Assessment of Servovalve Design for Special Fluid Usage

The benefits of fire resistant fluids in steel mill hydraulic systems are obvious and there are several types of fire resistant fluids on the market. The push to convert to a safer fluid does not always become a top priority until there’s a fire. This was the case last year at an Alabama steel mill. Much of the hydraulic system was destroyed and is now being redesigned in preparation for a switch to a water/glycol fluid. This very unfortunate incidence created a need for Moog to help with specialized technical assistance and service.

The original hydraulic system was designed for use with mineral oil and Moog worked with the pump manufacturer that the customer selected to review and revise the entire hydraulic system to provide robust operation on water/glycol. Moog Servo and Servo-Proportional Valves were used in the original system and had provided the customer with excellent service. The challenge was to evaluate whether existing designs would require modification for the new fluid. While most of Moog’s Valves products are designed with mineral oil systems in mind, field experience has shown they will work well with almost any hydraulic fluid. In most cases a seal compatibility and system pressure check are the recommended protocol to verify acceptable operation with a special fluid. The critical nature of this application meant that a special analysis was needed to ensure high performance and long service life of the valves.

One of Moog’s distributors (Flow Dynamics) is the first line of contact for this mill. Moog’s application engineers worked with the pump manufacturer and the distributor to create a plan for successfully testing the valve as part of the new system. In preparation for a meeting with the mill’s maintenance supervisor, Moog Applications Engineers polled several other steel mills across the country and collected pertinent information from other users of Moog valves on water/glycol fluids. Moog Design Engineering created a bill of material compatibility chart listing the materials of all of the wetted parts in each of the valve models used in the system and verified that they were acceptable. The meeting provided an open forum for the sharing of experience and opened up the communication channels for everyone involved. All of the companies left the meeting with a mutual understanding of each other’s product and can now proceed with confidence in the changeover effort.

To ensure the success of this changeover, Moog has recommended that the steel mill send in valves for periodic review and analysis to ensure the optimum running of the new system.

Related Products and Services

Moog has extensive experience in applying products and solutions for the unique needs of steel mills. In addition to the servovalves mentioned here, Moog’s RKP pump design is well-suited for special fluids in general and all sizes are suited for water glycol. For water glycol applications, we recommend a maximum pressure of 210 bar (3,000 psi) but experience in the field has shown that customers have run a water glycol application with 280 bar (4,000 psi) without problems.

Moog also provides customized products for the steel industry application including explosion-proof products. Other engineering services related to adapting valves to the demanding environment of steel mills include a range of recommendations to resolve potential issues of stability due to viscosity and stiffness when using non-standard fluids.


Mark Ludlow is a Design Engineer who is responsible for developments in Moog’s Mechanical Feedback Servovalves. He has been with Moog since 1984 working in applications, product and development engineering. He graduated from Buffalo State College in 1983 with a BT in Electro-Mechanical Engineering.