Talk:Net positive suction head

wrong project
''' This should come under mechanical engineering, or just irrigation or pump technology. It is not directly related to bioengineering and the only link to chemical engineering is the vapor pressure aspect. —Preceding unsigned comment added by Ozfreediver (talk • contribs) 08:12, 3 April 2010 (UTC) undefined '

Headline text
Hello, I need some examples of NPSH calculation.. can you please post them?

NPSH = Psuction - Psaturation

please explain... —Preceding unsigned comment added by Crackerbuzz (talk • contribs) 06:26, 20 September 2008 (UTC)

Atmospheric Pressure
"A very important and interesting thing to note is that if you have an NPSHA of say 10 Bar then the pump you are using will deliver exactly 10 bar more over the entire operational curve of a pump than its listed operational curve."

I think this is just the difference between gauge and absolute pressure....

Mikejens (talk) 22:45, 13 November 2008 (UTC)

That is correct. Another way of referencing it to pump curves etc. — Preceding unsigned comment added by Crackerbuzz (talk • contribs) 09:03, 17 October 2011 (UTC)

This article needs SERIOUS work
The definition of vapor pressure in the "somewhat simpler method of understanding NPSH…" section is wrong:

"Vapor pressure examples: (vapour pressure = the temperature in which a liquid turns into a gas, which may change under negative and positive pressure environments)."

The definition from http://en.wikipedia.org/wiki/Vapor_pressure is correct "Vapor pressure (also known as equilibrium vapor pressure), is the pressure of a vapor in equilibrium with its non-vapor phases."

Vapor pressure is a strong function of temperature, but what is described in the section is boiling point (closely related - The boiling point of a substance is the temperature at which the vapor pressure equals the pressure seen by the fluid).

I am at work now (pump sizing!), but will attempt to clean up more of this mess today when I get some spare time. I think the author knows what NPSH is, but he/she needs some help explaining it. —Preceding unsigned comment added by 66.60.230.62 (talk) 13:02, 22 June 2009 (UTC)

Wrong equation
The equation for the NPSH should ADD the delta Z (not a minus)

Furthermore, nothing is said about the velocity head in the flow. Total energy grade line (EGL) is the hydraulic grade line (HGL) plus the velocity head (V*V/2g), therefore, the "pressure" or the HGL that the fluid (and potential to cavitate) sees is the EGL - V*V/2g ..... or the Po + delta Z - Losses - V*V/2g

So if the velocity head is significant, the hydraulic pressure in the pipe is significantly LOWER and less NPSH is available. —Preceding unsigned comment added by 72.164.173.2 (talk) 18:55, 12 August 2010 (UTC) NPSH = the net (left over) positive pressure of suction force into a pump intake after friction loss has occurred. Liquid head height or liquid head pressure + gravity pressure, minus friction loss, leaves a net head pressure of force into the pump.

The bove statement contradict the equation mentioned ...

Please advise which is correct. — Preceding unsigned comment added by 125.22.37.243 (talk) 07:19, 11 October 2011 (UTC)

Authors
I have attempted to contribute some simple explanations of NPSH for readers that are without the relevant qualifications such as from a university etc. (Even then it is still a handful to read). The idea is to write it in a way for pump technicians and salesman to get a better understanding of the topic rather than just looking at NPSH curves and pretending to know what they are.

I have added only portions to assist. Even so it is a very difficult topic to explain without getting technical!

If anyone can edit or add to assist or add simple calculations please do so.

Regards — Preceding unsigned comment added by Crackerbuzz (talk • contribs) 08:58, 17 October 2011 (UTC)

Something About NPSHa and NPSHr
Please mention something about NPSH available and NPSH required. I don't think the technical description is adequate Kir360 (talk) 02:50, 13 June 2012 (UTC)

I agree, see comment inserted below. — Preceding unsigned comment added by 195.67.14.129 (talk) 15:05, 4 October 2016 (UTC)

Moved section
I have removed the section "A somewhat simpler informal way to understand NPSH" as definitely not in encyclopedic style. There may be useful information contained in it, but it needs reworking so I've moved it here. "Examples" needs work as well but I've let that stay. I will work on it further. Hyarmendacil (talk) 10:01, 24 April 2013 (UTC)

=== A somewhat simpler informal way to understand NPSH… ===

Fluid can be pushed down a pipe with a great deal of force. The only limit is the ability of the pipe to withstand the pressure. However, a liquid cannot be pulled up a pipe with much force because bubbles are created as the liquid evaporates into a gas. The greater the vacuum created, the larger the bubble, so no more liquid will flow into the pump. Rather than thinking in terms of the pump's ability to pull the fluid, the flow is limited by the ability of gravity and air pressure to push the fluid into the pump. The atmosphere pushes down on the fluid, and if the pump is below the tank, the weight of the fluid from gravity above the pump inlet also helps. Until the fluid reaches the pump, those are the only two forces providing the push. Friction loss and vapor pressure must also be considered. Friction loss limits the ability of gravity and air pressure to push the water toward the pump at high speed. Vapor pressure refers to the point at which bubbles form in the liquid. NPSH is a measure of how much spare pull you have before the bubbles form.

Some helpful information regarding atmospheric pressure; Atmospheric pressure is always naturally occurring and is always around us. At sea level, it equates to 101.325 kPa or approximately 14 Psi OR 10.33 meters of water pressure head. As we move to higher altitudes, the air gets thinner and the atmospheric pressure reduces. This should be taken into account when designing pumping systems. The reason there is atmospheric pressure is due to earths gravity and its position in our solar system. It is a natural phenomenon and we are very lucky to have it as water wells and bores with shallow aquifers allow us to use this atmospheric pressure to our advantage.

