about suction pressure again?

Question

thanks for explaining suction pressure. i wish to make my doubt more clear. i will explain my understanding.... more discharge pressure means more work done by the pump. my q is will the discharge affects suction pressure.. does more suction press means more work done by pump or less work. is high suction pressure is required for more discharge. onother thing.. the pump is intaking the same amount of water that it delivers. so why suction pressure and discharge pressure not the same?? may sound idiotic.. but please explain me from the basics.

Answer 1

Okay, basically any pump takes a suction on the inlet side and PRESSURIZES whatever medium you're pumping, so the discharge side of a pump should always be at a higher pressure than the suction; otherwise, it doesn't pump. This is true whether you're pumping water, oil, or air (that's all an air compressor is: a pump that sucks air in and discharges it at a higher pressure) If the pressure at the suction of a pump is raised (say, by pressurizing the supply tank, for instance), it just means that the discharge pressure should go up by some amount, not necessarily in a 1:1 linear relationship (depends on the type of pump and conditions you're working with). In same applications, like when you're pumping water into a tower, you need to have pumps run in series, so the first pump discharges a certain pressure that then supplies the second pump, giving that second pump the extra "oomph" it needs to push the water the rest of the way up, because it couldn't handle it on its own (the first pump is often referred to as a booster pump)

It's basic physics: things will always flow from an area of high energy to an area of low energy, unless you input more energy to combat that tendency. If the pressure on both sides of a running pump were the same, then it's not doing anything -- the pump is doing no work.

Answer 2

Well, if the suction and discharge pressure were the same, why would you be wasting the energy costs of running a pump?
Here's how a centrifugal pumps works:
A centrifugal pump consists of an impeller rotating within a casing. Liquid directed into the center of the rotating impeller is picked up by the impeller vanes and accelerated to a high velocity by the rotation of the impeller and discharged by centrifugal force into the casing and out the discharge. When the liquid in the impeller is forced away from the center of the impeller a reduced pressure is produced and consequently more liquid flows forward. Therefore, a steady flow through the impeller is produced unless something happens to break the vacuum at the inlet or disrupt the flow to the center of the impeller or unless the flow at the discharge is restricted by a pressure greater than the pressure head developed by the rotating impeller.

Answer 3

It may help to work with a particular type pump then expand that to other types. Centrifugal pumps are very popular and quite diferent from positive displacement (piston?) pumps. Centrifugal pumps create pressure by spinning the fluid in an impeller, much like you could do by spinning a bucket of water on a rope. The pressure depends on the diameter and rotary speed of the impeller. If there is a hole in the the bottom of the bucket, water will squirt out depending on the hole size and pressure and work is performed to move the fluid. If there is no hole, there is no work (useful work) even if you tire spinning the bucket.

The amount of work (useful work) done by the pump depends on the force on the fluid moved a distance. Lift a 100 lb weight one foot and you do 100 ft-lb of work (useful work not including lifting your own boby!). If there is a high suction pressure at the pump, water will move through the pump even if the pump is idle (and no work is done by the pump). If the discharge valve is shut and the pump is operating, maximum discharge pressure will result but no useful work will be done (friction losses will heat the water in the pump and the pump motor will draw minimum current). As you open the discharge valve, water will begin to flow and the pump does work increasing motor current. Only the work done as a result of the discharge pressure minus the suction pressure is performed by the pump. For example if two pumps were operated in series both would produce part of the pressure increase and both would share the work.

The suction pressure of the pump must be high enough to prevent cavitation (formation of vapor bubbles in the suction water that sound like pumping gravel and can damage pump walls and impellers when the bubbles collapse at higher pressure releasing energy as tiny shock waves that penetrate metal and pit it). Air bubbles are different and serve as cushions. If you vacuum up water from a vessel at the boiling point (212F) only steam vapor will rise because the water will flash to steam at any pressure below atmospheric. As you cool the water you can vacuum it to greater and greater height and the maximum height is about 34 feet for cold water. A centrifugal pump can suck cold water from a water surface no more than 34 feet below the pump if it is primed (to remove air) but usually the suction lift is much less for practical considerations. These are the basics and I hope it helps a little, especially if you read more about pump operations. Good luck.

Answer 4

Just a couple more points. Water and other liquids (not talking about compressors handling gases here) are pretty well incompressible (IE volume is the same at different pressures) so a pump must deliver the same volume as it takes in at suction. The work done by the pump is a function of the difference in pressure over the pump and the flowrate. The pump is imparting extra energy in the form of extra pressure to that which exists at the suction.

Answer 5

Think about this practical problem:
If you have a 100 ft horizontal pipe full of water, and both site a exposed to the atmospheric pressure, there must be no flow, do you agree? Look at the scheme below:
Patm ---------------------------- Patm (no flow)

Now think: for the water to flow there must be a Delta P, OK? Look at the scheme below:
Patm +Delta(P) ---------------------------- Patm (there's a flow)

If you want to have the water flowing with atmospheric pressure on both sides of the pipe, you must install a pump on it. Look at the scheme below:
Patm -------------(pump)------------- Patm (there's a flow)

If you come from the right side of the tube to the discharge flange of the pump, the pressure increases, cause it's Patm+(head loss).
If you come from the left side of the tube to the suction flange of the pump, the pressure decreases, cause it's Patm-(head loss).
So a pump always takes a fluid from a lower pressure and sends it to a higher pressure. That's why suction and discharge pressure are not the same.
A pump is used to "defeat" a head loss.
I hope I have helped you.

Answer 6

In addition, in some real world situations if the pump is attempting to establish a discharge pressure beyond it means, then this could in affect slow down the speed of the pump by an amount that can reduce the flow by a noticeable amount. Think of a pedal bike with only one pedal and how hard it can be on the down stroke compared to the return stroke, especially if trying to go uphill.

Answer 7

it fairly is conceivable that the jet nozzle in the pump head is jammed with airborne dirt and airborne dirt and dust. take some screws aside and get the pump head off the motor. there's a cone shaped element in there that acts like a water pistol that forces the water up into the pump. if it nozzle gets airborne dirt and airborne dirt and dust or a chew in it then it won't be ready to squirt. i'm assuming you have a shallow nicely jet pump. a million pipe in nicely. in case you have 2 pipes into the nicely it fairly is a deep nicely pump and the nozzle is on the backside of the nicely.