Pump Application: Building Water Pump
Pumps are devices used to move fluids from one point to another by creating differences in pressure between the two points. Pumps are powered by a prime mover, which can be either a diesel engine or an electric motor. The pump converts the energy supplied by the prime mover to potential energy by pressurizing the fluid it’s pumping. For example, a diesel pump has an engine that converts chemical energy in the fuel to mechanical energy in the form of torque. The torque from the engine is then picked up and transmitted to the pumping mechanism for fluid pressurization (Rishel 2002). On the other hand, electrical motors in electric pumps convert electrical energy to mechanical energy through electromagnetism. A shaft then picks up the torque from the electric motor, which is then used to generate pressure in the pumping system (Engineering Essebtials 2012). Hydraulic systems use pressure generated from pumps to lift or push fluids. This paper explores the type of pumps used in hydraulic systems, their efficiency and shortcomings. This is a paper on the application of a water pump to drive water up a building. Special focus is given to the pump selection and the calculation head losses, and
Objectives
Determine the fluid flow parameters associated with the pump
System Description
Modern high rise buildings are usually very tall, and as such, the piped water from the municipal supply cannot reach the top most floors due to lack of enough pressure. The pressure head due to the water column in a tall building overcomes the pressure of the piped water supply and hence it cannot climb up the building. Therefore, a pumping mechanism is required to drive water up the height of a building.
Normally, a temporary storage tank positioned at the top of a building is used to receive the pumped water. A plumbing system then delivers the water from this tank to the floors below it through the force of gravity. The pump operates intermittently such that water when the storage tank gets full, the pump goes off automatically (Forsthoffer 2005). Also, when the water level in the tank goes below a certain point, the pump is actuated and starts refilling the tank again.
Figure 1: Schematic drawing of a building water pumping system
As shown in figure 1 above, water is supplied to the building via a mains gate then channeled to the pump. According to Moss, (2003), the pump raises the water pressure such that it can overcome the force of gravity and climb to the top of the building. The pipes used in the water pumping system are made of mild steel, with an estimated roughness value (K) of 0.045mm. The building water supply system has a globe valve that regulates the flow of water into the pumping system. Also, the valve is used to cut out the water supply during maintenance operation of the pump or the plumbing system. In the pumping system, the suction side of the pump is adjacent to the valve while the discharge is on the house supply side.
The pump
In order to design a building water pumping system, in depth knowledge of how pumps operate with relation to pressure and flow rate is required. Also, knowledge of transmission of pressure in fluid is essential for such a design. Reid (2003) posits that there are two types of pumps used in the engineering industry, centrifugal pumps and reciprocating pumps. Centrifugal pumps produce fluid pressure, otherwise referred to as “head” in pumping applications, by swirling a fluid round a circular volute, through the use of an impeller (Burns 1993). As the fluid go round the volute, its kinetic energy increases and hence its rotation speed and pressure. The fluid is then tangentially ejected from the volute though an outlet at the required pressure. On the other hand, positive displacement pumps create fluid pressure by alternatively confining the fluid in cylinders and pressurizing it with pistons. The fluid is then ejected from the cylinders when the desired pressure is achieved. Positive displacement pumps have a constant output pear rotation cycle irrespective of discharge head and pressure (Effery 1992).
Figure 2: A residential water pump. Source: Speroni Water Pumps n.d.
The above water pump is a positive displacement pump and would be suitable for this application. The pump is manufactured by Seproni Water Pumps and it operates at a speed of 2850 rpm and delivers 40 liters per minute. The pump has a head capacity of 80 meters (Speroni Water Pumps n.d.).
Calculations
Figure 3: Piping schematics
Piping parameters
The water is supplied at a pressure of 6 bar (P1) while pressure at the tank inlet (P2) is 0 bar. The difference in height between the pump outlet and the tank inlet (z1 – z2), otherwise referred to as the head, is 95m. According to the Bernoulli’s principle,
P1ρg+u122g+z1+hA-hR-hL=P2ρg+u222g+z2
∴hA=u222g- u1 22g-P1ρg+hL+z2-z1
Also, the velocity of water flowing in the pipe is
u=QA
Where Q is the flow rate and A is the cross section area of the pipe
Since there are two pipes with two different diameters, 2 and 4 inches, the velocity in each pipe section is therefore different and so is the accompanying pressure. For example, the velocity at the 2 inch diameter section can be given by:
u2=QA1=0.00067π(0.05082)2=0.331m/s
Also, velocity at the wider section of the pipe, 4 inches, can be given as:
u1=QA1=0.00067π×0.101622=0.083 m/s
Calculation of the pressure losses
The Reynolds number for turbulent flow is given as
Re=ρudμ=1000×0.083×0.08431.3×10-3= 5382.231
Figure 4: The Moody chart
1f=-3.6log106.9Re+k3.71d1.11=-3.6log106.97651.8+0.0453.71×84.31.11
The above calculation gives the value of f as f=0.008463
The above frictional value can be used to determine the frictional head loss as shown below.
