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Can You Spec A Pump with Negative NPSHA/NPSHR
3

Can You Spec A Pump with Negative NPSHA/NPSHR

Can You Spec A Pump with Negative NPSHA/NPSHR

(OP)
I'm working on specifying a pump to transfer LPG from storage offsite.  The thing that I noticed is that the NPSHA that I'm coming up with is extremly low.  Infact, it is negative.  I've scanned a specsheet for another pump in similar service and I noticed that it was specified with a NPSHA of 0.  The NPSHR under the performance data is -2.  I've never heard of negative NPSHA or NPSHR before.  I was hoping that someone could advise if it is acceptable to specify negative NPSHA which would in turn require a negative NPSHR.  If so, what is the practical limit before I play games like placing the pump below grade etc. to overcome this issue?

Thanks,
Odom71

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Logically speaking NPSHA can be negative (damnit!oxymoron), but NPSHR should always be positive otherwise I don't find any point in specifying it. (and I think you are also talking about centrifugal pumps)

There was a similar discussion at this Thread407-51468.

If you are getting negative(?) NP(?)SHR then you should think of pumps otherthan centrifugals.

Best Regards,

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Since you aren't attaching a length unit, ft or m, to the NPSH reported values, I'm inclided to think the values aren't correctly determined, and think it would be advisable to recalculate them.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Transporting condensate often lead to such problems. Maybe you need another type of pump than a centrifugal or maybe a booster pump?

If this wont do you will have to look at your piping design. Maybe the pumps could located in a low place (to add extra head)?

Best Regards

Morten

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

NPSH is effectively margin before boiling. Negative NPSHA means that your liquid is boiling before it gets to the pump. It will then suffer a further decrease in pressure as it accelerates into the impeller eye. In reality if the flow goes two phase in the suction, your suction pressure drop will increase leading to a worse situation. At the very least you will not pump what the curve tells you. You may very well vapour lock your pumpp

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Boiling liquids (i.e, total pressure = vapour pressure) are usually pumped when there is sufficient positive static head (the difference in height between the level in the boiling suction reservoir and the pump center line) after deducting the friction drop in the suction line.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

You may very well have negative NPSH available and required values.  This would lead to 2-phase flow at the entrance to the pump.  The pump would then be doing two services, increasing the pressure to collapse the 2-phases to just liquid and then pressuring up the liquid.  The problem is the process of collapsing the gas to liquid will cause cavitation damage and also lower pump head and flow.

As with all things, you can solve the problem with extra money buying a fancy pump for this service and paying higher maintenance costs.  If the other pump is providing acceptable service and you can't install the pump in a hole, then buy a similar pump.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

We must look at all the facts.

When the NpshA is specified; where is it specified.  it is possible that the pressure in the LPG vessel is lower than the LPG vapor pressure and that in fact the liquid is boiling, thus it has a negative NPSHA in the vessel.  this would typically be the case in a vessel that is in equilibrium and from which LPG is being removed, a typical storage tank case.

However it is most likely that the pump will not be located there and that the pressure will be higher at the inlet of the pump suction and that NPSHA will be higher there. (Most likely the pump is installed under the LPG reservoir)

Therefore I do believe that the NPSHA = 0 on the other pump data sheet is correct.

NPSHR is another issue.  Pressure in MOST centrifugal pumps will drop between the suction flange and the inlet eye of the impeller.  Because the liquid velocity increases as the liquid passage diameter decreases and because of the friction.  Pressure will only go up when the liquid is already well entered in the impeller.  Once pressure starts to increase gas bubbles will implode and cause cavitation damage.

It is possible to manufacture a pump that has a pressure increasing device (inducer or screw pump) installed before the impeller and that will start increasing liquid pressure prior to the liquid entering the impeller.  It is even possible to design this part so that there is no or close to no pressure drop once the liquid enters the pump.  Hence the pump with NPSHR = 0 is possible.  It is even possible to manufacture a pump so that the pressure increasing inducer passes through the suction flange and increases the pressure prior to the liquid entering the pump, hence NPSHR < 0 is possible.

What is important is to make sure that all play by the same rules.  Both NPSHA and NPSHR must be determined at the suction flange of the pump and under dynamic conditions and the available must be larger than the required.

Best Regards.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Visit Thread407-51468 started by rww88.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Guys! You are forgetting one thing that liquids cannot be towed, but they should be pushed in the pipe line. As the maximum atmospheric pressure on earth is not more than 10.336 meters, if your net suction head (I don't use the word negative NPSHA) is negative pumping doesn't occur.i.e pump cannot lift the water.

