Going thru old links I still had saved in my computer and ran across this one. I hope this will answer a lot of question for everyone in the future. Thanks Doc for having this Information available for all of Us.
(now there is a way to cheat on these requirements to some degree by using a center tap at the end of a length of hose to build the psi up as you would be building a gravity fed nozzle) {origin
al link and posting as follows http://goldprospectingforum.com/viewtopic.php?f=28&t=1296}
ANSWER:
First... there is an online owner's manual for most of our units on the website. You should make sure you go there and watch the videos too.
There are two types of pumps.
Semi trash pumps and high pressure pumps. (HP often referred to as clear water.)
Semi trash pumps will move large amounts of water with not too much PSI. Most 2" semi trash pumps are rated about 35 - 42 psi. Rocks, sand and gravel, up to about 1/4" - 1/2" can pass through these based on the pump.
High pressure pumps have different "water moving" parts with tighter tolerances. (for lack of a better word) This gives them higher pressure. On a 2" it might range from 70 - 95 PSI. Often the GPH is about the same.
If you are dredging.... you always want to over power you unit with psi. This will allow you to run your engine less than full throttle and still have REALLY good suction. We have run our mini (1.5" dredge) with the same high pressure pump as our 4" dredges. We simply let the motor run just above idle. Running a 2" then move the throttle up more. Running a 3" more... etc. GPH is not the key issue with dredges. The gph is controlled both by the pressure and the size of intake on the dredge nozzle.
The Northern Tool Northstar 2" high pressure pump is a GREAT pump to use for dredging.
Just remember... MAKE A FINE SCREEN FOR THE INTAKE HOSE.
If you let gravels and large sands into the pump you will have to open up the impeller and clean them out.
The pump will stop working. Put a fine screen that uses 1/8" mesh around the pump.
Now the questions is........ can I use a high pressure pump on my unit as a highbanker?
Sometimes.... It is very hard to control the GPH on a high pressure pump. (Because it is is high pressure.)
If you do.... reduce the flow at the PUMP with a ball valve or gate valve. If you don't, and you are running lay flat hose to your highbanker, you could blow out your hose.
PSI needed for GOOD dredging?
These are "ball parks" for good performance.
1.5" ...... 30 - 40 psi.
2" ......... 40 - 50 psi.
2.5" ....... 50 - 70 psi
3" ......... 50 - 80 psi
4" .......... 80+ psi
5".......... We like to run dual 2" pumps that are rated at 90 PSI.
Note: We only have to run them at about 50% rpm for great suction.
This is on a DUAL hose jet log.
6".......... Same as above but pumps run at 100% RPM.
Or run the 3" high pressure pumps necked down to 2".
When you do this, the RPM can be 50% due to the increase
in GPH and PSI.
Here is a link for the 2" high pressure pump we like mentioned above.
Remember.... put a fine screening bag around the intake it comes with.
http://www.northerntool.com/shop/tools/product_200418131_200418131
Also here is a head lift to PSI conversion tool
http://www.convertunits.com/from/psi/to/foot+of+head
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