Orifice clogging experiment

Figure 1: Experimental Setup - Orifice Clogging

Overview

A linear dosing meter is currently being utilized in Honduras to control the flow of alum, based on varying plant flow rates. In this system, the alum flows from the stock tank to the constant head tank through a small 0.23 cm orifice. This orifice is becoming clogged with an unknown substance, preventing the flow of alum into the entrance tank and consequently reducing the effectiveness of flocculation. The constant head tank orifice has to be cleaned multiple times per day by the operator. A more ideal system would clog at most once per day. The objective of this experiment is to determine the cause of the clogging in Honduras (precipitant or foreign material) as well as estimate the frequency of clogging in the new non-linear dosing system. If the new orifice (0.1 cm diameter) clogs frequently with lab grade alum, the system will need to be redesigned to incorporate a larger orifice. If the source of the clogging is determined to be foreign material (little or no clogging occurs with lab-grade alum) a strainer will be included in the overall system design to remove the foreign material.

Experimental Setup

The experimental setup accurately mimics the conditions in the non-linear dose controller without incorporating the additional hassles of a large stock tank, constant head tank and entrance tank. The water circulates through a peristaltic pump to a small one liter reservoir intended to steady the pulsing input from the pump. The reservoir is connected to a tee, which connects a pressure sensor to the system just before the orifice. Water then drips through the smallest orifice, 0.1 cm diameter, into another one liter reservoir. The reservoir is connected to the pump, creating a closed system. The pump will be set to pump water at the same rate as the water flows through the orifice to maintain steady-state operation. The experiment will first be run with deionized water as a control. Then it will be run with the maximum concentration of alum to be used in a plant, 125 g/L. As the experiment runs, Process Controller will record the pressure directly upstream of the orifice. If the orifice clogs, the pressure will increase significantly and Process Controller will turn off the pump to prevent any damage.

Results

The first experiment using an alum solution ran continuously for four days. From the data, a 1 cm change in pressure was observed about 2.4 hours after the experiment began. Other fluctuations in pressure were observed throughout the remaining time, but they were deemed negligible. The blip seem in day four is a result of flow rate measurements made by the team.

Conclusions

For the first run, it appears that the orifice clogged after 2.4 hours. The clog was pushed through the orifice by a one cm increase in pressure. No other clogs were observed.