Tonyh! Correction to your post. The minimum condition for cavitation to occur is not negative NPSHA (?) but NPSHA<NPSHR.

Regards,

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

to Odom71, if you casn take a pressure gage reading at the pumps' suction flange you can determine the NPSH by the following formula:

NPSHa=(absolute-LPG vap. pressure)/sp. gravity)+v2/2g

The absolute pressure is the gage pressure + 1 atm.
Even when assuming the pressure difference is small you are still left with v2/2g. If, as an example, the linear velocity measured at the pump suction piping is 1.5 m/s, the vel. head would be 1.52/(2x9.81) = 0.11 m of fluid that should be added to the pressure difference to obtain the NPSHa.

I wonder whether your estimates of NPSH did take into account this small contribution, then voiding the statements of a zero or negative NPSH at the working pumps.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

I am certainly no pump guru, but negative NPSHA is used in the methane gas fields (with special permits)of Wyoming. Just thought I would add that.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Buzzp, you are right.

Negative NPSHA is possible.  It all depends, some liquids do no damage when they are cavitating.

As long as your pump can take the stresses that are inflicted on it by cavitation and does not get damaged all is OK.

I think of the casses where stellite is used to overcome cavitation issues.

Things that must be considered:

pumped liquid.  lighter hydrocarbons do less cavitation damage than heavier ones or water

Pump material.  the stronger the more cavitation it can take.

Pump operation point.  Lower speed puimps with less discharge head will have less cavitation damage.

Best regards.

Scalleke

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Ok, I have to throw my two bits in here too.

I think the topic of negative NPSHR has been well covered, one thing I'd like to add to the discussion of negative NPSHA.  In my opinion, negative NPSHA is impossible, and negative NPSHA does not cause cavitation.  Simple reason being, if you've got negative NPSHA, you've got a vapour, not a liquid.  Cavitation isn't when the liquid vapourizes, it's when bubbles form in liquid then implode on rising static pressure again.  In the case of negative NPSHA, you've just got vapour.  It doesn't have to be a vacuum, methane at 1000 psig still can be considered to have a negative NPSHA.  If you've got negative NPSHA, it means you need a compressor, not a pump!

The issue gets a lot more complicated when it comes to hydrocarbon condensates, especially with multiple paraffin fractions.  I recently installed a condensate pump off the bottom of the inlet separator to a compressor station.  Strictly speaking, the liquids in a separator, even hydrocarbon condensates, are at their vapour pressure, the only place you get NPSHA is from the static head between the liquid level and pump inlet.  Realistically speaking, however, depending on your condensate analysis 98% of your liquid could be C4+, with some saturated high vapour pressure methane, ethane and propane.  Their presence in the condensate artifically increases the apparent vapour pressure, but once they come out of solution in the form of bubbles, I don't believe they're likely to implode unless you happen to be operating right at that fraction's vapour pressure.  I have to admit though, that's just based off personal experience and observation with natural gas condensates, I've never been able to find any kind of authoritative literature to support the idea.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

If there was a way of cooling the suction line (i.e., subcooling the LPG below its bubble point) by a few degrees, the NPSHa would increase markedly. Depending on the LPG composition, one could gain more than 4 ft of water per oF.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Scipio,

I do agree that at negative NPSHA there is Vapor formation however depending on how negative NPSHA is there will be a little or a lot of vapor.  It is not so that as one drops below the vapor pressure of a liquid suddenly 100% of the liquid will become vapor.

Liquids such as LPG can be stored at pressures lower than vapor pressure.  True the LPG will be boiling but part will remain liquid.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

scalleke,

Good point - I suppose it depends on the time and what happens to the vapour as it boils out of the liquid.  In a closed volume, the liquid will boil until the vapour reaches the vapour pressure, but it could take anywhere from minutes to days depending on how 'negative' the NPSHA is.  I suppose it could be maintained by venting vapours faster than they're boiling out of the liquid phase, but again, wouldn't you be into compressor territory there?

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

This is indeed the vague area between compressors and pumps.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

I've read elsewhere that -statistically speaking- most centrifugal pumps cavitate in some degree.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

NPSH is measured in absolute head. 0 NPSH is a perfect vacuum. Negative NPSH is impossible, or at best left to the trekkies.
0 NPSHR is sales gimmickery. Negative NPSHA is asking too much from one pump. Boost it.

