Piston Pump

No.

Yes, if it drops below the suction connection of the pump. (And maybe even before then.)

If the pump suction is higher than the liquid level, the pump has to lift it higher to pump it. If the liquid level is higher than the pump, the head pressure on the pump gets lower and the pump has to lift the liquid higher. Pump pressures are a combination of the distance of the pump above or below the liquid and the height that the liquid is being pumped plus any frictional losses.

Good Luck, Old Salt

A positive displacement pump should maintain velocity, operating within design specifications.....The only thing that should vary is energy usage...but I guess it depends on the design....

How deep is your tank? You might not even get anything out if with suction you mean the pump is higher than the fluid level. So the pressure drop will be tremendous!

Otherwise: How deep can your tank possibly be to create more than 14 psi difference. Can you even see this on your manometer?

It depends upon whether anything in the suction tank or line can affect the equipment on the delivery side. Without seeing the equipment it is not possible to say.

The problem is a bit more complex.

Even if the suction level is over the rim of the suction pipe it can be possible that at pump entry the pressure drops so much that cavitation occurs. In such a case the aspirated volume decreases and according to it pump delivery, BUT the delivery pressure is always determined by the resistance of the space where it has to deliver.

Does "delivery pressure is always determined by the resistance of the space where it has to deliver' mean the total distance from the surface of the suction liquid to the end of the discharge piping (or if it is a subsurface discharge, the level of the discharged liquid) vs. from the level of the pump to the discharged liquid (same about subsurface discharge, etc)?

Good Luck, Old Salt

May be you mix up two kind of pumps. You are -I suppose- accustomed to CENTRIFUGAL pumps and have few experience with VOLUMETRIC pumps.

What you say is CORRECT for 1st type and NOT for the second.

Before making comments you should check if they are RIGHT.

I recommend to look in a book about pumps which you can find by a google search for free.

Old Nick Name

nick name-

Contrary to your presumption, I have extensive experience and knowledge about many types of pumps. Some of those in the classification of piston pumps include sterile packaging pumps with a volume/stroke of 1cc up to and including oil transfer pumps with a capacity of over ten gallons/stroke. That knowledge is perhaps why I may have asked a question that I don't need to check if I am right. As you suggested "Before making comments you should check if they are right". Yes, I also have extensive experience and knowledge with centrifugal, over 50 years of it. Perhaps I might assist you at some time.

My question was concerning the clarity of your posting. With a piston pump the distance between the pump suction and the level of the fluid being pumped requires energy to pump. The higher the distance, the more energy required. The more energy used for the suction the less available for the discharge side. This also gets to a point where the vapor pressure of the liquid causes the material to vaporize in the partial vacuum causing the pump to cavitate. The practical limit with water suction is approx. 23 ft

The second part of my question pertains to the statement ALWAYS. My question had one part which possibly you misunderstood. For both centrifugal and piston pumps the discharge pressure is not ALWAYS determined by the resistance of the space where it has to delivered. The frictional resistance of the transferring piping or hose most often has more influence than the resistance of the space where it has to be delivered.

For proof of this look into a historical book on firefighting. The old horse drawn steamers used steam driven piston pumps. The friction of the hose interior had more influence on the discharge pressure than the space where it had to be delivered. There is usually a few of these on display at antique fire truck gatherings. Modern fire trucks have centrifugal pumps because they have: larger capacities, usually between 1,250 to 2,000gpm; higher pressure capacities, a double stage pump will put out about 300 to 400psi; are easier to maintain; and especially to operate.

If you would like to I can suggest several excellent books on both pumps and fire truck pumps. Start with Pirsch, American LaFrance, Ahrens-Fox, and a Google search.

Maybe next time, if there is one, we should discuss air operated single or double diaphragm pumps. It has been a while but I still know a "few" things about them.

Also, I hate to read or write such terse communications. We are supposed to be here to help others who need assistance in finding their answers. "Any day on the GREEN side of the grass is a great day."

Good Luck, Old Salt

Thank you for the extensive teaching, if I will ever need a help be sure I shall NOT hesitate to come back and ask for it, I am always ready to learn since I guide myself according to the principle that the more you know the more you know that you know nothing!

Now coming to your question, your definition of the "space" is much more restricted than mine, for me the pump outlet is the start of the space where the fluid is delivered.

According to THIS definition any connection between pump and reservoir is part of the delivery space.

You have a good experience - no doubt- and you give many examples from the fire fighting domain, but I still think that the way you formulated the question is more related to the centrifugal pumps.

