Technically Speaking

Gravity is not always a Good ideaBy Product ManaGer ralf schoMäcker

Moving huge amounts of soil. Positioning drainpipes and moving sewer lines. Cutting through concrete floors. Traditional wastewater gravity draining can be very expensive, vulnerable to backwater and difficult – especially in building refurbishments.

A lifting station like the Grundfos Multilift is a much more flexible solution to the problems wastewater management can pose in a refurbished, re-configured or new building. Easy to install and maintain, a lifting station will give you decades of trouble-free operation - including backwater protection.

This time in Technically Speaking, we will be looking at the benefits of lifting stations, showing how pressurised sewage systems can be combined with commercial building gravity systems and giving you some pointers on correct sizing.

Challenge:Discharge wastewater from one or more basement levels in a building into a gravitational sewer main.

Traditional solution:Installation of a deep pumping pit outside the building or of inaccessible gravity pipes under the basement concrete, requires extensive digging in connection with pipe work below and around the building.

Traditional gravity solution with deep sewage pipes.

New solution:A Multilift lifting station is placed inside the building to collect and discharge all wastewater generated below ground level and to protect the building against backwater flooding. The pressure pipe should loop above the maximum backflow level, which is usually ground level. With this 100% safe solution, the building will no longer have to be evacuated during floods, since the Multilift will just keep pumping wastewater into the flooded sewage main. Wastewater generated above ground level (maximum backwater level) is led directly to the sewer main by gravitation. See illustration below.

Solution where a lifting station discharges wastewater into the aboveground sanitary system’s gravitational sewage pipe. The lifting station can be placed either in or below the lowest basement level.

Lifting stations like the Grundfos Multilift offer significant advantages in connection with many different types of building – both new constructions and refurbishments – because their easy installation can help to cut costs. Take, for example, a shop being converted into a restaurant with basement level sanitary facilities. Here, a lifting station would collect wastewater from all the new sanitary appliances below ground and discharge it into the existing gravity pipe under the basement ceiling. This saves the cost of cutting through the concrete basement ceiling

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or digging a well outside the building to install new drainpipes.

Note: The values in the table are given only for the purpose of calculation and are not related to discharge rates of sanitary appliances quoted in product standards.

Lifting stationsThe tank sizes and pump performances of lifting stations like the Grundfos Multilift can be tailored to fit any building. Designed for internal use in buildings, they come completely pre-assembled with a connected controller, which eases installation significantly. Place the unit, attach inlet and outlet pipes, mount the controller on the wall – and installation is complete.

Lifting station operation is fully automatic and starts

Appliance SYSTEM I System II System III System IV

DU DU DU DU

I/S I/S I/S I/S Wash Basin, Bidet 0,5 0,3 0,3 0,3 Shower without Plug 0,6 0,4 0,4 0,4 Shower with Plug 0,8 0,5 1,3 0,5 Single Urinal with Cistern 0,8 0,5 0,4 0,5 Urinal with Flushing Valve 0,5 0,3 - 0,3 Slab Urinal 0,2* 0,2* 0,2* 0,2* Bath 0,8 0,6 1,3 0,5 Kitchen Sink 0,8 0,6 1,3 0,5 Dishwasher (household) 0,8 0,6 0,2 0,5 Washing Machine up to 6 kg 0,8 0,6 0,6 0,5 Washing Machine up to 12 kg 1,5 1,2 1,2 1,0 WC with 4,0 L Cistern ** 1,8 ** ** WC with 6,0 L Cistern 2,0 1,8 1,2 to 1,7*** 2,0 WC with 7,5 L Cistern 2,0 1,8 1,4 to 1,8*** 2,5 WC with 9,0 L Cistern 2,5 2,0 1,6 to 2,0*** 2,5 Floor Gully DN 50 0,8 0,9 - 0,6 Floor Gully DN 70 1,5 0,9 - 1,0 Floor Gully DN 100 2,0 1,2 - 1,3 * per person ** not permitted *** depending upon type (valid for WC’s with siphon flush cistern only) - not used or no data

Table 2: Discharge Units (DU)

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as soon as the wastewater reaches a predetermined level in the tank. In a double-pump unit – which is recommended for all commercial building applications – the controller ensures alternating operation, so the wastewater is reliably discharged even if one of the pumps needs maintenance.

Multilift lifting stations and all station connections are completely water and pressure tight to prevent leakage. The robust tanks are made from wastewater-resistant single-mould polyethylene and a venting pipe runs into the building draining system to make the system completely odour-free. Once installed, a lifting station like the Multilift will provide trouble-free operation for decades.

Scheduled for launch in late 2011, the next generation of Multilift lifting stations will offer a lot of new features that will increase reliability further and make installation, service and maintenance even easier. And with tank sizes from 44 l to 1350 l and a performance range from 1.5 kW to 2x10 kW, the Multilift series will drain any building, be it a small one-family house or a large commercial building.

How to size a MultiliftWhen sizing a lifting station, you need to determine the wastewater flow rate, pressure pipe size and duty point.

We will take you through these calculations step by step.

