A numerical model can help design engineers incorporate the hydropower valve into conventional systems, potentially reducing energy costs.

Transcription

1 DISTRIBUTION SYSTEMS Hydropower valve: a new application foran olddevice A numerical model can help design engineers incorporate the hydropower valve into conventional systems, potentially reducing energy costs. Wesley P. James T The hydropower valve is a combination of a hydraulic ram and a pressure-reducing valve. Its intended use is to redistribute water between pressure zones in a water distribution system. Energy recovered in releasing water to a lower pressure zone can be used to pump water to a higher pressure zone. Using the hydropower valve can reduce energy consumption in the operation of a water distribution system. he hydropower valve is intended to conserve energy in the operation of a water distribution system with several pressure zones. It would act as a pressurereducing valve and a hydraulic ram. Energy recovered as water is released from the supply pressure zone to a lower pressure zone would be used to pump water from the supply pressure zone to an upper pressure zone. Application of the hydropower valve would be limited to water distribution systems in which water is released to a lower pressure zone and pumped to a higher pressure zone. The amount of water that can be pumped to the upper pressure zone is restricted by the amount For executive summary, see page VOLUME 9, ISSUE 7 JOURNAL AWWA

2 FIGURE 1 Schematic of hydropower valve installation High-pressure zone Stroke Spring adjustment adjustment Impulse valve Middle-pressure zone H s H d Q d Discharge line (high) High storage Low-pressure zone Supply storage Qs Supply line Supply air chamber Check valve Drive pipe Hydropower valve High air chamber Impulse valve Low air chamber Q r Discharge line (low) Low storage H s supply head, H d high-discharge line when the hydropower valve is operating, Q s flow rate in the supply line when the hydropower valve is operating, Q r flowrate in the low-discharge line when the hydropower valve is operating of energy recovered in releasing water to the lower pressure zone. Although the concept of the hydropower valve is theoretical and no installations exist, the valve has the potential for saving energy costs in the operation of some water distribution systems. Figure 1 shows a schematic drawing of a hydropower valve in a water distribution system. The power valve is connected to three pressure zones the high or upper zone, the middle or supply zone, and the lower zone. The power valve consists of three air chambers, an impulse valve, a check valve, and a drive pipe. Unsteady flow only occurs in the drive pipe that extends from the supply air chamber to the impulse valve. The high-pressure air chamber is located upstream of the impulse valves, whereas the low-pressure air chamber is located just downstream of the impulse valve. If the three air chambers are sized correctly, the flows in the supply line and two discharge lines are uniform. These lines do not need to run directly from or to storage tanks as shown in Figure 1 but can be tied into the distribution system at appropriate locations. The air chambers are only a few cubic feet in size and would include bladders to prevent loss of air. The impulse valve is used to create unsteady flow in the drive pipe. When the impulse valve slams shut, water-hammer pressure is generated in the drive pipe. The check valve located between the Application of the hydropower valve would be limited to water distribution systems in which water is released to a lower pressure zone and pumped to a higher pressure zone. drive pipe and high-pressure air chamber opens, and water is discharged into the high-pressure air chamber. At the same time, water is flowing from the high-pressure air chamber into the high-pressure JULY 1998 W.P. JAMES 75

