Coping with Specialty Fluids for Hydraulic Testing #1: Phosphate Ester

This is the first in a series of articles outlining the system requirements and considerations when using specialty fluids for product test stands.

Since first posting this blog on back in July of 2014, I have witnessed several applications which have developed issues relating to the use of Phosphate Esters. Therefore, I thought that I would post a quick update on some of the trials and tribulations of dealing with Phosphate Ester (PE) fluids witnessed over the last few years.

Fitting Compatibility– Many of the PE systems in use today require test pressures to 10,000PSI. While stainless steel tubing and fittings are the norm, there was one system that presented difficulties due to the material compatibility. The high pressure lines utilized stainless steel bite type and O-Ring Face Seal Fittings (ORFS), but there were also some larger diameter low pressure return lines required. The hydraulic pipe fitters utilized a common 90 degree flanging system on the smaller diameter tubes, but didn’t have the machine capacity for the 1-1/4” and larger tube sizes.  To solve this they utilized brazed-on face seal fittings on the larger diameter lines. Unfortunately, they elected to use the fitting manufacturer’s common pre-formed braze rings during assembly. The braze rings apparently contained a higher than expected amount of copper, which reacted almost immediately to the PE, resulting in several leaks. Needless to say, all of the brazed line had to be replaced.

Paint Compatibility– As anyone who has experience with a PE system knows, painted surfaces and PE’s are not compatible. I recently witnessed a system in which the pump to motor adapters, which were mounted vertically inside power unit, were supplied painted. The system manufacturer didn’t catch this, the paint failed upon the filling of the unit, and contaminated the entire system. While there are some manufacturers now touting PE resistant paint or powder coatings, we have yet to see a long term solution to this issue.

Material Compatibility– A system was built utilizing all PE compatible components, but after about three months of operation, rubber particles started to appear within the fluid. The system still seemed to operate correctly but the mystery of the rubber’s source was an ongoing concern. After several tear-downs and inspections, it was determined that the PE compatible accumulator was supplied with a Buna-N seal on the lower half of the poppet valve. While this was just an error of someone at the manufacturer pulling a part from the wrong bin, it outlines to what extent everything must be checked during assembly.

Viscosity– I recently saw a hydraulic system in which the manufacturer simply converted their standard petroleum unit design over to a PE compatible unit. Unfortunately, the change in viscosity of the PE fluid was never taken into account during this process. Upon the first system emergency stop, and the rapid venting of the accumulator’s fluid volume, the excessive back-pressure in the reservoir resulted in the end covers being blow-off the unit.

Experience with phosphate esters, and all specialty fluids, is essential when selecting a test system supplier. For more information please contact Genuen (formerly Wineman Technology) for all of your testing applications.

Fire-Resistant Phosphate Ester Aviation Hydraulic Fluids

Since the early 1940s, aircraft companies have been using phosphate ester (PE)-based hydraulic and lubricating fluids extensively in critical systems where fire-resistance is a requirement. While manufactured by several companies, and product branded under various names such as Exxon Mobil Corporation’s™ HyJet V™ or ICL Industrial Products’ (IP)™ Fyrquel®, this fluid is most commonly referred to as Skydrol® which is actually a product of Eastman Aviation Solutions™. Easily identified by its purple color, PEs present several challenges to the design, operation, and servicing of Servo Based Hydraulic Test Stands in today’s marketplace.

Health and Safety Requirements

Due to the acidic-based compounds, phosphate ester fluids are an irritant to human tissue. Gloves and goggles are recommended as a minimum in safety equipment when servicing PE systems. In addition, animal studies have shown that repeated exposure to some phosphate esters may cause urinary bladder damage.

Skin Protection

Skin contact should be avoided as PEs will dissolve away some of the fatty materials and can dry the skin. Contact with the skin can create an itchy, red rash with a burning sensation that feels similar to sunburn can occur. The effects generally subside within a few hours and castor oil applied to the affected area can neutralize the burning. Once the phosphate ester is removed by washing, the pain should subside. If left unattended, this could result in complications such as dermatitis or even secondary infection from bacteria. A glove material such as nitrile, nylon latex, or polyethylene "throw-a-ways" is recommended when handling these fluids.

