Specialty Item Flow Indicators
Flow indicators tell an operator if fluid is moving through a pipe. Flow indicators alert the operator to a flow problem before damage to equipment occurs. They allow the operator to see what is flowing through the pipe. In addition, flow indicators alert the operator if the fluid becomes solid at certain temperatures. This is very useful in refrigerant-oil sight glasses to determine if the oil is contaminated with any water moisture.
Two types of flow indicators operate using either spinners or hinged flappers. The spinner type flow indicator uses an offset spinner or paddle wheel which is enclosed in a body that has a clear-side-view port to allow one to see the spinner. The spinner is offset to allow it to rotate and indicate flow. This device has a low pressure drop, so it will not affect the system. A disadvantage of the spinner occurs when the speed of the flow causes the spinner to revolve so fast that flow direction cannot be determined. The spinner type flow indicator may be installed in a pipe in a position which is convenient for the operator to view the device.
The hinged-flapper type flow indicator is hinged on one side of the pipe. When there is flow in the pipe, the flapper swings out of the way as can be seen in the clear-view port. Unlike the spinner type, this device allows one to see the direction of the flow. This may be important for various reasons including prevention of backflow or accidental suction in a process line. The hinged-flapper type flow indicator must have gravity acting on it to keep it in the flow, thus the device must be installed in a horizontal pipe with the flapper hinge on top.
For a given system, it may be a requirement to monitor the condition or color of the fluid. To do this properly, the flow indicator must have a window on each side of the device to allow one to see through the fluid. Sometimes the only priority is to see the fluid color which can be accomplished with a simple section of clear pipe.