What is economic pipe diameter

Determination of the pipe diameter

According to the regulations of the European Committee for Standardization CEN, regulations contained in European standards may not be regulated in other forms in national standards. DIN EN 806-3 deals with the calculation of internal pipe diameters using a simplified procedure, so that the national supplement in DIN 1988-300 may only contain a differentiated procedure (Image 1). Compared to a simplified dimensioning according to DIN EN 806-3, a differentiated dimensioning according to DIN 1988-300 can capture all the influencing factors that are required for the most hydraulic, energetic, economical and hydraulically most favorable pipe diameter much more precisely and in more detail. The simplified dimensioning method according to DIN EN 806-3 should only be used to determine cold and hot water pipes in residential buildings with up to six apartments if certain criteria are met. The DIN / Beuth / ZVSHK commentary on DIN 1988-300 and DIN EN 806-3 serves as the basis for this specialist article. The contents of the ZVSHK membership edition and that of the DIN / Beuth edition are identical (Picture 2).

The calculation rules in DIN 1988-300 for consumption and circulation lines are based on the well-known and proven rules of DIN 1988-3 and DVGW worksheet W 553. They have been modified and adapted so that all the latest findings are taken into account. In DIN 1988-300, all rules have been aligned in such a way that functionality is ensured with the smallest possible internal diameter. The rules enable a differentiated calculation of all drinking water installations of single and multi-family houses, old people's homes, hotels, hospitals, schools, administrative buildings etc. as well as of commercial and industrial plants.

The calculation method in DIN 1988-300 can in principle be carried out “by hand”, but is so time-consuming that the use of calculation software is urgently recommended for practical applications. The simplified calculation method in DIN EN 806-3 is basically only suitable for the dimensioning of "normal installations", if z. B. the supply pressure is sufficient and other criteria are met, which are described in the commentary on DIN EN 806-3. The note to the standard therefore recommends using this method only for drinking water installations in residential buildings with up to six apartments. This method is unsuitable for all other applications, as essential hydraulic relationships are not taken into account.

Product data exchange

In technical building services, the planning and computational design of systems are increasingly taking place using computation software. This way of working makes it necessary to provide the product data in machine-readable form. The aim of VDI guideline 3805 is to create standardized data record descriptions that allow uniform product descriptions to be created for each product group. This standardization basically allows calculation programs to use product data from different manufacturers. The VDI guideline includes products and components for heating, air conditioning and sanitary technology. The normative regulations of DIN 1988-300 were formulated on the condition that the manufacturers provide their data according to VDI 3805 at least for all hydraulically relevant components. The software manufacturers must equip their calculation software with interfaces via which the manufacturer data can be conveniently read in and used.

Product-neutral tender

If the client requests a product-neutral tender, the comparable nominal widths for pipes and resistance coefficients for fittings and connectors as well as for fittings made of the different materials must be used in the pipe network calculation in accordance with Annex A of DIN 1988-300. Table A.1 provides a clear assignment of the nominal diameter DN to the inside diameter d across all material groupsi. The smallest inside diameter offered on the market was assigned to the respective nominal size. Tables A.2 to A.5 contain representative drag coefficients ζ for fittings and connectors of the respective material group as well as fittings. The use of the respective inside diameter of such a series of nominal widths and the associated representative drag coefficients in a hydraulic calculation provides the basis for a product-neutral tender. As a rule, the pressure loss calculated in this way in the drinking water installation is higher than with calculations based on the manufacturer's information. This applies in particular to materials where both the inside diameter and the resistance coefficients of the fittings and connectors can be different, depending on the system and manufacturer, as is the case with composite and plastic pipe systems. This results in functional drinking water installations in any case.

If the pipe system to be implemented with the respective inside diameters and the resistance coefficients deviates significantly from those of the product-neutral calculation, the drinking water installation must be recalculated using the manufacturer's data. This ensures that from a hydraulic, economic and drinking water hygiene point of view, the optimum pipe diameter is realized with the design.

Individual resistances

In addition to straight pipes, a pipeline system for drinking water installations also includes fittings and connectors, fittings, etc. These components generate pressure losses that are usually greater than the pressure losses of straight pipes of the same length. The difference between the pressure loss caused by fittings and connectors, fittings, etc., and the pressure loss in straight pipe sections of the same length is called the pressure loss due to individual resistances ΔpE. = Z denotes. A pressure loss from individual resistances is calculated from the resistance coefficient ζ and the dynamic pressure.

