Can You Spec A Pump with Negative NPSHA/NPSHR 3

[Odom71]

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

 

[quark]

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,

 

[25362]

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.

 

[MortenA]

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

 

[tonyh]

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

 

[25362]

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.

 

[pmureiko]

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.

 

[scalleke]

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.

 

[25362]

Visit thread407-51468 started by rww88.

 

[quark]

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,

 

[25362]

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

NPSH_a=(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 NPSH_a.

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.

 

[buzzp]

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.

 

[scalleke]

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

 

[Scipio]

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.

 

[25362]

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.

 

[scalleke]

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.

 

[Scipio]

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?

 

[scalleke]

This is indeed the vague area between compressors and pumps.

 

[25362]

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

 

[smckennz]

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