NPSHa 2

[rww88]

I have found the concept of NPSHa quite interesting and have been thinking through the classical definition. Will someone tell me if it is possible or likely that a NPSHa analysis could result in a negative quantity for NPSHa? I could refer to this concept as NNSHa (Net Negative Suction head available). If if fact this can physically occur, would this new term more corretly be referred to as Net Negative Suction head unavailable or Net Postive Suction head unavailable.

 

[quark]

There is no trick in it. Suppose if the net suction head is negative, it defeats the very purpose of the pump.

Generally where the net suction head is negative, we should make it positive (by some arrangement) to run the pump.

Say, you have to pump water from a tank which is
10 meters below the pump suction. The net suction head is negative and water will evaporate and the cavitation occurs. To reduce this effect, you have to submerge the pump by which you are increasing the net suction head.

Net negative suction head may be a useful term as far as language is concerned to provide an antonym for net positive suction head. But practically it bears no significance and is a useless term IMO.

 

[25362]

Interesting thoughts.
Negative NPSHa (independent from the pump itself) appears when the liquid is at boiling conditions and the pump elevation is higher than that of the free surface of the liquid.
Although the wording "positive" wouldn't be adequate when the NPSHa is indeed negative, I'd stick to the original terminology so one would immediately notice it.

 

[Kawartha]

rww88,

NPSH available or required, are both based on absolute pressure scale. 0 pressure absolute is equal to approx. minus 14.7 psig (gauge) and 14.7 psia (absolute scale) is equal to 0 psig (based on conversion of scale at sea level at 60 DegF).

I don't beleive that you can reduce pressure below 0 absolute, therefore you can never have a negative NPSHa. The negative aspect of NPSH occurs when NPSHr is greater than NPSHa and the difference (which is negative) is the shortfall in the NPSHa.

 

[TD2K]

NPSHA is essentially the pressure at the suction of the pump above the liquid's vapor pressure. If you rearrange the equation, NPSHA = (Pa - Pvp) + (Phead - Pline losses).

Pa is the system pressure, Pvp is the vapor pressure
Phead is the static head above the pump centerline and Pline losses are the line losses including any acceleration terms.

An NPSHA of 0 essentially means you have bubblepoint liquid at the pump inlet. If you tried to reduce the pressure further, you'd start to boil liquid and would still have an NPHSA of zero.

 

[DLANDISSR]

rww88:
As TD2K said the NPSHa is the pressure at the pump suction above the liquid vapor pressure.

The term "positive" refers to the fact that it must be above the vapor pressure not that it is above or below 0PSIG.

So the NPSHa (available)must always be above the NSPHr (required)or the pump will cavitate (cause the liquid to boil in the pump suction).

 

[25362]

Kawartha, NPSHa can be "negative". As said above when the vessel that contains the liquid to be pumped is located below the pump level, and the liquid is boiling (ie the vapour pressure equals the system pressure), you have a negative NPSHa.

TD2K, your formula is right, not so the interpretation. Static head can be negative if the pump is located above the liquid reservoir.

 

[Scipio]

25362, agreed. Liquid level doesn't even have to be located below the pump level (a suction lift scenario). Another example could be a steam condensate tank, with even a short suction line, losses can exceed the static head and result in a negative NPSHa. Amounts to the same thing though, whether your NPSHa is 0 or -100', you still aren't going to be moving much.

 

[25362]

Scipio, you are right. When speaking of NPSHa, I think people tend to err on two accounts:

a. By considering pressures instead of heads. Of course, there are no below-zero pressures. NPSHa measures head differences.

b. By assuming the particular centrifugal pump works and moves a fluid... No centrifugal pump would actually pump with a zero or negative NPSHa. I pointed before that NPSHa is a suction-side system characteristic and is independent of the pump in question.

 

[Kawartha]

25362,

Thanks for your comments. I have not looked at NPSHa in this light before, and your and other posts are very interesting. Your example of a boiling liquid with free surface located below the pump would result in a negative NPSHa only if the pump operated. But since the pump cannot operate, how can there be a negative NPSHa? Isn't the formula meant to measure NPSHa during pump operation? If there is no liquid at the pump suction, does NPSHa really apply?

Regards

 

[25362]

Kawartha, as I see it, NPSHa is independent from the pump itself, whether it is already in place or not.
It is a characteristic of the system to be allocated as suction side to a 'future' centrifugal pump.

 

[quark]

Damn it! Can I ever catch a TD2K or a Kawartha stepping in wrong shoes!!!