3 FIGURE 2 Pressure Head m FIGURE 3 Velocity m/s Pressure in the drive pipe at the impulse valve Velocity in the drive pipe at the impulse valve Pressure Head ft Velocity fps discharge line and from the low-pressure air chamber into the low-pressure discharge line. When the pressure downstream of the impulse valve is greater than the upstream pressure, the impulse valve opens and the check valve closes. During this phase of the cycle, water is released through the impulse valve into the low-pressure discharge line and air chamber. Water continues to flow through the impulse valve until the velocity in the drive pipe reaches the valve closure velocity (V c ). The stroke and spring of the impulse valve can be adjusted so that the valve will close at a specific velocity. When the water flows fast enough through the impulse valve, the drag force of the water on the valve disc causes it to slam shut, creating water-hammer pressure in the drive pipe. This cycle is repeated up to several hundred cycles per minute. Numerical model simulates hydropower valve The method of characteristics was used to develop a numerical simulation model of the hydropower valve. 1 The model was written so that the supply line and the two discharge lines can consist of any number of pipes in series, whereas the drive pipe is a single line. The two independent partial differential equations used to solve for the velocity (V) and pressure head (H) at any time (T) and location in the system are the Euler and continuity equations. d V dt 1 P g d Z f V V (1) s ds 2 D FIGURE 4 Velocity in the supply line, low-pressure discharge line, and high-pressure discharge line a 2 V 1 s d P (2) dt Velocity m/s Supply line Low-pressure discharge line High-pressure discharge line Velocity fps in which a is the wave speed, is the density of the water, P is the pressure, Z is the elevation, s is the distance along the pipe, f is the Darcy Weisbach friction factor, g is acceleration attributable to gravity, and D is the pipe diameter. The two equations are combined using a scaling factor. Selecting a scaling factor so that s V a (3) T s V a (4) T 76 VOLUME 9, ISSUE 7 JOURNAL AWWA

4 TABLE 1 Simulation results* for 15-mm (6-in.) hydropower valve H s Q o Q s Q d Q r Frequency E f m (ft) H d /H s V c /V o N L/s (gpm) L/s (gpm) L/s (gpm) L/s (gpm) Q s /Q o Q d /Q s T c /T p cpm percent (11) 2.9 (46) 3.5 (55) (25).85 6 (479) 11.5 (182) 4.6 (73) 6.9 (19) (81) 1.2 (19) 3.9 (62) (235) 2.6 (41) 12.2 (194) (67).5 (8) 3.7 (59) (182) 1.1 (17) 1.4 (165) (162) 4.6 (73) 5.6 (89) (5).85 4 (677) 18.8 (298) 6.4 (11) 12.4 (197) (162) 2.3 (36) 7.9 (126) (346) 3.7 (59) 18.1 (287) (54).4 (6) 3. (48) (358) 2. (32) 2.6 (326) (22) 6.1 (97) 7.8 (123) (1).85 3 (957) 28.4 (45) 9.4 (149) 19. (31) (171) 2.5 (4) 8.3 (131) (412) 4.7 (74) 21.3 (338) (125) 1.2 (19) 6.7 (16) (479) 3.6 (57) 26.6 (422) *H s supply head (see Figure 1); V c /V o valve closure velocity divided by steady-state drive pipe velocity with impulse valve open; N number of pressure surges in the drive pipe while the impulse valve is closed; Q o steady-state discharge in the supply line, drive pipe, and low-discharge line with the impulse valve fully open; Q s flow rate in the supply line when the hydropower valve is operating; Q d flow rate in the high-discharge line when the hydropower valve is operating; Q r flow rate in the lowdischarge line when the hydropower valve is operating; T c /T p fraction of the cycle when the impulse valve is closed; frequency frequency of valve operation in cycles per minute = 6/Tp; E f efficiency of the hydropower valve (E f = 1 X Q d X H d /(Q s X H s ) TABLE 2 Simulation results* for 3-mm ( 12-in.) hydropower valve H s Q o Q s Q d Q r Frequency E f m (ft) H d /H s V c /V o N L/s (gpm) L/s (gpm) L/s (gpm) L/s (gpm) Q s /Q o Q d /Q s T c /T p cpm percent (293) 7.9 (125) 1.6 (168) (25).85 8 (2,66) 7.5 (1,117) 21.1 (334) 49.4 (783) (478) 6.9 (19) 23.3 (369) (772) 9.7 (154) 39. (618) (185) 1.4 (22) 1.3 (163) (824) 5.7 (9) 46.3 (234) (64) 18.2 (289) 22.2 (352) (5).45 3 (3,76) 52.2 (828) 22. (349) 3.2 (479) (488) 7.4 (117) 23.4 (371) (1,16) 13.4 (212) 5.7 (84) (339) 2.3 (36) 19.1 (33) (1,244) 7.8 (124) 7.6 (1,12) (691) 2.5 (325) 23.1 (366) (1).45 3 (5,33) 8.6 (1,28) 33.1 (525) 47.5 (755) (691) 1. (158) 33.6 (533) (1,97) 22.3 (353) 11.9 (1,617) (426) 4. (63) 22.9 (363) (1,81) 14.8 (235) 99.3 (1,575) *H s supply head (see Figure 1); V c /V o valve closure velocity divided by steady-state drive pipe velocity with impulse valve open; N number of pressure surges in the drive pipe while the impulse valve is closed; Q o steady-state discharge in the supply line, drive pipe, and low-discharge line with the impulse valve fully open; Q s flow rate in the supply line when the hydropower valve is operating; Q d flow rate in the high-discharge line when the hydropower valve is operating; Q r flow rate in the lowdischarge line when the hydropower valve is operating; T c /T p fraction of the cycle when the impulse valve is closed; frequency frequency of valve operation in cycles per minute = 6/Tp; E f efficiency of the hydropower valve (E f = 1 X Q d X H d /(Q s X H s ) results in two linear characteristic equations. Each pipe in the system is divided into computational lengths ( s) so that s T (5) max a + V in which T is the computational time setup and max a+v is maximum absolute value of the sum of the wave speed (a) and the velocity (V). The two characteristic equations, when combined with appropriate boundary conditions, are used to compute the velocity and head throughout the system for each time step. Simulation results Results of computer simulation runs for a 15- mm (6-in.) hydropower valve and a 3-mm (12- in.) hydropower valve are shown in Tables 1 and 2, respectively. Line and pipe lengths were identical for both runs, but diameters were different. For the 15- JULY 1998 W.P. JAMES 77