In a piston pump (the notion of "piston" has to be taken in the general meaning since gear pumps have also "pistons") the fluid is aspired from a low pressure space and delivered to a high pressure space by "pushing" and not by giving it an amount of kinetic energy which is transferred to potential energy. The piston pump "pushes" the fluid against the pressure in the delivery space and at its outlet this pressure sums all "resistances" encountered by the fluid column from this outlet to the "user".

If we consider the fluid as incompressible and the leaks small the delivery is function ONLY of the aspired volume and of the RPM. The outlet pressure does NOT depend on the height of the column at inlet (or static pressure at inlet).

What could be influenced is the power the motor has to supply and since piston pumps are mostly used for high pressure this has almost no influence on the RPM even in case of asynchronous motors (the slip will change but not significantly).

To go with my explanation further. If the pump has (as one of the basic types) check valves at in and outlet the opening of the outlet valve (and thus the delivery pressure) depends ONLY on the pressure in the "delivery space" defined as I do.

As you see I do not insist on the way how it works at the inlet side since several gave already explanations of good quality.

Such explanations are good for others too so that it is positive to have a contradictory discussion even if for some it is not pleasant.

As a last comment with respect to pump experiences I designed (among other equipments) pumps for different media as water (for water jet cutting), drilling mud (for earth geological research) and hydraulic systems for pressures as high as 300 MPa. So that in the "piston" pump design I have not very much to learn.

As for the engineering experience I have it is spread over a couple of years more than yours.

nick name-

The abridged intention of my original question, refer to #8, was to obtain some clarification on a statement that was made in #7 which was not clear. I certainly didn't expect a such a presumptive reply as #9 about a subject concerning my knowledge. How you can conclude that from a few sentences is beyond my comprehension. I still would like to know your answer.

Also, I didn't make any statements in #8. I asked the question.

Good Luck, Old Salt

I am surprised that out of all the previous posters, yours is the only one that mentions the discharge connections of the pump, and that determines output pressure more than anything else, as long as supply to the pump is maintained. Most positive displacement pumps, whether piston or diaghragm will lift fluid to the suction as needed, as long as no air is entrained. No one should design a choked supply to a pump that would cause cavitation, that would just be foolish.

I wanted to give you GA, but some bozo already marked you as "off-topic", when you hit it dead center.

could not find a gold star emoticon.

Thank you for the comment I consider it as a full "GA".

I do not want to make any comment on other answers but as you will many times see answers are not related to question. Many times answers are given to demonstrate the "knowledge" of the answering person.

"Will delivery pressure of the piston pump goes down if the suction level in the tank drops?"

No, as long as the NPSH available is greater than NPSH required.

Piston pumps are used in either closed loop or open loop circuits. In a closed loop circuit the inlet to the pump is fed by a charge pump so the pump performance is independent of tank conditions as long as there is fluid available to the charge pump. In an open loop circuit the inlet to the piston pump must be "flooded" or the pump will starve and cavitate. "Flooded" means an unrestricted and sufficient supply of fluid to meet the output volumetric delivery of the pump at a given speed (rpm) and displacement. If the tank and connecting suction line to the open loop pump can meet the flow demands of the pump without creating a vacuum (or pressure below the vapor pressure of the fluid) at the suction/inlet port then the output of the pump (pressure and flow capacity) is maintained and is independent of the head (fluid level above the port) in the tank. So, the answer to your question is - as long as the piston pump has a sufficient supply of fluid at it's suction/inlet port to prevent "starving" or cavitation, then the suction level (head) in the tank does not effect the performance of the pump.

In general, I agree with Lyn's 1-word answer. When the piston is making its pressure stroke, the valve or port to the supply tank is closed, so the delivery pressure is determined solely by the friction of the discharge pipe and the flow in units of volume per unit of time.

--John M.

But when the pump makes its suction stroke, conditions can affect whether the next pressure stroke contains all liquid, or some entrained air or vapor. In those cases, the output pressure is affected.

Delivery pressure will not drop unless the pump cannot suck the oil from the tank if the tank level is below the pump intake, if that's what you mean by "suction level". If this happens, you will have a greater problem than loss of pressure, you will ruin your pump because there will be cavitation. Also have to watch for condition of any suction strainers that may be present and ensure they are clean - best way to do this is by proper filtration in your hydraulic system, with return filters and, where warranted, in line pressure filters for closed loop systems.

Normally no. Special case, yes. If the supply tank is not vented and is not under pressure, as vacuum develops in the supply tank, the tank will either collapse under vacuum, or the pump will cavitate when suction pressure is less than the vapor pressure of the liquid.

I think most of our answers are far too complex to fit the flavor of the OP's simple question. That's pretty normal for us.

Agreed. Yours is the best answer and the highest word economy, too!

Yes