Valve Stiction Problem Cured by Soft Particle Removal

Mark Beesley, Fortum O & M (UK) LTD
Tags: hydraulics

Westferry Printers in London prints The Daily Express, The Telegraph, The Observer, The Star and The Financial Times, among others. Westferry is Europe’s largest newspaper printing facility, producing 18 to 20 million newspapers and supplements each week. For some time, Westferry faced a hydraulic problem that set off a failure chain reaction. It started with a short pressure loss in the power pack after start up. This pressure loss tripped a safety device and brought the printing process to a halt ultimately costing the company thousands of dollars in lost production. To keep the presses running until a permanent solution was found, a three-second delay was programmed into the machine’s computer to allow the operating pressure to be restored before the interlock cut in.

Results from the oil analysis tests showed the oil to be clean and within acceptable limits. Hydraulic experts were then consulted but to no avail. In a final effort to eliminate the pressure-loss problem before replacing up to 200 valves, Westferry decided to try an electrostatic liquid cleaner (ELC). The ELC was connected to the tank as an offline system. Within one month, the pressure loss had reduced so much that only a two-second delay was required to keep the presses running. Within two months, the pressure-drop problem was remedied and the delay was no longer required.

The pressure loss was attributed to slow servovalve operation, caused by increased friction due to oxidation buildup (varnish) and the subsequent reduction of clearance between valve spool and bore. The ELC removed the insoluble oxidation products in the oil that lead to varnish and potential valve stiction. Westferry was so impressed with the results of the ELC and the improved performance of the servovalves that it began implementing a program to install the ELC systems throughout the site. ELC systems have been commercially available for some time. Used by many of the largest and best-known companies in the world, electrostatic liquid cleaning deals with oil contamination differently from conventional methods of contamination removal. The ELC targets one of the most common causes of problems of hydraulic system reliability.

Soft Contaminants
To understand how the ELC operates, one must first understand oil concentration contaminants, and their effect on system reliability. There are two basic types of solid contaminants within a hydraulic fluid: Hard particles (wear metals, dust and fibers, etc.) and soft particles (oxidation products, sludge and depleted additives).

In many cases, significant portions of contaminants are soft (Figure 1).

Figure 1. Relative Concentrations of Soft and Hard Contaminants

Many organizations commonly report only hard contaminants by particle counting (ISO or NAS). In reality, even though system users keep their oil’s solid contamination below the maximum levels recommended by OEMs, they may still experience reliability problems. This is because a low particle count doesn’t always mean the oil is contaminant free; in fact, the chances are it isn’t.

Gravimetric Analysis
It is possible to measure insoluble oxidation products and other very small soft and hard particles in oil by gravimetric analysis (ASTM F313-78, ASTM D2387-65T or ASTM D4055-97). This method uses a submicron membrane, which collects contaminants (both hard and soft) and measures their collective weight. First, a known volume of oil is measured and mixed with a solvent such as petroleum ether, which decreases the viscosity of the oil to allow it to pass more freely through the membrane. Once this is done, the insoluble contaminants remain on the patch, which can then be dried and weighed, giving a contamination level in milligrams per 100 milliliters.

Monitoring and removing the soft contaminants is a major part of a preventive and proactive maintenance program. Contamination control is vitally important to the long-term health of the machine, but this should not fail to include the control of submicron soft particles.

Varnish and Stiction
The lacquers or varnishes for instance, can have a major effect on the performance of valves, particularly as today’s modern servovalves have clearances of less than three microns. The layers build up over a period of time, reducing the valve clearances so that oil cannot pass between the body and spool to lubricate the surfaces (Figure 2).

Figure 2. Silt Particles Migrate into the Clearances Between the Spool and Bore, Increasing Friction when the Valve is Actuated.

This causes erratic valve operation and ultimately even seizure. In terms of a system this leads to:

This process is not aided by many of the small particles, which are often too small to be filtered. As they flow through the system, they can become glued to the sticky varnished surfaces. Interference occurs between the sliding surfaces, causing control problems. Many people who have taken apart servovalves for inspection or cleaning see the very fine scoring on the body and spool of the valve caused by these contaminants.

Other problems caused by insoluble oxidation products and other soft particles are:

Electrostatic Cleaning
It is a good maintenance strategy to remove oxidation products as they are produced, before they build up within the system. One option for removing insoluble soft contaminants is electrostatic separators. This process has been quietly gaining a reputation around the world as an effective solution for varnish and sludge related reliability problems.

Electrostatic separators are effective because mineral, synthetic and vegetable oils have low electric conductivity and dielectric constant. Also, most insoluble contaminants suspended in oil have polarity or electric charge, regardless of their size or composition. By passing an electrical current through the oil, these contaminants can be removed by electrostatic precipitation onto collection plates or other suitable media. The process has been reported to remove single contaminants less than 0.05 microns, but more importantly it allows the oxidation products to agglomerate, cleaning the oil at a molecular level.

This method of separating contaminants has been shown to effectively remove oxidation byproducts, silt-sized particles and used additive floc. Tests suggest that it does not remove active, dissolved additives. Because the electrostatic field is high voltage/high potential, the units are equipped with shut-off mechanisms that are achieved by current at just a few milli-amps (Figure 3).

Figure 3. Charged Particles are Attracted to the Plate with the Opposite Charge in the Electrostatic Field.

As a result, the units can’t operate when water contamination is present, due to its poor dielectric properties.

With today’s rising oil costs and disposal costs, coupled with the environmental impact, this type of technology can significantly extend the oil life as well as provide a modern oil cleaning system for enhanced machine reliability.