Maintaining water glycol fluids: Follow the formula for success

Measuring viscosity and pH levels is just part of the process.

By Peter Skoog June 10, 2016

Water glycol hydraulic fluids, also known as HFC, occupy an important space in the overall fire-resistant fluids market. On a pounds-produced basis, water glycol fluids are the number-one fluid in the marketplace. They offer excellent fire resistance and are relatively inexpensive as compared with the various water-free fire-resistant hydraulic fluid technologies.

Because HFC fluids have a relatively low pressure ceiling (roughly 3,000 psi, depending on the type of equipment and the original equipment manufacturer, or OEM) they are widely used in the steel continuous casting and die casting segments of the fire-resistant fluids market. Because HFC fluids have a significant market share and market presence, it is important to understand both the formulation and maintenance parameters that will keep your water glycol fluid and your equipment operating efficiently.

Fluid formulation

Water glycol fluids are complex solutions. The product classification clearly indicates that the fluid contains water and glycol. Other chemical building blocks are required to produce a finished fluid capable of operating in today’s commonly used hydraulic hardware. When mixed together, water and glycol have the viscosity of water. In an HFC fluid, a water-soluble polyalkylene glycol (PAG) thickener is added to give the fluid viscosity.

When mixed together, these three components yield a fluid with an oil-like feel. Unfortunately, this fluid has no lubrication value and will not protect components against rust and corrosion. Additional chemical building blocks include fatty acids and amines for lubrication, amines for ferrous corrosion protection, yellow metal passivators for brass and bronze and a dye for fluid identification.

While ingredients like amines, lubrication additives, polymeric thickener and even water content can vary from one manufacturer to another, the final fluid formulations are roughly the same. Optimum fluid performance is obtained when all of the fluid components are kept in balance. It is, therefore, necessary to have a good fluid-maintenance program in place.

Key maintenance parameters

Here are a few of the variables within the maintenance process:


The viscosity of a water glycol fluid is directly related to the water content. The glycol and PAG thickener contained in an HFC fluid are not volatile and will remain in the hydraulic reservoir regardless of fluid temperature. As such, one would expect to see the viscosity of the fluid increase over time as water evaporates. The rate of viscosity increase will depend on such factors as ambient temperature, reservoir temperature, air flow across the reservoir breather, amount of make-up fluid added, etc.

A knowledgeable hydraulic fluid supplier can supply their HFC users a graph that directly relates the viscosity to the water content. Having such a graph eliminates the need to run actual water contents and allows for easy maintenance. It should be pointed out that a falling viscosity is related to excess water in the water glycol fluid. This excess water can only come from a leaking heat exchanger or over-addition of water in a water adjustment. Quaker’s suggested viscosity ranges are:

  • Normal operating viscosity range: 180 to 250 SUS
  • Viscosity below 180 SUS: Check for source of excess water.
  • Viscosity 250 to 500 SUS: Adjust with soft, distilled or deionized water according to charts.
  • Viscosity above 500 SUS or below 160 SUS: discard fluid and refill system with fresh water glycol fluid.

Should a water adjustment ever be necessary, adjustments will need to be made using soft, distilled or deionized water. Divalent metal ions, such as calcium and magnesium found in tap and spring water, will cause the lubrication additive in the fluid to become solid and ineffective.


The pH of a water-based fluid is a measure of the total corrosion-inhibiting properties of a fluid. In a water-based fluid, the pH of the fluid must be more than 8.0 to inhibit rust. Additionally, alkanolamine chemistry must be used to raise the pH; caustic materials like sodium hydroxide will not inhibit rust. For example, Quaker’s Quintolubric water glycol fluids have a pH of between 9.4 and 9.8.

The pH of the fluid in use will drop due to loss of the vapor-phase corrosion inhibitor. This is to be expected, as the vapor-phase corrosion inhibitor is volatile; it must leave the body of the fluid to inhibit rust in the vapor space above the fluid in the reservoir. A pH of less than 8.0 indicates that the system has been contaminated or diluted.

Alkaline reserve

The alkaline reserve (AR) is a measure of the combined corrosion-inhibitor package—liquid phase and vapor phase—in a water glycol fluid. AR will drop with use because the vapor-phase inhibitor is slowly being removed from the fluid. The rate of evaporation will depend on reservoir temperature, ambient temperature and airflow across the reservoir’s breather. An AR of 90 or more is needed to inhibit vapor-phase rust. Small additions of make-up fluid can have a significant positive impact on the alkaline reserve of a water glycol fluid. Depending on the amine used as the vapor-phase corrosion inhibitor, there may never be a need to adjust the AR of a fluid in service.

Particle count

Particle counts are a measure of dirt, silt and wear debris suspended in a fluid. To maximize component life, particle counts need to be managed. Pump and valve OEMs have recommended particle counts for fluid depending on pump type, operating pressure, whether or not servo or proportional valves are used, etc. Identifying the critical hardware in your hydraulic system will allow you to set particle-count targets for a given piece of equipment. This applies to fluid supplied in drums and intermediate bulk containers.

Obtaining accurate counts using ISO 4406 requires crystal-clear fluid. Hazy fluid will introduce errors in the particle counts. Other methods, such as particle resuspension, can provide better results but are not completely accurate. This method is good for trending analysis, however, a review of OEM fluid-cleanliness guidelines as well as dialogue with your fluid supplier will help in establishing cleanliness limits for fluid in service.

Peter Skoog is technical manager of the fluid power and grease division of Quaker Chemical Corp.,  a provider of process fluids and chemical specialties.