Dimensions and types of flanges – what do the DN and PN standards mean for pumps

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Dimensions and types of flanges – what do the standards DN and PN mean for pumps

When a customer orders a flanged circulation pump for the first time and receives a delivery note with markings such as DN 50 PN 6 or DN 80 PN 16, most people simply look at the pipe diameter and hope it fits. In practice, this usually ends either in a prolonged installation or – in the worst case – in returning the product, because the flanges physically do not match the opposite flanges of the existing system. This article therefore examines the standards DN and PN in depth – not only what these abbreviations mean, but also why they are key for flanged circulation pumps and how to work with them correctly when selecting, designing and installing.

What is a flange and why it exists

A flange (in English flange, in German Flansch) is an extended plate or edge at the end of a pipe, valve or pump, which allows a solid, tight and detachable connection between two components. Unlike threaded connections (where components are screwed into each other), a flanged connection is realized by placing two opposite flanges against each other, inserting a gasket between them, and fastening the whole with bolts.

The reason why flanged connections dominate in higher performance classes of circulation pumps for heating and water is simple: detachability. The pump can be removed from the system without the need to cut the pipe. In service interventions, bearing replacement or overall replacement of the unit, this is a crucial advantage. Therefore, all larger circulation pumps for industrial buildings, boiler rooms and larger heating networks are standardly equipped with flanged nozzles.

Cross-section of a flanged connection: two flanges fastened with bolts with a gasket between them.

Standard DN – nominal diameter (Diamètre Nominal)

The abbreviation DN comes from the French Diamètre Nominal, in Slovak we speak about the nominal diameter or nominal bore. It is a dimensionless number that indicates the approximate internal diameter of the pipe or valve in millimeters. The word "approximate" is key here – DN is not the exact internal diameter, but a standardized class that ensures compatibility of flanges in that class regardless of the manufacturer.

Standard EN ISO 6708 defines a range of nominal diameters, which we most often encounter in heating systems in practice:

DN 25 – corresponds to approximately 1" pipe, common in smaller systems
DN 32 – corresponds to 5/4" pipe
DN 40 – corresponds to 6/4" pipe (1½")
DN 50 – corresponds to 2" pipe, very common in medium-sized boiler rooms
DN 65 – a range between standard inch dimensions, common in European systems
DN 80 – corresponds to 3" pipe, typical for large boiler rooms and industrial buildings
DN 100 – corresponds to 4" pipe, used in central heat sources
DN 125 – higher performance, distribution networks
DN 150 and higher – industrial applications, heating plants, hospitals, large shopping centers

It is important to understand that with the same DN, the actual outer diameter of the flange can vary depending on the PN class. Flanges DN 50 PN 6 and DN 50 PN 16 have the same bore in the nozzle, but the flanges differ in thickness, pitch circle diameter (PCD – Pitch Circle Diameter) and number of bolt holes. This fact is one of the most common causes of confusion on construction sites.

Standard PN – nominal pressure (Pression Nominale)

The abbreviation PN comes from the French Pression Nominale, which means nominal pressure. It indicates the maximum allowable overpressure (in bars) at a reference medium temperature of 20 °C. Standard EN 1092-1 defines the following standard classes:

PN 6 – maximum overpressure 6 bar at 20 °C; typical for low-pressure domestic water distribution, floor heating
PN 10 – 10 bar at 20 °C; common heating systems in single-family homes and apartment complexes
PN 16 – 16 bar at 20 °C; the most widespread class in boiler rooms, commercial buildings and industrial heating systems
PN 25 – 25 bar; high-pressure industrial applications, steam systems
PN 40 – 40 bar; special industrial and energy applications

When selecting a flanged circulation pump from the category flanged circulation pumps, it is essential to always verify that the PN class of the pump corresponds to the pressure class of other valves and pipes in the system. For example, if the designer designed a distribution system with PN 16 flanges and the pump is delivered with PN 6, the flanges physically do not match – the pitch circle diameter of the bolt holes has a different diameter.

