Pumping Station Marisa Handajani
Function To lift or to elevate the liquid from a lower elevation to an adequate height at which it can flow by gravity or overcome the hydrostatic head Applications: 1. Raw or treated wastewater 2. Grit 3. Grease and floating solids 4. Dilute or well thickened raw sludge, or digested sludge 5. Sludge of supernatant return 6. Dispensing of chemical solution Each of the various pumping applications is unique and requires specific design and pump selection considerations.
Pump Types Classification Kinetic Positive displacement Type Centrifugal Peripheral Rotory Screw Diaphragm Plunger Airlift Pneumatic ejector Application Raw wastewater, secondary sludge return and wasting, settled primary and thickened sludge effluent Scum, grit, sludge and raw wastewater Lubricating oils, gas engines, chemical solutions, small flow of water and wastewater Grit, settled primary and secondary sludges thickened sludge raw wastewater Chemical solutions Scum, and primary, secondary and settled sludges, chemical solutions Secondary sludge circulation and wasting grit Raw wastewater for small installation (100-600 l/menit)
Pumping Stations Type Wet-well station: Employ either suspended or submersible pump. Dry-well station: Employ either dry-well of self-primming centrifugal pump. The dry-well centrifugal pump operate within the dry-well adjacent to the wet-well
Wet-well Dry-well
Hydraulic Terms and Definitions Head Capacity Work Power and Efficiency
Head Hydraulic energy (kinetic or potential) equivalent to a column of liquid of specified height above datum. Unit : meter water column Consisted of: Static suction lift (static suction head) Static discharge head And total static head
Scheme of Head Term in Pumping
Total Dynamic Head (TDH) sum of total static head, the friction head (including minor losses) and the velocity head. TDH = h stat + h f + h m + h v h f = f. (L/D)(v 2 /2g) h f = 6,82 (v/c) 1.85 (L/D 1,667 ) h m = K (v 2 /2g) hh v = v 2 /2g (DW) (HW) h L = h f + h m + h v
Capacity (Discharge of Flow Rate) Volume of liquid which is pumped per unit of time Unit : m 3 /s, liter per second (l/d)
Work Power and Efficiency The work done by a pump is proportional to the product of the specific weight of the fluid being discharge and the total head against which the flow is moved. Pw = K Q (THD) γ The pump efficiency is The ratio of the useful pump output power to the input power Ep = Pw/Pp (60-85%) Ee = Pp/Pm (90-98%) Ep = pump efficiency Ee = motor efficiency Pw = power output of the pump (water power) Pp = power input of the pump (brake power) Pm = power input of the motor ( electrical power)
Centrifugal Pump The head is developed principally by centrifugal force. The inlet to pump is axial and the outlet is tangential. The flow accelerated by rotating impeller which imparts to it both radial and tangential velocity depends on the design of the impeller. The increase in cross section of the volute (casing) produces the change from velocity head to pressure head. www.engineersedge.com
Specific Speed The pressure and dischage of pump varies with pump speed. Specific speed is a number characterizing the type of impeller in a unique and coherent manner. Specific speed are determined independent of pump size and can be useful comparing different pump designs. The specific speed identifies the geometrically similarity of pumps. Specific speed is dimensionless and are expressed as N s = ω Q 1/2 / h 3/4 where N s = specific speed ω = pump shaft rotational speed (rpm) Q = flow rate (m3/h, l/s, m3/min) at Best Efficiency Point (BEP) h = head rise (m, ft) Specific speed is an index number descriptive of suction characteristic of a given impeller. High specific speed pumps are more efficient than with low.
Pump Characteristic Curve BEP
Best Efficiency Point - BEP The best operating conditions will in general be close to the best efficiency point - BEP. Special consideration should be taken for applications where the system conditions change frequently during operation. This is often the situation for heating and air conditioning system or water supply systems with variable consumption and modulating valves. Carry Out When a pumps operates in the far right of its curve with poor efficiency - the pumps carry out. Shutoff Head Shutoff head is the head produced when the pump operates with fluid but with no flow rate. Churn A pump is in churn when it operates at shutoff head or no flow.
