31.1 NPSHr . . . too close for comfort?
31.1 NPSHr . . . too close for comfort?
(OP)
Pump application has me at 31.1' NPSHr for a 50hp, or $4500 more for a 60 hp variety with 18'.
Application is a hydronic 35% glycol loop recirculating out of an open tank. Level in the tank from pump centerline is atleast 2', adjustable up to 4'. Suction is ample at 4 ft/s up to pump connection.
Is this too close for comfort? At sea level, 33.9' plus 2' = 35.9' NPSH available. Should be good, right? Only concerns may be long term operation, wear, and if this could lead to cavitation issues.
Thanks
Application is a hydronic 35% glycol loop recirculating out of an open tank. Level in the tank from pump centerline is atleast 2', adjustable up to 4'. Suction is ample at 4 ft/s up to pump connection.
Is this too close for comfort? At sea level, 33.9' plus 2' = 35.9' NPSH available. Should be good, right? Only concerns may be long term operation, wear, and if this could lead to cavitation issues.
Thanks





RE: 31.1 NPSHr . . . too close for comfort?
Always remember that cavitiation limit / curve is higher than NPSH, commonly by 3-6 feet. See if the vendor can supply that curve or absolute head for your duty point
NPSH is measured at the point where the differential head reduces by 3% as this is measurable.
Your margin could easily come down to 3 feet or less.
Too close for me. Can you lower the pump even by 3 feet or so. Makes no difference to the pump differential, but might be cheaper than a different pump?
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: 31.1 NPSHr . . . too close for comfort?
I would agree with Little Inch that its a bit close for comfort and i would normally reject a design if it was that close. But if the consequences of failure are low and or its very difficult to do something else it would be worth proceeding with the current pump. Otherwise consider some alternative design or pump.
I have previously posted a reply to a different problem and included pictures of a pump that had a cavitation failure. I dont remember the numbers now but it had been designed with a margin about as slim as this one and had two major impeller failures in about 12000 hours of operation.
http://www.eng-tips.com/viewthread.cfm?qid=407008
The first failure occurred after about two years and 6500 hours of operation. There was close to 10 years of operation and about 12000hrs when the second failure occurred. The failures were unexpected and the exact cause unknown. However the design was very close to the NPSH limits.
Regards
Ashtree
"Any water can be made potable if you filter it through enough money"
RE: 31.1 NPSHr . . . too close for comfort?
Can you elevate the tank by a few feet?
Can you post the curve?
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: 31.1 NPSHr . . . too close for comfort?
See pump curves attached. Sounds like we're a bit too close for comfort. . .
RE: 31.1 NPSHr . . . too close for comfort?
RE: 31.1 NPSHr . . . too close for comfort?
Worth running power costs over a year based on efficiency, you might get a surprise as to the savings, cheapest initial pump cost is not always the best selection.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: 31.1 NPSHr . . . too close for comfort?
50hp (45.46 hp rated power)
$19,984/year
60hp (49.26 hp rated power)
$21,655/year
So the 60 hp variety cost $1670 more per year to operate versus the 50 hp. The 60 hp costs $4500 more than the 50 hp.
I don't understand what point you're trying to make regarding "cheapest initial pump cost not necessarily being the best selection". 50 hp variety has lower OPEX and CAPEX. Your comment makes it seem as though you thought the 60 hp pump would have a lower OPEX, thus it's able to make up the difference in CAPEX.
RE: 31.1 NPSHr . . . too close for comfort?
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: 31.1 NPSHr . . . too close for comfort?
As Artisi says, other pump suppliers exist and may well have a pump more suited to your particular needs.
I will say that 84% efficieny is very high - if you could get the 50hp to work then it would be best, but the margins are too tight given that the inlet head is effectively fixed by the sound of it.
Have you asked the pump vendor if he can put an inlet screw (inducer) on the pump inlet to lower the NPSH? Often offered as an option if you ask.
Like this.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: 31.1 NPSHr . . . too close for comfort?
RE: 31.1 NPSHr . . . too close for comfort?
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)