ZERO NPSH PUMP 1

[SShiva]

We have to install a pump to pump out gasoline with traces of water from an open sump. Density of the liquid is around 600 cu. m/kg. The NPSH is zero and the discharge pressure is 6 bar abs and the flow is 5 cu. m/hr . The pump should should not be a vertical type which is a constraint. Like to have a suggestion to what type of pump is most prefferd to this application and any vendor's name to contact. It can be either positive displacement or centrifugal type.

 

[butelja]

I don't know of any pump that can operate at 0 NPSH. However, you may be able to use a pump/eductor combination (otherwise known as a jet pump) to solve the problem. Basically, a self priming centrifugal pump is installed above the sump. An eductor is located at the bottom of the pit. A portion of the discharge off the centrifugal pump is used as the motive fluid for the eductor. The discharge from the eductor leads to the centrifugal pump suction. The pressure rise across the eductor creates a positive NPSH for the centrifugal pump. The overall efficiency of this system is not high, but it may solve your problem.

 

[Wesspo]

Are we talking 0 psi gage or absoulte? Pumps can operate at 0 gage not probably zero absolute.

 

[MJCronin]

P&M,

Consider a self priming pump for this application. Goulds Pumps makes the 3796 model for process duty, Ingersoll Dresser and Gorman Rupp also make self primers.

Self Primers draw from a reservior just adjacent to the impeller. Some require a "suction tank" for startup.You will have to "leave some liquid" in the sump for the pump to work.

You may have to provide a liquid supply line and a startup procedure to start the pump if use is infrequent.

Try the vendor's & talk to a local rep...... MJC

 

[tstead]

Consider a self-primer????? NO WAY!! Not with zero NPSH!

First of all, you do not have zero NPSH - but let me get to that shortly.

NPSH is one of the most misunderstood terms in the pumping world. Most people equate NPSH with height of fluid above the pump which is ONLY true when you have large suction piping (i.e. no friction loss) and the fluid is at saturation temperature for the gauge presure. You will only have zero NPSH if the gas/water mixture is hot and the fuel is vaporizing - and if it is, why are you using an open sump with such a dangerous fire hazard just waiting to destroy your building and kill people?

With zero NPSH, you are trying to pump a vapor and, MJCronin, I am sorry but self-priming pumps still need some amount of NPSH to operate and, generally, they are greater than equivalent sized centrifugals - not always though. Like all centrifugals, self-primers need to pull the liquid into the impeller eye. When the pressure in the impeller eye reaches saturation pressure, cavitation will ensue. I dont care if you have a centrifugal, (self-priming or not), rotary, lobe, gear, screw, whatever...when the pressure drops to saturation you have problems - either with vapor binding or cavitation.

As to wheter to use a self-primer or not...all that type of pump can do for you is to maintain the vacuum to keep the siphon effect going. You fill a tank with fluid, the pump evacuates it which draws more fluid from below the pump into the reservoir and allows the pump to contiune working. If your fluid is already at zero NPSH, you will neither be able to start nor maintain the siphon.

Now, to your problem. You have an "open sump" with liquid in it. That means that you have atmospheric pressure helping you with your NPSH (there's an automatic 10m of NPSH). Without knowing your temperature (probably around 22 oC) you probably have about 7-8m of NPSH. I haven't worked the calc - that is just an arm's length guess.

Want to solve your problem easily? Use a submersible pump.

What country are you in?

 

[MJCronin]

tstead,

You are right in stating that the problem described does not appear to be a "ZERO NPSH" situation...

However, you and I are both guessing without an adequate description of the problem, for example what is the sump contents, vapor pressure at operating conditions and what is the sump depth.

The truth is that, using your figures, a self primer with 7-8 meters of NPSHa is a workable design (given that the lift and liquid temperature are modest)You and I, tstead, are both guessing...

Various pump designs exist because no "one size fits all" There are limitations with your submersible pump designs also. A submersible must reject heat to the surrounding liquid, and if the liquid is too hot, the pump will trip off on high temperature. Most manufacturers of submersibles require special designs above 110F and above 140F, as I recall, you run out of manufacturers...

I suggested to "pipingandmechanical" that he contact a larger vendor to evaluate his particular solution...on what basis are you so sure that a submersible design will be appropriate ? Any references or websites that you can offer to help the questioneer ?

MJC

 

[tstead]

I estimate that the motor hp requried on a submersible pump would be around 3 hp. I doubt the gasoline is very hot which would mean that a submersible pump would probably do fine - just make sure it is explosion-proof.

As with any application, you have to make sure that the pump will indeed work as it is intended - but you should have some sort of idea as to what type of pump you are looking for. If, after reviewing the application, a submersible is not a good idea, then another design could be used.

As far as heat rejection, this is correct - submersible pump motors reject heat to the fluid they are in. Depending on the design and materials, upwards of 60-70% of the heat generated by the motor can be rejected into the fluid which is pumped.

104 oF is the practicle limit for most submerisble pump desings because it has to do with the requirements for explosion-proof ratings as per the NEC. Some manufacturers go further and de-rate the motor for temps above 104 F up to 140 F (while remaining explosion-proof). Above 140 F, those de-rated motors might be able to be used up to a maximum practicle temperature of about 190 F. That, however, depends on how much the motor can be de-rated, how large a motor can be bolted to the casing, and how much juice is required by the impeller.

I have not seen a "hot gasoline being pumped from an open sump" application yet - but that does not mean that they don't exist. As with many things, you should first go with your gut and then confirm that your gut is correct.

My post may have come across as a little critical of using self-primers in gerneral but what I intended to do was to reinforce the fact that self-primers are not suitable in low NPSH applications.

My suggestion on using a submersible is based solely on seeing many applications with "waste sumps" where gas, oil, and debris accumulate and must be pumped out (such as drains from fuel pumps) and submersibles worked great (using vortex type impellers). This may not be such a case, but it is worth looking into.

If the sump is shallow, then a submersible might not work too well because the motor would not be submerged all the time and - in which case - a self-primer would be better.

 

[SShiva]

MJCronin/tstead,
I am placed in Brunei. We are contemplating to install motor driven double diaphragm positive displacement pump (electro-mechanical)from ABEL. They claim the pump is a self priming and can run under dry conditions. The temperature is ambient around 35Deg C. The depth of the sump is 2meters. The sump is a separater and partially covered.

 

[harveyd]

Interesting dialogue. However, it occurs to me that this application is not unlike that of every gasoline service station - an underground, vented gasoline tank with an above ground pumping unit pulling a suction lift.

Granted, many stations now use submersible pumps, there are hundreds of thousands of them using positive displacement rotary vane type pumps - that I would recommend be considered for this service.
One manufacturer worth considering is Blackmer Pump - I believe they are at www. blackmer.com. Contact them for more help and their local rep in Brunei. I know they routinely handle volatile hydrocarbon pumping applications like yours.

Good luck.

 

Rent a "Godwin" pump which uses an air compressor w/ a venturi to prime the pump - Seen them do some fairly phenomal things

 

Rent a dri-prime pump which uses an air compressor w/ a venturi to create a vacuum which then will prime the pump -
I have ween this type of pump lift from dry 26 ft.

 

[1973]

Try a positive displacment pump or a Self Priming pump. I know a company called "WASTECORP. PUMPS" , they make both types. These pumps consider all of the variables and allow for any change in the system. If the liquid changes in any way, it will effect the NPSH. For this reason,you need this selection of pump.

Regards Joe Massa