Pump Performance Curve The pump characteristic is normally described graphically by the manufacturer as a pump performance curve. The pump curve describes the relation between flowrate and head for the actual pump. Other important information for proper pump selection is also included - efficiency curves, NPSHr curve, pump curves for several impeller diameters and different speeds, and power consumption.
System Head Curve A fluid flow system can in general be characterized with the System Curve - a graphical presentation of the Energy Equation. The system head visualized in the System Curve is a function of the elevation - the static head in the system, and the major and minor losses
Pump Selection A pump can be selected by combining the System Curve and the Pump Curve: The operating point is where the system curve and the actual pump curve intersect
Pump Combinations Pump may be connected in series or in parallel Parallel for a given head the total discharge is added up for all the pumps Series for a given discharge the total head for all pump is added When two or more pumps are arranged in parallel, the procedure for determining the pump-operating points is based on development of modified pump head capacity curve
Pumps in Serial - Heads Added When two (or more) pumps are arranged in serial, their resulting pump performance curve is obtained by adding heads at the same flow rate. For two identical pumps the head will be twice the head of a single pump at the same flow rate. If one of the pumps stops, the operation point moves along the system resistance curve from point 1 to point 2 - head and flow rate are decrease
Pumps in Parallel - Flow Rate Added When two or more pumps are arranged in parallel, their resulting performance curve is obtained by adding their flowrates at the same head. For two identical pumps the flow rate will be twice the flowrate of a single pump at the same head. If one of the pumps stops, the operation point moves allong the system resistance curve from point 1 to point 2 - head and flow rate are decreased.
Net Positive Suction Head - NPSH For a centrifugal pump to operate, the liquid must enter the center of the impeller under pressure, usually atmospheric pressure. This pressure is reffered to as NPSH. The Net Positive Suction Head - NPSH - can be expressed as the difference between the Suction Head and the Liquids Vapor Head and expressed like NPSH = hs - hv NPSH = ps / γ + vs 2 / 2 g - pv / γ There are two values of NPSH: Available NPSH depends on the location and design intake system (calculate by engineer) Required NPSH determined by pump manufacture based on extensive tests.
Available NPSH - NPSHa NPSHa is the minimum suction head required at the inlet of the impeller to prevent boiling of the liquid under reduced pressure condition created at the impeller and smooth operation of the impeller without cavitation.. The available NPSHa can be calculated with the Energy Equation. NPSHa = H abs + Hs hl Hvp For a common application - where the pump lifts a fluid from an open tank at one level to an other, the energy or head at the surface of the tank is the same as the energy or head before the pump impeller NPSHa = p atm / γ + hs - hl - pv / γ The absolute pressure on the surface of the liquid in the suction well 101.33 kn/m2 The absolute vapor pressure of water at temperature 20 oc is 2.3 kn/m2.
Available NPSHa - the Pump is above the Tank If the pump is positioned above the tank, the elevation - hs - is positive and the NPSHa decreases when the elevation of the pump increases. At some level the NPSHa will be reduced to zero and the fluid starts to evaporate. Available NPSHa - the Pump is below the Tank If the pump is positioned below the tank, the elevation - hs - is negative and the NPSHa increases when the elevation of the pump decreases (lowering the pump). It's always possible to increase the NPSHa by lowering the pump (as long as the major and minor head loss due to a longer pipe don't increase it more). This is important and it is common to lower the pump when pumping fluids close to evaporation temperature.
Required NPSH - NPSHr The NPSHr, called as the Net Suction Head as required by the pump in order to prevent cavitation for safe and reliable operation of the pump. The required NPSHr for a particular pump is in general determined experimentally by the pump manufacturer and a part of the documentation of the pump. The available NPSHa of the system should always exceeded (1 meter or more) the required NPSHr of the pump to avoid vaporization and cavitation of the impellers eye. The available NPSHa should in general be significant higher than the required NPSHr to avoid that head loss in the suction pipe and in the pump casing, local velocity accelerations and pressure decreases, start boiling the fluid on the impeller surface. Note that the required NPSHr increases with the square capacity.