hf=4fld×u22g=4×0.00846×80. 1016×0.08322×9.81= 0.00093m
The contraction of the main supply pipe as joins the main valve also leads to head losses. The losses associated with the contraction can be calculated as below:
A2A1=π×0.050822π×0.101622=0.25
Therefore, the consequent head loss due to the contraction can be given as
hcont=u2 22g1CC-12=0.33122×9.8110.632-12=0.00325 m
Also, the frictional losses along the length of the two inch pipe can be calculated as
Re=ρudμ=1000×0.331×0.05081.3×10-3= 12934.461
In smooth pipes, the frictional factor is given by the formula below
f=0.079Re-0.25=0.079×12934.461-0.25=0.00741
The head loss accompanying the turbulent flow can be given as
hf=4fld×u22g=4×0.00741×1600.0508×0.33122×9.81=0.5213m
The head losses due to the equivalent length can be calculated as:
The equivalent length can be given mathematically = [(2 x30) d = 60d] + 340d = 400d
0.00741 respectively. Therefore, the equivalent head loss can be determined as
hT=4fld×u22g=4×0.00741×400×0.05080.0508×0.33122×9.81=0.066m
Finally, the total losses in the system can be given by the addition of the losses calculated above. Therefore,
hl= hf + hcont +hf+ ht
Substituting
hl=0.00093m+0.00325 m+0.5213m+0.066m=0.5915m
The total head required from the pump to drive the water up the building can be given by:
hA=u222g- u1 22g-P1ρg+hL+z2-z1
Substituting for the figures gives:
hA=0.33122g- 0.08322g-600x103ρg+0.5915+95=34.43m
The power output of the pump can be calculated as
PA=ρghAQ=1000×9.81×34.43×0.00067=226.328 Watts
Conclusion
Pumps are used to drive a fluid round a closed loop or move fluids from one point to another at a given amount of pressure. There are two types of pumps used in engineering applications, centrifugal pumps and positive displacement pumps. Positive displacement pumps ensure a higher pressure and a constant flow rate. To pump water to the top of a 90m tall building, a positive displacement pump with an output flow rate of 40 liters per minute is used. Pumping of water is accompanied by pressure losses such as pipe contraction losses and frictional losses. When the losses are factored into the total head required to deliver water at the top of the building, the overall head becomes 34.43 meters. On the other hand, the pump proposed in this project has a head output of 80m. Therefore, assuming the pump operates at the rated efficiency, it has enough capacity to drive water up the building to the storage tank.
References
Burns, M. (1993). Cottage water systems: an out-of-the-city guide to pumps, plumbing, water purification, and privies. Toronto, Cottage Life Books.
Effery, T. D. (1992). Hydraulic ram pumps: a guide to ram pump water supply systems. London, Intermediate Technology Publications.
Engineering Essebtials, (2012). Fundamentals of hydraulic pumps. [Online] (updated 2016). Available at: <http://hydraulicspneumatics.com/200/TechZone/HydraulicPumpsM/Article/False/6401/ TechZone-HydraulicPumpsM>[Accessed 21 March 2016].
Forsthoffer, W. E. (2005). Pumps. Oxford, Elsevier Advanced Technology.
Reid, R. N. (2003). Water quality and systems a guide for facility managers. Lilburn, Ga, Fairmont Press. http://www.crcnetbase.com/isbn/9780824740108.
Rishel, J. B. (2002). Water pumps and pumping systems. New York, McGraw-Hill.
Moss, K. (2003). Heating and water services design in buildings. London, Spon Press.
Speroni Water Pumps. (n.d.). Positive-displacement pump / distribution / water. [Online] (updated 2016) Available at:< http://www.directindustry.com/prod/speroni/product- 37897- 545796.html> [Accessed 25 March. 2016].
