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Post-start-up venting

Post-start-up venting

Post-start-up venting

I know that it may seem trivial to someone, but this question is rather interesting to me in recent period.

In machines built 5-7 years ago we still have habit of venting hydraulic system after major repair or oil change, especially in applications where stable and precise stroke is critical.

In new systems there is no any note in manufacturers' documentation regarding venting. I made contact to several manufacturers and some of them are stating that their system is self-venting, i.e. pump withraw all air to tank and breather does rest of the job.

But the position in system where we use to vent is dead spot at the end of the cylinder, and "primitive" venting by loosening cylinder connection and pushing piston toward end of stroke allways realeses at least a little air.

I cannot find any reference in literature as well. Suggestions are welcome.

RE: Post-start-up venting

When you say "venting", do you mena "bleeding"? Are you trying to remove air that is trapped in the system.

RE: Post-start-up venting

yes, the right term is bleeding! my english sometimes fails...

RE: Post-start-up venting

Drazen...Forget about the breather...It has nothing to do with air in the system. It is there to filter air that is drawn into the tank as the oil level drops.

I have found that the cleanest and safest way to remove air is to install test points in the pipework and cylinders. Then use a hose to slowly bleed the air off. This can be done in a controlled way and with the system under pressure.

If this is not possible...Then fully stroke all of the actuators and then loosen the fitting that is highest or in cases with dead zones...the fitting closest to the air pocket.

Bleeding systems with dead zones in it can be a messy business. If you can I suggest you use a testpoint and hose.

There are no rules for bleeding hydraulic systems. There are  
only guidelines to follow. Each type of hydraulic system will trap air in a different place...It's down to the person doing the bleeding to track the air down and get it out.


RE: Post-start-up venting

Why bleed?  The answer to this question answers some of the others.  There are two problems with some air in a hydraulic system.  The first is a "sponginess" that will occur because of the compression of the gas and the second is the potential for damaging the pump and/or motors.

Generally speaking, a hydraulic system will remove any trapped air and bring it to the reservoir where it is dissipated.  The normal turbulence in the system will entrain any free air and move to the reservoir in the form of bubbles.  A properly designed reservoir will allow the hydraulic fluid to sit idle for about two minutes before being drawn into service again.  This provides enough time for the bubbles to float to the surface and be released through the breather.  By allowing the hydraulic fluid to dwell in the reservoir for at least two minutes, we are sure that the air bubbles will not be drawn into the pump.

If air bubbles are drawn into the pump, they cause two problems; one is a lack of lubrication and consequent wear; the other is noise when the air bubbles suddenly collapse as they enter the pressure side of the pump.

With new installations or major repairs, there may be enough air in the system to warrant the bleeding process as a matter of saving time.  If the actuators move relatively slowly, there may not be enough turbulence to capture the air.  Or, if the connectors to the cylinders are so long that the hydraulic fluid from the cylinders never gets back to the reservoir, there may be cause to bleed the system.  Otherwise the air will literally work its way out.

RE: Post-start-up venting

Couple cautions:
1. Loosening the hose fitting, as done for years with flare fittings, should be avoided with ORFS face o-ring seals. The o-ring can partially extrude and cause leaks. I definetely prefer putting a short hose on a test port quick coupler to bleed air.

2. Bleeding air by operating the functions back anfd forth must be carefuly thought through. Most control devices (meter out flow control, counterbalance, servo valve, etc) depend on metering an incompressible liquid flow, in compression, out of the load to maintain control. If there is air in the system the cylinder can move dramatically by compressing the air. The flow controls have no effect.

If the load is resistive load only (cylinder is always pushing against something resisting motion) cycling the function back and forth in both directions may work easily.

If the load is overrunning (load will move on its own in same direction the cylinder is moving, say lowering a weight) it will generally be already in the down position when started up and the air will be compressed when the circuit is operated to lift. The lift side will get filled (or at least the air is compressed and stays compressed) when raising the load, and control is maintained when the valve shifts to lowering.

The most dangerous scenario is when a load starts out resistive but goes past center into overrunning. This could be a cylinder, crank or toggle mechanism folding a conveyor or implement attachment. One side of the cylinder starts to pressurize to raise the load, the air is compressed, and it moves slowly. Once it goes past center, the other side of the cylinder must resist the overrunning load. If there is only air in that side, it will compress as the load slams uncontrolled. The air must be removed, or the load controlled with some external method.


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