Cheers

Steve McKenzie

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

So Steve,  What is the NPSH of a liquid while it is boiling in a reservoir that is at a pressure below vapor pressure?

Phase changes do not happen instantaneously.  Pulling liquid out of a vessel that is at vapor pressure will make the vessel pressure drop below vapor pressure and depending on the flow rate of the liquid being pulled out and depending on the evaporation rate of the liquid in the tank it will take a certain time before vessel pressure is at vapor pressure again.



Best Regards
Scalleke

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Steve, 0 NPSH is NOT absolute vacuum.  It means the fluid is at its boiling point, whether that is -40F or 500F.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Scalleke & TD2K,
Agreed completely - smckenne, check your formula for NPSH.  It has nothing to do with a vacuum, it has to do with the difference between vapour pressure and absolute pressure of liquids.  As I've stated, you can have a pressure of 1000's of psig and still have a negative NPSH.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Oops forgot to say ignoring vapour pressure, and for that matter the inlet velocity head.
NPSH is referenced to an absolute vacuum,but the vapour pressure is subtracted from it.
Imagine a tank with 10m H2O absolute head on the surface, negligible vapour pressure and friction loss. If the takeoff was 10m above the tank liquid level, the NPSHA would be 10-10 = 0m or a perfect vacuum. This could be measured near enough with a manometer or barometer. NPSH is measurable. Thats how they produce NPSH curves. You cant measure a negative Net Positive Suction Head because it doesnt exist.
Negative NPSH? Nah. Impossible real-world condition.
NPSHR > NPSHA? Possible but expect cavitation and decreased liquid flow rate.

Cheers

Steve McKenzie


RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Don't know why this post keeps comming up?

Yes you can!

But I wouldn't do it.

Rule of thumb that I have been using is you add a couple of feet to the NPSHR for safety/comfort/screw-ups.

So why would one start out tryng to screw up?

How much air is allowed into the system from leaks? Is there a min operating pressure? Have you looked into a canned pump, it is somtimes necessary.

If you have a two phase pump I would like to learn about it.
Good luck in your search.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

To 25362

You sent me into some reviews because we/I have always calculated NPSHa on the non-recoverable losses.
Looking at one of the more recent texts "Transport Phenomena A Unified Approach" by Brodkey and Hershey, 1988, Chapter 10, page 451 when they calculate the available pressure they do not consider the kinetic energy term.
In my mind it stads to reason because if the pressure is taken at the wall and it is lower than the vapor pressure then one will have a vapor.

Taking the stagination pressure is not the lowest pressure in the system.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

To Assumptions, the NPSHa estimated at the pump suction nozzle as described above is the recognized formula, which is mainly used for evaluating "test results".
The v2/2g term, however small, is a component of NPSH when measured at the pump suction reference date line.

The other formula, mostly used in the design stages to estimate NPSHa, is:

[(P over liquid surface-Vap. press.)/density] +/- Static elevation-friction drop.

Those are the definitions accepted in the pump business.

As for NPSHr the requirements may be based on different criteria, depending on the specific pump duties.

I can only repeat the phrase attributed to Galileo: Eppur si muove.


RE: Can You Spec A Pump with Negative NPSHA/NPSHR

It happens for some applications. Vertical canned pump is usually the answer. The pump is installed inside a pit/tank and length is decided in such a way that enough NPSH is made available because of the static head above the pump suction.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Not enterig into the NPSH debate, but there are mixed phase pumps availible that have compressor vane type primary stages that concentrate the gas back to a liquid that is then pumped by an ordinary closed shrouded impeller final stage.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Going through the huge amount of messages this issue has commanded here and in other threads, I'm driven to think the "perpetuum mobile" (aka perpetual motion machine) has been finally discovered.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

NPSH is defined as the excess of inlet  total head over the head equivalent of the vapour pressure of the pumped liquid.
NPSHa is defined as the NPSH available at the pump inlet.
NPSHr is defined as the NPSH required at the pump inlet for operation free from unaccptable deficiencies caused by cavitation.

To achieve acceptable operation of a centrifugal pump the value of NPSHa must be greater, or at the very least equal to the value of NPShr.

Since the value of NPSHr is always > 0, the value of NPSHa can never be negative.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

To escheir:

Then why are there dual phase pumps available?

