Section 4 Page 2. Flow in Pipes

Transcription

1 Flow in Pipes

2 Section 4 Page 2 Flow in Pipes 1.0 INTRODUCTION In this module we are focusing on the fundamental principles of headloss and we learn how to use headloss charts to estimate the headloss in a simple pipe system. HEADLOSS CALCULATION / PIPE SELECTION 2.1 Using the Headloss Charts Once a fluid starts to flow in a pipe there will be a loss of head in the pipe. This shows up as a drop in pressure or a difference in head. Manometer Tubes Headloss Pressure Gauge WATER FLOW Pipe manufacturers make available tables or charts of headloss for different pipes. The information contained in the chart is: the pipe size and usually the pressure rating. the flow rate in litres per second. the rate of headloss in metres of headloss per 1000 m of pipe.

3 Section 4 Page 3 If we know any two of these three items we can determine the third. Thus to determine the pipe size we need to know what flow we want and our allowable rate of headloss. Alternatively if we know the pipe and the flow we can work out what headloss will occur. Exercise 1 A flow of 1 litre per second in a DN40 PN9 Rural Redline pipe will result in what rate of headloss? [The answer is 39 m/1000 m.] Exercise 2 We have 2500 m of DN32 PN9 Polyethylene with a flow of 40 litres per minute, what is the loss of head which occurs? [The answer is 150 m.] Exercise 3 Water is flowing at 70 l/min from a pump. The pump pressure is 455 kpa and the pipe is DN50 PN9 Rural Redline. What will be the head at the end of 350 m of pipe? [The answer is 39.2 m.]

4 Section 4 Page 4 Exercise 4 An existing system has DN25 PN12 Rural Redline pipe. If the headloss is 10 m/1000 m what flow will occur? [The answer is 0.11 litres per second.] 2.2 Head loss in fittings The head loss in fittings is often referred to as minor losses although in some cases this is misleading since the head loss in fittings can be greater than the head loss in the pipe itself. The easiest way to deal with the headloss in fittings is to compute the equivalent length of a fitting ; this is defined as that length of straight pipe which gives the same head loss as the fitting itself. The table below gives equivalent lengths in pipe diameter for each fitting. To use this data we must know what pipe diameter we are working with. For example, if we have a 25 standard elbow the equivalent length = 30 x = 0.75 m. The following table is from an American source and uses some different terminology; therefore set out after it are diagrams of the different fittings mentioned in the table.

5 Section 4 Page 5 Globe valves Angle valves Gate valves Check valves REPRESENTATIVE EQUIVALENT LENGTHS IN PIPE DIAMETER (l/d) OF Conventional Y-pattern Conventional VARIOUS VALVES AND FITTINGS Description of Product Conventional wedge disc, double disc, or plug disc Conventional swing In-line ball Foot valves with strainer Butterfly valves (6inch, and larger) 90 standard elbow 45 standard elbow 90 long radius elbow Fittings 90 street elbow 45 street elbow Square corner elbow Standard tee Standard tee With no obstructions in flat, bevel, or plug-type seat With wing or pin-guided disc. No obstruction in flat, bevel, or plug-type seal. With stem 60 from run of pipe line With stem 45 from run of pipe line With no obstruction in flat, bevel, or plug-type seat With wing or pin-guided disc. With poppet lift type disc With flow through run With flow through branch Fully open Fully open Fully open Fully open Fully open Fully open Equivalent Length in Pipe Diameters (l/d) Fully open 13 Fully open 135 Fully open 150 Fully open 420 Fully open

6 Section 4 Page 6 Figure 1: Types of Valves

7 Section 4 Page 7 Example: A piggery water system consists of 150 m of DN25 PN15 upvc pipe and includes 25 elbows, 6 gate valves (fully open) and 5 tees (with flow through run). If the system was to carry 40 litres per minute what would be the total headloss? First calculate the equivalent length of straight pipe. Equivalent length = Straight pipe : 150 m 25 elbows : 25 x 30 x : 18.8 m 6 gate valves : 6 x 13 x : 2.0 m 5 tees : 5 x 20 x : 2.5 so, equivalent length = 173 m