The determination of the drag coefficients on the basis of DVGW worksheet W 575 follows according to the section definition. The drag coefficients must be used as they were determined in this procedure so that there are no mathematical misinterpretations. In the graphic symbols in the tables in Appendix A of DIN 1988-300, the flow velocity to which the specified drag coefficient relates is marked with v. The pressure loss due to individual resistances is to be understood as an "allowance" to the pressure loss of the straight pipeline.

As part of the development of DIN 1988-300, a uniform basis for determining the drag coefficients of fittings and connecting pieces was created using DVGW worksheet W 575. The resistance coefficients contained in DIN 1988-3 could no longer adequately describe the fittings used for the new materials and pipe systems. Therefore, it was mutually agreed with the manufacturers that for DVGW-certified pipe systems the drag coefficients for fittings and connectors must be specified in the product documents in future. The drag coefficients are to be determined on the basis of DVGW worksheet W 575.

Usage unit

A usage unit is a sanitary room with tap fittings in residential buildings, such as bathrooms, guest toilets, kitchens or utility rooms, or in non-residential buildings, if it can be assumed that it will be used similarly to an apartment, such as bathrooms in hotels, old people's homes or hospital beds. It is assumed that a maximum of two extraction points are opened at the same time. When determining the total volume flow, sanitary fixtures and fittings in a usage unit (NE) that go beyond a fixture standard are not taken into account. picture 3 shows an example of the application of this rule in the relevant usage units of a single-family house. The urinal in the guest toilet and the second washstand in the bathroom as well as the shower in addition to the bathtub are not taken into account when determining the total flow rate.

Peak flow

In addition to the available pressure gradient, to select a suitable diameter in a section, at least the flow rate of the calculation case - the peak flow rateVs - be known. The peak flow to be expected in a section is largely determined by the number of extraction fittings to be supplied, the respective calculated flow rate of the extraction fittings VR. and the use of the building. The distinction between cistern and flush valve installations, which was still contained in DIN 1988-3, has been abandoned, since today drinking water installations hardly use flush valves to any significant extent (VR.> 0.5 l / s). With the respective peak flow in the sections, a pipe network load is assumed, which in reality is very rare and which then only occurs over a very short period of time (a few minutes). Picture 4 shows an example of the flow in a main distribution line for cold drinking water over a measurement period of one hour. It can typically be seen that flow peaks only occur briefly here due to the superimposition of withdrawal processes. The measured flow peaks are still well below the peak flow rates calculated according to this standard. Even if a frequency distribution is only related to the peak volume flow in the measurement period (VS. = 2.86 l / s) it can be seen immediately that peak values ​​of this magnitude only occurred in 0.1% of all measured values. The load on the pipe network predominates in the low partial load range.

In Pic 5 It is shown that the basis for the curves for the calculations of the peak volume flow was already a DVGW research project from the 1990s for DIN 1988-3 from 1988. On this basis, the practical experience gained since then has now also been taken into account in DIN 1988-300 and the calculation curve has been flattened somewhat in order to come closer to today's conditions.

Minimum supply pressure

According to AVBWasserV §4, the water supply company is obliged to deliver the water under the pressure that is necessary to properly cover the demand. The tapping valves in apartments are to be taken as the standard for the usual requirements. According to DIN EN 806-1 section 4.3, the water supply company must provide the supply pressure at the transfer point on request. Before starting the planning for a drinking water installation, the planner on behalf of the connectee must inquire about this with the water supplier. The responsible water supply company determines the place for specifying the minimum supply pressure. Information that relates to the following places is common:

Connection lines and water meters are the responsibility of the water supply company, based on DVGW worksheets W 406 "Volume and flow measurement of cold drinking water in pressure pipes and W 410" Water demand - parameters and influencing variables ", measured according to different criteria than drinking water installations according to DIN 1988-300 .

Because of the different assessment bases for the connection line, the water meter and the drinking water installation, DIN 1988-300 provides that the water supplier preferably specifies the minimum supply pressure behind the water metering system (point C) that is available at peak volume flow, calculated in accordance with DIN 1988-300 stands. If the minimum supply pressure is specified by the water supplier on the connection device (point A) and the pressure losses in the connection line and the water meter are not specified, the planner assumes that the pressure losses as specified in DIN 1988-300 do not exceed 850 hPa.

