I need to replace a 5210C Hypro Big Twin piston pump.  The metal crankshaft parts inside have too much slack in them now.  They usually have pressure pulses that need to be damped, but now even with a liquid-filled gauge and the surge dampener in the system the needle jumps around so badly that it's almost impossible to tell what pressure it's running at.  Looks like the freeplay is causing water hammers in the system.  Anyway, this pump style and capacity works well with my sprayer and the gas-powered pump that runs it. 

 Something like the Hypro 6500C-R or the longer-lasting 6500XL-R (both the reverse rotation clockwise models because of the gas engine) would be an easy replacement to get away from the Big Twin pulsing, but I don't like the wear rates of the 6500C rollers.  Anybody have any experience with the 6500XL?  Are they worth triple the price of the 6500C and almost the price of the Hypro Big Twin?

I've thought of switching to either a centrifugal or diaphragm pump, but the flow rates of the centrifugals I've found are way too high for my sprayer (4.5 gals/min is what usually comes out of the spray tips, plumbing is mostly 1/2", and tank size is 100 gallons).  Diaphragms are nice but pretty spendy.  Also, the intake/output ports are usually up high on the pump, and as you can see from my trailer design below, a lower intake port would be nice to keep it below the bottom of the spray tank.  Any thoughts on a physically tough (cast is nice, plastic output hose barbs scare me a bit) alternative to a 6500XL or Big Twin piston design?

http://www.rittenhouse.ca/asp/product.asp?PG=188

http://www.deindustrial.com/HYPRO-6500XL-R-PUMP-ASSY-p/6500xl-r.htm

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 Most of what goes through that pump is Velpar DF, a dry flowable but not what I consider a powder.  I'm assuming that it wouldn't be terribly hard on rollers, but still want to stay away from something that requires rebuilding often.  Some of the chemicals foam pretty badly (Spartan DF with Agral 90 does because the Agral is almost like adding foam marker soap to the tank), so I was hoping to stay away from a high-flow pump which would make the foaming worse.  Never thought of throttling a centrifugal pump waaaay back with a ball valve on the output side or something similar because I figured that would cause lots of foaming and/or heating too.

 I don't remember seeing any clockwise rotation centrifugal pumps though.  I'll have to look again.  It'd be quite a bit more expensive and time consuming to mount a normal bare CCW one beside the gas engine and have two shafts, 4 pillow blocks, and two pulleys to get the rotation right.  Hhhhmmm, now that I think of it, I did see some gas-powered centrifugal pumps with a gear-reducer on the pump.  Maybe there's a CW rotation version somewhere that'd work with the gear reducer on the engine?  And this might be an idea- a high-flow gear-reduced centrifugal pump mounted on a gear-reduced engine might drop the flow down to near the level I want without a ball valve on the output.  High pressure isn't needed (usually 15-30psi at the tips).

 Anybody trying to talk me into the 6500XL-R roller pump because they're cheaper, simpler, and saying that they last a long time between rebuilds or manual adjustments because of pressure dropping??

 Pressure washers?  Interesting.  I know it'll develop enough pressure to blow a pretty good hose (specs say it'll do 400psi).  It came as a standard pump on JB&D 3pt sprayers around these parts.  It was a pretty popular model around here. We've bought three used JB&Ds like in the picture and they all came with that pump.

 Just curious by how many running hours you mean when you say a year?  That one gets probably a minimum of 200 running hours/year, and the max might be 300-350 or so.