Sizing Suction & Discharge Pipe for a ZWL Pump

You can buy the right pump and still get poor results if the pipework fights it. On a ZWL direct-coupled self-priming sewage pump, the suction and discharge lines have a big say in how fast it primes and whether solids keep moving. Here’s how to size them sensibly.

The suction line — priming depends on it

The suction side is where most trouble starts, so:

• Use full port size or larger. Never neck the suction down below the pump’s inlet — a restricted suction slows priming and starves the pump.
• Keep it short. Aim for a suction pipe run of up to about 10 m, and keep the vertical lift within 4.5–5.5 m. The shorter and lower, the faster and more reliably it primes.
• Minimise bends and avoid high points that trap air.
• Make every joint airtight. An air leak on the suction side is the number-one cause of “won’t prime.”
• Fit a strainer with an open area about 2–3 times the pipe’s cross-section, so it doesn’t restrict flow.

The discharge line — keep solids moving

On the discharge side, the goal is enough velocity to carry solids without wasting head to friction:

• Don’t undersize it. Too small a discharge means high friction (lost head) and, at low flow, solids can settle and block the line.
• Don’t grossly oversize it either — too low a velocity lets solids drop out. A discharge around the pump’s port size is usually the right starting point.
• Support the pipework independently. The pump body must never carry pipe loads — that causes leaks and cracks.

Pipe-sizing reference

• Suction: full port size or larger, ≤10 m run, ≤4.5–5.5 m lift, airtight, few bends
• Strainer: open area ~2–3× pipe cross-section
• Discharge: around port size — enough velocity to carry solids, not so small it chokes on friction
• Pipework: independently supported, no load on the pump
• Range: 8–800 m³/h, DN25–DN300

Send us your layout — lift, pipe length and bends — and we’ll confirm the pipe sizes along with the pump.