5 FIGURE 5 Pressure Head m FIGURE Pressure in the drive pipe near the center node Discharge from high-pressure air chamber Pressure Head ft mm (6-in.) valve, the supply line was 91 m (3 ft) in length and 127 mm (5 in.) in diameter, the drive pipe was 46 m (15 ft) in length and 152 mm (6 in.) in diameter, the low-pressure discharge line was 91 m (3 ft) in length and 125 mm (5 in.) in diameter, and the highpressure discharge line was 183 m (6 ft) in length and 1 mm (4 in.) in diameter. For the 3-mm (12-in.) valve, the supply line was 91 m (3 ft) in length and 25 mm (1 in.) in diameter, the low-pressure discharge line was 91 m (3 ft) in length and 25 mm (1 in.) in diameter, and the high-pressure discharge line was 183 m (6 ft) in length and 15 mm (6 in.) in diameter. For the simulation runs listed in Tables 1 and 2, the pump head (H d H s ) ranged from 7.6 to 152 m (25 to 5 ft), and the pump discharge rate (Q d ) ranged from.4 to 33 L/s (6 to 524 gpm). As the valve closure velocity ratio (V c /V o ) increased for a specific installation, the discharge increased, but the efficiency decreased. The efficiency (E f ) is defined as Discharge L/s Discharge gpm d E f H X Q X 1 (6) H Q s From the tables, it can be observed that d s Q d Tc (7) Q T s in which (T c /T p ) is the fraction of the cycle during which the impulse valve was closed. Combining Eqs 6 and 7, p FIGURE 7 Discharge from low-pressure air chamber T p X H s X E f = T c X H d (8) Discharge L/s Discharge gpm The left side of Eq 8 represents the hydropower valve driving impulse, whereas the right side of Eq 8 represents the output impulse. The fraction of the cycle during which the impulse valve was closed is related to the head ratio Tc Hs = E T f X (9) H p A series of plots was prepared to show changes in velocity, pressure, and discharge as a function of time at selected locations in the power valve system. The plots in Figures 2 7 were prepared for the simulation run listed on the second line of Table 1 for a 15- d 78 VOLUME 9, ISSUE 7 JOURNAL AWWA