Scalleke

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

To Scalleke,

I was talking about a centrifugal pump.
What are dual phase pumps? do you mean vapor ejector?

escheir

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Dual phase or mixed phase.  Pumps that can handle liquid mixed with gas.

Scalleke

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Isn't centrifugal vs. dual phase a redundant point since NPSHA is independant of the pump in question?

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

I agree, a canned vertical turbine pump is probably your best bet. I'd also look real hard at all of your piping line losses, you may be able to get an increase in NPSHA by reducing line losses.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

PE/Steel Gas Transition Fitting can sustain negative pressure?

Anybody knows if a 50mm dia. PE100-steel gas transition can sustain negative pressure (7 bar from outside surface of the pipe / fitting) at a prolonged period? What is the effect? Any literature / information about it?

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Odom,

I have not seen these many replies on any other post.   Obviously you tickled something special in us.

Your calculations are wrong. Sorry to be blunt. By definition NPSHA cannot be negative. Your calculations are wrong does not mean you do not have a problem. Usually, when dealing with fluids such as LPG (at close to boiling point), you would be requiring a vertical pump in a can. Your calculations (though wrong) are proving this point. With a canned pump, you will ‘create’ adequate NPSH, by simply selecting right length of the can. If someone is selling you a horizontal pump, please be prepared to leave  some change in the bank to change the pump later.



Regards,

Guru

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

(OP)
Dear RotaryGuru,

Thank you so much for your comment.  Unfortunately, I must return the favor and advise you that you are WRONG.  My calculations are spot on.  It may not be sensible to have a negative NPSHA, which is why is posted the question since I am not a pump specialist.  However, my calculations were correct in pointing out the fact that there is insufficient pressure available to efficiently get the material to a standard centrifugal pump.  The bottom line is we simply have to modify the suction piping, or increase the pressure on sphere.

Thanks again for your reply.

Best Regards!

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Interesting thread!  Didn't read everyone's question, but thought I'd comment.  Cryogenic pumps often end up with a 'negative' NPSH when actually put into service.  The result is decreased 'volumetric efficiency' due to the gas that is ingested.  Pumps can be damaged by this, but not always.  Some cryogenic recips are designed to be used in 2 phase lines, with vapor fractions as high as 90%.  I've not heard of centrifugal pumps like this though.  Can you improve the suction line insulation?  Increase diameter?  Do you know which affect contributes the most to the loss of NPSH (ie: elevation, heat, or pressure drop)?  Have you focused your solution on the largest contributor to the loss of NPSH?  

Best of luck,
Dave.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Odom,

The definition of NPSHA is simple: Static head + surface pressure head - the vapor pressure of your product - the friction losses in the piping, valves and fittings.

See for reference http://www.mcnallyinstitute.com/11-html/....


For NPSHA to become ZERO:

Static head + surface pressure head= the vapor pressure of your product + the friction losses in the piping, valves and fittings.

Assuming that in the worst case scenario, there is NO externally applied pressure in the suction tank, then

surface pressure head (in the suction tank) = the vapor pressure of your product


In that case:

For NPSHA to become ZERO:

Static head = the friction losses in the piping, valves and fittings.

For NPSHA to become negative

Static head< the friction losses in the piping, valves and fittings.

If Static head< the friction losses in the piping, valves and fittings that indicates that there is NO flow. If there is no flow there is no friction loss.

Hence, NPSHA can never become negative. Instead of going through all the above let us just see what the acronym NPSHA stands for in the first place: "Net POSITIVE Suction Head".

You are getting negative value since you ignored the physical principle that without DP across a piece of pipe there will not be any friction loss. We all make mistakes. No one is immune. This forum helps us in understanding those so that we may not repeat them.

Regards,

Guru

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Guru,
I think you need to step back from the equation to understand what Odom is saying.  Note first he's referring to a cryogenic liquid, but that's not terribly important as it applies to any fluid.  One generally knows the conditions of a liquid in a tank (ie: pressure and amount of sub cooling or temperature) and one can generally calculate the amount of heat transferred to the product as it passes through the pipeline.  One can also calculate the frictional losses in the pipe/fittings and effect of elevation change.  So in addition to the parts of the equation you cite, you really need to consider the affect of heat transfer in this case, which changes the vapor pressure of the fluid as it passes down the pipeline.

