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A work in progress
New User
Posted
Hello all;

I have a question concerning the size of a suction line on a system. I work in the mobile equipment industry, therefore we deal with axial piston pumps, and cavitation is always a concern. It is common practice to oversize the line that goes from the hydraulic oil tank to the inlet of the open circuit pump. I understand that this is to prevent a lack of oil at the pump inlet and therefore avoid cavitation of the pump.

My question is this:

Sizing a line larger then the Size of the inlet port on the pump means less friction in the suction line and therefore less back pressure to your tank and less constriction of flow to the inlet? Even though the inlet will create a constriction being a smaller cross sectional area then the line from tank to pump. IE: Does the line diameter really matter as long as it is bigger then the inlet port?
Of course it does or people wouldn't oversize it. so... Why does it matter?

Also,

Is there a standard for how big you should go relative to the size of the pump inlet? Or is it a combination of a bunch of factors, ie: pressure in hydraulic tank, Head pressure from oil level and height of tank, and size of pump used/size of inlet port?

Dazed and confused
 
Posts: 2 | Registered: 23 July 2008Reply With QuoteEdit or Delete MessageReport This Post
Bud T
Bourdon
Picture of Bud T
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To make a long writeup short, take a look at what I learned over the years on sizing Hydraulic Flow Lines in the Basic Ebook here:
http://www.hydraulicspneumatics.com/200/eBooks/
Look in Chapter 3 Pages 4-5 and Chapter 8 Page 19.


Bud Trinkel
FP Consultant Retired
"Fame is a vapor, popularity an accident, riches take wings. Only one thing
endures, and that is character." -- Horace Greeley
 
Posts: 1271 | Location: Newburgh, Indiana | Registered: 07 October 2005Reply With QuoteEdit or Delete MessageReport This Post
Josh Cosford
Pascal
Picture of Josh Cosford
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Bud's own words on suction line:
quote:
Fluid velocity should not exceed 2 to 4 fps. The reason for this recommendation is that the highest possible pressure drop in the pump inlet line is one atmosphere. Actually, no type of hydraulic pump can even come close to this, so most inlet lines never see much more than 3- to 4-psi vacuum. Using velocity higher than 2 to 4 fps dramatically increases pressure -- causing cavitation and pump damage. It is best to use a suction line equal to or larger than the size of the pump inlet being plumbed. There are circumstances when a smaller suction line is satisfactory, but only do this when absolutely necessary and with the supplier's approval.

The suction line should be full size; as straight as possible; have no or the minimum number of fittings; never include a standard pipe union; and be completely sealed. Using hose in place of pipe or tube can overcome many possible suction-line problems. Hose is a viable alternative and is quite satisfactory if certain precautions are addressed. Always use hose designed and specified for suction (vacuum) use. Hose normally used for pressure may be rated at 3000 psi but is not suited for suction lines. The reason for this is pressure hose uses an inner lining like a tube in a tube-type tire. The outer layers are strong but they are porous and would leak high-pressure fluid except for the inner tube. High-pressure hose as a suction line sees constant negative pressure trying to collapse the inner tube. After some time, it is possible for the inner liner to be drawn in, restricting flow and causing pump cavitation. This phenomenon may not happen immediately, but usually does cause problems in time.


Josh Cosford
FPH
 
Posts: 217 | Location: Hamilton, Ontario | Registered: 04 June 2008Reply With QuoteEdit or Delete MessageReport This Post
Josh Cosford
Pascal
Picture of Josh Cosford
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Also, to calculate velocity:

ft/sec=(.3208 X GPM of pump)/Area of suction hose


Josh Cosford
FPH
 
Posts: 217 | Location: Hamilton, Ontario | Registered: 04 June 2008Reply With QuoteEdit or Delete MessageReport This Post
A work in progress
New User
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Thanks guys,

Yeah, next time i won't be so quick to post a question before doing a little more research, try not to waste everyones time if the answer is readily available, thanks again for replying, i appreciate the patience.

DAzed and Confused
 
Posts: 2 | Registered: 23 July 2008Reply With QuoteEdit or Delete MessageReport This Post
IceStationZebra
Darcy
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Plus remember that the pressure(vacuum) drop is cumulative. A larger inlet pipe equates to a lower total inlet vacuum.

You also need to remember that off-road vehicles need to deal with cold weather start-up. Hopefully the operator is smart enough to let the oil come up the temp, but not always!

ISZ
 
Posts: 17 | Location: Wisconsin (currently) | Registered: 15 February 2007Reply With QuoteEdit or Delete MessageReport This Post
kevin j
Pascal
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And it is frustrating that the pump manufacturers often provide inlet ports smaller than their own recommendations call for....