Flanges DN 50 PN 6 and PN 16 have the same bore diameter, but different outer diameters and bolt circles – they are not interchangeable.

Dimensional tables of flanges according to EN 1092-1

Specific dimensional values are standardized and binding. For the practice of boiler rooms, the following values are most relevant (all dimensions in mm, bolts in metric threads):

DN	PN	Flange outer diameter D (mm)	Bolt circle diameter PCD (mm)	Number of holes	Hole diameter (mm)	Bolts
DN 40	PN 6/10	150	110	4	18	M16
DN 40	PN 16	150	110	4	18	M16
DN 50	PN 6/10	165	125	4	18	M16
DN 50	PN 16	165	125	4	18	M16
DN 65	PN 6/10	185	145	4	18	M16
DN 65	PN 16	185	145	8	18	M16
DN 80	PN 6/10	200	160	8	18	M16
DN 80	PN 16	200	160	8	18	M16
DN 100	PN 6/10	220	180	8	18	M16
DN 100	PN 16	235	190	8	22	M20
DN 125	PN 16	270	220	8	26	M24
DN 150	PN 16	300	250	8	26	M24

Note: Dimensions are approximate according to EN 1092-1 types 01 and 02. Always verify the dimensional documentation of the specific pump manufacturer, as some manufacturers (especially for PN 6 and PN 10) provide identical flange dimensions for both pressure classes, while others differentiate them.

Types of flanges according to EN 1092-1 – not all flanges are the same

The standard EN 1092-1 distinguishes several types of flanges according to their structural design and method of connection to the pipe or pump body. For heating technology, the following are mainly relevant:

Type 01 – flat face flange (Flat face): The gasket is placed on the entire front surface of the flange. Used for low pressures (PN 6, PN 10) and for connecting to cast iron opposite flanges. Typical for castings.
Type 02 – raised face flange (Raised face, RF): The center of the flange is slightly raised (by 2 mm for PN 6–16, by 7 mm for PN 25 and higher). The gasket lies only on this raised ring. This is the most widespread type in European practice for PN 16 and higher.
Type 05 – ring-type joint (Ring-type joint, RTJ): A special application for high pressures and temperatures, not encountered in standard heating practice.
Loose flange with ring: The flange is not welded or bolted to the pipe, but is held by a collar. It allows rotation – so always align the holes during installation. Advantageous in tight installation conditions.

For flanged circulation pumps, it applies that the pump body has cast or machined flanges directly as part of the casing. The type of flange (shape of the face) is defined by the manufacturer and must correspond to the type of mating flange in the pipe. The combination RF + FF (raised face) is theoretically possible with suitable gaskets, but in practice, it is recommended to maintain the same type for heating systems.

How to correctly identify an existing flange at the installation site

This is a practical situation that an installer encounters when replacing a pump in an existing boiler room. The procedure is as follows:

1. Measure the outer diameter of the flange – using a caliper or tape measure, measure the outer edge of the flange. Using the EN 1092-1 table, you can determine the likely DN.

2. Measure the bolt circle diameter (PCD) – the distance from the center of one bolt hole to the center of the opposite bolt hole in 4-hole flanges, or the angle in 8-hole flanges. For 4 holes: PCD = distance between the centers of opposite holes. For 8 holes: PCD = distance between the centers of bolt holes × 1/sin(22.5°).

3. Count the number of bolt holes – four holes typically correspond to smaller DN up to DN 80, while eight holes are common from DN 65 PN 16 and higher DN.

4. Verify the internal diameter of the bore – measure the opening in the center of the flange and compare it with tabulated DN values.

5. Verify the PN – if there is no documentation or label, the PN can be inferred from the combination of the outer diameter, PCD, and hole diameter. If the PCD and outer diameter match the table for PN 10, but the system was originally designed for a boiler with a maximum working pressure of 3 bar, it is likely PN 10, which is a safe choice for standard heating.

