How to Size a Bypass Pump for Sewer Line Maintenance

How to Size a Bypass Pump for Sewer Line Maintenance

Bypass pumping is one of the most critical operations in municipal sewer maintenance. Get the pump sizing right and the job runs smoothly. Get it wrong and you're dealing with sewage overflows, regulatory violations, and emergency calls at 2am.

Here's a practical guide to sizing a bypass pump correctly every time.

Why Pump Sizing Matters So Much in Bypass Pumping

In most construction dewatering applications an undersized pump is an inconvenience — water accumulates a little faster than you'd like but the job continues.

In bypass pumping an undersized pump is a crisis. Sewage has nowhere to go. It backs up into homes and businesses, overflows manholes, and creates a public health emergency. The consequences are immediate, serious, and expensive.

Bypass pump sizing is not a place to guess or cut corners.

Step 1 — Determine the Flow Rate of the Line

Before you can size your bypass pump you need to know how much flow the sewer main carries. This is expressed in gallons per minute or gallons per day.

Your local municipality or utility should have flow data for the line. If flow data isn't available you can estimate based on:

• Pipe diameter — larger pipes carry more flow
• Service area — how many homes and businesses drain into this section
• Time of day — flow rates vary significantly throughout the day

Never size for average flow. Always size for peak flow.

Peak flow in a residential sewer main typically occurs in the morning between 7am and 9am and in the evening between 6pm and 8pm. Peak flow can be 3 to 4 times the average daily flow rate.

If average flow is 200 GPM your bypass pump needs to handle 600 to 800 GPM during peak periods.

Step 2 — Calculate Total Dynamic Head

Your bypass pump doesn't just need to move the flow — it needs to push it through the bypass line from the upstream manhole to the downstream discharge point.

Total dynamic head includes:

Elevation change — if the discharge point is higher than the pump location you need additional head to overcome the elevation difference. Every 2.31 feet of elevation equals 1 PSI of pressure.

Friction losses — water moving through a hose or pipe creates friction that the pump must overcome. Longer bypass lines and smaller diameter hoses create more friction loss.

A practical rule — for most municipal bypass pumping applications add 20 to 30% to your calculated head as a safety factor for unexpected friction losses and elevation variations.

Step 3 — Select Your Pump

With your required GPM and total dynamic head confirmed you can select the right pump. Key considerations:

Match GPM to peak flow — your pump's rated GPM at your operating head pressure must meet or exceed peak flow. Check the pump curve — not just the maximum GPM rating.

Solids handling — municipal sewage contains solids, rags, and debris. Your pump needs adequate solids handling capability. Minimum 2 inch passage, 3 inch preferred for active mains.

Continuous duty rating — bypass pumping runs 24 hours a day. Your pump must be rated for continuous duty operation.

Power source — electric submersible pumps are the standard choice for most bypass applications. Diesel surface pumps for larger flows or sites without reliable power.

Step 4 — Plan for Redundancy

This step is non-negotiable on municipal sewer work.

Always have a backup pump staged and ready to deploy immediately if the primary pump fails. The backup pump should be sized to handle the full flow on its own — not just partial flow.

For critical mains or extended bypass operations some contractors run two pumps simultaneously at reduced capacity, with a third on standby. If one fails the remaining pump handles full flow while the failed unit is replaced.

Step 5 — Set Up and Monitor

Once your pump is selected and on site:

• Install the suction connection at the upstream manhole
• Route the bypass line to the downstream discharge point
• Size the bypass hose correctly — undersized hose creates back pressure that reduces pump performance
• Test the system before disconnecting the live main
• Monitor flow rate and pump performance throughout the operation
• Check fuel levels every 4 to 8 hours for diesel equipment
• Have your backup pump connected and ready to start instantly

Quick Reference Sizing Guide

Note: These are estimates only. Always obtain actual flow data from the utility before finalizing pump selection.

How Flowcor Equipment Supports Bypass Pumping Projects

Flowcor Equipment supplies bypass pumping equipment to municipal contractors and public works departments across the U.S. Tell us your pipe diameter, estimated flow rate, bypass line length, and project duration and we'll source the right equipment and get you a quote within 1 business hour.

Submit a quote request at flowcorequipment.com or call us directly at 610-241-6770.