LTA - Tryckavlopp för småhus - Xylem US · PDF fileLTA - Tryckavlopp för ......
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LTA - Tryckavlopp för småhus
Standarder
Krav enligt EU-standard SS-EN 1671
SS-EN 1671
Pumpar
Min hastigheter
Nödåtgärder
Max uppehållstid
Driftpunkter
Lednings- dimensionering
Presenter
Presentation Notes
Following are an extract from the EN standard exemplifying PSS design consideration needed to optimize a reliable and cost-efficient wastewater transportation system.
5.4.2 Minsta hastighet
“För att minska risken för sedimentation i rörledningar skall en minsta hastighet om 0,7 m/s uppnås minst en gång per dygn. Hastigheter kan accepteras under vissa driftförhållanden under förutsättning att ovanstående villkor är uppfyllt. När pumparna inte är kapabla att uppfylla villkoren ovan skall användandet av ett system med tryckluft för regelbunden renspolning av systemet övervägas.”
EN:1671
Pump units
Minimum velocities
Emergency
conditions
Maximum retention
time
Duty point
Pipe sizing
Presenter
Presentation Notes
The minimum velocity 0.7m/s should be reached to ensure scour at least once every 24 hours. The maximum pipe inclination is estimated to ensure that air pockets are transported away from the pipe. It is calculated with the eq.: Vmin 0 1.23 x SQRT(g x d x sin alfa)
5.4.3 Maximal uppehållstid
”För att begränsa bildandet av gas i systemet skall uppehållstiden för avloppsvatten ej överstiga 8 timmar. Denna tid kan variera beroende på lokala föreskrifter och lokala förutsättning.”
EN:1671
Pump units
Minimum velocities
Emergency
conditions
Maximum retention
time
Duty point
Pipe sizing
Presenter
Presentation Notes
Sewage is purified at a treatment plant with bacteria, which uses the "Dirt" as food. These same bacteria also develop in the PSS systems and live in a slime layer on the pipe wall. By "eating" the dirt the bacteria are using (so called aerobic bacteria) oxygen that dissolves in the water. If the retention time of the sewage water in the PSS systems is a long period (rule of thumb; longer than 8 hours) then the oxygen runs out. Another type of bacteria (anaerobe family) who are able to get oxygen from protein molecules will take over. By protein degradation H2S occurs (hydrogen sulphide or rotten eggs air). Until the sewage is encased in the tube, the H2S simply dissolved in the water, but at the outlet (and this is often a free gravity sewer) the H2S gas occurs and that does smell badly, is toxic and affects concrete. The latter is done by the H2S dissolves in the water of the wet walls and reacts with water to H2SO4. That we know better as sulfuric acid. The acidity on the concrete walls can be up to a pH of 2 to 3. The concrete wall is not resistance against this acid. That stench and degradation, we obviously would rather not have. There is a relatively simple solution to this. The accompanying drawing shows a so-called indirect discharge. The purpose of this outlet construction is meant to separate the H2S from the sewage water, so there will be no acid damage in the free gravity systems. At least 20 meters from the free gravity sewer we let's the PSS system end in a tightly closed plastic (i.e. acid proof) pit. That can be a regular inspection pit, but be sure that the cast iron cover, concrete walls are coated polyester. Obviously the cast iron cover must be close to prevent bad odour. The pit has to be installed with a 200 mm diameter pipe which is connected above the convex pit floor, always 30 cm water must be remains in the pit. The PSS pipe should be as high as possible in the pit installed, preferably with a 90gr bend upwards so that the sewage water falls on the bottom. The more turbulence the better because this would bring the H2S to the outside and that’s the intention. This process is called "stripping". The H2S creates, against the wet weather pit walls, H2SO4, the sewage water on the bottom has been sufficiently free of H2S and can safely drain to the sewers. To ensure that no H2S gas will flow into the gravity system, the connection to the sewer must be at least 20 meters.
4.4.1 Pumpar
”Exempel på pumpar som används i tryckavloppssystem är:
–Pumpar med flerkanaliga öppna pumphjul och skärenhet (tugger)
–Skruvpumpar med tugger
–…..”
EN:1671
Pump units
Minimum velocities
Emergency
conditions
Maximum retention
time
Duty point
Pipe sizing
Presenter
Presentation Notes
ITT has a long history of producing pumps for professional usage and consumer usage. Pump design requirements are exemplified here.
5.4.4 Nödsituationer
”Buffertvolymen för nödsituationer, t ex vid strömavbrott kan utgöras av uppsamlingstankarna och eventuellt även lämplig självfallsledning. Buffertvolymen för nödsituationer skall motsvara minst 25% av det totala genomsnittliga dagliga inflödet, och skall rymmas ovanför den normala startnivån.
Om buffertvolymen för nödsituationer inte är tillräckligt stor…”
EN:1671
Pump units
Minimum velocities
Emergency
conditions
Maximum retention
time
Duty point
Pipe sizing
Presenter
Presentation Notes
The volume from the top of a 3068 to the inlet pipe is approximately 160 l which is 25% of the inform from a std household in Europe (3.5 persons using 180 l each) Different monitoring equipment can be selected to alarm in different ways (according to house owner needs) at emergency conditions .
6.4 Ledningsdimensionering
”6.4 Genom att använda principerna för 6.1 *… skall optimala ledningsdimensioner beräknas … minsta flödeshastighet (0.7 m/s) uppnås…”
*”6.1 …antal pumpar i samtidig drift…”
EN:1671
Pump units
Minimum velocities
Emergency
conditions
Maximum retention
time
Duty point
Pipe sizing
Presenter
Presentation Notes
The pipe sizes are selected from standard dimensions to fulfil minimum velocity of the wastewater to ensure scour of sediments and suspension of the slurry. The number of pumps running simultaneously are estimated to check that minimum velocity is obtained once every 24 hours.
