Dry Well PS

8/11/2019 Dry Well PS

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Dry well installation pumping station:

1. The dry well installation type consists of two separate underground chambers, thesump or the wet well containing the liquid to be pumped and the dry well where the

pump, motor and associated pipe work are located. This type usually has anassociated superstructure where the control gear is located.

2. Two pumps connected in parallel. One Duty and one stand-by.

Under normal operating conditions the pumps operate automatically, being brought intooperation by water level detecting devices. These range from electrodes and mercury tiltingswitches to ultrasonics. The operating levels are determined from consideration of inflow,outflow and sump capacity.

All the electrical switchgear is connected to a control panel which can be used to control thestation manually if required. The panel will normally keep a record of operationalinformation such as hours run etc.

Dry well pumping station


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Pump Performance curves:

System resistanceThis curve is obtained by computing the total static head (H TS) and the total friction head (h fs+ h fd) for a range of flow rates. The results are presented graphically.

Duty pointThe duty point may be found by plotting the two curves (System resistance and Pump curve)on the H/Q axes:-

Flow rating for sewage pumps

This is related to hypothetical dry weather flow which for domestic foul sewage can only bedetermined in litres/sec from

DWF =60x60x24

dh/2001/property x/persons3xpropertiesof No

Number of pump units

This will generally depend upon the size and variability of the flow. For low to mediumflows two 100% (ie required capacity) units should be used. For larger inflows, three 50%flow units (or a greater number if correspondingly smaller inflows) are suggested to giveflexibility in respect of variable inflow rates. However, this is subject to a single unit beingable to produce self-cleansing velocities of 0.75 m/sec in the rising main.


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Sizing of rising mainThere are three major factors influencing the rising main sizes:

1) Cost of pipeline: Generally the smaller the diameter the less the cost.2) Frictional resistance: The smaller the diameter the greater the friction head to be

overcome (as H f 2 v ) and hence the larger (and more expensive) pump

required.3) Self-cleansing velocity.

Factors influencing selection of station typeIn general there are five main areas of consideration which are listed below:

1) Civil Construction CostsThe main parameter that affects civil costs is the area and depth of excavation.Secondary considerations are the loadings on the floors and the size of thesuperstructure. Dry well stations are the most expensive being the largest in planarea and usually have substantial superstructures, incorporating lifting beams andhoists.

2) Initial plant costs:Generally speaking there isnt much to choose between the actual pumpsthemselves, submersible pumps tending to be marginally more expensive. Howeverextension to drive shafts, intermediate support, and level detecting devices,additional valves and bends will influence costs.

3) Running costs (power consumption):Generally the types of pumps found in dry well installations tend to operate mostefficiently (this is mainly related to the energy losses in the thrust bearing) thus theytend to have the lowest running costs.

4) Reliability:The degree of reliability of an installation is not directly related to the plantarrangement. It is influenced more by the hydraulic, mechanical and electricaldesign of the equipment. In the endeavour to keep down costs a design may beadopted that will not have a degree of safety sufficient to permit variable operatingconditions, possibly resulting in failures. Overall dry well installations tend to bemore reliable, perhaps because of factor more than any other reason.

5) Maintenance:Dry well stations have advantages in terms of accessibility and ease of maintenance.


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6. Stop level higher than suction pipe inlet by inlet velocity head or inlet borewhichever is greater.

7. Dissolved oxygen level must be maintained, min 0.5mg/Litre. Inlet drops and largesurface area can assist.

9. Sulphide problems must be minimised and septicity avoided.

10. Inlet design either free fall or submerges depending on sulphide content of incomingsewage.

Pumping station design stages:

1. Determine peak and dry weather flows to station

2. Select station type factors above

3. Select rising main diameter and determine friction and static head lift

4. Select Number and configuration of pumps

5. Determine sump plan area for chosen configuration

6. Determine maximum water level in station based on incoming pipe invert level

7. Determine volume required in sump for various operational scenarios based

8. Work out stop and start levels for control purposes

9. Determine exact chamber configuration

10. Check station & rising main for night flows

Design Basis:

The below parameters are arrived:

Estimated Peak design Flow (Metres cubed per second)Minimum Discharge Level (Meters)

Maximum Discharge Level (Meters)

Suction Level (Meters)

Minimum Suction Level (Meters)

Maximum Suction Level (Meters)

Pump Axis Level (Meters)


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Pump Selection:

The following details listed below are the basis for Pump calculation:Pipe roughness coefficient is considered to be 0.3mm.

One Duty and one Stand-by pump.

Minor Loss Factors for fittings (K) are taken from the Perry Data Hand book.

Frictional Factor (f) is calculated using Swamee & Jain equation which is thesimplified form of Colebrook-White equation. Friction factor can also be arrived fromMoodys chart.

Vendor data:

Manufacturer

Pump Model No.

Pump curvesPump quotation.

Dry Well PS

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