5 replies to this topic

[sskumar]

Guest:
When air/vapour enters suction of a pump, pump discharge head remains same as that with the liquid which the pump is supposed to pump. But the discharge pressure (Pressure=head*density*g/gc), which depends on density of the medium, is far below with air/vapor as compared to the discharge pressure with liquid. This is because the density of liquids is far greater than that of vapours. Hence with air/vapor in the suction line of the pump, the discharge pressure will be far less than the system pressure and hence air/vapor can not be pumped into the system. As continuously supply of energy is there to the air/vapor , it gets heated up. Maximum 6% by vol of gases are allowed in the liquid, above which, pump will cavitate.

[Art Montemayor]

Guest:

This is such a basic-basic question, it really belongs in the Student Forum. I can readily answer it with the common sense response that pumps are designed and manufactured to pump liquids and compressors are designed and manufactured to compress (and transport) gases. The two fluids should not be mixed in either machine simply because that is not its design.

If you persist in feeding a pump a liquid mixed with gas, you will loose prime to that pump and the pumping action will cease. This is because the gases are compressed within the pump chamber and then expand again when the pump tries to go into suction mode. Gases are compressible and this feature defeats the workings of a liquid transport machine like a pump.

When dealing with compressors - especially reciprocating types - liquids cannot be allowed to enter the suction of the machine because the liquids will literally destroy the machine due to the fact that it is designed to compress gases with a resultant "clearance" volume that does not allow for liquids.

In the future, when writing about compressors or pumps, you should specific about what type of compressors or pumps you are relating to. Different types react differently and have distinct characteristics that distinguish them from other types. Compressors can be positive displacement or Dynamic type. Pumps can be positive displacement or centrifugal. If you expect to receive a definite and clear answer to your query, you should be specific and clear in your question. It continues to astound me how people can expect quality, accurate, and detailed responses to queries that contain almost no basic data and little or no detail.

[Art Montemayor]

Guest:

You have to visualize or go to the cross-sectional view of a reciprocating compressor to see what is physically happening in order to bring about the compression and subsequent transport of gas in order to understand what I already explained to you. But, since you obviously have not done this, I'll go into a primer explanation in order to prove to you that you can destroy a reciprocating compressor with the introduction of liquids. Why you would want to do this - in contradiction to all manufacturer's operating instructions - is beyond my understanding.

Gas is introduced into the cylinder (compression chamber) via inlet valves in the suction port of the compressor. These valves are nothing more than "check" valves that allow gas in but do not allow it out. Once gas is in the cylinder, the piston compresses the gas charge and the compressed result exits the cylindert via discharge valves in the discharge port. These valves are also check valves that are uni-directional: they only work in one direction. As the piston retreats to its original position, more suction gas is allowed to enter through the suction valves for subsequent compression. The process is repeated and you get gas transport from a low pressure to a higher pressure level.

Note that the above description does not mention that the piston must physically replace the empty cylinder volume - all the way to the end of the cylinder wall, called "top dead center" - in order to compress and evacuate ("push") the cylinder of the compressed product. Theoretically, the piston has to accomplish a feat where piston metal meets cylinder end wall metal. This is OK on a theory basis, but in actual practice this is disastrous because the slightest clearance difference would mean the piston metal mass would beat against the cylinder end wall! So, what actually happens in the real world is that an "end clearance" has to be built-in into the machine's operation. This means then that there is a "volumetric efficiency" - not all of the theoretical cylinder displacement is used effectively; there is some residual, pressurized gas left in the cylinder after the compression stroke and this pressurized gas expands upon the subsequent suction action of the piston. The effect reduces the amount of suction gas entering the cylinder.

The above mechanical design means that if there is enough liquid in the suction gas to fill the cylinder's clearance volume, then the piston will pound into this liquid "wall" (remember, liquids are essentially NON-COMPRESSIBLE) and the result will be a mechanical failure due to rupture or breakage of metal parts. The compressor valves are designed to handle gas - not liquids - as I previously stated, and therefore the liquid cannot escape sufficiently fast enough to avoid such a disaster. YOU DON"T WANT TO DO THIS AND YOU DON'T WANT TO GO THERE!

I hope the above compressor basics serve to explain to you the importance of understanding and respecting the effect of introducing liquids into a compressor. I also hope that by now you also understand why I previously said this was a basic-basic question and really belonged in the Student Forum. Additionally, please also note that you failed to state what type of compressor you are dealing with and as such, I have had to assume a reciprocating type.

I hope this serves to clear up your understanding of the subject question.

[djack77494]

Guest:
While Art's explanation goes into quite a bit of depth, his early statement that pumps are designed to move liquids and compressors are designed to move gases really covers the situation. Art apparently felt that such a simple response would be insufficient. You have to think about the features needed to design both types of machines and the differences between the features. But I'm approaching the problem of why a pump is not a compressor and vice versa. Your question, I think, leans more to why gases/vapors are undesirable in pumps and liquids are undesirable in compressors. I think Art touched on both, but let me go a bit further and expand the discussion to include a centrifugal compressor. Compressor rotors typically move at very high velocities - far higher than typical pump impeller tips. They need apply relatively small forces to accelerate the low density gases through the machine, and so need not be nearly as robust as would be needed for a high speed part moving through a much higher density fluid (e.g. a liquid). Such a "flimsy" high speed blade/impeller could easily be damaged by the impact of (undesigned for) liquid drops, and therefore liquids should be removed before the gas enters the compressor.

The situation with pumps is not normally quite a serious, since pumps can often tolerate small quantities of gas/vapor. However, performance degradation can be severe, even with only fairly small quantities of noncondensibles. Mechanical damage is not out of the question either. Besides cavitation, slugs of a low density fluid moving through a pump would generate unbalanced forces that could damage the equipment. I hope this helps you understand the differences between these two types of machine.

(Note that there are some pumps designed to handle appreciable amounts of vapor, though even these pumps do suffer from performance drops when such vapors are present.)

Doug