The planner can only then calculate the pressure losses in the connection line ΔpHAL and in the water meter ΔpWZ responsible if the water supplier provides all necessary information for the calculation, such as the type of connection device, material, nominal diameter and length of the connection line (HAL), geodetic pressure difference, type of main shut-off device (HAE), type and nominal size of the water meter, etc. in a binding manner represents. If the water supplier specifies the minimum supply pressure behind the HAE (point B) in individual cases, only the pressure loss in the water meter needs to be determined. When coordinating with the water supply company, not only the level of the minimum supply pressure but also the transfer point to which this information refers must be clarified, as there can be considerable pressure differences between the individual transfer points depending on the situation.

Circulation systems

In a circulation system, the flow paths begin in the pressure port (1) of the circulation pump, run from there via a connection of the circulation to the consumption line and then end again in the suction port (2). The flow paths close to form a circle in the circulation pump and thus conceptually become the so-called "circulation circles" (Pic 7). In all sections of the circulation system, the circulation volume flow must be able to transport the amount of heat that is lost through the surface of the pipeline system at a given water temperature. Only if this state of equilibrium can be ensured at every point in the circulation system can the desired temperatures ≥55 ° C be maintained in the pipe system. The heat loss via the surface of the pipeline is directly related to the circulation volume flow required to maintain the temperature and therefore forms the basis for the volume flow calculation.

Degree of admixture

In DIN 1988-300, a so-called degree of admixture "η" was newly introduced into the equation for calculating the circulation volume flows. With a degree of admixture ηη = 0, the mathematical results are the same as in the calculation method according to DVGW worksheet W 553 "Dimensioning of circulation systems in central drinking water heating systems". A degree of admixture> 0 means that the circulation volume flows are somewhat larger in the short circles and somewhat lower in the longer circles (Picture 8). However, the change in the circulation volume flows is relatively small, so that it has no significant influence on the dimensioning of the pipe network or the dimensioning of the thermostatic valves.

The "correct" choice of the degree of mixing depends on the type of pipe network and the characteristic curves of the fittings of the thermostatic circulation regulating valves used. If no simulation calculations are carried out for larger installations, the valve manufacturer should make recommendations on the correct choice of the degree of admixture. Regardless of whether the pipe network has been calculated hydraulically with an admixture degree of 0 or> 0 and the resulting volume flow distribution, thermostatic circulation regulating valves with factory settings always set the same operating points on the valve characteristics and thus the same operating conditions. The quality of the hydraulic calculation can be improved with the correct selection of the degree of admixture. When using thermostatic valves, however, the improvement is of a purely theoretical nature and has no effect on the real pipeline operation.

Precisely because now all calculations for the heat losses and the pressure losses have to be carried out "differentiated", the results of a pipe network calculation for the pump pressure difference should not be overinterpreted. The energetic optimization should therefore always be carried out in larger circulation systems with a speed-controlled circulation pump! In this way, deviations between theory and practice can usually be compensated for without any problems.

Hydraulic balancing

The pump pressure difference determined for the most unfavorable circulation circuit is available in all other circulation circuits to overcome pressure losses. When dimensioning the circulation lines for comparative calculations, the primary goal must be to set the "available" pressure difference of the pump against the pressure losses in the pipelines. The selection of the diameter for the circulation pipes to be measured is based on the one hand on the inside diameter of the selected row of pipes and on the other hand by the requirements for compliance with minimum inside diameters (DN 10) and maximum speeds (vMax = 0.5 (1.0)) limited.

Due to these conditions, the “available pressure difference” cannot be fully represented as a pressure loss in pipelines. In each circulation circuit, there are more or less large differences between the pump pressure difference and the pressure losses in the lines. These deviations from the ideal state - the "hydraulic balancing" - are particularly large in the sections close to the pump. The circulation volume flows specified from the calculation via the circulation connections to the consumption lines are only set in the installed system if the so-called "hydraulic balancing" is ensured."Hydraulic balancing" means the state of equilibrium between the pump pressure difference and the pressure losses in each circulation circuit for a given circulation volume flow. For this reason, the pressure difference that is still missing for "hydraulic balancing" must be generated selectively in circulation regulating valves (Pic 9).

