The bubble extractor will purge air from a line, tank or filter cartridge so your pump will not lose its prime.
Equipment like air conditioning cooling running on raw water will often pick up bubbles entrained under the hull that collect in or are sucked into raw water intakes. The next time you try to run the A/C it overheats because the pump has lost its prime. This is particularly troublesome in the case of watermakers where any air will prevent the high pressure pump working. The bubble extractor is located at a high point in the lines or on top of a filter so any air will be removed. It vents air but blocks liquid. To do this it needs a positive pressure to atmospheric so the high point from which the bubbles are extracted needs to be below the water line or on the outlet side of a pump so the liquid level can reach the extractor and remove all bubbles.
Typically this bubble extractor will be mounted on the top of a strainer or filter where air will collect. It can also be positioned at a high point in the plumbing using a TEE fitting. Since it vents to air, there has to be a positive pressure so if it is on the suction side of a pump, the pump will need to be turned off to allow it to vent. However by providing a vertical tube up to the extractor that will fill to sea level, bubbles can collect in this tube for some time before venting is necessary.
The extractor can be mounted any distance above the TEE, above or below the water line. Increasing the hight above sea level does not increase the collection capacity however if mounted above the water line it will operate dry and still prevent liquid venting if positive pressure is applied.