J_J
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Punp volume will try and remain the same at a set rpm.
Pumps, flow and restrictions.....
- Thread starter AKKAMAAN
- Start date
- Views: 12132
SPYDERLK
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- JD2010, Kubota3450,2550, Mahindra 7520 w FEL w Skid Steer QC w/Tilt Tatch, & BH, BX1500
Yes, except you are not able to decrease the output of a steady speed positive displacement pump. The restricted line causes V and P to increase in order to forcethe fixed output of our "perfect" pump thru that restriction. The fact that something will finally fail with greater and greater restriction is not part of the thot problem with the given boundary conditions [which are defined to exclude heat and pump and fluid imperfectness]. The amount of pressure rise with restriction is good to have a mental handle on tho so you can sorta "feel" the circuit in your head.
larry
Yes.
Also in a closed loop system as the aperture gets smaller the fluid may heat up, change characteristics, and result in a loss of NPSH to the point no fluid is being pumped.:thumbsup:
I do not like to use NPSH in in systems were we use positive pumps....NPSH belongs in "garden hose systems".....pressure supply from a water tower....CONSTANT PRESSURE as long as we have constant head (NPSH)....
In our hydraulic we use a positive pump, which is pressure supplied (suction) from the atmospheric pressure or lower (cavitation) or higher (pressurized reservoir).
A normal system will not get any substantial changes in fluid characteristics from "normal over heating"....so lets leave that part of the discussion out. It only confuses more than helps to understand the basics of this
Pump flow will react different to orifice restriction, depending on if it is a fixed displacement pump or a pressure compensated variable displacement pump.
If we condition that the prime mover can stay on constant rpm, and also leave all distractions about temperature and internal leakage, out of the discussion.....
A pump with a fixed constant flow (constant flow system, open center system usually) trough a restriction, will increase supplied pressure (flow will stay constant). To add an orifice in a constant flow system, to regulate speed, means that excessive flow have to be diverted, by cracking the PRV.
A pump with a pressure compensated variable displacement (constant pressure system have to be a closed center system) will always try to provide a constant pressure to the restriction, when restriction opens up a bit, pressure drops for a millisecond or two, pump compensator will react by increasing pump flow (adding length of piston stroke by turning the swash plate)....If restriction narrows, pump compensator will react by decreasing pump flow.
To add an orifice in a constant pressure system, to regulate speed, means that pump compensator will react by decreasing flow.
EDIT:To add an orifice to a constant flow system, to regulate speed, means that flow will stay the same through orifice, until pressure raises to the level that flow will start diverting (hopefully through the PRV
)
Diverting pressurized flow means LOSSES, the prime mover have to use energy to "waste" pressurized flow back to reservoir. Fluid will heat up!!
There are losses in constant pressure systems too, but we take those on another thread
If we condition that the prime mover can stay on constant rpm, and also leave all distractions about temperature and internal leakage, out of the discussion.....
A pump with a fixed constant flow (constant flow system, open center system usually) trough a restriction, will increase supplied pressure (flow will stay constant). To add an orifice in a constant flow system, to regulate speed, means that excessive flow have to be diverted, by cracking the PRV.
A pump with a pressure compensated variable displacement (constant pressure system have to be a closed center system) will always try to provide a constant pressure to the restriction, when restriction opens up a bit, pressure drops for a millisecond or two, pump compensator will react by increasing pump flow (adding length of piston stroke by turning the swash plate)....If restriction narrows, pump compensator will react by decreasing pump flow.
To add an orifice in a constant pressure system, to regulate speed, means that pump compensator will react by decreasing flow.
EDIT:To add an orifice to a constant flow system, to regulate speed, means that flow will stay the same through orifice, until pressure raises to the level that flow will start diverting (hopefully through the PRV
)Diverting pressurized flow means LOSSES, the prime mover have to use energy to "waste" pressurized flow back to reservoir. Fluid will heat up!!
There are losses in constant pressure systems too, but we take those on another thread
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My whole point with this discussion have been to show that it is a "myth" that we always can restrict flow (reducing actuator speed) with an orifice.....IT can be done, but there are conditions that have to be met....
I can see many posts where arguments about using larger fittings, hoses etc will increase flow....in a constant flow system, it can only happen by diversion of flow.....