The case here is a classic example of Kasparov vs Deep Blue. A calculation by a computer can yield negative pressures below absolute vacuum. But human intelligence just stops at absolute vacuum and tries to reduce the net effect. That is why I said we should make it positive . It is very much true that there cannot be any pressure below absolute zero. I just wanted to trivialize the original post, in my first post.

The same thing occurs with Barometric condenser of steam jet ejector system. Absolute vacuum can hold a maximum height of 29.92 ft or 10 meters of water column at sea level. Like wise the theoretical net suction head can be any thing, but the pump cannot lift more than 10meters or 29.92 feet at sea level. (I am still talking about Utopia)

Net negative suction head (once again I say, an impractical term) is independant of pump characteristic as no pump can suck above that and if we don't use a pump there is no suction head either.

 

[25362]

To quark: It may seem pure philosophy, but I think the closing line on your last paragraph cannot be generalized.
The fact that some pumps cannot successfully suck when a positive NPSHa << NPSHr, doesn't make the NPSHa cease to exist, does it ?

 

[d23]

My two cents:

NPSHa is the available &quot;liquid&quot; at the pump intake in terms of head in feet. This number cannot be a &quot;Negative&quot; number. If it is there is simply no fluid available to the pump intake.

25362:

In most industrial pump applications considering NPSHa it terms of feet or meters is the proper consideration. There are a few applications where pressure is a more appropriate consideration. Suppose you have a natural gas in solution that you wish to pump. Keeping the gas in solution at the pump intake will require a high pressure. To convert that number to feet or meters will yield very large hard to manage numbers. In this type application there will be a “pressure” switch at the pump intake to assure liquid is available.

 

[25362]

d23, there is nothing wrong in using pressures as long as the vendor (NPSHR) and the client (NPSHA) understand each other.

 

[Kawartha]

Threads that develop keen interest are a little rare and are worthy of a STAR. I also found 25362's comments very interesting, also a STAR.

 

[zdas04]

D23,
NPSHa is a physical thing and Mother Nature just doesn't care what set of arbitrary units we mortals choose to use. The pump sees stuff at the eye of the impeller and it is either enough to prevent cavatation or not enough - the motive force to get the stuff to that location could have come from a column of liquid, the discharge of a booster pump, or a gas/liquid mixture in an oil field reservoir. It only matters that there is enough stuff to allow the pump to remove the amount of stuff it is designed to move and leave enough stuff behind to prevent phase change. We can measure that difference in stuff in feet, meters, kPa, or Newtons/square furlong. The only differnce between a head switch and a pressure switch is the markings on the face of the gauge.

When I see a statement that &quot;head must be measured in feet&quot;, I feel that the author of that statement really thinks the way we report physical characteristics matters. It matters that we use consistent units in calculations, it matters that we communcate the units that we use, but I can't see that it matters what units we used to record a physical parameter.

David

 

[raja011]

The concept of NPSHA is purely for meeting the NPSHR to ensure the Pumps operation within the specified rated points and also for the safety of pump impeller / casing.

The Negative NPSH or Suction Lift is another parameter where the water level or water tank level is lower than the pump cL.
To overcome this problem the pumps are provided with Footvalves for non-self priming pumps and other CF pumps are available which can self-priming for negative suction lift from the certain specified depth.

However, I faced the similar problem of suction lift from a height of 9 mtr in one of my projects, where we were advised to go for Multistage Centrifugal pumps which is now serving the intended purpose.

If anyone have, more clarifications, you're welcome.

 

[TD2K]

Suction lift does not equate to negative NPSHA. The 'lift' has to be compensated by the difference between the source pressure and the vapor pressure of the fluid.

 

[packdad]

Should velocity head be included in the calculation of NPSHA? Intuition tells me no; however, in Karassik et. al.'s Pump Handbook, Equation 2-20 reads as follows:

hsv = (Pa-Pvp)/SW + Ps/SW + Zps + V^2/(2g)

Pa...atmospheric pressure surrounding suction gauge
Pvp...fluid vapor pressure
Ps...suction gauge pressure
SW...specific weight of fluid
Zps...Gauge/pump suction elevation difference
V...Average suction velocity

While I certainly don't question the validity of the Pump Gods' equation, it just seems to me that fluid STATIC pressure, not TOTAL pressure, would be more appropriate for looking at available NPSH. My (misguided) mental picture is of water MOVING through a pipe, with static pressure dropping until vapor pressure is reached. However, I guess the NPSHA equation is special to pump design, where the designers take the total fluid energy into account when figuring whether cavitation will occur somewhere within the pump's internal flowpath. Thoughts?