For example, hydrostatic charge suction ports of 1 inch. Pump manufacturer, on another page, recommends 2-5 ft/sec, when the 1 inch port would already mean 6-8 ft/sec.

Most hydros we end up with jump size or specially welded up fittings to get larger hoses than the port size.

For mobile equipment in cold temps I am especially conservative and make large suction lines.
 
Posts: 151 | Location: Minneapolis MN USA | Registered: 02 November 2005Reply With QuoteEdit or Delete MessageReport This Post
Bud T
Bourdon
Picture of Bud T
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Kevin;
Maybe we need more TRAINED/DEDICATED persons in the Fluid Power Industry??????????????

The Mechanical and Electrical fields have pretty much eliminated these lack ofknowledge problems due to TRAINING and often CERTIFICATION.

Oh well, Someday Maybe.


Bud Trinkel
FP Consultant Retired
"Fame is a vapor, popularity an accident, riches take wings. Only one thing
endures, and that is character." -- Horace Greeley
 
Posts: 1271 | Location: Newburgh, Indiana | Registered: 07 October 2005Reply With QuoteEdit or Delete MessageReport This Post
John73
Bernoulli
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Is it safe to say that if the manufacture choose a particular suction port size for their pump and you sized this pump to fit your system that the corresponding suction hose size should be appropriate?
 
Posts: 3 | Registered: 20 October 2008Reply With QuoteEdit or Delete MessageReport This Post
Bud T
Bourdon
Picture of Bud T
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quote:
Is it safe to say that if the manufacture choose a particular suction port size for their pump and you sized this pump to fit your system that the corresponding suction hose size should be appropriate?

Generally speaking and for short runs that works well.

Then there is the person who runs a pump at twice the rated speed or who mounts the pump 20 ft. from the tank or 15 ft. above it or has 6 elbows to route the suction line around some in the way obstructions.

I've seen all the above, especially on mobile circuits. After all it's just an old pump and can take care of itself.


Bud Trinkel
FP Consultant Retired
"Fame is a vapor, popularity an accident, riches take wings. Only one thing
endures, and that is character." -- Horace Greeley
 
Posts: 1271 | Location: Newburgh, Indiana | Registered: 07 October 2005Reply With QuoteEdit or Delete MessageReport This Post
whywhyzed
Bernoulli
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As the original post was referring to mobile applications, I'll add that on some mobile applications where the diesel is turbocharged, boost is fed into the pressurized hydraulic tank. Of course a check valve is needed to prevent the diesel from running away on hydraulic oil....
They tend to do this where the series of pump being used is at or near it's maximum displacement.
Cat 345B excavator is one machine where this is done. They used the Rexroth A8V0 with the max angle screws backed right out.
An earlier model Cat exc. actually used a 24V air compressor to boost the tank.
 
Posts: 7 | Registered: 11 November 2008Reply With QuoteEdit or Delete MessageReport This Post
AKKAMAAN
Reynolds
Picture of AKKAMAAN
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quote:
Originally posted by whywhyzed:
As the original post was referring to mobile applications, I'll add that on some mobile applications where the diesel is turbocharged, boost is fed into the pressurized hydraulic tank. Of course a check valve is needed to prevent the diesel from running away on hydraulic oil....
They tend to do this where the series of pump being used is at or near it's maximum displacement.
Cat 345B excavator is one machine where this is done. They used the Rexroth A8V0 with the max angle screws backed right out.
An earlier model Cat exc. actually used a 24V air compressor to boost the tank.

This is another option to pressurize the tank
Need at least one frequent actuator in system with cap/rod area ratio <>1


Per A
Retired Swedish forest engineer
 
Posts: 54 | Location: Port Angeles WA USA | Registered: 24 September 2008Reply With QuoteEdit or Delete MessageReport This Post
Peter Nachtwey
Pascal
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So how much force will there be when you multiply 4 psi times all the surface area of the tank? It would need to be much stronger. We made an electrical cabinet once and just pressurized it to 1/4 psi to keep the dust and moisture out. The cabinet bowed outwards and was deformed.

I like the idea but one can't just pressurize a tank without taking some care. I don't know that much about hydraulic tanks and what pressures they are rate for but I would find out before I tried this trick again.


Peter Nachtwey
Delta Computer Systems, Inc.
http://www.deltamotion.com
"Living is easy with eyes closed, misunderstanding all you see..." John Lennon
 
Posts: 325 | Location: Vancouver, WA | Registered: 09 August 2006Reply With QuoteEdit or Delete MessageReport This Post
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