Eye Exposure

PEs contain mild organic acids that give a slight burning sensation and, when in contact with the eyes, can cause severe pain. First aid is recommended by washing with tap water or a standard eye irrigation solution. 15 minutes of washing with water will usually be enough to remove the fluid and cause the pain to cease. Secondary treatment should be sought by a medical professional, as possible infections could occur. Always remember to wear safety glasses or chemical goggles to prevent eye exposure when working around phosphate esters.

Inhalation of PE Fluids

Upper respiratory tract irritation, including nose and throat irritation and tracheitis and/or bronchitis, can occur from inhalation of PE fluid mist. People with asthma may demonstrate a more marked reaction. When mist or vapor is possible because of high-pressure leaks, or from any fluid hitting a hot surface, a respirator capable of removing organic vapors and mists should be worn. In all cases, users should review the fluid’s Material Safety Data Sheet (MSDS) and fully understand the effects and treatments required by the specific fluid being used.

System Design Requirements

System design is always a challenge when phosphate ester fluids are utilized. As PE fluids react adversely (and sometimes dramatically) with many materials, care should be taken in all aspects of component selection. This includes secondary components such as wire coatings, electrical connectors, grommets, gaskets, and seals.

Component Materials

PE fluid reactions can be quite extreme and can occur quickly with many materials used on standard petroleum hydraulic systems. Care should be taken to avoid selecting components with external or internal materials such as copper, acrylic, most thermoplastics, vinyl, PVC, cork, leather, or anodized aluminum.


Seal material selection is critical in hydraulic components. Components utilizing butyl, nylon, PTFE, EPR, or Kalrez® seal material should always be used. Because of the limited amount of components in today’s market, many components must be converted for use with PE fluids by secondary sources. 


Epoxy primers are compatible with PE fluids, as well as two-part polyurethane top coats. Generally, most manufacturers who build test systems utilizing PE fluids will use stainless steel reservoirs and structures to avoid the ultimate failure of painted surfaces. In controlled lab environments, unpainted carbon steel is also commonly used, but should be maintained with a light oil coating on a regular basis. 

Reservoirs and Conductors

Phosphate esters, which are mildly hygroscopic, must be protected from medium- to high-humidity atmospheres. In order to keep moisture in the air from absorbing into the fluid, fluid reservoirs should be designed as sealed systems, or desiccant breathers utilized to absorb moisture from the air. Flat-faced O-Ring fittings seal material must also be changed out to be compatible and avoid potential failures. As many standard hydraulic hoses contain some form of Buna-N (NBR), products specifically manufactured for EP fluids must also be selected.

Because phosphate ester can degrade products quickly and can catalyze further degradation, systems containing PEs require a rigid preventative maintenance and condition monitoring plan as part of the lab’s regular routine.

Other Requirements

Other areas to consider when using phosphate ester fluids are items normally not taken into account when exposed to standard hydraulic fluid systems. These can be compatibility issues related to plastic or coated tools, synthetic fiber clothing worn by employees, shoe or boot sole materials, and computers located with the immediate exposure area.

Regular fluid testing should also be conducted on a compressed timeline basis. The percentage of acid contained within the fluid should be closely monitored along with particle contamination. Frequent particle counts and contaminate material make-up can provide an indication of possible component breakdown and immanent failures associated with PE fluids.

For more information on hydraulic servo-based test systems for specialty fluids, please contact Genuen for a system audit and recommendations today.

 Interested in learning more? Read the next post in this series, Coping with Specialty Fluids for Hydraulic Testing #2: Automatic Transmission Fluids.

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About Genuen

Our goal is to improve time to market without compromising product quality or safety standards. With experience in mission-critical applications and regulatory compliance, Genuen creates custom test systems across the product lifecycle, including hardware-in-the-loop (HIL), fluid power test, and electromechanical test. Headquartered near Kansas City, we have offices across the United States and serve clients in aerospace, transportation, national security, and beyond. The company's Quality Management System (QMS) is certified to ISO 9001.