Pressure gauges exist on pumping systems and other machines to give us an indication of what performances are being achieved. We also use known pressures versus known performance in order to create a reference for system designs. An example would be an experienced pump technician or plumber knowing that a pressure of between 300 kPa and 500 kPa will provide adequate and comfortable pressure for household use.

A typical pressure gauge reads what is known as 'Gauge Pressure,' or pressure relative to atmospheric pressure. An 'Absolute Pressure' gauge displays atmospheric pressure (typically 100 kPa or 14 psi or 10.33 meters of water pressure head) before any system had been connected. Manufacturers set typical gauge pressure gauges to read ZERO at sea level as a standard, assuming designers will make allowances for the atmospheric pressure calculations themselves. Knowing this simple fact can make NPSH easier to understand.

If we now know that there is 100 kPa or 10.33 meters of water head pressure, plus or minus whatever the gauge pressure gauge shows, then we can safely see that this gives us an instant advantage of 10.33 meters of water head pressure at sea level. This means we can borrow against this and drop a maximum of 10 metres into or under the ground (or below sea level) reducing the gauge to zero and still get natural 'push' into our pump. Great for wells and bores with shallow aquifers within this depth! It is important to note that to get to exactly 10 meters may be difficult, but with the correct pipework and system design, it is possible to get very close.

Once NPSH is fully understood, sizing and controlling pumps and pumping machines is a much simpler task.

NPSH is the liquid suction force at the intake of a pump. In other words, the force of a liquid naturally “pushing” into a pump from gravity pressure plus liquid headpressure only - into a single pump intake.

This means; NPSH = the net (left over) positive pressure of suction force into a pump intake after friction loss has occurred. Liquid head height or liquid head pressure + gravity pressure, minus friction loss, leaves a net head pressure of force into the pump.

If we want to pump some amount of liquid, we have to ensure that this liquid can reach the center line of the suction point of the pump. NPSH represents the head (pressure and gravity head) of liquid in the suction line of the pump that will overcome the friction along the suction line.

NPSHR is the amount of liquid pressure required at the intake port of a pre-designed and manufactured pump. This is known as NPSHR (Net Positive Suction Head Required). The pump manufacturer will usually clearly have a NPSH curve to assist you in the correct installation. -- //Comment: The definition of NPSHR is in some places given as the pressure required at the intake port _above_ the vapor pressure of the fluid used during the test procedure. Which one is correct?? --

NPSHA is the amount (A = available) to the pump intake after pipe friction losses and head pressures have been taken into account.

The reason for this requirement?

When the pump is receiving liquid at intake port and the impeller is pushing the liquid out the discharge port, they are effectively trying to tear each other apart because the pump is changing the liquid movement by a pressure increase at the impeller vanes, (general pump installations). Insufficient NPSHR will cause a low or near-vacuum pressure (negative NPSHA) to exist at the pump intake. This will cause the liquid to boil and cause cavitation, and the pump will not receive the liquid fast enough because it will be attempting to pump vapor. Cavitation will lower pump performance and damage pump internals.

At low temperatures the liquid can "hold together" (remain fluid) relatively easily, hence a lower NPSH requirement. However at higher temperatures, the higher vapor pressure starts the boiling process much quicker, hence a high NPSH requirement.

Water will boil at lower temperatures under lower pressures. Conversely its boiling point is higher at higher pressures.

Water boils at 100 degrees Celsius at sea level and an atmospheric pressure of 1 bar.

Vapor Pressure is the pressure of a gas in equilibrium with its liquid phase at a given temperature. If the vapor pressure at a given temperature is greater than the pressure of the atmosphere above the liquid, then the liquid will boil. (This is why water boils at a lower temperature high in the mountains).

At normal atmospheric pressure minus 5 psi (or -0.35 bar) water will boil at 89 degrees Celsius.

At normal atmospheric pressure minus 10 psi (or -0.7 bar) water will boil at 69 degrees Celsius.

At a positive pressure of +12 psi or +0.82 bar above atmospheric, water will boil at 118 degrees Celsius.

Liquid temperature greatly affects NPSH and must be taken into account when expensive installations are being designed.

A pump designed with a NPSHR suitable for cold water may cavitate when pumping hot water.

Missing variable definitions
Whats Hf, z? Further why is subscript 0 bolded, but i is itallic? — Preceding unsigned comment added by 203.27.186.78 (talk) 23:02, 23 February 2021 (UTC)

$$\text{NPSH}_A = \left( \frac{p_i}{\rho g} + \frac{V_i^2}{2 g} \right) - \frac{p_{v}}{\rho g}$$ should be $$\text{NPSH}_A = \left( \frac{p_i}{\rho g} + \frac{V_i^2}{2 g} \right) - \left( \frac{p_{v}}{\rho g} + h_f \right)$$ where h_f is the pressure loss associated with the flow of water through the pipes up to the pump. I feel this is more thorough than the original equation. To answer your questions, OP, I believe $$\ h_f $$ referred to the pressure loss associated with the flow of water in an enclosed surface, i.e. the "drag" that any surface causes when a fluid moves over it.--IanVG (talk) 13:43, 23 September 2021 (UTC)


 * good work, your linked pdf is a much clearer explanation than the wikipedia article Effgee123 (talk) 02:25, 19 April 2023 (UTC)