The result of the fluid pressure dropping below it's vapor pressure is not that flow stops, the result is that the liquid begins to boil.  As it does, the vapor fraction of the fluid increases.  A vapor fraction of 0 means the fluid is 100% liquid.  A vapor fraction of 1 means the fluid is 100% gas.  So as the pressure of the fluid continues to decrease below the vapor pressure, the vapor fraction increases from 0 and rises depending on how low the pressure continues to drop.  So the meaning of "negative NPSH" is generally understood to mean the fluid pressure has dropped below it's vapor pressure.

For example.  If a fluid has a vapor pressure of 100 psig at ambient temperature, and the fluid sits in a tank at 110 psig, we have 10 psig to play with before the fluid starts to boil.  If we put a long pipe on this tank and take it over to this pump, and the pressure decays below 100 psig because of flow losses (let's assume the pipe is horizontal so we can neglect elevation changes), then the end result is not that the flow will stop when it gets to 100 psig, the end result is the pressure continues to drop and the vapor fraction increases from 0 and goes up such that the vapor fraction stays in equilibrium with the pressure and temperature.  Flow through a pipe is estimated as an isenthalpic process, with the exception of heat flux which changes the enthalpy.  But for simplification, neglect heat transfer for the moment.  If the enthalpy of the product is known in the tank (we know fluid pressure, and vapor pressure which gives us enthalpy) then we also know the enthalpy at 90 psig (10 psig below the vapor pressure) which gives us the vapor fraction when the pressure in the pipe decays 10 psi below the fluids vapor pressure.  Hence the bubbles/gas in the fluid.  That, I believe, is what Odom is referring to.  Hope that clarifies things.

Take care,
Dave.


RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Just a clarification to the above, as I don't see any way of editing these posts.  Perhaps if there's a way of editing a post, someone can point it out for me.

For the case of a saturated fluid (ie: one at it's vapor pressure), as the fluid pressure drops traveling through the pipe, the vapor pressure (by definition) drops along with it.  

I think that's where the confusion arrises.  You can talk about NPSH assuming tank conditions and come up with a negative value, but calculating the NPSH at a pump, depending on your interpretation, may result in a value of zero since the two phase fluid at the suction of the pump is in fact saturated at it's vapor pressure.  But this interpretation, IMHO, is very misleading.  I've never liked the concept of NPSH, especially as it applies to cryogenic pumps, and perhaps this discussion helps to explain why.  In the past, I've often wanted to take the acronym over to the cement wall out back and put a few bullets through it!  Argh…

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

iainuts, you got me totally confused when you say: "...as the fluid pressure drops travelling through the pipe, the vapour pressure (by definition) drops along with it".

Do you mean to say the VP increases due to friction and heat, and there is some vaporization all along ?

There is no reason to get angry at a concept that was, and still is, very useful in the design and application of pumps in general, and centrifugal, in particular. Right ? Would you prefer changing it, for example, to MSCPC (Minimum Suction Conditions to Prevent Cavitation), thus  deleting the word "positive" ?

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

A similar liquid to LNG that's caused me headaches when it comes to NPSH calcs is liquid hydrocarbon condensates separated out of natural gas streams.  Just try to make a sensible calculation with a room-temperature liquid at two or three hundred psi, that's 98% C5+, but is saturated with dissolved methane & ethane gas.  I've never been able to figure out how to make a sensible estimation of vapour pressure with multiple-fraction liquids, so typically just to make life bearable assume vapour pressure = surface pressure.  Not quite right, but I figure close enough.  Always wondered though, if methane & ethane fractions bubble out of solution at the pump inlet because of reduced pressure (line losses, etc.), will they actually implode again causing cavitation?  My experience, at least in recips, is it tends to stay out of solution and vapour lock the pump, but I've never been 100% sure on the mechanics.

Now, you really want to see a value for NPSHA (or NPIP, Net Positive Inlet Pressure, for the recip purists) that'll screw with your head?  Take into account the acceleration head of a single-acting simplex plunger pump.  I've got one vapour locking right now (until I get the stabilizers fitted) because vapour pressure assumed = surface pressure, static head is about 15 feet, and without even considering friction losses I've got a calculated acceleration head in excess of 50 feet ;)

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Hi 25362.  Got a laugh out of that response about one shouldn't get mad at a concept!  That's a chuckle.  Thanks, you got me smiling here.  :)

Regarding "...as the fluid pressure drops travelling through the pipe, the vapour pressure (by definition) drops along with it".  Let's say the fluid is at it's vapor pressure at some point in the pipe as 100% liquid.  This is the same as saying it's a saturated liquid.  It's on the verge of boiling.  Any additional heat will force it to boil.  Similarly, any reduction in pressure will also force it to boil  

If we reduce pressure on the fluid, the liquid boils and the 2 phase stuff that remains is still saturated somewhere below the dome.  ie: if the fluid is at it's saturation point, then regardless of what quality or vapor fraction the fluid has, it is still in equilibrium at it's vapor pressure.  Thus as the pressure drops, so does (by definition) it's vapor pressure.  

