Flow Controller Design and Construction (Spring 2008)

Design Objective

The purpose of the Flow Controller is to deliver a calibrated and easily varied dose of clay and alum into the plant.

Theoretical Design

The Flow Controller for the demonstration plant consists of bottles of alum solution and clay suspension attached to an upright pipe. A certain amount of tubing directs flow from each bottle into an inflow column before it flows into the first channel of the flocculator. There are multiple holes in the inflow column, and both the alum and clay tubes can be inserted into holes at varying heights to achieve different flow rates. In order to determine tube lengths and where they should be plugged into the mix inflow column for a certain flow rate, the FCM design tool was used. The theory behind the design tool is located on the Flow controller theory page, including formulas and application. The tool utilizes the Hagen-Poiseuille equation, explained on the theory page, for a relationship between head loss and tube length. We determined that with the water level in the bottles even with the top of the mix column and a head loss distance of about 5 cm between the water level and the outflow at the end of the pipe into the mix column, the design length for a tube with an 0.17" inner diameter is 73.5 cm to achieve a flow rate of 100 mL/min for the clay. For the bottle and outflow levels and a tube with an inner diameter of 0.063", the design length is about 95 cm to achieve a flow rate of 5 mL/min.

Construction

The bottles of the FCM are attached to a vertical piece of sturdy pipe. Larger stock bottles of both the clay and alum are at the top, and they flow into smaller bottles with float valves to keep a constant head and drive constant flow. These bottles are then connected to the inflow column by the lengths of tube given above for specific head losses to achieve certain flow rates. An arbitrary length of tubing connects the bottom of the inflow column to the first flocculator channel.

Differences from Previous Demo Plant FCM

• In the previous plant design, the rapid mix column attached directly to the flocculator via an elbow pipe fitting that screwed into the bottom of the first channel of the flocculator. To avoid the leaking that eventually occurred at the connection with wear on that plant, we decided that for the current design, the inflow column would be attached to the FCM column with the alum and clay bottles, and water would flow into the top of the first channel of the flocculator from a tube connected to the bottom of the inflow column.
• For ease of setup, we also more permanently connected the bottles to the column, instead of intending them to be put on and removed each time the plant was used.
• To make packing the plant for transport more convenient, the PVC pipe of the column was cut into three pieces that could be taken apart and would more easily fit into a box or suitcase for packing.
• In order to allow for greater visibility and understanding of the float valves in the intermediate clay and alum stock bottles, clear plastic peanut butter jars were used instead of opaque plastic labaratory bottles.

Materials

The main column of the FCM is three pieces of 2" white PVC pipe connected with 4 cm PVC connectors. The bottom segment is 18 cm long, the middle segment is 41 cm, and the top segment is 25 cm. The stock bottles are a 4 L bottle for the raw water suspension and a 1 L bottle for the alum solution. Hose ring clamps on the bottles are also clamped around thin stainless steel strips clamped to the PVC, wchich attaches the bottles to the column securely. The level control bottles are clear plastic peanut butter jars connected to the middle segment of the PVC pipe in the same manner. (All bottle and jar lids are kept loose or off while the plant is running.) These jars have two holes in them, the top one of which is the inflow, with a float valve to control the water inflow and keep the level constant, and the bottom of which is the effluent. For the clay jar, the effluent is connected to the inflow column for the flocculator via 0.17" i.d. transparent plastic tubing. The alum jar is connected with 0.063" i.d. transparent plastic tubing. The inflow column is a 25 cm long 1" clear PVC pipe plugged on the bottom with a plastic cap. There are two sets of holes drilled on the front half of the top section of the pipe, each to fit one of the tubes from the clay and alum jars. There are 5 of the holes for the larger tube, and they are spaced with 2 cm between the centers of the holes. There are 8 of the smaller holes, and they are spaced with 1 cm between the centers of the holes. The cap on the bottom of this column has a hole drilled into in it. A plastic tube connection is glued into the hole, into which a piece of 0.17" i.d. clear plastic tube is connected. This tube is inserted into the first channel of the flocculator.

Figure 1. Demo plant flow control module

Post Construction Modifications

Once the FCM was set up and the bottles were attached to the PVC and connected to each other with the lengths of tube calculated by the FCM design tool, we measured the resulting flow rates manually to determine whether the tube lengths provided appropriate flow rates. (The intended flow rates were 100 mL/min for the clay and 5 mL/min for the alum when the tubes were both plugged into the middle hole on the inflow column.) We found that the clay flow rate was about 120 mL/min, so we increased the tube length and continued to measure the flow rate until it was as close as possible to 100 mL/min. The alum flow rate with the design length of tubing was a little bit slow, so the tube was cut to 75 cm for a flow rate of . The actual tube lengths for each of the 3 existing flow controllers are as follows:

These results were interesting because they implied that in the case of the larger diameter tube, the head loss equation over-estimated the head loss in the tube, and in the case of the smaller diameter tube, the equation under-estimated the head loss in the tube.  We also observed that it significantly impacted (slowed) the flow rates through the tubes to add bends by looping them around, which was done in order to avoid the inconvenience of very long tube lengths and save space.  The added friction of the bends in the looped tubes caused the flow rates to slow, so that it was necessary for them to be adjusted accordingly in order to achieve the desired flow rates. 

The modifications necessary after the construction of the flow controllers unfortunately make it difficult to have a concrete plan for their design in the future. We were not able to observe a distinct enough pattern in the inconsistencies to allow us to create a method for making standard adjustments to the design values without all of the manual flow measuring and adjustment that took place.