6 mm (6-in.) power valve with a supply head of 7.6 m (25 ft), a head ratio (H d /H s ) of 2, and a valve closure ratio (V c /V o ) of.85. Figure 2 plots the pressure in the drive pipe at the impulse valve. When the impulse valve closed, the pressure head increased from 39 to 65 m (128 to 218 ft). After six pressure surges in the drive pipe, the pressure dropped to 38 The maximum discharge into the highpressure air chamber occurred when the impulse valve slammed shut. m (125 ft), and the impulse valve opened. Figure 3 is a plot of the velocity in the drive pipe at the impulse valve. The velocity in the drive pipe was zero when the impulse valve opened. The velocity gradually increased to the valve closure velocity (V c ) of 1.47 m/s (4.82 fps). When the impulse valve closed, the velocity decreased in steps. Based on the simple waterhammer equation, the velocity decrease in the first step was approximately V = a g Hd =.24 m/s (1) Theoretically, the velocity decrease during each of the succeeding surges would be twice that given by Eq 1. Figure 4 shows that the velocities in the supply line and two discharge lines were nearly uniform. Figure 5 shows pressure head in the drive pipe near the center node. Pressure in the drive pipe varied widely, primarily when the impulse valve was closed but also when the impulse valve was open. Figures 6 and 7 show the discharge from the highand low-pressure air chambers, respectively. The maximum discharge into the high-pressure air chamber occurred when the impulse valve slammed shut, whereas the maximum discharge into the low-pressure air chamber occurred just before the impulse valve slammed shut. The efficiency of a conventional turbine and generator for power generation ranges from 3 to 8 percent, and the efficiency of a conventional pump and motor for pumping water ranges from 6 to 7 percent. The overall efficiency of the conventional system of power generation, transmission, and motordriven pump is approximately 2 5 percent. A properly designed and operated power valve would be nearly twice as efficient. Use of a hydropower valve in a water distribution system could reduce power costs in three ways: (1) it is more efficient than conventional systems, (2) it uses energy that would normally be lost in a pressurereducing valve, and (3) when used in conjunction with the conventional system, it adds flexibility for scheduling pumps in the conventional system to take advantage of off-peak power rates. The numerical model simulated power valves with diameters of 15 and 3 mm (6 and 12 in.) and pump-discharge rates ranging from.4 to 33 L/s (6 to 524 gpm). The valve closure velocity (V c ) can be adjusted to control the number of pressure surges in the drive pipe when the impulse valve is closed. The larger the number of pressure surges in the drive pipe, the lower the frequency and efficiency. If the impulse valve is adjusted to operate at a high frequency, the velocity in the drive pipe might be too low, and the water-hammer pressure might not be great enough to open the check valve between the drive pipe and the high-pressure air chamber. The frequency of the impulse valve can also be modified by changing the length of the drive pipe. The simulation model based on the method of characteristics can be used to size hydropower valve components and to evaluate their performance at a specific site. References 1. WATTERS,G. Analysis and Control of Unsteady Flow in Pipelines. Butterworths, Ann Arbor Sci., Boston (1984). Summary and conclusion The hydropower valve, a combination of a hydraulic ram and pressure-reducing valve, may be used to redistribute the water between pressure zones in a water distribution system. Energy recovered as water is released to the lower pressure zone is used to pump water to the upper pressure zone. The power valve offers several advantages for pumping water. It requires no additional power source; has a simple design, with only two moving parts; is inexpensive to operate and maintain; is energy-efficient over a range of flows and heads; and does not raise environmental concerns about power generation and transmission. About the author: Wesley P. James is president of H2 WR Inc., POB 197, College Station, TX A member of the American Society of Civil Engineers (ASCE), James has a BS from Montana State University in Bozeman, an MS from Purdue University in Lafayette, Ind., and a PhD from Oregon State University in Corvallis. He has previously published in the American Water Resources Association Bulletin and the ASCE Journal of Water Resources Planning and Management. For 26 years, he taught water resources engineering at Texas A&M University, College Station. JULY 1998 W.P. JAMES 79