Really bad idea?

If you can connect a valve on the output of the pump, this will allow you to control the amount entering your tank.  You can adjust until the you hit the threashold amount.

 

Fawn,
Put a valve on the return from the Mag...never mind, got beat to the answer.
You can also undersize the return to take some of the load away from the valve.  For instance, using a return of 1" would yield approx. 976 gph whereas .75" would give you a return of 757 gph, obviously .5" would restrict further.  Used a guesstimate on your tank of 5' return with 5 rt angles.
Still a lot of pump!
-Eric

 

I think with going to a pump that big, Fawn you'll need to "T" a section in your pump. I don't think you'll be able to close a valve enough with that much flow.  I put a Mag 12 on my 72 and it was too much with 1 HOB overflow with just a single line going into my tank.  In fact, I bought a second HOB and both couldn't handle it without making me feel comfortable.  So I did like what I sketched out. 
About 8 inches above your pump, put in a T and run that anywhere in your sump and put a valve at the end of it also. So you will have one valve each exit. Then play around with the valves until you're comfortable with how much is going into your tank.
Sorry for the crude illustration, but I don't have any of the fancy stuff like microsoft paint even.
X is place to put ball valves.
                    Return
                        l
                        l
                        l
                        X
                        l
                        l
                        l
                        l
                        l
     _X ________l   

 

I would do what reefer madness suggested. This way you are not putting unneeded strain onto the pump

 

I had mine setup like reefer madness was saying you can kind of see it in the picture. The main line is going straight up out of the pump with a ball valve on it and then it tee's back into the sump with another ball valve.

 

Posted By Bela on 06/12/2009 09:23 AM
I would do what reefer madness suggested. This way you are not putting unneeded strain onto the pump
I don't believe that it would put undue strain on the pump - it's an impeller spinning in water.  Mag 18 shuts off @ 21' as the weight/pressure from the water is too much for the pump to overcome, therefore if I ran a 21' vertical lift I would destroy the pump?  And why wouldn't you undersize the return?  The output for the Mag 18 is only 3/4"?  A 3/4" return with some extra elbows would effectively reduce the output without worrying about a valve.  If a 3/4" return caused "unneeded stress" on the pump then it would either destroy itself or dramatically reduce the useful life and Danner would have used something different.
Let's face it, even with a "T" and two valves, this is too much pump for the application unless as a band-aid for a short time (which it sounds like it is) as they'll be several hundred gallons per hour dumping back into the sump.

-Eric

 

Another option you can do it to use some lockline on the return. Split into 2 or 3 returns with some lockline & nozzles on that. It will restrict the flow somewhat & help you increase the flow pattern within the tank eliminating dead spots.

I would then put a valve on the return & then adjust it to where you need.

I dont think you need the t return either

 

I am only going from what I had read about putting more strain on the pump than is necessary. Perhaps I don't really understand how pumps work but, to me, it stands to reason that the more work the pump is doing, generally the shorter the expected lifespan of that pump can be expected to be? Again, I am new to this so maybe I am incorrect in these assumptions?