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Revision as of 13:20, 14 June 2021
Appendix E: Supply Pumping Systems
It is important to understand the different types of pumps, design procedures, source of pumping power, motor starting, machine protections and economics of electric power systems.
Rationale
The running and the economy of a water production line relies mainly on the
success of the following project planning sub-components: intake and plant design,
pumping system design, equipment type design, equipment selection, plant,
pumping system and equipment protection, accuracy and comprehensiveness
of erection, operation and maintenance instructions, economics of electrical
power systems or other power systems, energy considerations, compliance with
instructions, observation of the factory ordinance. The main goal of any water
pumping plant and pumping system is to lift water from a lower to a higher level.
Common Types of Pumps used in water supply
There are two main pump types used in the water supply projects which are different in
design and application. Table 3.15 shows the most commonly used pump types.
Rotordynamic Pumps
In the rotordynamic-type pump water while passing through the rotating element
(impeller or a rotor) gains energy which is converted into pressure energy by an
appropriate impeller casing and consists of three types: centrifugal, peripheral
and special pumps.
Centrifugal Pumps
Centrifugal pumps are available in a wide variety of arrangements for mounting
both above and below water. They are available as single or multi-staged units,
and can be arranged for either horizontal or vertical mounting and for above
water use come with fully enclosed or split casings. They can be direct coupled
to a prime mover or in vertical mode driven via a shaft from a motor mounted
above. The capacity of the centrifugal pump is greatly influenced by the pressure
it works against, and also by the speed, form and diameter of its impeller. Low
speed centrifugal pumps wear less and last longer than high speed pumps.
Generally, speeds selected for raw water pumps should be limited to a maximum
of 1500 rpm (Source: Water Supply Design Manual 2nd edition, Uganda.).
Standard (dry mounted) Centrifugal Pump Sets
Standard, above ground centrifugal pump sets will either be horizontally or
vertically mounted with the prime mover either at one end or immediately above
the pump. For general waterworks purposes, the maximum pressure normally
developed by a single stage pump will be 80 -100 m so that for heads greater
than this, a multi-stage pump is usually required although increased head can
also be achieved by increased speed and/or larger impellors. However, and as a
general rule, the lower the speed, the longer the life of the pump.
With single stage horizontal pumps, end entry suction with side or top outlet is offered by some manufacturers whilst for vertically mounted units and for multistage pumps either side or top entry and exit ports are necessary. An end entry, single stage pump is illustrated in Figure E.2and a multi-stage pump in Figure E.3.
Multistage unit casings can be either axially split (Fig. 3.12) or radially split. Multistage pumps can be either horizontally or vertically disposed. A section through a typical multistage pump is illustrated in Figures 3.12 and 3.13.
The characteristics and applicability for the different types of centrifugal pumps include:
Single-stage: the usual depth range is 20 – 35 m. it requires skilled maintenance; not suitable for hand operation, powered by engine or electric motor;
Multi-stage shaft-driven: the depth range is 25 – 50 m. it requires skilled maintenance; the motor is accessible, above ground; alignment and lubrication of shaft critical; it has a capacity range of 25 – 10,000 l/min; and
Multi-stage submersible: the depth range is 30 – 120 m. its operation is smoother but maintenance is difficult; repair to motor or pump requires pulling the unit from the well; it has a wide range of capacities and heads; subject to rapid wear when sandy water is pumped.
Axial- and Mixed-flow Centrifugal pumps
An Axial-flow propeller pump consists of a propeller which thrusts rather than
throws the liquid upwards. Impeller vanes for mixed low centrifugal pumps
are shaped to provide partial throw and partial push of the liquid outward and
upwards. Axial and mixed-flow designs can handle large capacities but only with
reduced discharge heads. They are constructed vertically. Axial flow pumps are
used mostly for high-capacity and low-lifting pumping. They can pump water
containing sand or salt. Axial flow pumps are the nominal choice for high-volume,
low head raw water pumping. They are available in a wide range of capacities and
sizes. They are usually installable to a depth range of 5 – 10 m.
Specific Speeds
Specific speed (Ns) is the parameter which characterizes the rotordynamic pumps
more explicitly and is given in Table E.2.
Centrifugal pumps can be either self–priming or nonself–priming and can have open, semi-open, or closed impellers depending on the specific requirements of the particular pump.
Positive Displacement Pumps
Positive displacement pumps are essentially rotary or reciprocating machines in
which energy is periodically added by application of force to movable boundaries
of enclosed fluid containing volumes, resulting in a direct increase in pressure.
The various types are illustrated on Figure E.5.
Reciprocating Pumps
The reciprocating pump utilizes the energy transmitted by a moving element
(piston) in a tightly fitting case (cylinder). Frequently in reciprocating pumps, a
piston or plunger is used in a cylinder, which is driven forward and backward
by a crankshaft connected to an outside drive. The reciprocating pumps can be
divided into the two main categories: suction pumps and lift pumps.
Types of reciprocating pumps:
Suction Pumps
Suction pumps are used in shallow wells. In a suction pump, the pump element (cylinder and plunger) is positioned above the water level, usually within the pump stand itself. A suction pump relies on atmospheric pressure for its operation. They lift water through a vacuum (sucking) action. All the moving parts are above the ground. Typically in the form of cast iron pumps, but also they come in different forms such as the plastic Rower pump and Diaphragm pumps. The suction pumps can be installed up to a depth of 7 m.