Three key dimensions of a flange: outer diameter D, bolt circle PCD, and internal bore d.

Why the PN class is important not only for pressure, but also for temperature

This is where many installers and designers make a mistake. The nominal pressure PN is defined at a medium temperature of 20 °C. At higher temperatures – a common situation in boiler rooms – the maximum allowable working pressure decreases.

Example: A cast iron flange PN 16 can operate safely at only around 10 bar at a medium temperature of 120 °C, and only 6–7 bar at 150 °C. This factor is included in pressure-temperature diagrams, which pump and valve manufacturers include in their technical documentation.

For standard low-temperature heating (heat transfer medium up to 90 °C), the PN 10 class is usually sufficient from a temperature perspective, but most boiler rooms today are designed for PN 16, because:

Pressure reserve in case of transient conditions (start-up, failure of expansion tank)
Compatibility with the most common valves and shut-off valves on the market
Ability to connect boilers with higher maximum pressure (10–12 bar)
The price difference between PN 10 and PN 16 is minimal for standard sizes

Practical examples of projects – how it looks in the field

Example 1 – Apartment building, replacement of an existing pump: The building manager found that the pump in the boiler room had reached the end of its 20-year life. The flanges on site had no label at all. The installer measured an outer diameter of 185 mm, a bolt circle of 145 mm, 4 bolt holes with a diameter of 18 mm, and an internal opening (bore) of approximately 63 mm. The table says: DN 65, PN 6/10. The system is old, low-temperature, and the pressure certainly does not exceed 4 bar. A new pump was ordered as DN 65 PN 10 – it fit perfectly without any pipe modification.

Example 2 – Industrial hall, new installation: The designer planned a distribution system for a production hall with a boiler operating at a maximum pressure of 12 bar. All valves in the project are PN 16. The pump was ordered as DN 80 PN 6, which is an absolute mistake – not only insufficient from a pressure standpoint, but the flanges would not fit during installation (the PCD difference can be up to 15 mm). The project was halted, and the pump was replaced with the correct DN 80 PN 16. This is a classic case where saving on a lower pressure class caused delays and additional costs for transportation back and forth.

Example 3 – Boiler room reconstruction in a school: During the reconstruction, the pump DN 50 was being replaced. The existing flanges were cast iron, type 01 (flat face). The new pump came with flanges of type 02 (raised face). It was necessary to use a special full-face gasket instead of a standard ring gasket to avoid having to grind the raised face. An alternative solution – purchasing transition gaskets – was cheaper and faster.

Example 4 – Hotel, pump replacement during the season: The hotel manager wanted to replace the pump as quickly as possible. The pump was in a system without shut-off flanged valves directly at the pump (there was only a remote shut-off). Disassembling the flanged pump without short shut-off cocks or flanged shut-offs immediately after the pump flanges required draining the entire system. Conclusion: for every flanged pump, it is a professional standard to install shut-off valves of the same DN as the pump directly before and after the pump – precisely for the sake of easy future replacement.

Compatibility with American standards ANSI/ASME B16.5

This is a more marginal topic in the Slovak and Czech environment, but when importing technologies from overseas markets or in special industrial projects, it becomes relevant. The American standard ANSI/ASME B16.5 uses instead of the DN/PN system, inch nominal pipe sizes (NPS – Nominal Pipe Size) and pressure classes marked as Class 150, Class 300, Class 600, etc.

There is no direct conversion – Class 150 is not identical to PN 16, although they are similar. The bolt hole spacing dimensions are different, and the flanges are not directly compatible. In any case of doubt about the origin of existing flanges, it is necessary to verify whether they are EN or ANSI standards, otherwise there is a risk of a leaky or even mechanically unsuitable connection.

