Pet Waterer: Constant Head Device
Design Documentation Report
Research Project for CEE 454: Sustainable Small-Scale Water Supplies
FluiDivas: Lindsey Ehinger, Laura Mar, Stephanie Wedekind, Kim Wilson
December 8, 2004
Table of Contents
Abstract……………………………………………………………………………….….3
1 Project Goals and Context……………………………………………….……...3
1.1 Literature Review……………………………………………………….…….…4
2 Design Development Process………………………………………….………...4
2.1 Constant Head Device………………………………………………….………..5
2.2 Pet Tank Modifications………………………………………………….……....6
2.2.1 Modification Flaws……………………………………………………………….8
2.3 Constant Flow…………………………………………………………………...10
2.4 Experimental Procedures……………………………………………………...11
2.5 Test Results.……………………………………………………………………..11
2.6 Problems……………………………………………………………………..…..14
2.7 Operation…………………………………………………………………….….14
2.7.1 Start-up Procedures.…………………………………………………………....14
2.7.2 Refilling the Pet Tank…………………………………………………………..15
2.7.3 Refilling the Storage Tank …………………………………………………….15
2.8 Maintenance…………………………………………………………………….15
2.8.1 Unclogging the IV valve………………………………………………………..15
2.8.2 Cleaning the System……………………………………………………………15
2.8.3 Material Degradation…………………………………………………………..16
2.9 Expenses…………………………………………………………………….…...16
2.10 Design Benefits…………………………………………………………….……16
2.10.1 Alternative Applications………………………………………………….…….17
2.11 Design Flaws…………………………………………………………………….17
3 Recommendations for Modifications………………………………………….18
3.1 Recommendations for Further Research……………………………………..18
4. Conclusion………………………………………………………………………18
Works Cited…………………………………………………………………………….19
Abstract
The Pet Waterer is a device that we designed and built to maintain a constant fluid flow for a variety of water treatment systems. Since electricity in target regions is often not reliable, our system is not dependent on an electrical power source. We also realized that in order for this system to be feasible, it must be inexpensive and easy to make and operate. The cost of our prototype is feasible considering most of the materials, aside from the actual waterer and the IV drip, are readily available resources in most communities and thus cost very little. These include flexible tubing, linen or wire mesh screen, and storage buckets. Our device relies on simple fluid mechanics principles to create a constant head of water. This constant head, when combined with an easy to adjust IV drip roller, maintains a constant flow that can be adjusted. Because clogging of constant head devices is often a large issue, we tested our system with clean and dirty water over several hours to determine how often cleaning would be necessary. The results indicate that cleaning and flow reduction over time would not be a large hindrance in our design because it is so simple to clean. However, the frequency of cleaning to maintain a constant flow of dirty water would need to be every one to two hours. Since our actual as-built pet waterer system has some flaws, we recommend a simpler design for testing and actual implementation in the developing world. This design will be built before the project presentations.
1 Project Goals and Context
The availability and accessibility of clean drinking water remains a severe problem for much of the developing world. The lack of clean water causes numerous health and welfare issues including severe illness and death. The death toll is especially high among young children. Environmental engineers are working to resolve this issue by creating individual point of use (POU) systems and small-scale community water treatment systems to increase the number of people with access to clean water. One major issue being addressed is the irregular and intermittent flow often experienced by these systems. Proper flow control is imperative for a highly effective water treatment system.
Flow control is a critical parameter for small-scale water systems. Valves are used commonly to control large flows, but they are not accurate for the flows required to deliver water constantly to a small community or household. Electronic devices such as peristaltic pumps are a reliable way to deliver low flow rates; however, in most developing countries where the water needs are on a smaller scale, electricity and the money to buy expensive equipment is not available. Because of this, flow control for small systems is a challenging and critical obstacle to delivering clean water to poor rural communities in the developing world.
Flow control can be used in various places in the water treatment train. In a community system that uses a large storage tank, a flow control device can be used to accurately monitor and maintain the chlorine dose. Further, a flow control device that can be adjusted depending on the influent water characteristics would allow the operator of the system to maintain a constant chlorine residual. Similarly, if a community system is using coagulation and flocculation to remove particulate matter, a flow control device must be used to add the coagulant (often aluminum sulfate, i.e. alum) into the system. This device should also allow for a variety of flow rates so that the alum concentration can increase with increased turbidity of the influent water. On a household level, many point of use devices would be more efficient if water was delivered at a constant rate rather than receiving one days worth of water at one time. Thus, a constant flow device would deliver a small flow rate over the course of the day to a point of use (POU) system such as a slow sand filter.
Our system is designed to function independent of the application. It can be used to administer a fluid into a flocculation tank, a storage tank or a POU system. However, we are focusing on the application into a POU system by testing our device with dirty water. We are testing how well the device maintains a constant flow rate with clean and dirty water. For alum and chlorine dosing further tests would be needed to determine the duration before clogging in order to recommend a maintenance schedule. Additionally, materials chosen would have to be compatible with chemicals so that corrosion and material degradation does not negatively impact the life and success of the system.
1.1 Literature Review
The basis for the pet waterer is the chicken waterer and are both controlled by atmospheric pressure (Northrop, 2004). The chicken waterer can be easily constructed using an old tin can and a pie dish. By drilling a hole, ½ in. to ¼ in., near the rim of the tin can, placing the pie tin on top of the rim, and quickly inverting the whole thing, a simple chicken waterer can be made (Kim, 2004). The Marriot bottle works in a somewhat similar way using the difference in atmospheric pressure and the pressure inside the bottle to generate a constant head. The resulting constant flow out of the Marriot bottle is extremely desirable and has been used in many experiments (Abu, 2004).