Lift Pumps
Lift pumps are used in shallow wells. In a lift pump, the pump element (cylinder and plunger) is located below the water level in the well. Lift pumps create lift of the water, most commonly using a piston with leather, rubber or plastic washers (cup seals) located in a pump cylinder below the water level. The piston travels in an up and down motion at the pump head (direct action), a lever type handle, or a circular motion handle. Other mechanisms include spiral or helical stainless steel rotors encased in a rubber stator in the cylinder, and rubber diaphragms actuated hydraulically. The depth ranges18 of the lift pumps are as follows:
Hand pumps are the most common used reciprocating pumps and, in most cases, the only economically feasible water lifting device for community needs (UNICEF, 1999). Yield depends on the depth and design, normally in the range of 600 to 1,500 litres per hour during constant use. The most important design criterion for a hand pump is its maintainability. Some typical maintenance programmes for Hand pumps include: periodic lubrication of above-ground components, replacement of washers and seals, replacement of plastic bearings, occasional replacement of individual rising mains. The maximum pumps (lifts) for comfortable operation of hand pumps are shown in the Table E.9
Rotary Pumps
Positive displacement rotary pumps work on the principle of rotation. Rotary pumps generally consist of gears, screws, vanes or similar elements enclosed within a casing. The rotation of the pump creates vacuum which draws in the liquid. The need to bleed the air from the lines manually is eliminated in rotary pumps because the air from the lines is naturally removed by the vacuum created. Positive displacement rotary pumps also have their weaknesses. Because of the nature of the pump, the clearance between the rotating pump and the outer edge must be very close, requiring that the pumps rotate at a slow, steady speed. If rotary pumps are operated at high speeds, the fluids will cause erosion, and thereby showing signs of enlarged clearances, which allow the liquid to slip through and detract from the efficiency of the pump. Rotary pumps are usually low in cost, require relatively small space, and are self priming20.
Helical rotor pumps are the most commonly used type of rotary pump. A helical rotor pump consists of a single thread helical rotor which rotates inside a double thread helical sleeve, the stator. It is the meshing helical surfaces which force water up to create a uniform flow. Water delivery by rotary pumps is continuous and therefore smoother. However, internal losses in rotary pumps are normally higher through slip (internal leak-back). Slip increases with increasing pressure, making rotary pumps unsuitable for use in high pressure systems.
Maximum Suction Lift Calculation
In order to ensure continuous and smooth operation of any rotor dynamic pump such as a centrifugal pump, there is a limit to the net positive suction head available (NPSHA) and hence the suction lift that can be achieved. The maximum suction lift shall be determined in accordance with the following formula:
Hs1 (Pepperberg) = B + Hsuc- Hfs (3.29) Where,
Pa = atmospheric pressure
Vp = vapour pressure at the given temperature of the water source
Hfs =friction losses in the suction line NPSHR =net positive suction head required
Hfs = a value dependant upon the altitude and the water temperature and thus on the barometric pressure and vapor pressure of the water in meters head of water
Hsuc = static height difference on the suction side of the pump in m (i.e. between the centre line of the pump element and the water level on the intake side of the pump)
Turbine Pumps
The turbine pump motor is usually placed above the water level, but submersible types are available depending on the design requirements. Generally, turbine pumps have a constant head, and water flows uniformly at high pressure. The stages can be connected in series to increase the head capacity of the turbine pump. Two common types of turbine pump are submersible turbine pumps and deep well turbine pumps, which are also known as vertical turbine pumps
Submersible Pumps
The submersible pump, an illustration of which is shown in Figure 3.19, is a pump which has a hermetically sealed motor close-coupled to the pump body. The whole assembly is submerged in the fluid to be pumped. The advantage of this type of pump is that it can provide a significant lifting force as it does not rely on external air pressure to lift the fluid. The pump is installed just above the motor, and both of these components are suspended in water. Submersible pumps use enclosed impellers and are easy to install and maintain. These pumps run only on electric power and can be used for pumping water from very deep and crooked wells. Moreover, they are unlikely to be struck by lightning and require constant flow of water across the motor.
Notes:
(1)The NPSHR curve should be provided by the pump manufacturer or his agent. Otherwise the NPSHR value must be obtained from the manufacturer‘s catalogue or if even provided therein, it must be confirmed from the relevant manufacturer who should give a written guarantee as to the value appropriate for the design head-flow point of the pump.
(2)The term (Pa - Vp + Hp + Hfs + Hsl) = NPSHA, which for continuous operation must be at least 1 m.
Example of Calculation of Head loss through the Strainer and the Foot valve
A pump is to be located 4 m above the minimum water level in an inlet sump and is to be used to pump 50 l/s. The suction pipe is 200 mm dia., 10 m long and has two 45o bends. There is a strainer with a foot valve at the inlet of the pipe at the altitude is 1,100 masl. Question: What should be the NPSH of the pump?
Solution: Hsuc = - 4 m Hfs = 0.40 m (strainer & foot valve) + 0.07 m (bends) + 0.20 m (10 m of pipeline) = 0.67 m B (From Table 5.5, MoW 3rd edition Design Manual; see also Appendix A) = 8.3 m Hence, NPSHR = 8.3 – 4 – 0.67 = 3.63 m. Therefore it is necessary to select a pump which has an NPSH of 3.63 metres or less for the capacity of 50 l/s.
Pumping System Setup
When setting up the pumping system, carefully calculate the driver HP required based on the data on the flow, pressure and efficiency of the pump. Check the pump RPM and drive RPM and select the proper size pulleys to achieve the desired flow. Review the maximum horsepower per belt to assure that the pump receives adequate power to deliver the desired flow. The correct belt length and centre distance must be established to achieve the proper HP. If in doubt, consult your pump and/or drive supplier for their recommendations.
