How Is Pump Duty Calculated for an Adoptable Pumping Station?

Getting the pump duty right is one of the most important parts of designing an adoptable pumping station.

The calculation determines whether the selected pumps can deliver the required wastewater flow against the hydraulic conditions of the system. If those conditions have not been fully established, a pump that appears suitable on paper may not perform as expected once installed.

For developers, drainage consultants and groundworkers, this can create problems beyond the pumping equipment itself. An incorrect duty can affect rising main performance, wet well design, operating efficiency and ultimately technical approval.

That is why pump duty needs to be considered as part of the wider hydraulic design, rather than simply selecting a pump based on the number of properties being served.

What is pump duty and how is it calculated?

Pump duty is the flow rate a pump must deliver against a specified total dynamic head (TDH). It is normally expressed in litres per second (l/s) at metres head (m).

For example, a duty of 10 l/s at 13 m head means the pump needs to deliver 10 litres of wastewater per second while overcoming 13 metres of total hydraulic head.

Establishing that duty requires two main calculations:

• Design flow: How much wastewater the pumping station needs to transfer.
• Total dynamic head: The hydraulic resistance the pumps must overcome to achieve that flow.

Both depend on the characteristics of the development and the proposed drainage arrangement.

For adoptable pumping stations, the relevant Water Authority or NAV requirements must also be considered, as these can influence the required flow rate, rising main design and acceptable operating conditions.

Establishing the design flow

The starting point is understanding what the pumping station needs to handle.

For residential developments, this includes the number of dwellings, expected wastewater flows, any future development phases and the applicable foul drainage design criteria.

However, the average incoming wastewater flow is not necessarily the flow the pumps need to deliver.

For foul pumping stations serving fewer than 500 dwellings, Water UK’s Design and Construction Guidance (DCG) specifies that the design pump flow should be at least the greater of half the incoming peak design flow or the flow needed to achieve the required minimum rising main velocity.

The proposed adopting organisation may also have specific requirements that need to be considered.

This is why the number of houses alone is not enough to establish pump duty. The designer also needs to understand the rising main, discharge conditions and any restrictions on the permitted flow rate.

Calculating total dynamic head

Once the required pumping flow has been established, the next step is determining the total head the pumps must overcome.

For a typical sewage pumping station, this includes three main elements:

Static head is the vertical hydraulic lift between the relevant wet well level and discharge level.

Friction losses occur as wastewater travels through the rising main. These depend on the pipe’s length, internal diameter, material and flow rate.

Local losses arise from valves, bends, fittings and other components within the system.

For a simplified pumping arrangement:

Total Dynamic Head = Static Head + Friction Losses + Local Losses

Where applicable, discharge pressure and other hydraulic conditions must also be included.

Example pump duty calculation

Consider a proposed pumping station where the hydraulic assessment establishes:

• Required pumping flow: 10 l/s
• Static head: 8 metres
• Rising main friction losses: 4 metres
• Valve and fitting losses: 1 metre

The total dynamic head would be:

8 + 4 + 1 = 13 metres

The preliminary pump duty would therefore be 10 l/s at 13 metres head.

This is a simplified example rather than a project-specific calculation, but it shows how the duty point is established.

The next step is checking that the selected pump can deliver the required performance across the expected operating conditions.

Why the rising main matters

The rising main has a significant influence on pump duty.

Its diameter, length, route, levels and fittings all affect the hydraulic resistance the pumps must overcome.

A smaller rising main will generally produce higher velocities and friction losses at the same flow rate. A larger main may reduce friction losses but increase wastewater retention time, potentially creating septicity concerns.

This is where problems can arise if the pump selection and rising main design progress separately.

A change to the proposed rising main route or diameter can alter the calculated duty. If pumps have already been selected, the change may require the equipment and associated design to be reviewed.

At Advantage, we consider the pumps and rising main as part of the same hydraulic system. Our manufacturer-independent approach also gives us flexibility to select equipment around the actual requirements of the development rather than working within a predetermined product range.

Selecting pumps for the calculated duty

Once the required duty has been established, the manufacturer’s pump performance curve needs to be checked against the system curve.

The point where those curves intersect determines the pump’s expected operating flow and head.

It is also important to consider how the pump will perform as conditions change, including variations in wet well levels and rising main characteristics.

For adoptable stations, the selection must also account for matters such as efficiency, solids handling, duty and standby arrangements, starting frequency and the requirements of the future asset owner.

A pump that can achieve the required duty point is not necessarily the most appropriate pump for the project.

The aim is to select equipment that will operate reliably and efficiently throughout the pumping station’s intended life.

Adoption requirements can affect the duty

For adoptable pumping stations, the hydraulic design also needs to satisfy the requirements of the organisation that will eventually own and operate the asset.

Water Authorities can apply approved Local Practices alongside the national Design and Construction Guidance, while NAVs may have their own technical and operational requirements.

These differences can affect rising main velocity, pump selection, operating arrangements and the required discharge flow.

If the intended adopting organisation changes after the hydraulic design has been developed, the pumping duty may need to be reviewed.

That can become particularly difficult where equipment has already been ordered or discharge parameters agreed.

Advantage works with developers, consultants and groundworkers to identify these requirements early and coordinate the pumping station design around the proposed adoption route.

Explore Water Authority & NAV Pumping Station Guidance

Getting the duty right early

Pump duty is one calculation, but it influences much of the wider pumping station design.

An assumption made about wastewater flows, rising main levels or pipe diameter can affect the pumps, wet well, controls and electrical requirements.

The earlier these factors are resolved, the easier it is to develop a coordinated design before civil works progress or equipment is ordered.

That does not mean every detail must be finalised before involving a pumping station specialist. In many cases, an early technical review can help identify missing information, test the proposed hydraulic arrangement and highlight any requirements that may affect the design.

For Advantage, the aim is to make sure the pumping station is designed around the real project requirements, rather than adapting the project later to suit an already selected pump.

See how this works in practice

At Allison Homes’ Witheridge Phase 2 development in Devon, Advantage provided the adoptable pumping station design for 56 homes under Independent Water Networks Limited (IWNL) requirements.

The design required coordination between the pumps, emergency storage and rising main retention. As the development layout changed, our engineers revised the hydraulic calculations and design information to maintain the required technical criteria.

The project illustrates why hydraulic design decisions need to be considered together, particularly when site layouts and drainage requirements evolve.

Read the Witheridge Phase 2 Case Study

Working on an adoptable pumping station design?

If the pumping duty has not yet been fully established, or the rising main or drainage arrangements are still changing, this is a good point to involve Advantage.

Our engineers can work alongside your consultant and groundworker to review the hydraulic requirements, assess suitable pumping equipment and help coordinate the design around the intended adopting organisation.

Discuss Your Pumping Station Design

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