NPAH & NPEH 1

[PUMPDESIGNER]

I hope I do not look like an idiot, but this website has me stunned.
They use the terms:
NPAH, "Net Positive Available Head"
and
NPEH, "Net Positive Exhaust Head"

My first thought was to figure the guy was off base.
But then I thought I had better check here first.
I have never heard of such things, has anyone else?

Here is the complete quotation from the website:
"Cavitation in pumps occurs when the Net Positive Available Head (NPAH) coming from the pump is equal to the Net Positive Exhaust Head (NPEH) leaving the pump. When used in combination with a Cycle Stop Valve the Available Head (NPAH) from the pump will always be slightly higher than the Exhaust Head (NPEH) leaving the pump, eliminating the possibility of cavitation."

This statement seems rediculous, but what is this manufacturer trying to say?

Richard Neff
Irrigation Craft

 

[TD2K]

I have no idea, have you got the URL to post?

It does sound like doublespeak to me.

 

[BobPE]

PUMPDESIGNER:


YIKES!!!!!Did they say what planet they were from?

BobPE

 

[sprintcar]

I'd be looking at another pump vendor - one that understands industry standards or makes pumps instead of fans Keep the wheels on the ground
Bob
showshine@aol.com

 

[PUMPDESIGNER]

The company sells pump stations and controls
The URL is:

http://www.cyclestopvalves.com/how_cycle_stop_valves_effect_pumps_&_motors.htm

When I get the time I will be nice and send him an email alerting him to his error. Richard Neff
Irrigation Craft

 

[d23]

PUMPDESIGNER:

I made a quick look at the reference site. It looked like all motors are continuous duty devices and all pumps require less HP the more you restrict flow. If you would like a few pump curves to address that let me know.

davidlsus@yahoo.com

 

[d23]

PUMPDESIGNER:

I have looked a little closer at your favorite web site. If you want info for the:

Soft Start Equipment?

section I can supply that. To say sub-pumps need to increase speed quickly due to the T-bearing may leave a little room for error. I wonder if they have heard of the affinity laws and their effect of thrust loading?

They also talked about a start that is at 65% then switches to a 100% run. A Y-D start is 57% plus require 6 wires, not a good thing in a well. Does this statement mean they are talking about auto-trans start? End users may have a different view, but most manufactures want to avoid both types of starters due to torque loading on the shafts not water hammer problems.

To explain this a little further; on a large HP unit something as innocent as a auto-reclosure on the distribution system can break a shaft if the time is set too short for the motor starter relay to clear.

The section on:

Minimum Flow?

I doubt they have ever heard of "floater" pump designs. Pumps do have a min flow rate. The other thing about min flow is liquid flow past a motor. What do they think cools the motor?

As a sub-pump manufacture I should like a web page like this. It makes sales for me.

If you want info for your reply please let me know. I would be happy to jump in the middle of this!!!

If you should decide to leave this alone let me know I may want to do this myself.

 

[PUMPDESIGNER]

d23 - You are really funny with the "your favorite web site"
Go ahead and contact the guy, he needs you.

I studied the site and what he was doing.
He has one unique idea that I have never thought of ... thankfully.
Using his own terms:
He sets his cut-out pressure setting above the pressure control valve setting. Now pressure in the mainline cannot rise to the cut-out pressure until all flow stops or almost stops. He has a small by-pass tube around the control valve. When flow stops this by-pass line can bring mainline pressure above the control valve setting to the cut-out pressure and turn off the pump.
Problems with this are:
1. Different systems will require different by-pass.
2. Unpredictable results ...
Oh heck, I'm tired and do not feel like analyzing this anymore. Richard Neff
Irrigation Craft

 

[d23]

Richard:

This will be a Friday night email....

I may need a little liquid courage to write the proper comments!!!

I do believe that pressure cut-out settings would be more in your line! If need be I'll buy you a beer.

 

[saxon]

Pumpdesigner, Just remember, garbage in = garbage out. Or as the Harvard MBA said, "What the Heck! We'll just let Marketing write the Technical Sales Documents. That way we don't have to put competent Technical Writers on the staff and I can pocket the difference for bringing the Department in under budget! What a concept!"

saxon

 

[PUMPDESIGNER]

saxon - If you have that quote put it on the board.
Did you guys here the joke about the marketing executive that got lost and asked an engineer for directions? Richard Neff
Irrigation Craft

 

[25362]

I think the wording is mistaken, the website writer probably thought of centrifugal pumps used as turbines or cases when NPSH problems can never appear. In this case the common NPSH requirements to avoid cavitation by internal circulation on discharge restriction, are changed to TREH and TAEH, total required exhaust head and total available exhaust head, respectively.

There are graphs that give the cavitation constant sigma = TREH/H as function of the turbine's specific speed. See Hicks Standard Handbook of Engineering Calculations, Ed II, page 3.376, MgGraw Hill.

 

[d23]

PUMPDESIGNER:

Just so you know:

I sent an email asking if they were discussing sub-pumps or turbine pumps. I personally wanted to know.

If they are talking about sub-pumps as described on the site you will owe me a beer or few tomorrow night!

I'll let you know how life in the sheltered world does with this!

 

[PUMPDESIGNER]

25362 - Thank you for your comments, you are correct about his intent. I was too aggravated to think clearly when I read it, you must be a pretty calm peaceful guy.


d23 - He considers them all to be the same. He discusses on his site centrifugals, line shaft turbines, and submersible turbines. All three pumps in his market have the same power to flow characteristics.
Richard Neff
Irrigation Craft