphragm wear.

Recommended limits

Rule-of-thumb conversion: 10 m of water ≈ 1.0 bar (≈ 14.5 psi). So a tank level 3 m above the pump gives ~0.3 bar (~4.4 psi) static inlet pressure (ignoring friction).

How to measure inlet pressure correctly

1. Gauge location: Install a liquid-filled pressure gauge (or transducer) as close as practical to the pump inlet.

2. At rest vs running:

   • Static (pump off): reads the head/blanket gas.

   • Dynamic (pump on): includes friction losses; trend both.

3. Pipe strain & air pockets: Mount the gauge vertical, avoid high-point air traps, and support piping to prevent vibration damage.

System set-ups and what to do

1) Flooded suction (tank above pump)

• Fit an isolation/throttling valve on the inlet so you can tame strong head.

• Avoid collapsing hoses: use reinforced suction hose or rigid pipe.

• Keep the suction line short, straight, and ≥ pump inlet size.

• Provide some discharge back-pressure (part-closed valve or BPR) to help seat check balls and reduce chatter.

2) Pressurised source / gas-blanketed vessel

• Sum the static head + vessel pressure—this can exceed safe inlet pressure quickly.

• Control with inlet throttling and discharge back-pressure.

• Re-check that air set-point + inlet ≤ pump rating (and hose/fitting ratings).

3) Suction lift (drawing from below the pump)

• Start with low air pressure so chambers fill gently, then increase.

• Seal the suction side: no leaks, minimal elbows, properly sealed joints.

• Consider a foot valve or priming aid if repeated starts are required.

Piping and accessory checklist

• Suction size: Same as or larger than the pump’s inlet.

• Length & fittings: Short runs, large radius bends, minimal restrictions.

• Strainers: Use only where required; choose a large basket and keep it clean.

• Hose selection: Reinforced/rigid on suction to prevent collapse.

• Instrumentation: Inlet and discharge gauges help you verify the pressure ratio.

Troubleshooting: symptoms → likely cause → fix

• Check-ball chatter / surging flow
  Likely cause: Inlet pressure too high vs discharge (poor differential).
  Fix: Add discharge back-pressure; throttle inlet to ≤ 0.7 bar.

• Premature diaphragm wear / frequent failures
  Likely cause: Running with ~1.0 bar inlet or spikes from pressurised source.
  Fix: Reduce inlet pressure; fit a back-pressure regulator; verify ratings.

• Slow prime / poor suction lift
  Likely cause: Air leaks or restrictive suction piping.
  Fix: Reseat/retape joints, shorten suction, use larger line, slow the start-up.

• Flow lower than expected
  Likely cause: Check-ball sealing issues from high inlet or too little discharge head.
  Fix: Increase back-pressure slightly, reduce inlet pressure, verify valve condition.

Worked examples

Example A – Flooded suction with tall tank

• Tank level above pump: 5 m → ~0.5 bar inlet.

• Air set-point: 4.5 bar.

• Expected discharge ≈ 4.5 + 0.5 = 5.0 bar, subject to losses.

• Check ratio: inlet/discharge = 0.5/5.0 = 10% (good). If chatter occurs, add a touch of discharge back-pressure and verify suction line design.

Example B – Pressurised vessel

• Static head: 2 m (~0.2 bar); vessel blanket: 0.6 bar → inlet ~0.8 bar.

• Air set-point: 4.0 bar → discharge target ≈ 4.8 bar.

• Ratio: 0.8/4.8 ≈ 17% (borderline ok), but 0.8 bar is high—throttle inlet or add discharge back-pressure to bring inlet closer to 0.5–0.7 bar.

Frequently asked questions

Do AODD pumps need NPSH like centrifugal pumps?
Yes—especially for hot or volatile liquids. Keep NPSHa > NPSHr with a margin; reduce suction losses and avoid high temperature drops that push the fluid to vapour.

Can I run an AODD to atmosphere on discharge?
You can, but some back-pressure usually improves check-ball seating and flow stability. A slightly closed valve or a back-pressure regulator often helps.

Is more inlet pressure always better for priming?
No. Excessive inlet pressure can unseat the checks and shorten diaphragm life. Use just enough to flood the pump—≤ 0.7 bar (10 psi) is a good ceiling.

Quick calculator (rules of thumb)

• Static head to pressure: p[bar]≈height (m)10p \text{[bar]} \approx \frac{\text{height (m)}}{10}p[bar]≈10height (m)​ (water at ~20 °C).

• Discharge estimate: pdischarge≈pair+pinletp_\text{discharge} \approx p_\text{air} + p_\text{inlet}pdischarge​≈pair​+pinlet​ (stay within ratings).

• Ratio check for stability: pinletpdischarge<0.25\frac{p_\text{inlet}}{p_\text{discharge}} < 0.25pdischarge​pinlet​​<0.25.

Need help sizing or troubleshooting?

Global Pumps can review your tank elevations, hose runs, media properties (viscosity, solids, temperature), and duty, then recommend a stable, long-life configuration—including inlet control and discharge back-pressure setup.

Talk to our team for:

• On-site or remote troubleshooting

• Piping layout and accessory selection

• Pump sizing for viscous and abrasive duties

• Start-up procedures and maintenance planning