8 Section 4 Page 8 For DN25 PN15 40 litres per minute the rate of headloss is : 53 m/1000 m so total headloss = 53 x 173/1000 = 9.18 m. Exercise 5 A pipeline consists of 15 elbows, 2 gate valves (fully open) and 700 m of DN40 PN12 upvc pipe. If the flowrate is to be 60 litres/minute what will be the total headloss? (57m) 2.3 Residual Head When water flows from a pipe its pressure is the same as that of the atmosphere; therefore all the pressure (or head) is used up as it flows through the pipe. That is to say, when we turn on a hose the flow increases until it reaches a level whereby all the available head is used up by the time the water reaches the end of the pipe. Now if there is a ballcock, or any sort of valve, on the end of the pipe then some head will be lost when water flows through it. To accommodate this requirement the concept of residual head is used, it is the head which must be left over at the end of the pipeline so that it can be used up as the water flows through the fitting. The following residual heads are recommended:

9 Section 4 Page 9 Water Outlet Fitting Flow l/min Residual Head m. Trough valve 45 3 Trough valve 82 7 Trough valve Yard washdown (Nozzle size must match pump) 180/ Domestic shower 9 5/7 Garden hose 12 15/ SELECTION OF PIPE SIZE In designing a water system, the prime tasks are to decide what size and type of pipe to use and what pump to use. The method used does depend on what sort of system we are designing for. 3.1 Natural Gravity Systems To determine the pipe type and size we need to know the flow, the allowable rate of headloss, and the maximum head which will occur. The maximum head will be the difference in height between the highest and lowest points of the pipeline; with the maximum head occurring in the pipeline when there is no flow. We can then determine the pressure rating of the pipe required. To determine the maximum allowable rate of headloss we first work out how much head there is available for headloss and then divide it by the number

10 Section 4 Page 10 of 1000 m of pipe. Then using this number and the desired flow we go to the headloss chart and determine the pipe size. Example: A tank on a hill supplies water to 3 troughs. The lowest trough is 35 m below the base of the tank. It is also the farthest away at a distance of 900 m. The flow must be 60 litres/minute. What size and type of pipe should be used? Solution: The maximum working head is 35 m, this is low so we will try MDPE. (Greenline) The head available for headloss = 35 m The maximum allowable rate of headloss = 35/(900/1000) = 35/0.90 = 38.9 m per 1000 m go to headloss chart for MDPE (Greenline) Enter at 1.0 litre per second go up until you meet the line for 38.9 m/1000 m. Take the next biggest pipe below this point. The head loss for a 40mm pipe is approx 39m., so we must go to next size of pipe, but notice from the chart how the pressure rating of the pipe changes with size. What should be done? Exercise 1 A tank is situated on a hill 73 m above a cowshed. If the flowrate to washdown the yard is to be 5 l/s and the pipeline is 850 m long, what size of Black n Blue pipe should be used?

11 Section 4 Page 11 [The answer is 63 mm] Exercise 2 A tank is to deliver water to a house sited 25 m below the level of the tank. If the house is 450 m away and it requires a peak flow of 30 litres/minute and a residual head of 7 m what pipe would you install? Check all three common types, then justify your decision. [The answer is DN32 PN6.3 Greenline, DN32 PN9 Rural Redline,] 3.2 PUMPED GRAVITY SYSTEMS Falling Main In the case of the falling main system we have two pipelines to design. Firstly, the pipe the water is pumped to the tank in, and secondly, the pipe the water goes from the tank to the outlets in. The pipe from the tank to the outlets is in fact a gravity system and so the procedure in 3.1 above is used. For the pipe from the pump to the tank we use the following procedure: determine the static head, this is the head which exists when no water is flowing. It is usually the difference in height between the two ends of the pipe. It represents the height through which the pump must push the water; determine the design flow; determine the head which the pump delivers at the design flow; determine the residual head required, if any; determine the head available for headloss: head available for head loss = pump head static head residual head. determine the maximum allowable rate of headloss in m/1000 m. go to the head loss chart to select a pipe.

