|
Holy turbo chargers! When it’s a boost valve! Cavitation is a term which is often used in relation to pump suction conditions. However, it can occur at other system locations, one being at the actuator where negative loads cause a partial vacuum because pump flow cannot keep up. Air and contamination can be drawn into the cylinder rod seals. This aeration causes sponginess in the system, can damage the seals causing external leakage and can cause pitting of cylinder bore resulting in premature cylinder failure. Here we look at ways to help prevent this situation from occurring and provide a trouble free system. |
|
Aeration is usually addressed by employing counterbalance or suitable flow control valves which inherently reduce efficiency. Sometimes, it may be desirable to allow the load to run relatively freely while valuable pump flow is diverted to another function. In this case, fluid must be drawn from both the annulus and the tank where the negative load extends the cylinder. However, the potential for cavitation is increased unless care is taken to boost the tank line.
A pressure reducing valve normally limits pressure to a specific function while retaining higher pressure upstream of the valve. The function is however reversed when they are used in boosting circuits. They then act as a pressure maintaining valves.
The ERXX-S30 is a pressure reducing logic element which normally requires a pilot relief valve for it to function as a high flow pressure reducer.
However, it can be used as a stand alone valve, or with a specifically rated CV08 valve in its pilot line to maintain minimum pressure in exhaust lines, and thus prevent damaging cavitation, or to add stiffness to the circuit.
In this hypothetical loader circuit application the valve spends most of its life closed, as the normal downstream tank line pressure is above the valve's setting. The ER valve only opens when the tank line pressure decays.
It’s important to understand the particular physical circuit with respect to directional valve pressure drop, pipe lengths and sizes, and reservoir position.
Reservoirs are sometimes also pressurized to help prevent cavitation.
The circuit below illustrates where this boost feature may be applied in a loader circuit, where gravity can be utilized to allow the pump flow to be directed to other simultaneous functions.
During part of the cycle the bucket needs to be dumped as quickly as possible (also to shift the load at ‘rap-out’/bucket shake). The geometry of the bucket is such that the tensile load varies with vehicle inclination, bucket position and bucket content. It will go over centre at some indefinable point dependant on these factors.
Flow to the full bore side needs to be maximised in this runaway condition.
It can be regenerated from the rod side of the cylinders. This, combined with pump output, may not satisfy the high flow demand. High pressure drop in your directional valve and pipe line during recirculation and cold start-up can also cause cavitation.
The ERXX-S30 will act as an anti-cavitation safety valve, only opening when needed to boost the short-circuit path.
See circuit below.
To download the above boost circuit, CLICK HERE.
The pressure setting used here (90 psi) is determined by the pressure drop which is expected between the tank gallery and the cylinder inlet port. The 80 psi spring value is effectively increased by inserting a suitable CV08 in the dynamic pilot line.
In this circuit EP1 (EPxx-35) ensures that the Arm cylinder is not starved of flow when the bucket is initially actuated.
This circuit is a theoretical Loader example. Cylinder orientation, (extend or retract to dump) in loader systems vary due to the different linkage method (parallel or Z bar). Directional valves will often include a regenerative position but this can be quite restrictive due to spool flow limitations.
ER can be replaced with ECxx-32 where additional CV loading is not required.
The above circuit illustrates the following application enhancements:
1) It can prevent cavitation within the cylinder, increasing rod seal and cylinder life
2) It can allow pump flow to be fully utilized when a negative load does not require pump assistance. Calculated pump size may therefore be reduced.
3) It can provide stiffness to a system, improving response and positional accuracy.
At next month's NEW PRODUCT TRAINING, I will be presenting a related load-sense droop prevention application that will enable you to achieve faster response in dynamic systems, or cold start scenarios. I hope to speak with you then.
____________________________________________________________About the Author:
Jim Eaton is an Applications Engineer at HydraForce LTD in the U.K. He has been designing hydraulic systems for over 30 years.