In the case of a liquid flowing through a pipe with pressure dropping as it goes, this process is governed by the 1'st law of thermodynamics.  One also needs to add the affects of elevation change and velocity change.  This is all 1'st law energy balance with the difficulty being calculating pressure drop due to frictional flow of a fluid who's composition keeps changing (vapor fraction continues to increase as pressure drops).  This all makes for a relatively difficult 2 phase flow through a pipe problem that I prefer to leave to a computer program.  

Hope that helps.
Dave.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Yet another clarification (wish I could edit posts here!)

The above statements I made are only true for a pure component, ie: pure LNG, pure liquid hydrogen, pure liquid nitrogen, etc...  I'll differ to a ChemE to answer the question proposed by Scipio where the fluid is a mixture of multiple components.

Thanks,
Dave.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

Dissolved gases have a half-life of evolution shorter than for solution. That's one reason for their incomplete collapse. The remaining bubbles are recirculated to the impeller inlet passages to join with freshly-cavitated bubbles. This leads to an accumulation of gas blocking part of the impeller inlet.

Many pumps, especialy those working on level control, operate with flow rates well below their BEPs. These flow reductions promote recirculation.

Besides, an impeller can act as a centrifugal separator, evolving gas and holding it near the eye periphery. The accumulated gas may even start backing up into the top of the horizontal section of the suction pipe.

Venting the pump at the top of the volute while running is usually useless. Venting near the suction nozzle from its top, may do some good while running to avoid vapour lock. Anyhow, the normal procedure is to stop the pump and vent it.

In order to estimate the "right" NPSHa for cases like  those mentioned by Scipio, I think one could get help from an article in Chem. Engineering of July 26, 1982 by Mao J. Tsai, titled Accounting for dissolved gases in pump design

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

I'm not a mechanic engineer, but an operation engineer and I was wondering if someone considered that,even when NPSHa<NPSHr, at the pump suction you cannot vaporize all the liquid even if it is LPG . Simply, because there is not enough energy to vaporize it .Once reached the liquid boiling temp at given suction pressure, it will no more vaporize so you can have negative suction head and still have a lot of liquid and a bi-phasic flow in the pump eye. At that low temp, the gas bubbles will easily collapse in the impeller causing cavitation.

I didn't find a detail explanation of the dynamics of this process, but I believe that the vaporization enthalpy is reconverted thereafter, when the gas is re-liquefied  but this happens somewhere after impeller, so that this energy is transferred only partially,  by conductance, back to the suction nozzle, providing some additional energy for vaporizing...

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

16347,
You bring up a valid point.
As with many natural processes, a momentary equilibrium is achieved.  Some of the liquid goes gaseous, which momentarily and in small localities raises pressure back above vapor pressure so that not all liquid is phase changed.

Pressure varies throughout the liquid column, which means that not all liquid "sees" vapor pressure.  Once an area falls below vapor pressure, some fluid in that area vaporizes, which then raises pressure in that area and thereby prevents furhter phase changing, but only in that small area.

PUMPDESIGNER

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

This thread is unusual.Where is the definition of NPSHR?
I thought it was the point where there is a 2 or 3% drop in TDH (I forget which) over the expected value if the fluid pumped remains liquid.

The original question said the NPSHR was -2.

Would someone be so kind as to refer me to the pump manufacturer who provides pumps with negative NPSHR ratings?
After all the question is about specifying.

Cheers

Steve McKenzie

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

just wanna add that in the refinery i worked for, we used to store LPG in horton spheres at around 12 barg. It avoids two phase flow thru pumps and keeping LPG at high pressure also minimizes loss due to vaporization.

RE: Can You Spec A Pump with Negative NPSHA/NPSHR

To sb00, one is inclined to think that the 12 barg were cancelled out by the vapour pressure of the LPG (=12 barg) thus leaving your NPSHA equal to the elevation difference less the friction drop in the suction lines. Am I right?

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