Waste water drainage

The wastewater discharge Qww according to DIN EN 12056-2, is determined from the sum of the connected loads (DU), taking into account the simultaneity, where K is the guide value for the discharge coefficient. It depends on the type of building and results from the frequency of use of the drainage objects. Qww – wastewater discharge (calculated flow) K – discharge coefficient DU – connection value of unit Qtot – total wastewater discharge Qs – continous wastewater discharge (without simultaneity reduction) From the sum DU, the wastewater discharge Qww can be calculated with the above formula, taking into account the corresponding discharge coefficient K. If the determined wastewater discharge Qww is smaller than the largest connected value of a single drainage object, the latter is decisive (limit value).

Rainwater drainage

r5/2 Five-minute rainfall, which statistically must be expected once in 2 years r5/100 Five-minute rainfall, which statistically must be expected once in 100 years

DIN 1986-100 lists the values for several German cities as examples. The values differ from r5/2 =200 to 250 l/(s ha) or r5/100 = 800 l/(s ha) [1 ha = 10,000 m²]. Information on rain events is available from the local authorities or alternatively from the German weather service. Reference values are given in DIN EN 1986-100 Annex A. If no values are available, rT(n)=200 l/(s ha)should be assumed. For economic reasons and in order to ensure the self-cleaning capability, pipe systems and the associated components of the rainwater drainage system shall be dimensioned for a mean rainfall event. In the scope of DIN 1986-100, the calculated rain is an idealized rain event (block rain) with a constant rain intensity over 5 minutes. The annuality (Tn) to be used for each design case is determined by the task definition. Rainfall events above the calculated rainfall (r5/2) are to be expected as planned. Qs – Rainwater drainage (calculated flow) rT(n) – Dimensioning value according to statistical events C – Characteristic coefficient for type of drainage area A – Size of drainage area

Calculating the System H-Q Curve

The required pumping head in a branchless pipeline is determined from BERNOULLI’s equation for one-dimensional, stationary flow of incompressible fluids: pin, pout = pressures on suction respectively discharge liquid levels ρ = fluid density g = gravity (9.81 m/s2) Hgeo = static height difference between suction and discharge liquid levels Hl,tot = total pipe friction loss between inlet and outlet areas vin, vout = mean flow velocities at inlet and outlet liquid level areas The mean flow velocities at the inlet and outlet areas are, based on the Continuity Law, mostly insignificantly small and can be neglected, if the tank areas being relatively large compared to those of the pipe work. In this case, above formula will be simplified to: The static portion of the system H-Q curve, that part that is unrelated to the rate of flow, reads: For closed circulating systems this value becomes zero. The total friction losses are the sum of the frictional losses of all components in the suction and delivery piping. They vary, at sufficiently large REYNOLDS numbers, as the square of the flow rate. g = gravity (9.81 m/s2) Hl,tot = total friction loss between inlet and outlet areas vi = mean flow velocities trough pipe cross-section area Ai = characteristic pipe cross-sectional area ζi = friction loss coefficient for pipes, fittings, etc. Q = flow rate k = proportionality factor Under the above stated premises the parabolic system H-Q curve can now be drawn: The proportionality factor k is determined of the specified duty point. The intersection of the system H-Q and the pump H-Q curves defines the actual operating point.    

Pump Selection

It is highly recommended to always select the smaller pump if the specified system duty point is between two possible pump curves. The resulting capacity reduction has, in heating systems, no appreciable effect on the effective heating performance. The positive effects are lower noise levels, lower investment costs and improved economy. For heating installations it is customary to undersize pump capacities up to 10% below the specified duty. To avoid Cavitation (vapour formation within the pump) it is necessary to maintain at the pump suction port an adequately high positive pressure (static head) in relation to the vapour pressure of the fluid being handled. The minimum required inlet heads for Glandless pumps are generally listed in pressure charts. Glanded pumps require calculations in accordance with the NPSH information.

Dimensioning criterias

The top four criteria are: WHAT for a medium? –> Pumping medium HOW MUCH Amount? –> Flow rate WHERE, how far, how high? –> Pipe geometry WHICH pump unit should be used? –> Delivery unit If the flow rate and pipe geometry are known, the head can be calculated with the help of the pressure loss calculation. Flow rate and head together form the duty point for the pump design.

Duty Point

The point is composed of the volume flow Q and the flow rate H. To calculate the design point, the required volume flow (flow rate of the pump) is first determined. Depending on the application, this can depend on various variables (e.g. heat requirement for heating systems, volume of wastewater produced, etc.). The calculated volume flow is then used to determine the frictional losses of the pipeline, which together with the static head then gives the total head of the pump. If a minimum flow velocity is specified for the application and this is not reached for the calculated flow rate, the rated flow rate is adjusted so that the minimum flow velocity is reached. The pump then runs in off mode (intermittent). The duty point of the system is the required operating point for the pump selection. The standard pumps usually have a deviation between the desired duty point and the actual operating point. The permissible deviation depends on the field of application and is partly regulated by applicable standards. With speed-controlled pumps, the speed of the pump is modified so that the set operating point is approached exactly. Especially in systems that are operated in different load conditions (e.g. heating system), this enables efficient operation. Depending on the design of the pump, there are further possibilities for adapting the pump performance curve to the duty point. In addition to changing the speed, the following methods are widely used:
  • Impeller trimming
  • Blade angle adjustment for axial flow pumps
  • Throttling
  • Bypass

Required Pump Shaft Power

The shaft power requirement or the power input of the pump are, as the hydraulic performance, also shown in a graph.
  • It demonstrates the dependence of power input on the flow rate.
  • Max. shaft power requirements are reached at max. flow for many pump types.
The drive motor is to be sized to suit that point of the pump. Small pumps (e.g. heating circulators) are typically equipped with motors that allow operation over the entire characteristic curve. This reduces the number of types, and as a result, easier stocking of spare parts is guaranteed. For larger pumps, several motor options are usually offered so that the appropriate drive can be selected according to the operating conditions. If the specified duty point of a pump is located on the left hand portion of the duty curve with a corresponding lower power input it is feasible to select a smaller size motor. In such case however exists the hazard of overloading that size motor if the actual duty point allows a higher flow rate than that calculated (a more flat system curve). As in practice there is always the danger of the duty point having shifted it must be recommended to select the motor to drive a pump with power reserve of approx. 5 to 20% above that theoretically required. To determine the operating costs of a pump it must be principally distinguished between the required shaft power P2 [kW] of the pump and the electrical power input of the motor P1 [kW]. The latter is the basis for determining the operating costs. It can, In case that only the shaft power requirements P2 are given, be determined by dividing that value by the motor efficiency. The electric power input P1 is stated where pump and drive motor are an integrated unit such as submersible pumps. Here it is even customary to state both P1 and P2 values. The required shaft power P2 is generally given in the case of aggregates, where pump and motor are coupled or rigidly connected to non-submersible pumps. This is necessary to allow the use of most distinctly different types of motors – beginning from IEC-standards up to the special design motors – with their varying electrical criteria such as power inputs and efficiencies.


The flow rate for the duty point of a pump is determined from the application, for example for heating systems from the heat requirement calculation or for wastewater systems from statistical parameters for the maximum expected wastewater volume. National and international standards exist for many applications. The performance curves of a centrifugal pump (e.g. head, power consumption, efficiency) are given as a function of the flow.