Flow Control

1. Flow Control Creativity without a trip Variations on a drip Giving head loss the slip

2. Overview • Why is constant flow desirable? • If you had electricity • Hypochlorinators in Honduras – Hole in a Bucket • Constant head devices • Overflow tanks • Marriot bottle • Floats • Float valve • Orifices and surface tension • Flow Measurement

3. Applications of Constant Flow • POU treatment devices (Point of Use) • UV disinfection • clay pot filters • SSF (slow sand filters) • arsenic removal devices • Reagent addition for community treatment processes • Alum for ____________ • Calcium or sodium hypochlorite for ____________ • Sodium carbonate for _____________ • Could you make a flow control device that increased the dose in proportion to the main flow? coagulation disinfection pH control

4. Why is constant flow desirable for POU treatment devices? • Slow constant treatment can use a smaller reactor than intermittent treatment • It isn’t reasonable to expect to treat on demand in a household • Flow variations are huge (max/average=_____) • System would be idle most of the time • Use a mini clearwell so that a ready supply of treated water is always available 40

5. If you had electricity… • Metering pumps (positive displacement) • Pistons • Gears • Peristaltic • Valves with feedback from flow sensors • So an alternative would be to raise the per capita income and provide electrical service to everyone… • But a simpler solution would be better!

6. orifice Supercritical open channel flow! VERY Flexible hose Constant Head: Floats (variation on hypochlorinator) Head can be varied by changing buoyancy of float Unaffected by downstream conditions!

7. Floating Bowl • Adjust the flow by changing the rocks • Need to make adjustments (INSIDE) the chemical tank • Rocks are submerged in the chemical • Safety issues

8. Float 1.0 m 1.5” PVC overflow tube Transparent flexible tube (0.5”) 1.05 m 1.78 m PVC needle valve 0.5” PVC tube Water in the distribution tank Chemical Metering (Hypochlorinator) What is the simplest representation that captures the fluid mechanics of this system?

9. Hole in a Bucket Vena contracta Orifice

10. Use Control Volume Equation: Conservation of Mass volume h0 Orifice in the PVC valve Integrate to get h as f(t)

11. Finding the chlorine depth as f(t) Separate variables Integrate Solve for height

12. Finding Q as f(t) Find Aor as function of initial target flow rate Set the valve to get desired dose initially

13. Surprise… Q and chlorine dose decrease linearly with time! Linear decrease in flow Relationship between Q0 and Ares? Assume flow at Q0 for time (tdesign) would empty reservoir

14. Effect of tank height above valve Case 1, h0=50 m, hres = 1 m, tdesign=4 days Depth in reservoir Case 1, h0=1 m, hres = 1 m, tdesign=4 days

15. Constant Head: Overflow Tanks Surface tension effects here What controls the flow?

16. A simple constant head device Why is pressure at the top of the filter independent of water level in the Marriot bottle? What is the head loss for this filter? Disadvantage? ___________ Constant Head: Marriot bottle batch system

17. Constant Head: Float Valve Float adjusts opening to maintain relatively constant water level in lower tank (independent of upper tank level) NOT Flow Control! ? Describe sequence of events after filling

18. Flow Control Valve (FCV) • Expensive • Work best with large Q and large head loss flow rate • Limits the ____ ___ through the valve to a specified value, in a specified direction • Calculate the sizes of the openings and the corresponding pressures for the flows of interest

19. Float valve and small tube Raw water reservoir and SSF Flow control device Small diameter tubing Clean water reservoir • This is a winner! • Controls the flow of clean water to reduce clogging • Unaffected by clogging of filter or water level in reservoir

20. Floating Ball Valve Float valve Small diameter tube What are the two components and their roles that make this work?

21. Float valve with IV drip

22. Floating Bowl with Orifice Construction is difficult Not modular (must be built in tank) Slow response to adjustments

23. Driving pressure for sand column HJR Upflow prevents trapped air Holding container (bucket or glass column) Sealing pipe Pong pipe Sand column (keyword: “prevent”)! What is the purpose of the sand column?

24. Flow Control Competition Results from CEE 454 in 2004 • What are the two essential elements of gravity powered flow control? • Constant head (float valve wins!) • Head loss elements • ____________________________________ • ____________________________ • ____________________________ • Can use flexible tube to facilitate adjusting the head Orifice i.e.. small hole or restriction Long small diameter tube Porous media

25. Float valve and small tube (Gravity dosing system) chemical stock tank Flow control device If laminar flow! hl Small diameter tubing Neglecting minor losses

26. Long small tube head loss • Laminar flow • Turbulent Flow Flow proportional to hf

27. 8 d 2.5 d Dh d Orifice flow Solve for h and substitute area of a circle to obtain same form as minor loss equation Kor = 0.63 therefore K=2.5 D

28. Porous Media Head Loss: Kozeny equation Velocity of fluid above the porous media Laminar flow assumption k = Kozeny constant Approximately 5 for most filtration conditions

29. Tube vs. Orifice Minor losses Major losses • Clogging • Adjustability

30. Fs= Fp= Surface Tension Will the droplet drop? Is the force of gravity stronger than surface tension? 2prs

31. 0.080 0.075 Surface tension (N/m) 0.070 0.065 0.060 0.055 0.050 0 20 40 60 80 100 Temperature (C) Surface Tension can prevent flow! Solve for height of water required to form droplet

32. Design constraint for flow control devices: Surface Tension Delineates the boundary between stable and unstable Flow control devices need to be designed to operate to the right of the red line! No droplets form to left of line

33. Hypochorinator Fix http://web.mit.edu/d-lab/honduras.htm What is good? How could you improve this system? What might fail? Safety hazards?

34. Modular Flow Control

35. Identify the Flow Controller Failure Modes • Moving parts • Wear • Corrosion (especially with corrosive chemicals) • Precipitation (e.g. calcium carbonate) • Incompatible materials • Don’t forget sunlight has UV rays! • Clogging • Design errors…

36. Flow Measurement Devices • Orifice in the side of a pipe • Pipe vented through water surface • Jet of water must free fall inside the pipe • Korifice is due to the vena contracta and has a value of approximately 0.6.

37. Free Surface with Orifice limitations • The head loss from making the measurement is wasted (likely on the order of 20 cm) • Ability to include this type of flow measurement depends on availability of excess potential energy • The useable measurement range doesn’t include the range where the orifice is only partially submerged • Thus large diameter orifices aren’t ideal because they limit the measurement range • For reasonably small head loss the flow per orifice can’t be much greater than 100 Lpm • Use multiple orifices for larger flow rates

38. Alternative Flow Measurements • Block the effluent port from a small tank and measure the rate of depth increase • The grit chamber at the head of a water treatment plant could be used for this purpose • But this causes a major flow disturbance for the plant • open channel weirs for very large flow rate measurements • Orifice plates in a pipe (use manometer to measure pressure drop) • If you have access to electricity, then there are a large number of measurement techniques available