Suction Line Pressure/Cavitation

[reelfish]

We have a positive displacement pump and was told in a training class that we should have around 3 psi on the suction line side of pump.
Is oil pushed into the pump or sucked in? We hear alot either way but dont know who to believe.
We hooked a gage to te suction side and pulled a small vacuum of 3.
Were having pumps going out thinking cavitation prolems. This is just one place for us to start. Any suggestion?

Thanks

 

[Artisi]

Pumps DON'T suck. A pump will /can lower the pressure at it's inlet to allow atmospheric pressure to PUSH the fluid into the pump.

 

[25362]

IMHO, suction pressures in rotary pumps are needed to fill the teeth or thread cavities quicker than the cavity travel speed. The fluid viscosity dictates the conditions needed to satisfy that requirement for a given pump speed.

 

[MortenA]

You fail to mention to things:

-Pumped media (and temperature)
-Specific pump type

So we can do as 25362 and guess the type - or you can tell us the above mentioned data. Cavitation in a PD pump and in a centrifugal pump is not the same - and the likelyhood will wary from suvtype to subtype. The "3 psi" rule of thumb would most like only refer to e.g. water. Other liquids have higehr boiling point thean 3 psia (another thing- try to remember specifying gauge or abs. Its a source for frequent mistakes).

Best regards

Morten

 

[micalbrch]

reelfish,

As MortonA wrote it would be good to know what PD pump you are talking about. That makes a big difference.
Artisi's explanation is physically 100 % correct although all users (and manufacturers, too) talk about suction capability of a pump.
I never heard of the "3psi rule" but I'm European. What is perhaps ment is the 3 % drop in flow that is defined as the point where a pump starts to cavitate and which is used to determine the required NPSH value of a pump.

regards

Michael

 

[reelfish]

Thanks for the replys guys. Its is a Parker PAVC 100 rotary piston variable displacement pump. Max 50gpm 120 degree run temp.

 

[micalbrch]

That is a hydraulic pump (for oil hydraulic circuits). These pumps can "suck" (= lift a head) but are very limited in this and should have a positive suction head, especially if the fluid is warm or hot. As far as I know these pumps must be initally filled before start-up.

 

[25362]

When there is vacuum at the suction, entrained and dissolved gases are released and expanded reducing the delivered capacity. The noisy and inefficient resulting operation is referred to by many users as cavitation.
Recommended reading:Pump Handbook[/b[ by Karassik et al. Chapter 3. Ed.: McGraw-Hill.

 

[tech56]

We are having pre-mature hydraulic pump failures with our hydraulic driven equipment
We are concerned that these failures are due to our NPSHa is less then our NPSHr.
Our pump is a rotary piston type pump, variable displacement; the oil temperature is maintained at 120 degrees.
Question: Can a combination gauge on the suction side of the pump confirm this, and what kind of pressure should we be seeing?

 

[micalbrch]

Installing a gauge on the suction definitely helps to get an idea what happens. But what the gauge must show is something nobody can answer without have the following information: Is the oil tank above the pump? If not, how far below the pump's inlet is the oil level in the tank? What size and how long (vertical and - if applicable -horizontal) is the suction pipe?

 

[CH5OH]

good operating practice would be to keep a header tank well above suction of a hydraulic rotary piston pump.Also the suction line should be oversized and kept as short as possible(bigger diameter than pump inlet).As the pressure inside the piston bores rapidly goes from vacuum to discharge pressure,entrapped air/oil vapour will boil out on suction stroke, the bubles then collapsing at discharge stroke(cavitation).Problems of such kind will show pale spots (micro pitting) on the shiny material on piston heads and rotary valve plate (on the edge of discharge port).Also good practice is to keep oil temperature in suction header tank between 60 and 80 degC (cooler in return line hydraulic system)
Although you can dismantle a pump like that for failure analysis, to overhaul them I would not recommend such action

 

[reelfish]

I attached a photo of our tank and pump, I hope it shows up. The suction line on the tank is 24" from the deck. The top of pump is ref 11" from the deck.
The strainer in the tank sets about 1" from bottom. Oil level is 20" from deck. Tank size is 19"X32"x25"h
We dont think were getting air into the suction line after pump is shut down. The line and pump should stay primed? After machine is down for a day or two, when starting up it makes alot of noise(cavitation?)at start up. We dont know if our lines are getting air, air getting in after shutdown, pulling to vacuum?

 

[25362]

Not necessarily entrained air, just dissolved air would serve to create the "cavitation" effect when the suction is under vacuum.

 

[CH5OH]

what a crappy installation(excuse my French)
my advice:
cut the skid next to the tank
weld four legs besides the pump and motor
make a hole in the bottom of the tank and weld a pipe in the hole in such a way the pipe protrudes the tank about 3inch (this way, if you have sediments in your tank, they don't get sucked in the pump).install the tank above the motor and pump.On the outside mount a ball valve on that pipe.next connect the ball valve to the flexible suction hose (the suction line should be a straight line going down to your pump inlet).the two little flexibles mounted besides the suction hose are leakage lines, I assume.They can stay there.the suction strainer, get rid of it (the oil you buy from the supplier should be clean).you have a HP filter to protect your installation if the pump decided to loose some internals.the installation return line going back to the tank should have a filter ( I assume its the grey part behind the tank breather?This way you catch dirt, if the installation decides to loose some internals.
The way its built now you always end up with an air pocket (the piece of suction pipe which goes higher than the oil level in the tank).I hope the squaire lit is just an inspection cover or for filling up the tank.If it is the cover of some sort of suction strainer.....well than you really asking for it.

 

[reelfish]

Thanks for response!
You say I will have a n air pocket in the highest part of the hose above tank. Wont atomspheric pressure keep line full, isnt there know way air can get into line?

 

[CH5OH]

atmospheric pressure=level of tank (communicating barrels)
when torricelli turned a tube filled with quicksilver into a bucket of quicksilver, he didn't create vacuum, he created a pressure equal to the vapour pressure of quick silver .in hydraulic system there is always an amount of air dissolved in the oil. this will boil out when pump is stopped,and fill the air pocket (air pocket being the term used for piping if that goes up and down again)

 

[hydtools]

Plumb the pump low into the side of the tank. That keeps the oil level above the pump suction. Flooded suction. Get rid of the suction strainer. Is the return flow baffled from the pump suction so that return flow does not go directly to the pump intake.

If there is an air leak in the current suction line, that will allow air to enter. Both when the pump is running and not running.

Ted