If the "hydraulic balancing" is not achieved through adjustment measures, the volume flows of the calculation case cannot be set in the installed system. However, the circulation volume flow must be able to transport the amount of heat that is lost through the surface of the pipe system. This means that a specifically specified water temperature can only be maintained if the described state of equilibrium is ensured at every point in the circulation system. The "hydraulic balancing" is the basic requirement for a drinking water hygienically correct functioning of a circulation system.

Simplified calculation method according to DIN EN 806-3

The European standard DIN EN 806-3 regulates a simplified calculation method that is not suitable for all drinking water installations in larger buildings. Therefore, in the scope of DIN EN 806-3 and in Section 4.2, it is described that the simplified procedure should only be used for "normal installations". However, the information in Section 4.2 is not sufficient to delimit a "normal installation" so clearly that the optimum can always be achieved for functional operation, taking hydraulic, hygienic, energetic and economic concerns into account, especially in taller or widely ramified larger buildings. The experience in Germany with simplified calculation methods also contradicts the statements in Section 5.1 General of DIN EN 806-3, according to which the simplified method can be used for the vast majority of all buildings. For this reason, the responsible standards committee decided that the differentiated calculation method should generally be used for all types of building (Section 1 of DIN 1988-300).

The exception to the principle was then regulated in the note belonging to the section of DIN 1988-300. According to this, the dimensioning method for cold and hot water consumption lines in residential buildings with up to six apartments according to DIN EN 806-3 can be used if the supply pressure is sufficient and hygiene is ensured. The minimum supply pressure provided by the water utility is one of the most important decision points as to whether the simplified procedure can be applied. Hygiene is ensured if all tapping points are operated as intended, i. H. regular water exchange takes place. This can usually be assumed for residential buildings with up to six apartments.

In DIN 1988-300, only residential buildings were described as an exception, but other buildings in which a comparable use with regular water exchange can also meet the hygienic requirements of the Drinking Water Ordinance. The hydraulic conditions can e.g. B. in commercial or industrial buildings may be different than in residential buildings and can therefore not be used there. Before using the simplified dimensioning procedure, it must be demonstrated that the minimum supply pressure is sufficient to ensure the minimum flow pressure at the most unfavorable tapping valve.

The minimum supply pressure can be regarded as sufficient if the available pipe friction pressure gradient in the most unfavorable flow path is greater than 10 hPa / m. This finding results from comparisons with differentiated calculations based on DIN 1988-300. In the simplified dimensioning procedure, the following criteria influence the pipe diameter:

  • Minimum supply pressure
  • Pressure loss from geodetic height difference
  • Select pressure losses in the apparatus, devices, drinking water heaters, water meters, filters, with as little pressure loss as possible
  • Select extraction fittings with low minimum flow pressures, max.0.1 MPa (1bar)
  • Withdrawal flows of the withdrawal fittings are not greater than according to Table 2 of DIN EN 806-3
  • thermally controlled mixer taps or electric or gas flow water heaters can have a higher pressure loss ΔP TE , Table 4 of DIN 1988-300 (800-1000 hPa)
  • no extraction valves as long-term runners (& gt; 15 minutes)
  • The ratio of peak flow Q D in l / s to total flow Q T in LU must correspond to Figure B1 of DIN EN 806-3.
  • Stress values ​​- LU - are to be determined according to table 2 of DIN EN 806-3
  • With the calculated load values, the respective pipe diameters for the corresponding section are to be selected according to the pipe material used according to Tables 3.1 to 3.8. The largest individual load value and, for the smaller pipe diameters, depending on the pipe material, the maximum permissible pipe length must be observed.

The calculation example carried out in the commentary on DIN EN 806-3 shows that with smaller drinking water installations the result is sufficiently accurate and comparable pipe diameters as with the differentiated calculation method according to DIN 1988-300.


Sample calculations show that both the simplified calculation method according to DIN EN 806 can only be used for smaller, simple building structures and the influencing criteria described, and in particular the differentiated calculation method according to DIN 1988-300 can be used for all drinking water installations. The user / planner must have the specialist knowledge and decide when to use which design method.

The next issue will deal with the subject of DIN EN 806-4 "Installation".