However, larger fittings will reduce pressure, increase efficiency and save energy. Vice versa with smaller fittings....
When it comes to trouble shooting a system, this misunderstanding (myth), can easy put you on a totally wrong "path"....
I can see many posts where arguments about using larger fittings, hoses etc will increase flow....in a constant flow system, it can only happen by diversion of flow.....
However, larger fittings will reduce pressure, increase efficiency and save energy. Vice versa with smaller fittings....
When it comes to trouble shooting a system, this misunderstanding (myth), can easy put you on a totally wrong "path"....
Egon
Epic Contributor
It does happen out there in the big wide world.:thumbsup:
NPSH- well I don't really know what that means. Seen it somewhere though and thought it sounds good. Maybe it reminds me of the odd -35C evening and a 8 in. intake and a buried tank!!:laughing:
sorry to hear that your part of the "big world" have so rough conditions....LOL
So thats how you got your 13000+ posts....talking about stuff you don't know anything about.....
...head pressure might become a factor when fluid is closer to solid (frozen) or gas (boiling), than liquid....:thumbsup:
Egon
Epic Contributor
Actually its the type of fluid!:thumbsup:
What head pressure? Pump was above tank. I was cursing at the thingy that attached the hose to the pump and it was a heavy hose.:thumbsup:
There's a plethora of things I know a very little about, by Gar!
See Wikipedia for NPSH, head pressure
Great quote....Applies to me too, but I do not want to talk about those things.....
Love you mmurphy....you have the right attitude to my posts....yes, I try to scramble the wiring in your heads a little......and then I'll try rewire the "right way".....:laughing:
pushing oil (pump flow), is nothing complicated....just plain mechanics....just liquids instead of solids....add up ten cars on the street and push them from behind....1 car per second through the intersection (flow rate)....you can use the same force (pressure) as long as resistance is the same....if some one applies the brakes in the front car, you need to increase force to make 1 car per second through the intersection.....if you (prime mover or engine) "bog down"....less cars per second (decreased flow), but if you push harder, add another "horse" aka throttle the engine (increased pressure), car per second (flow) will stay the same.
More confused now??
Egon
Epic Contributor
Tried to pump some ice once!
Egon
Epic Contributor
Not only that the Beer was froze too!
Not only that the Beer was froze too!
Found this at DynaFlow website
Which means that is NOT the case (throttling flow with orifice) with a positive fixed gear pump flow.....
Link to DynaFlow Gear pump page....good reading thx Egon!!
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wdchyd
Elite Member
Totally, Totally Agree!!!!
Most people adding or designing hyd sys's think too much about $ when buying fittings, hoses and components but don't consider pressures,efficiency and heat issues.....(until problems come up).....
battling one now in my new Kubota B2320 w/BH.....New, dealer installed, all factory hoses and fittings......Darn thing heats up way too much after 1-2 hrs of digging ...I know it's wrong, Dealer tells me word from Kubota is 150 to 170 degrees is normal......
What's wrong with it is all the hoses are plumbed with 1/4 ID hyd hoses...(in & out & work ports).....You can hear them hissing from the pressure drop and WPR's prematurely kicking off...
This weekend I brought home a Flow Meter, QC's,& hoses to do some troubleshooting on my own......
My suspicion is the hoses are too small or crimped too tight.....Took a new Gates 1/4" ID hose end out of stock and measured the hole ID at .152.....this is before crimping!!!.....ARGGGG......(have not removed OEM hose yet)
Maybe you guys can help and do the math for me to see what the FPS velocity is going though these hose ends after flow readings are taken......the adapters look OK cuz the orifices are larger than .250"......
Sorry to ramble but this goes along with what AKKAMAN's been saying about slowing thing's down with restrictors.....I don't like them and don't install them....(accidentally or intentionally):2cents:
If your Kubota have a constant pressure system, restrictors wont be that "bad"...constant pressure systems with variable pump, do not divert flow...
I general I agree that manufacturers never over size any lines...always at low end of sizes...
Calculate pressure drop in hoses and fittings
or use two identical gauges...one in the beginning of the line and one at the end of the line.....problem though is to be sure they are calibrated the same....there are also special pressure drop gauges