Flange seals – material and selection

A flange without proper sealing is worthless. For heating systems with clean water, the most common seals are:

Nitrile rubber (NBR) – resistant up to 100–120 °C, suitable for hot water, does not tolerate mineral oils or glycol (not suitable for glycol mixtures in solar or cooling systems)
EPDM (ethylene-propylene-diene rubber) – resistant up to 150 °C, excellent for hot water and steam, compatible with glycol mixtures, but not suitable for hydrocarbons
Graphite seals – for higher temperatures (steam, hot water above 150 °C), chemically inert, excellent compressibility
PTFE (teflon) – highly chemically resistant, for aggressive media, higher price

The thickness of the seal for standard PN 6–16 flanges is typically 2–3 mm. With raised face (type 02) flanges, the seal is placed only on the raised ring, not on the entire surface. It is important to set the tightening torque correctly – over-tightening can crush the seal, while under-tightening causes leakage. Bolts are tightened in a cross pattern, not in a circle, in several steps using a torque wrench.

Correct bolt tightening sequence prevents uneven compression of the seal and leaks.

Relationship between DN/PN and pump performance and flow

The choice of DN directly affects the flow velocity of the medium through the pump nozzle, and thus also hydraulic losses and noise. For circulation pumps in heating systems, it is recommended that the flow velocity in the nozzle does not exceed 1.5–2.0 m/s at full power. At higher velocities, noise and cavitation risk increase.

A simple calculation: Q (m³/h) = v (m/s) × A (m²) × 3600, where A is the cross-sectional area of the nozzle. For DN 50 (internal diameter approx. 54 mm, area ≈ 22.9 cm²) at v = 1.5 m/s, we get a flow rate Q ≈ 1.5 × 0.00229 × 3600 = approx. 12.4 m³/h. For DN 65 (internal diameter approx. 70 mm, area ≈ 38.5 cm²), it is Q ≈ 20.8 m³/h at the same velocity.

This is why large flanged pumps with high flow rates are not installed with too small a DN nozzle – the velocity would increase, hydraulic losses would be unacceptable, and cavitation would quickly destroy the pump. More about the relationship between flow and pressure can be found in the article What flow and pressure do I need for my flanged pump.

Flanged pumps and spacers (Montagestücke)

In practice, when replacing pumps, it often happens that the new pump has the same DN and PN but different structural lengths (Baulänge – total length of the pump between the flange seating surfaces). For example, the original pump had a structural length of 450 mm and the new one 420 mm. In such a case, the solution is:

Spacer (distance insert) – a steel or cast iron cylinder of the correct DN/PN, which extends the assembly to the required spacing
Shifting the pipe – if it is possible to shift one nozzle (flexible pipe mounting)
Wave compensators (expansion joints) – rubber or metal compensators that also absorb the difference in structural length

Therefore, when ordering a replacement pump, always measure not only DN and PN, but also the structural length L (face-to-face dimension between the flange seating surfaces). Larger manufacturers specify structural lengths according to the standard EN 1151 or EN ISO 2858 and it is possible to find compatible replacements of different brands at the same DN/PN/L.

Standards relevant to flanged circulation pumps – overview

For completeness, we provide an overview of standards that we encounter with flanged circulation pumps:

EN ISO 6708 – definition of nominal diameter DN
EN 1092-1 – flanges and flanged joints, steel flanges (types 01 to 15)
EN 1092-2 – cast iron flanges
EN 1092-3 – flanges made of copper alloys
EN 13480 – industrial piping systems
EN ISO 2858 – dimensional requirements for single-stage centrifugal pumps (applies to some types of flanged pumps)
EN 1151-1 – circulation pumps for potable water and heating, structural lengths
DIN 24255 – older German standard for structural lengths (still relevant for older installations)

More about the process of selecting the right pump for a specific system can be found in the article How to choose a flanged circulation pump for a heating system, where the selection criteria are covered in detail including hydraulic parameters.