2. Design Development Process
In our project proposal we had decided to look into float valves, IV drip valves, coffee pots, and pet waterers. In evaluating these different methods we decided against some of these options. Although using a 20 L storage tank with float valve and orifice is a viable option, it was disregarded. While in use, it had clogging problems, and we thought we could come up with a better solution. The coffee pot drip was another method that was discussed and researched. However, we found that the coffee pot produces a drip by boiling the water and producing steam. Since we are trying to maintain a constant flow, we would need to continuously boil the water. This seems impractical for a long-term device because of the amount of raw materials and energy needed; thus the idea of modeling our system after a coffee pot was discarded.
The unique design of the pet waterer system was further researched, and since we were unaware of how these waterers worked, we purchased one at the local pet store, specifically the Petzazz Pet Self Waterer. This model holds up to 3 liters of water and costs eight American dollars (Figure 1). The original purpose of the waterer is to allow owners to leave their pets at home with a full water trough. This system is similar to the Marriot bottle because it makes use of pressure differences to maintain a constant head of water (Weber-Shirk,2004). The main difference is, in a Marriot bottle, water and air travel through separate tubing, and in the pet waterer, water and air both travel through the same hole. The pet waterer maintains a constant level of water in the bowl, and will be used to deliver a constant head for our system.
Figure 1. Petzazz Pet Self-Waterer (Buy Pet Supply, 2004).
The pet waterer purchased is made of two parts: a storage tank (referred to as pet tank) and a trough where the storage tank sits. The lid atop the pet tank shown in the figure is not removable; it is simply for picking up the tank. The curvature on the lid helps prevent deformation due to low pressures inside the pet tank. The pet tank has a hole with a diameter of 2.5 cm on the bottom. This hole is used to fill up the tank and will be referred to as the fill hole. There is a second hole (feed hole) that is on the side of the pet tank that has a diameter of 0.5 cm. The water from the pet tank spills through the feed hole into the trough. Besides these two openings, the pet tank is airtight. In order to fill the system, you must pick up the pet tank and add water through the fill hole. Once filled, the pet tank is turned upside down and placed beside the trough.
2.1 Constant Head Device
After examining and testing the system and discussing the mechanics with three hydrology graduate students, we were able to determine what forces keep the trough at a constant water level. We realized that air above the water in the pet tank is under a partial vacuum when the feed hole is submerged. The pressure is atmospheric at the height of the feed hole, thus the height of the feed hole determines the height of the water in the trough. When the trough is not full, air is let into the system via the feed hole and the added air pressure above the stored water pushes water into the trough until pressures are equalized. More air is let into the system and the process continues until the feed hole is submerged. At this point no air can enter the pet tank and the system remains stagnant. Once the water level in the trough drops, more air is let into the tank, and more water is fed into the trough. If air is allowed into the pet tank (perhaps because of a leak) the system will fail. If this occurs, the pet tank and the trough will both be at atmospheric pressure and the water head in the pet tank will cause flow from the pet tank to the trough. The trough will overflow creating a mess and wasting water.
The critical piece of this system is the airtight seal. In a small system such as the pet waterer, this is easy because flipping the tank upside down and placing it in the trough does not require a lot of effort. However, a twenty liter storage tank needed for our system would be inconvenient to refill and maneuver. Additionally, every time the tank was refilled productive time of our constant flow device would be lost, and it would disrupt the constant head as any water left in the tank would flow into the trough. Therefore, we needed to devise a system that will allow the pet tank to be refilled without detaching it from the trough.
2.2 Pet Tank Modifications
In evaluating the different ways to refill the tank, we discussed running a tube from a storage tank into the top of the pet tank. However, if the tube leads to another tank that is open to atmospheric pressure or if the tube itself is open to atmospheric pressure, the pressure in the pet tank will become atmospheric. This will result in all the water in the pet tank flowing into the trough, disrupting the constant head, and causing water to overflow.
Thus, in order to account for these problems we will drill two additional holes to the pet tank (Figure 2 and Figure 3). One hole will be the air hole and it will have an airtight plug (plug #1). During operation this plug will always be closed. The other hole will have a pipe and an on/off valve leading from the larger 20 L storage tank. The storage tank will be elevated above the pet tank so that when filling, gravitational forces will drive the water into the pet tank. We used bulkhead fittings with O-ring seals to create a durable airtight seals. The tubing that connects the storage and the pet tank is 3/8” tubing. It is connected with bulkhead fittings and an on/off valve. The air hole uses a standard ¼” bulkhead fitting with plug.
The original holes drilled resulted in various flaws. When the feed hole was drilled in the bottom of the pet tank and fitted with a bulk head fitting an air tight plug (plug #2), the surface tension effects prevented air and water from traveling in the same tube. We attempted using a 3/8” rather than a ¼” bulkhead fitting, but the problem was not solved. We then removed the bulkhead fitting so that surface tension would no longer be a problem. When we drilled a hole for plug #2 we made the new opening too high without the fitting. Thus when the fitting was removed it would cause overflow from the reservoir. Filling half of the hole with putty so that the opening was at the original height solved this problem. This hole must still be fully plugged when refilling the waterer. Although not recommended, we have been using putty to plug the hole during testing.