You never need a vented loop on a pump inlet.  Vented loops let air in to break a siphon on a pump OUTLET so that when the pump stops the exit plumbing will drain and not siphon waterin reverse through the pump.  In particular to prevent water siphoning back into your bilge if the discharge outlet is below water line. The commercially available vented loop valves are badly designed - there is no need for it ever to have liquid contact.  See this article, we used 4 of these valves for 18 years with zero maintenance and zero failures. http://www.yandina.com/hints.htm#Siphon. They sell the parts for $8.95 or there is a list of parts to purchase your own. What you MAY needon a pump INLET is a bubble extractor.  (See following article from the above.)  Air picked up by the pump inlet can prevent a centrifugal pump priming or lose prime while running. When underway raw water cooling pumps or a watermaker can pick up bubbles entrained through the inlet that get trapped at a high point such as a straineror filteror even the top of the centrifugal pump housing.  These places typically have a manual valve you can turn on to let the air out.  The bubble extractor does this automatically every time the pump is turned off.