Most common errors in the selection of DN and PN

From practice, we know that most problems in the installation of flanged pumps stem from a few recurring errors:

Mixing up PN 6 and PN 16 – for the same DN, in some sizes, flanges may have the same dimensions (e.g. DN 50 PN 6 = DN 50 PN 10 = DN 50 PN 16 has the same outer diameter 165 mm and the same PCD 125 mm), which leads to a false sense of security. In other sizes (e.g. DN 100), the dimensions differ.
Ignoring structural length – the pump fits in pressure and diameter, but "shortens" the system by 3 cm, which causes stress in the piping or the need to extend the piping.
Wrong type of seal – using an NBR seal in a system with glycol (solar system, antifreeze heating) causes rapid degradation and leakage.
Omitting a torque wrench – bolts tightened "by feel" lead to uneven sealing and immediate or later leakage.
Mixing up American and European standards – with imported pumps from Asian markets, which are sometimes certified according to ANSI, not according to EN.

A more detailed description of typical installation errors and their solutions can be found in the article Installation of a flanged circulation pump – procedure and common errors.

How to Read the Nameplate of a Flanged Pump

Every flanged circulation pump has a nameplate that lists not only power and flow rate but also flange parameters. A typical entry looks like this:

DN 65 / PN 16 / Baulänge 340 mm / T_max 130 °C / p_max 10 bar

This entry means: nominal bore DN 65, flange class PN 16, construction length (face-to-face) 340 mm, maximum medium temperature 130 °C, maximum working pressure 10 bar (which is less than PN 16 = 16 bar, because at higher temperatures the pressure limit of the pump is lower than the nominal flange class – the pump body limits it, not the flange itself).

Note that the maximum working pressure of the pump may not be identical to the PN flange value. The PN flange is the upper limit for the flange itself at 20 °C; the pump may have a lower limit due to the construction of the impeller, bearing seals, or the pump body. Always the lower of the two values applies.

Summary – What to Remember About DN and PN Standards

To conclude the technical part, let's summarize the key points that are immediately applicable in practice:

DN = nominal diameter (approximate clear opening), not the exact internal diameter. It ensures dimensional compatibility of flanges from different manufacturers.
PN = nominal pressure at 20 °C. At higher temperatures, the maximum working pressure decreases according to pressure-temperature tables.
Flanges of the same DN but different PN are not always dimensionally different – it depends on the specific DN and the relevant standard.
When replacing a pump, always measure DN, PN, and construction length L.
The type of flange (RF, FF) must be compatible with the opposite flange or you must use the correct type of gasket.
For standard heating in boiler rooms, PN 16 is a safe and universal standard.
The flow velocity in the pipe must not exceed 1.5–2 m/s – when selecting DN, check this with a calculation.

Frequently Asked Questions (FAQ)

Can I connect a DN 50 PN 16 pump to an existing pipe with DN 50 PN 6 flanges?

In many cases, yes – for size DN 50, the dimensions of PN 6 and PN 16 flanges according to EN 1092-1 are identical (outside diameter 165 mm, PCD 125 mm, 4 holes M16). The flanges physically fit. However, the system must be operated only up to the maximum pressure that is safe for the weaker component – i.e., the PN 6 flange (6 bar at 20 °C, less at higher temperatures). If the system operates up to 3–4 bar, it is safe. Always check dimensional tables for the specific DN size, because for DN 100 and larger, the dimensions of PN 6/10 and PN 16 differ.

What does it mean when a pump catalog lists DN 50/32?

This designation means that the pump's suction (inlet) has a diameter of DN 50 and the discharge (outlet) has a diameter of DN 32. This is common with certain types of pumps with radial or axial impeller designs, where the inlet channel is intentionally widened to reduce velocity and minimize the risk of cavitation at the suction side. When ordering flanges, shut-off valves, and gaskets, you must order two different sizes.

Is there a maximum temperature up to which I can operate a flanged pump with

Do you have a question about this topic?

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