Pump Seal Fail Detection Methods
The first type is a "float" type sensor, where a sealed float with an internal switch is mounted inside the seal chamber of the pump. As liquid enters the seal fail chamber the float will rise and active the switch. The switch is typically a normally closed type, and the contact opens as the float rises.
The advantage of having a normally closed switch is that a bad connection or broken wire will be detected by the controller as a seal fail condition, prompting maintenance to determine if there has been an actual seal fail or if there is a connection problem to the float switch.
Disadvantages of this type of sensing is that although we can be alerted to a broken or loose wire, maintenance personnel will not be sure of the cause of the fault until further investigation. In addition, the float can only be used in a "dry" seal fail chamber, as it would tend to float if the seal fail chamber were lubricant filled. Typically this appears to be a preferred method of seal failure monitoring, especially on larger pumps.
The second type of detection is accomplished by using one or two conductive probes inserted into the seal chamber through the pump housing. The seal chamber is typically filled with oil. The probes are secured in the pump housing and they are electrically insulated from the pump housing. The resistance, either between one probe and the pump housing or between two probes, is monitored. Since liquids such as oil have a known conductivity, a sensor can detect a change in resistance and alarm a lowered resistance caused by water entering the seal cavity and diluting the oil, thus changing the resistance.
Disadvantages of the resistance type probes are that a broken wire, or incorrect connections can be virtually undetected and render the seal fail circuit useless. We can compare these sensors to a normally open type switch, which provides no way to electrically verify that the circuit is working.
Several companies sell devices that are called a Seal Failure Detector or a liquid level control that can be used with the probes or floats to indicate a seal failure. Their operation is based on the amount of current flow between two points. The voltage presented at the probe(s) or float is typically from 10 to 24 VDC with a current of up to 20 to 30 mA maximum. These circuits monitor the amount of current flow and if the current increases beyond the set-point level of the control, an internal relay will change state to indicate that a seal failure has occurred.
The current between the probe to pump housing ground (or between two probes in the pump housing of a two probe type system) will increase as the resistance between the points decreases. Specifically, more current will flow due to less resistance in the circuit. In the case of an oil filled seal chamber, the resistance of the oil is high enough that current flow is limited until water is mixed with the oil, changing the conductive properties.
When using a float type seal fail sensor, the normally closed switch will usually have the seal fail controller energized, thus the relay will change state (drop out) if the float switch opens. This has the advantage of also insuring that both the control and the float are operating properly.
Quality Manufacturing Company