System sucking in air - suggested fix is convert to closed - looking for more info

[John.g]

They can't make the minimum lower than the theoretical, but poor design can certainly raise it.
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I think I am back on track now.....I was actually correct in post #11 but lost my way after that.
I found a reference to the static head required for a Wilo Yonis Pico (like my own) and it requires 3M @ 95C.
Vapour pressure @ 95C = 8.63M Atmos pressure = 10.34M
NPSHA = Static Head(M) + atmos.press (M. absolute) - vapour pressure (M.absolute)
NPSHA = 3+10.34-8.63 or 4.71M
Even if the static head were 0 then the NPSHA would still be 1.71M.

Going back to the old selectric which requires 1.4M @ 85C
Vapour pressure @ 85C = 5.9M Atmos pressure = 10.34M
NPSHA = Static Head(M) + atmos.press (M. absolute) - vapour pressure (M.absolute)
NPSHA = 1.4+10.34-5.9 or 5.84M
Even if the static head were 0 then the NPSHA would still be 4.44M.

 

[doitmyself]

Grundfos states the following for the current Alpha1 and Alpha3 Pumps

Liquid temperature	75 °C	90 °C	110 °C
Inlet pressure	0.5 m head	2.8 m head	10.8 m head
	0.005 MPa	0.028 MPa	0.108 MPa
	0.05 bar	0.28 bar	1.08 bar

For the UPS3 the numbers are:

Liquid termperature	75 °C	95 °C
Inlet pressure	0.005 MPa	0.05 MPa
	0.05 bar	0.5 bar

 

[SJB060685]

3m static head plus atm is roughly 1.3 bar minus saturation pressure of.0.8461 bar for a temperature of 95°C gives NSPHA of 0.45 bar, which is below the requirement?

 

[John.g]

A static head of 3.3M is required IF 0.5 bar is the NPSR?

Vapour pressure @ 95C = 8.63M Atmos pressure = 10.34M
NPSHA = Static Head(M) + atmos.press (M. absolute) - vapour pressure (M.absolute)
NPSHA = 3.3+10.34-8.63 or 5.01M (0.5bar)

 

[SJB060685]

I think we're getting a bit muddled between us. I will attach an image if not for your benefit but others.
My understanding is simple. The NPSHA must be equal to or greater than the saturation pressure. If I'm reading you right from your example above the 3.3m static head is not enough for a flow temperature of 95°C to avoid cavitation. By my calculations you would need a static head of roughly 8m above centre line of pump, whether you raise header tank or lower position of pump and of course altitude will have an effect as well on atmospheric pressure.

 

[John.g]

I'm happy enough with my interpretation of it, At 95C the vapour (saturation) pressure is 8.63M BUT the atmospheric pressure alone is 10.34M and any pressure > the vapour pressure of 8.63M will stop the vapour forming, Even with zero static head the atmospheric pressure is giving a NPSA of (10.34-8.63) or 1.71M but Grundfos have a NPSR of 5M so a tank with its water level 3.3M above the pump CL must supply the remainder, (static head of 3.3M.)
so the NPSHA=NPSR?

 

[SJB060685]

NPSHR is minimum pressure required to avoid cavitation, so in essence equal to saturation pressure.

 

[John.g]

The Americans generally explain things in simple terms, try this very old explanation that I have had for years, It talks about NPSH, NPSHA and NPSHR.
The first few paragraphs explains it well, I think.

 

[SJB060685]

Link appears to be broken

 

[John.g]

Try this, back later.