First, you need to determine the volume of wastewater that your lifting station will have to drain. International standard EN12056-2 offers a table and a formula that helps you calculate the wastewater volume generated in a building by means of the connected sanitary appliances.

Wastewater discharge is not constant but varies over the course of a day. Remember that your wastewater disposal system needs to be designed so that it is also able to cope with peak flows. The values in the EN

12056-2 table take peak flows into account.Add up the volumes of all your discharge units (DU) in

table 2. The end result goes into the formula below:

The frequency factor K can be determined by consulting the table below:

The frequency factor and the calculated wastewater volume is used to determine the wastewater flow rate Qww that enters the lifting station and needs to be discharged.

In step 2, the pressure pipe needs to be sized.

Wastewater fl ow

20

0 2 4 6 8 10 12 14 16 18 20 22 24

40

60

80

100

120

140

160

180

200

Hour

t

% Q

Qww = K DU

Qww = Waste water fl owrate (I/S) K = Frequency factor ΣDU = Sum of Discharge Units

Usages of appliances k Intermittent use e.g. in Dwelling, Guesthouse, O� ce 0,5 Frequent use e.g. in Hospital, School, Restaurant, Hotel 0,7 Congested use e.g. in Toilets and/or Showers open to public 1,0 Special use e.g. Laboratory 1,2

Table 3: Typical frequency factors (k)

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This requires knowledge of the length, height and dimensions of the pressure pipe needed. The figure

shows the correlation:You only need to consider those parts of the discharge pipe where the wastewater does not flow by gravity. Note also that friction loss is reduced by 2/3 each time you go one pipe diameter up, which means that you can save money on pumping by going for a bigger pipe. But don’t go too far – to maintain a self-cleaning effect in the pressure pipe, the wastewater must flow at a minimum required velocity of 0.7 m/s.

In step 3, the duty point is determined. The calculated wastewater flow rate Qww, the pressure pipe dimensions, length and geodetic height lead to the so-called pipe curve (curve 1 in the following diagram). Computer-based programs like Grundfos WebCAPS automatically calculate the pipe curve based on the above parameters and identify the best pumps for the job, indicated by curve 2 in the following diagram

The duty point is the point where pump curve (2) and pipe curve (1) intersect. Remember that the drain capacity at this point must always match or exceed the calculated wastewater flow rate Qww.

Calculating tank volumeA lifting station has a limited tank volume and tank-external motors that are designed for intermittent duty, since they are not submersed in a cooling medium - and this also needs to be taken into account.

Pump size is influenced by the tank’s effective volume between start and stop level (Veff) and the pump’s intermittent operation time. If the intermittent operation time of a pump is given as S3 50% 1 min., this means the pump is designed for 60 starts/stops per hour and 30 s operation followed by 30 s pause.

Similarly, if the S3 specification is S3 30% 1 min, this would mean 60 starts/stops per hour and 18 s operation followed by 42 s pause.

Note that actual operational values are usually lower

10,0 m

Hgeo = 4,0 m

Htot

geo

v

p

HH

H

1

2

3

H

Q

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than the theoretical data sheet values.Within its accumulated hourly operation time, the pump must be able to discharge the hourly volume of wastewater generated by the system.

To avoid over-sizing and thermal switch-offs caused by too frequent pump starts & stops, the tank volume needs to be calculated correctly.The minimum required tank volume Veff can be

calculated by

Where Qp is the duty point pump volume (m3/h) and z (1/h) is the number of pump starts per hour.

The tank volume should match the wastewater flow rate and the pump performance. By avoiding over-sized tanks, you can ensure that the wastewater is stored in the tank for the shortest time possible and thus minimize wastewater fouling.

Remember: Double does not mean doubleA double pump station does not provide double drain capacity – each of the pumps must be able to discharge all wastewater at peak flow in order to be a 100% backup in case of breakdown. During sizing, parallel pump operation should only be considered for extreme situations, such as peak flows. Parallel operation does NOT double drain capacity since the resulting higher

flow increases friction loss in the discharge pipe significantly.

Easy sizingGrundfos is currently developing an easy calculation and sizing tool specifically for Multilift lifting stations. Until that tool is ready, you can use these few simple basics to correctly size a lifting station and ensure cost-savings for wastewater management in both your refurbishment and new-build projects.

Compact performanceLifting stations offer compact performance that can help to cut investment costs compared to traditional gravity draining. And even the smallest lifting stations can carry out larger jobs.

The smallest Grundfos lifting station for commercial application - the Multilift MD double station with 92 l effective volume and a footprint of just 0.6 m2 – can drain up to 5,500 l wastewater per hour if just one of the two pumps is sized for the full load. This ensures that there is full backup if one pump switches off. But the small lifting station can discharge up to 11,000 l/h in peak flow situations – the equivalent of 72 bathtub drains in an hour – when its two pumps are used in alternating operation.

Veff =Qww • (Qp - Qww)

Qp • Z

H (m)

Q (l/s) 50%

Each

pum

p in

pa

ralle

l

One

pum

p

Two

pum

ps

in p

aral

lel

New pipe system

60-80% 100%

2 pumps

1 pump

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