Determining the maximum inlet shut-off pressure

Experimental Methods

A peristaltic pump was connected to the inlet of the flow controller, along with a pressure sensor. The outlet tube of the flow controller was plugged. The peristaltic pump was then used to pump water into the flow controller. The pressure of the water in the inlet tube, or the pressure the float valve could resist, was then measured using process controller. The maximum inlet pressure was determined using three orientations, with the float straight down giving the lever arm a 90 degree angle, partially down for a 45 degree angle, and straight out at zero degrees. This was to determine which orientation provided the greatest resistance. The pressure was also measured as a function of the height of the water level inside the flow controller. This relationship was determined for each of the different orientations by marking the water level in the flow controller at random intervals while simultaneously inserting a text flag in the Process Controller data. The text comments allowed us to show what the exact pressure was for the corresponding flow controller water level.

This data was analyzed in Excel to determine maximum shut-off pressures and change in flow controller water levels. The data for each float angle orientation was analyzed separately to determine an ideal angle to use in the AguaClara plants. The data was also manipulated to see if the water level in the flow controller would vary greatly enough to significantly impair flow controller function as a stock tank would drain in a plant.

Experimental Results

The maximum inlet pressure that the float valve could resist was determined. Float orientations of 90, 45 and zero degrees all resisted a pressure of at least 8m. A float orientation of zero degrees resisted pressures of at least 11m. All of the float orientations have a maximum inlet pressure well above the 2m needed for the AguaClara plants.

This data was further analyzed to also determine the amount of variation in flow controller level with increasing pressure build up on the float valve.

The level in the module changed by many centimeters during the experiment, but this was over the entire range of inlet pressure build-up. For the range that is pertinent for designing AguaClara plants, up to at most 2m, the level changed in the flow controller much less. The table below shows the total change in flow controller water level at both the highest back pressure reached, and at approximately 2m of pressure.

Positioning the float straight out from the valve caused the most variation in water level, while positioning it at an angle of 45 degrees or 90 degrees had about the same effect. Currently the floats are positioned at an angle somewhere between 45 degrees and 90 degrees to best fit inside a small flow controller. Assuming this positioning is kept, the variation in water level is large enough to influence the outflow by effectively causing error in the expected head loss in the system of up to 0.4cm. While this does create a discrepancy between assumed and actual head loss in the system, it is much smaller than the uncertainty caused by a draining stock tank.