Low flow pump options/recomendations?

[Fifty7]

Hi all!

Long time reader, first tim eposter here. I currently working on a project at our combined cycle powerplant which includes looking for other pump alternatives to our current aqua ammonia forwarding system. We are currently using and older Milton Roy "G" series positive displacement pulsation pumps to pump 19%-29% aqua ammonia @ 100psi controlled using a PRV. Flow is controlled downstream with a fisher flow control valve. These Milton Roy pumps, even though they are robust have been giving us issues for years due to there "pulsating" nature. Even with variuous pulsation dampner installations, etc.. the pulsations are too great and we can fine tune out ammonia for emissions control due to this.....not to mention the pulsations are hard on instumentation, gauges etc. It's also a cost issue....the miltons roys are 3 times the cost of other centrifugal pumps not to mention pulsation dampner failures/maintenance...

I'm currently looking into replacing these Milton Roys with a Centrifugal or Regenerative Turbine style pump, preferably sealless for a smooth constant pressure. The problem is, I'm having trouble finding a pump that will pump at really low flows. My system runs at .22 - .77 gpm. I have found a regenerative pump by MTH but the manufacturer recommended it flow no less than .5 gpm ...........that being said I will have to install a minimum recirc of some sort. Any suggestions? Should I install a good centrifugal or regenerative turbine and just recirc? What about a positive displacement gear style? Any help will be greatly appreciated.

 

[gr2vessels]

With centrifugal pumps you'll have NPSH problems, you might need to install the pump in the cellar. A PD pump will always be easier to use, but you need to select a very slow pump, perhaps a progressive cavity pump. Talk to your supplier, he should be able to help, provided you confess to him all your problems...except the private ones...
Cheers,
gr2vessels

 

[Fifty7]

Hi! Thanks for the reply.

So you think the NPSH from the tank head alone wont be enough? I briefly looked into some progressive cavity pumps and those my be a good option......a PD pump without pulsation : )

 

[ccfowler]

I had to deal with a somewhat similar problem many years ago (similar flow range but different, corrosive, low viscosity fluid), and I recall finding an excellent potential candidate pump. It was a chemically compatible spur gear pump. If I recall correctly, the brand name was "ECO." In your situation, a suitable adjustable speed drive may provide sufficient control to allow removal of the control valve. The flow fluctuations associated with this type of pump will almost certainly be too small to be of any concern.

(In my situation, I eventually continued the use of a grossly oversized centrifugal pump with much recirculation because it was already in place and the relatively short projected service life of the system did not justify the costs of a more elegant solution.)

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[bimr]

Both the duplex and triplex positive displacement pumps provide a smoother flow than a single postive dispacement pump.

Suggest you contact Cat pumps for a recommendation.

http://www.catpumps.com/

 

[moltenmetal]

At 0.22 gpm you can forget about regen turbine pumps.

You're stuck with a PD pump of some sort.

Gear pumps running at low flows with high back pressures pumping thin fluids are a recipie for even more wear and maintenance. The reasons are rather obvious if you understand the construction. Vendors try to mix up the gear and wear plate materials to prevent those problems but they don't ever go away entirely. They work very well pumping lubricants, but my experience with them even in clean aqueous and solvent services (numerous vendors) is poor, though some swear by certain brands as being relatively bulletproof.

If pulsation is your primary concern, you might consider a Hydracell. It's a mechanically rather than hydraulically coupled triplex diaphragm pump, hence much lower pulsation than a simplex hydraulic diaphragm. If you can use an elastomeric diaphragm compatible with your service you should be able to get good life out of them. I'd be suspicious about lifetime if you need a Teflon diaphragm, so don't default to Teflon.