Cavitation Cavitation is a common problem in pumps and control valves - causing serious wear and tear and damage. Under the wrong conditions, cavitation reduces the component life time dramatically. Cavitation may occur when the local static pressure in a fluid reach a level below the vapor pressure of the liquid at the actual temperature. This may happen under the following design and operating conditions: 1. When impeller under high speed, travels faster than the liquid can enter of move 2. When suction is restricted 3. When NPSH available NPSH 4. When specific speed is too high for optimum design parameter 5. When the temperature of the liquid is too high for suction conditions 6. When the pump operates at extreme capacities below or above the BEP
Cavitation causes reduction in flow and under serious conditions the pump may lose its prime, cause pitting on the impeller surface, rattling or pinging noise and eventual breakdown of pumping equipment. Cavitation can in general be avoided by increasing the distance between the actual local static pressure in the fluid - and the vapor pressure of the fluid at the actual temperature This can be done by: reengineering components initiating high speed velocities and low static pressures increasing the total or local static pressure in the system reducing the temperature of the fluid
The Effect of Pressure Change Low pressure at the suction side of a pump can encounter the fluid to start boiling with reduced efficiency cavitation damage of the pump as a result. Boiling starts when the pressure in the liquid is reduced to the vapor pressure of the fluid at the actual temperature. To characterize the potential for boiling and cavitation, the difference between the total head on the suction side of the pump - close to the impeller, and the liquid vapor pressure at the actual temperature, can be used.
Design Pumping Station 1. Characteristic of the liquid which must be pumped (SS, Floating solid, density, temperature, pressure, ect.) 2. Expected flow range (min, average, max flow) 3. Site plan, piping scheme, and hydraulic profile from wet well to the receiving facility 4. Min and max water surface elevation in the wet well and receiving facility 5. Type pumping station and suction condition including NPSHa 6. System head capacity curve 7. Drive unit and expected power requirements 8. Design criteria for pumping station 9. Equipment manufacture and equipment selection guide
Example 1. Design the pumping equipment for peak, average and minimum design flow of 1.321; 0.440; and 0.220 m3/s 3m 15m 2. Base on the preliminary examination of the plant layout, piping and hydraulic consideration, the following elevation have been assigned: Floor level elevation of wet and dry-well are assumed = El. 0.00m Minimum water surface elevation in the wet-well = El. 2.44m Maximum water surface elevation in the wet-well = El. 3.66m Normal water level elevation in the grit chamber = El. 13.57m Pumping station level elevation = El. 4.5 m Pipe : ductile iron pipe 0.92 m in diameter, therefore all fitting are compatible with this pipe (C HW =100) A. Draw the pump system diagram, included the EGL and HGL B. Determine the NPSH a ( Assume the Pabsolute 101 kn/m2 and the vapor pressure 2 kn/m2; fluid density 1000kg/m3)
As an initial selection, provide a total five pumping units of equal size including one unit as a stand-by. Select dry-well centrifugal pumps with variable-speed drive. Calculation: A. Preparation of System Curves Compute system losses for valves, fittings Compute head loss in the force main Compute total head losses Develop system head curve B. Preparation of Pump Characteristic Curves (find Pump Curve) Prepare modified pump Curve Prepare pump combination curves Determination pumps operating head and capacity
Design of Pumping Station 1. Site selection 2. Pumping Station Selection 3. Pump and Control Selection
NPSHr Pumps with double-suction impellers has lower NPSHr than pumps with single-suction impellers. A pump with a double-suction impeller is considered hydraulically balanced but is susceptible to an uneven flow on both sides with improper pipe-work.
Sludge Pumping Common pumps: Nonclog centrifugal Peripheral (torque-flow or vortex) Screw Plunger pumps Headloss in pipelines Hazen Williams equation CHW is decresed because sludge is more difficult to pump CHW value: 1. Waste activated sludge (total solid up to 1%) C=90 2. Primay sludge (total solids up to 4.5%) C=55 3. Thickened or digested sludge (total solids up to 8%) C=35 Sludge piping > 15cm (6 ) in diameter For small sludge flow, pumps are operates on a time cycle to achieved larger flows and velocities during pumping cycles A liberal numer cleanouts and hose gate should be provided in the piping for cleaning the stopper.
Centrifugal Pump
Screw Pump
Diaphragm pump
Peripheral pump
Piston Pump