The pace of technological change, along with manufacturing cost structures, combine to place a wide number of skills and technologies beyond the practical experience of machine shops and similar operations. Hydraulics, widely used to power production machinery, is one of those areas of specialization that challenges operators’ expertise. Here is a helpful guide to understanding hydraulic fluids and their performance capabilities.
Although new technologies are continually improving the performance and safety of hydraulic fluids, the most common base for hydraulic fluids remains petroleum oil because of its low cost and high performance. Many hydraulic components are designed with petroleum oil in mind, and often the performance criteria of other fluids are measured against that of petroleum oil, as oil is generally regarded as the standard for hydraulic fluid performance.
In many applications petroleum oil-based fluids cannot be used because of their high flammability. For any hydraulic system that operates close to open flames and hot surfaces that can provide a source of ignition, a fire-resistant fluid is the best choice.
When compared to petroleum oil, fire-resistant fluids need to be selected based on required system performance, cost and stability. Selecting the right fire-resistant fluid often means striking a compromise between system needs and fluid performance characteristics to arrive at a solution that both minimizes cost and maximizes the desired health and safety benefits.
There are four major groups of fire-resistant hydraulic fluids (FRHFs): HFA or high-water containing fluids; HFB or invert emulsions; HFC which are water glycols; and HFD or water-free fluids, including synthetic esters, natural esters and polyalkylene glycols (PAGs).
HFA fluids are also called high-water-content fluids (HWCF) or 95/5 fluids, because they were originally to be a 5% emulsion of oil in water. Oil adds lubrication and corrosion protection, but the bulk of the fluid is water, so the strengths and weaknesses of water are still largely present with HFA fluids.
For example, HFA fluids are extremely fire-resistant, but they require equipment that has been specifically designed to run with water. Therefore, HFA fluids cannot be substituted for petroleum oil in typical hydraulic equipment.
HFA fluids are used widely in steel mills and coal mines, where the equipment is designed with HFA fluids in mind. Traditional oil-based HFA fluids are still in use, but they are being replaced with synthetic products that offer better lubrication, consistency, corrosion protection and resistance to microbiological growth.
Emulsions, Glycols, Synthetics
HFB fluids are also emulsions, in which water is dispersed in petroleum oil, which makes up the bulk of the product (about 60%.) These fluids offer better lubrication and corrosion resistance than HFA fluids, and in some cases offer performance approaching that of petroleum oil at operating pressure around 1,000 psi or 70 bar. The water provides the extinguishing mechanism should a fire occur. HFB fluids require regular maintenance to ensure safe and trouble-free performance. The system temperature should be kept low so that the water does not evaporate.
Likewise, the stability of the fluid should be checked regularly to maintain fire-resistance. Because of the maintenance requirements and inherent instability of water-in-oil emulsions, HFB fluids are not widely used.
HFC fluids, or water glycols, are the most commonly used fire-resistant hydraulic fluids. They contain 35-45% water, a glycol similar to those used in antifreeze, and special thickeners that improve viscosity. As in HFB fluids, the water content provides fire-resistance, so it needs to be monitored and maintained to appropriate levels. HFC fluids typically have good lubrication and anti-wear properties.
HFC fluids can be used in most equipment designed for oil, but pump speeds, temperatures, and pressures need to be considered during the fluid selection process. In general, water glycol fluid will perform in most applications where pressures are 3,000 psi (210 bar) maximum. Readily available equipment literature can aid in determining the suitability of equipment for use with HFC fluids.
HFD fluids as a group contain several different types of products that are all considered synthetic because they contain neither petroleum oil nor water.
Phosphate ester fluids — Once widely used, they were the first HFD fluids. Their use declined due to poor environmental performance, limited compatibility, and high cost. Some phosphate esters have very high auto-ignition temperatures so they are still used in specific applications, such as aircraft and power generation.
Polyolesters — Phosphate esters largely have been replaced by polyolesters. Based on organic esters, polyolesters, including natural esters, offer good inherent fire-resistance, good compatibility with system materials, easy conversion from petroleum oil, and excellent hydraulic fluid performance. In addition, the organic nature of these fluids gives them good environmental performance in biodegradability and aquatic toxicity. Polyolesters can cost more than twice as much as petroleum oil, so they are still only used when fire-resistance or biodegradability are high priorities.
PAG fluids — Polyalkylene glycol fluids are anhydrous synthetic fluids that, when formulated correctly, can be classified as HFD fluids. These types of products are noted for thermal and oxidative stability. PAGs are priced higher than polyolester fluids and have found limited market acceptance.
When to Select HFC
While water glycol additives have remained nearly unchanged over the last three decades, they can meet the demands of today’s hydraulic systems. HFC can provide these benefits:
• Excellent fire-resistance due to the high water content
• Exceptional thermal and hydrolytic stability
• A good safety-to-dollar spent ratio.
HFCs are very versatile fluids and can be used in almost any hydraulic application. Some ideal applications are continuous casters, electric arc furnaces (EAFs) applications, rolling mills, coilers, die casting machines, permanent mold machines and heat-treat furnaces.
While HFC fluids were developed decades ago, they remain the most commonly used fire-resistant hydraulic fluids. Nearly 20 years ago, an article appeared in a respected hydraulics magazine, touting the merits of water glycol hydraulic fluids. The author opened the article by saying that water glycol fluids were the closest things on the market to a permanent hydraulic fluid. This assertion was based on the fact that all the components of a water glycol fluid have exceptional thermal and hydrolytic stability. These properties are essential to the fluid and its performance (as water is the key fire-resistant component of a water glycol fluid) and fire-resistant hydraulic fluids are constantly exposed to heat.
The author went on to say that water-glycol fluids, because they contained water, provided the greatest safety per fluid dollar spent. That’s high praise for technology that hadn’t changed much since the early 1960s.
Despite the many benefits and inherent safety of HFC, to get consistently good performance, they need to be formulated properly for your situation and monitored carefully. Here are some issues to consider:
• HFC fluids need to be monitored and maintained at appropriate levels due to high water content and some volatile components. A process engineer can help you tailor a maintenance plan that considers your equipment and operating conditions.
• They have a lower maximum operating pressure than oil (due to the reduced film strength of water). While HFC fluids can be used successfully in most equipment designed for oil, pump speeds, temperatures and pressures will most likely have to be adjusted.
• If biodegradability and low aquatic toxicity is required in your application, you might consider a HFD-U fluid instead.
• HFC fluids provide less lubrication and anti-wear capabilities than oil and poly ester fluids (HFD-U). This can lead to reducing the service life of seals and bearings.
• HFC additive systems vary from manufacturer to manufacturer. These additives can impact lubricity, corrosion, and fluid life.