12 Section 4 Page 12 Example: A pump delivers 60 litres/minute at a head of 55 m. The water is to be delivered to a tank 35 m above the pump and 700 m away; a residual head of 5 m is required, select a suitable pipe. The static head = 35 m Design flow = 60 litres/minute = 1.0 l/s Pump head = 55 m Residual head = 5 m Head available for head loss = = 15 m Maximum allowable rate of head loss = = 21.4 m/1000 m The pipe has to withstand a head of 55 m. for Rural Redline use DN50 PN9. Exercise 3 A pump is to deliver 50 litres/minute to a tank 50 m above the pump and 1200 m away from the pump. The pump head is assumed to be 75 m and a residual head of 5 m is needed at the tank, determine the pipe to be used. [The answer is DN50 PN9 Rural Redline]

13 Section 4 Page 13 Exercise 4 You are to select a pump to deliver water to a tank, there are no off-takes along the pipeline. The pump is to be at an elevation of 42 m and the tank is at an elevation of 88 m. The pump will be controlled by probes in the tank. The tank is 2 m high. The pipeline is DN40 PN9 Rural Redline, and is 2.4 km long. The elevation of the water source is 42 m. The desired flow is 30 l/minute. What head must the pump produce at the desired flow? [74.4m] Rising Main In this case we do two design checks. One when the rising main has the pump working and one when it is not working (and is therefore a gravity pipeline). Example: A rising main is to deliver a flow of 40 litres/minute when the pump is running and is to deliver a peak flow of 60 litres/minute as a gravity pipeline. The static head is 50 m and the pump head is 65 m; no residual head is required and the pipeline is 750 m long. Select a suitable pipe. Maximum working head = 65 m (when pump is running) for gravity pipeline: Head available for headloss = 50 m Maximum allowable rate of headloss = = 66.7 m/1000 m. at 60 litres/minute and 66.7 m/1000 m

14 Section 4 Page 14 pipe selected is: DN40 PN9 Rural Redline, or, DN32 PN12 upvc Now check these pipes for the pump running situation. Design flow = 40 litres/minute. Head available for head loss = = 15 m Maximum allowable rate of headloss = = 20 m/1000 m at 40 litres/minute and 20 m/1000 m pipe selected is: DN40 PN9 Rural Redline (very close to being on the line). NOTE: the reason for the difference in flows is that the pump can fill the tank over a longer period of time than the peak flow occurs for. Exercise 5 A rising main pumped gravity system is to deliver 40 litres/minute when acting as a gravity system and 30 litres/minute when the pump is running. If the tank is 600 m away and 30 m above the pump, (pump head = 45m) what pipe would you install?

15 Section 4 Page 15 Exercise 6 You are to replace an existing piston pump and pipe in a rising main system. The piston pump was a MacEwans H3 delivering 800 gph with a cut-in pressure of 105 psi and a cut-out pressure of 125 psi. The static head is 40 m. The pipeline is 750 m long. A 20 mm trough valve is installed at the inlet to the tank; the water source is another tank immediately beside the pump site. Select a new pipe and specify the pump head that will be required. 3.3 Hydropneumatic Systems For these systems the method is essentially the same as for the pipeline from the pump to the tank in a falling main system. Thus the steps are: determine the static head between the pump and the highest outlet. determine the design flow. determine the pump head, this is usually the cut-in pressure. determine the residual head required. determine the head available for headloss head available for headloss = pump head static head residual head. determine the maximum allowable rate of headloss in m/1000 m. determine the maximum working head; this is the cut-out pressure if the pump is at the lowest point in the system. If the pipe runs down hill from the pump, the maximum working head is the cut-out pressure (expressed as a head) plus the static head between the pump and the lowest outlet. go to the headloss chart to select a pipe.

16 Section 4 Page 16 Exercise 7 A very independent dairy farmer recently needed to upgrade his water supply because he had purchased a neighbouring farm. He purchased a bigger pump believing that this would solve his problems, which it did not. You have been asked to solve his problem but he has indicated that he doesn t want to buy yet another new pump. The design flow in the mainline to the cowshed is 146 l/min. The pump is 2 m lower in elevation than the cowshed and a residual head of 35 m is required at the cowshed. Assume that the new pump will deliver the design flow at a pressure of 650 kpa and that the cut-out pressure is 700 kpa. Select a mainline pipe to run from the pump to the cowshed; the pipeline is 650 m long. [The answer is DN63 PN9 Rural Redline] Exercise 8 A hydropneumatic system is to be used to deliver 65 litres/minute peak flow on a farm which rises from a river (the water source) to an elevation 20 m above the river flats. The pump is on the river flats. It has been determined that the main pipeline will be 2400 m long and that the cut-out pressure will be 80 psi and the cut-in pressure will be 60 psi; it is assumed that the pump will deliver 65 litres/minute at the cut-in pressure. A residual head of 5 m is required. Select a suitable pipe for the mainline and then select a pump for this application. [The answer is DN63 PN9 Rural Redline.]