Correct valve sizing affects how the valve performs as part of the process. However, the valve size should not be selected based on the pipe size alone.
When sizing a valve, factors such as flow rate, differential pressure, temperature, and the medium being handled need to be considered. The intended use and valve design also affect which type of valve is suitable for the application.
What does DN mean in valve sizing?
In industrial valve applications, size is usually given as DN, or nominal diameter. For example, DN25, DN50 and DN100 are common valve sizes.
DN does not directly indicate the exact diameter of the valve or its flow opening in millimetres. For example, DN50 refers to a nominal size that roughly corresponds to a 50 mm pipe size – it does not mean that every internal diameter of the valve is exactly 50 mm.
The internal design of the valve also affects the size of the opening available for the flow.
Why does the full-flow opening matter?
In a ball valve, the flow passes through the bore in the ball. In a full-port ball valve, the flow opening through the ball is close to the nominal size, which minimises restrictions to the flow.
This affects flow resistance and pressure drop. That is why the DN size alone does not tell the whole story about a valve’s flow characteristics.
What needs to be considered when sizing a valve?
The valve should be sized according to the application. At least the following factors should be considered:
Flow rate
How much medium needs to pass through the valve?
Flow rate affects the required valve size. If the valve is too small for the required flow, it may restrict the flow and increase pressure drop.
Pressure and differential pressure
What is the operating pressure of the system, and what is the differential pressure across the valve?
The valve must withstand the operating conditions of the application. Differential pressure also affects valve operation and sizing.
Temperature
At what temperatures will the valve be used?
Temperature affects, for example, the choice of materials and seals. The same valve solution may not be suitable for all temperature conditions.
Medium being handled
What will flow through the valve?
The medium may be water, chemicals, steam, or another process medium. The properties of the medium affect the selection of materials and seals, as well as which valve design is suitable for the application.
Intended use
Is the valve primarily used to shut off or open the flow, or does it also need to regulate the flow?
Ball valves are well suited for opening and closing flow. If a valve is intended for continuous and precise flow control, it is worth considering whether a ball valve is the right valve type for the application.
Connection type and installation environment
The pipe connection type, available space and installation environment also need to be considered.
The valve needs to fit not only the piping but also the space where it will be installed and maintained.
Pipe size alone is not enough
For example, a valve used in a DN50 pipeline is not automatically determined by the DN50 designation alone.
The pipe size is a useful starting point, but it needs to be considered together with flow rate, pressure, temperature and the medium being handled.
Incorrectly sized valves can lead to, for example:
- excessive pressure drop
- insufficient flow
- unnecessary wear
- increased risk of leakage
- problems with process control
- additional costs
On the other hand, a larger valve is not automatically a better solution. The aim of sizing is to find a valve that suits the application – not simply the largest valve possible.
What information is needed for valve sizing?
When selecting a valve for a new application or replacing an existing one, the following information provides a good starting point:
- pipe DN size
- required flow rate
- operating pressure and differential pressure
- operating temperature
- medium being handled and its properties
- intended use of the valve
- required full-flow opening
- valve materials and seals
- connection type
- installation environment
- frequency of operation and serviceability
When this information is available, the valve can be selected as a whole rather than based on a single measurement.
At Maper, sizing starts with the application
At Maper, valve selection starts with understanding where and how the valve will be used.
The DN number alone does not tell you which valve is suitable for an application. Once the operating conditions and requirements are known, the valve design, materials, and size can be selected accordingly.
If you need help with valve sizing or selecting a suitable valve solution, contact Maper. We can go through the requirements of the application together.
Read more about valve selection:
Valve Selection Guide: Choosing the Right Valve
Ball Valves: a Reliable Solution for Versatile Applications
15.9.2026
