What is a Orifice

An orifice is a precisely sized opening or constriction within a pipe or chamber used to control the flow rate of water or wastewater. By reducing the cross-sectional area available for flow, the orifice creates resistance, which limits the volume of fluid that can pass through it over time. Orifices are a fundamental part of flow regulation in both stormwater management systems and wastewater infrastructure.

They are widely used in attenuation tanks, stormwater control structures, treatment works, and flow balancing systems to manage discharge rates, prevent flooding, and optimise treatment efficiency.

Function and purpose

The primary function of an orifice is to limit the rate of water flow from one part of a drainage system to another. This is especially important in managing peak flows during and after rainfall events, when large volumes of surface water can quickly enter a system.

An orifice allows for:

• Controlled release of stormwater from storage systems into receiving drains or watercourses

• Flow balancing between treatment stages in wastewater facilities

• Protection of downstream pipework or treatment units from overloading

• Regulation of flow into soakaways or infiltration systems

By throttling flow at a predictable rate, orifices ensure that systems operate within design capacity, reducing the risk of flooding, surcharge, and structural damage.

How an orifice works

When water flows through a pipe or structure containing an orifice, the reduced diameter increases the velocity of flow but decreases the volume that can pass through in a given period. The flow rate depends on several factors:

• The size and shape of the orifice opening

• The height or pressure of water upstream of the orifice

• The type of flow regime (free-flowing or submerged)

• The smoothness and geometry of the surrounding pipework

Flow through an orifice is usually calculated using hydraulic equations based on orifice theory, such as the Torricelli equation or variations of the Bernoulli principle. These allow engineers to size the orifice accurately for desired flow rates under specific head conditions.

Types of orifices

Orifices come in various forms, depending on application, flow conditions, and structural requirements. Common types include:

• Circular orifice
  The most common shape, typically drilled or cut into a steel or plastic plate. It provides consistent and predictable flow control.

• Rectangular or slot orifice
  Used in applications where flow needs to increase gradually as water levels rise. These are often used in flow control chambers or treatment works.

• Adjustable orifice
  Designed with a movable plate or sleeve to fine-tune the flow rate after installation. Useful in systems requiring operational flexibility.

• Multi-orifice plates
  Contain several small openings instead of one large one. This can help to reduce the risk of blockage and improve discharge characteristics.

• Bell-mouth orifice
  Features a flared entrance to reduce energy losses and turbulence. Often used in submerged flow conditions.

Applications in drainage and water systems

Orifices are found throughout civil and environmental engineering projects. Typical applications include:

• Stormwater attenuation tanks
  Orifices are installed at the outlet of tanks to control the discharge of stored rainfall, releasing it slowly into the public sewer or watercourse to prevent downstream flooding.

• Flow control chambers
  Used to manage discharge from SuDS features such as swales, detention basins or ponds.

• Sewage treatment plants
  Regulate flow between stages of treatment, such as from screening to settlement, or from balancing tanks to biological reactors.

• Rainwater harvesting systems
  Help manage the release of excess water when storage tanks reach capacity.

• Industrial effluent control
  Used in pre-treatment systems to maintain consistent flow into chemical or physical treatment units.

Design considerations

Correct design of an orifice is essential for system performance. Engineers must consider:

• Required flow rate under maximum and minimum head conditions

• Tolerance for blockage, sediment or debris

• Accessibility for inspection and cleaning

• Compatibility with surrounding materials and hydraulic structures

• Whether the orifice operates under free or submerged flow conditions

Many orifices are installed within prefabricated or in-situ concrete chambers, fitted to a steel or plastic plate mounted across the outlet pipe. Flap valves or grilles may be added to reduce the risk of obstruction or backflow.

Maintenance and operational issues

Orifices are passive devices, meaning they operate without mechanical parts or power supply. However, their performance depends on regular inspection and maintenance.

Typical issues include:

• Blockage from leaves, silt, litter or fat, which can reduce flow and cause upstream flooding

• Wear or corrosion of metal orifices in aggressive environments

• Damage from high flow pressure or mechanical interference

• Poor installation resulting in turbulence or bypassing

Maintenance teams should ensure that orifices are accessible, regularly inspected and cleaned as part of routine drainage system checks.

Advantages of using orifices

Orifices are a widely adopted flow control solution because they offer:

• Simple, robust design with no moving parts

• Predictable hydraulic behaviour

• Low maintenance requirements when properly managed

• Cost-effective implementation in both large and small systems

• Adaptability to a wide range of applications

When installed as part of a wider flow control strategy, orifices contribute significantly to network resilience and environmental protection.

Conclusion

An orifice is a simple yet highly effective device for controlling the flow of water in drainage and treatment systems. By restricting flow through a designed opening, it helps manage peak discharges, protect infrastructure, and improve the performance of stormwater and wastewater networks.

Its value lies in its simplicity, durability and ability to operate without mechanical systems. With careful design and ongoing maintenance, orifices remain a key component in efficient and sustainable water management.