positive displacement pump 2

[142846]

Hello everyone,
I have a question about positive displacement pump or compressor.
what does mean the word of positive in sentence of positive displacement pump/compressor. The word of positive really refers to what? what or which operation or action or ... is positive in positive displacement pumps/compressors. Is there any negative displacement compressor or pump?

 

[Strong]

https://www.bing.com/search?q=defin...90d97cb006d32823&pglt=299&FORM=ANNTA1&PC=U531

Walt

 

[142846]

Thanks Strong,
I have reviewed many pump handbooks, sites and asked this question from my colleague, but finally i could not understand the exactly meaning of positive in "positive displacement pump or compressor". POSITIVE refers to fluid? refers to function of pump? refers to pump curve?refers to direction of flow? I appreciate if you know this meaning, summery explain it.
For example first site definition is as follow:
“Positive displacement pump (PDP) is a type of pump in which a moving fluid is captured in a cavity and then discharges that fixed amount of fluid. The displacement of fluid takes place by some parts like plunger, piston, diaphragm etc. some of these pumps have expanding cavity at the suction side and a decreasing cavity at the discharge side.”
from which part we can extract/find the positive meaning?

 

[hydtools]

It may be clearer to think of the pump as positively trapping and delivering enclosed volumes, a particular amount of fluid as oppose to rotodynamic pumps which move fluid by non-positive means of transfer of energy from impeller to fluid.

A bucket on a rope could be a positive displacement pump.
Ted

 

[142846]

Hydtools Agreed, but I think still you are explaining “displacement pump” not “POSITIVE displacement pump”. What is the role of positive in positive displacement pump/compressor? honestly i did not understand "positively trapping and delivering ..." i appreciate for more explaining.

 

[LittleInch]

It really means positive as in forward or in the direction of flow.

Without a NRV, other types of pumps can have negative diaplcement, i.e flow goes back ward.

SOme PD pumps also need an NRV

SOme people might say negative displacement for e.g. a piston pump is when the piston is on the down stroke allowing fluid inot the cylinder.

But PD really just means that for each complete revolution of the pump there is positive displacement of fluid in the forward direction.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[FACS]

Mechanically force material out of a cavity results in a positive displacement into the atmosphere.

Install a vacuum pump, and mechanically force material out of a cavity to form a vacuum; you have a negative displacement into the atmosphere.

 

[bimr]

These pumps are commonly called constant volume pumps where the fluid is positively moved through the pump.

 

[hydtools]

Or fixed displacement, specific volume per revolution. Unless it is a variable displacement version.

Ted

 

[TiCl4]

Rather than the term "positive" meaning "forward flow direction", I submit that it actually means "certain" or "prescribed". From Merriam Webster, postive, adj. can mean:

1

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a : formally laid down or imposed : prescribed positive laws
b : expressed clearly or peremptorily her answer was a positive no

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c : fully assured : confident positive it was her book
2

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a : of, relating to, or constituting the degree of comparison that is expressed in English by the unmodified and uninflected form of an adjective or adverb and denotes no increase or diminution

​

b

​

​

(1) : independent of changing circumstances :

​

​

(2) : relating to or constituting a motion or device that is definite, unyielding, constant, or certain in its action


A positive displacement pump is supposed to pump a particular volume per revolution regardless (within limits, of course) of the conditions downstream. Thus, the displacement is definite, constant, certain, and independent of changing circumstances, i.e. "positive".

 

[142846]

Every body thank alot
LittleInch: All type of pumps are forward direction exept in a abnormal conditions such as surge and shutdown , ....
bimr: ... positively moved..?
hydtools: I do not understand relation between your text and my question. As per my previous comment please explain a little more.
TiCl4: OK. This PD pumps are constant volume at each conditions and certain and independent from downstream, is it definition of positive?

 

[SandCounter]

I think TiCI4 nailed it with the definition. I might add that not only is a PD pump unyielding and constant in moving fluid, it maintains a great attitude while doing it.

 

[142846]

Every body thanks a lot
LittleInch: All type of pumps are forward direction except in a abnormal conditions such as surge and shutdown , ....
bimr: ... positively moved..?
hydtools: I do not understand relation between your text and my question. As per my previous comment please explain a little more.
TiCl4 and LittleInch: i appreciate again your respond. I know This PD pumps are constant volume and independent from downstream and independent of changing circumstances at each conditions , but i have not seen EVEN one reference document or handbook mentioned and write that with these reason this type of pumps called positive displacement. Do you have another phrase containing positive in technical and engineering affairs that means similar to your definition above? for example in NPSH "net positive suction head" positive is exactly equal a number greater than zero because we use absolute pressure (head).
excuse me , this is edition of my upper text that i could not edit that.

 

[TiCl4]

Dararia,

I don't believe that the term "positive" has any higher technical meaning than the fact that the displaced volume "is independent of changing circumstances". It may have started as a slang word for that type of pump, and has since become common parlance.

There are other common names for items that have started that way - i.e. the power supply box thingie going to an electric motor I know is commonly called a "pecker head", and I know of no particular technical basis behind the name. I also don't know if it has another, more "correct" name.

 

[142846]

TiCl4- THANKS ALOT.
What is wrong if we use "DISPLACEMENT PUMPS" instead of " POSITIVE DISPLACEMENT PUMPS" from now?

 

[hydtools]

Might prefer constant volume pumps.
Or fixed displacement pumps.

Ted

 

[TiCl4]

Dararia,

The issue with simply saying "displacement pump" is that the term is non-specific. True, the term will likely be interpreted as a positive displacement pump. However, ALL pumps could be called displacement pumps, as they all displace fluids in some form or fashion. You might as well just call it a "pump" because saying "displacement pump" provides no additional information over simply "pump".

In the interest of concision, one would say "pump" when referring to a generic pump or an unknown pump type, and specify "positive displacement", "fixed volume", or "fixed displacement" pump when referencing that style of pump. In those terms, "positive" and "fixed" are adjectives that modify the word "displacement" (or "volume"), and all denote what specific type of displacement is occurring with these pumps.

Out of curiosity, why does the origin of the word matter? I feel like this thread should be in the language/grammar section.

 

[LittleInch]

OK positive means + or plus, i.e. more than zero.

Centrifugal pumps can dead head (zero displacement) at a fixed head pretty close to their operating pressure but still happily rotate for a considerable time

A positive (more than zero) displacement pump will try to push fluid out until the shaft stops rotating at pressures much higher than operating.

So a PD pump can be regarded as creating positive (more than zero) displacement at all times that it able to go round or operate. Very high pressures can occur very quickly if this positive (more than zero) displacement is not allowed.

Will that satisfy your curiosity?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hydtools]

My last contrbution:

"A non-positive-displacement pump produces a continuous flow. However, because it does not provide a positive internal seal against slippage, its output varies considerably as pressure varies. Centrifugal and propeller pumps are examples of non-positive-displacement pumps."

Ted

 

[FACS]

What is wrong if we use "DISPLACEMENT PUMPS" instead of " POSITIVE DISPLACEMENT PUMPS" from now?

No, all pumps "Displace".

Positive displacement pumps have a specific meaning, despite all of the dictionary definitions and trying to make them stick.

It means that the output is linear to the speed.

So if you want to change the world, you would use "Linear Displacement Pump" because that is what a positive displacement pump is.

I spent 5-years designing positive displacement gear pumps; and not once did we waste any time on the definition of "positive". Instead, we got things done ...