Suction Filer Sizing for Gear pumps - Any advice ?

[Losty]

How do we size up a suction filter for gear pumps ??

Key concerns may include:

1. Pressure drop across the filet
2. Maximum vaccume pressure on the inlet of pump

What implication the above factors have on the filter sizing ??

Thanks in advance

Losty

 

[maytag]

Losty, I have had nothing but BAD experience with any suction filter regardless of pump configuration. Filtering the new oil as it is added,return filters and a kidney loop filter systems are a lot better solution-at least that has been my experience. Maytag

 

[budt]

Losty;;

When you say "suction filter" I trust you are talking about "strainers." The only good way to use a Suction Filter is to have another pump of higher volume pushing oil through the filter into the inlet of the working pump. This is usually done with a cheap and dirty gear pump with its outlet going to the filter and the filters outlet going to the working pumps inlet. A relief valve, set at 5-15 PSI, is teed into this circuit between the filter and the working pump. This is a type of Off-Line filter with its outlet feeding the working pump. This circuit must have a pressure switch in the line between the gear pump and the working pump so the working pump cannot run unless there is some minimum pressure in the filter circuit.

Normally I use a 140 micron Suction Strainer with a 3 PSI Bypass on the working pumps inlet, except on Rexroth axial piston pumps since they will not replace any failed pump of that type if it had a suction strainer on it, to protect the pump from large particles left in the tank during assembly of the circuit. This is only meant to protect the pump from catastrophic failure from debris that may have been left in the tank for any reason. A Vacuum Pressure Switch can be added to signal increasing vacuum in the suction line.

My reasoning for this setup is, the pump is protected from startup problems and component failure and the suction strainer never gets clogged since the system is being contiually filtered with an Off-Line 3 Micron element.

I don't use Return Filters except in certain applications since flow through them is not continuos and smooth. Surging flow from oversize rod cylinders while retracting can open the bypass valve and send large particles into the tank and in some cases flow surges can destroy paper elements.

Filtration is very important for any hydraulic circuit and extremely important for Proportional and Servo operated actuators. Don't go the cheap rooute when specifying filters.


Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[95yukon]

Losty,

I agree with maytag. Kidney loop it, if you are getting heavy with more EH valving that is sensitive, it's wise to go with high pressure filters ahead of the components. Inlet screening on systems with any pump has given me nothing but headaches (and reworks).

Try to get the system kidney looped before startup if possible. New fluid is not always clean, but a procedure where you include a filter cart system to cycle the fluid through prior to startup can get a system cleaned up before you start it.

 

[danhelgerson]

Losty

Some years ago, Vickers published a booklet on preventative maintenance with filtration. It was designed to promote the sale of their filters but it contains some very useful information. It is a 33 page document in .pdf format. It is available on the Eaton web site at

http://hydraulics.eaton.com/products/pdfs/561.pdf

. I also have it on my web site where I have added bookmarks to make it a little easier to find information. My link is

http://www.cfpsos.com/Filtration.pdf

.

The primary mental adjustment that needs to be made is to not think in terms of the filter but in terms of the fluid cleanliness. When I ask people if their fluid is clean, I invariably get an answer describing the quality of the filters. Very few have taken the step of actually monitoring the actual cleanliness level of the fluid. Even fewer are aware of the ISO standards established for protection of the various hydraulic components.

This is all covered in the booklet. It would be wise to become familiar with this material before setting up any filtration system.


Dan Helgerson CFPS, AFPI, AJPP

http://www.cfpsos.com

 

[hydromech]

On the subject of cleanliness...

It does appear that most people that use this forum are from the U.S...you may be able to answer a question for me.

As a company based in the U.K we sell many hydraulic systems into the U.S.

When specifying cleanliness, are people in the U.S more likely to recognise the ISO 4406 standard or the NAS 1638 standard.

On suction filters/strainers...I always try to avoid them as I regard them as pump killers...If you have a customer that demands you install one...then use one with a by-pass.

Hydromech
Hydraulic Systems Engineer