Pumps, flow and restrictions.....

   / Pumps, flow and restrictions..... #41  
Punp volume will try and remain the same at a set rpm.
 
   / Pumps, flow and restrictions..... #42  
Something is still not right here. If you start decreasing the output of the hose/line, with smaller orifice, the pressure will increase, and the velocity through the orifice will increase. The smaller the orifice, more pressure is developed, until you close it off completely, and the pump develops burst pressure. Is this correct?
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
 
   / Pumps, flow and restrictions.....
  • Thread Starter
#43  
Something is still not right here. With a constant rpm, on a gear type pump, the volume through a tube will be the GPM's for the pump, with no or little pressure. If you start decreasing the output of the hose/line, with smaller orifice, the pressure will increase, and the velocity through the orifice will increase. The smaller the orifice, more pressure is developed, until you close it off completely, and the pump develops burst pressure. Is this correct?
You Nailed it J_J!!:thumbsup:
 
   / Pumps, flow and restrictions.....
  • Thread Starter
#44  
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
 
   / Pumps, flow and restrictions.....
  • Thread Starter
#45  
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:D
 
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   / Pumps, flow and restrictions.....
  • Thread Starter
#46  
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"....
 
   / Pumps, flow and restrictions..... #47  
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

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:
 
   / Pumps, flow and restrictions.....
  • Thread Starter
#48  
It does happen out there in the big wide world.:thumbsup:
sorry to hear that your part of the "big world" have so rough conditions....LOL

NPSH- well I don't really know what that means.
So thats how you got your 13000+ posts....talking about stuff you don't know anything about.....:D...

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:
head pressure might become a factor when fluid is closer to solid (frozen) or gas (boiling), than liquid....:thumbsup:
 
   / Pumps, flow and restrictions..... #49  
sorry to hear that your part of the "big world" have so rough conditions....LOL

Actually its the type of fluid!:thumbsup:


head pressure might become a factor when fluid is closer to solid (frozen) or gas (boiling), than liquid....: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!
 
   / Pumps, flow and restrictions..... #50  
Egon, NPSH is very important when pumping water at 7000 GPM!!!!! Them pumps scare the **** out of you when that is lacking!!!!!:laughing::laughing:
 

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