Valve Failure Modes

Failsafe spring lever on ball valve.
(Kinetrol)

An important design parameter of a control valve is the position it will “fail” to if it loses motive power. For electrically actuated valves, this is typically the last position the valve was in before loss of electric power. For pneumatic and hydraulic actuated valves, the option exists of having a large spring provide a known “fail-safe” position (either open or closed) in the event of fluid pressure (pneumatic air pressure or hydraulic oil pressure) loss.

Available Failure Modes

Valve fail mode may be shown in instrument diagrams by either an arrow pointing in the direction of failure (assuming a direct-acting valve body where stem motion toward the body closes and stem motion away from the body opens the valve trim) and/or the abbreviations “FC” (fail closed) and “FO” (fail open). Other failure modes are possible, as indicated by this set of valve symbols:

In order for a pneumatic or hydraulic valve to fail in the locked state, an external device must trap fluid pressure in the actuator’s diaphragm or piston chamber in the event of supply pressure loss.
Valves that fail in place and drift in a particular direction are usually actuated by double-acting pneumatic piston actuators. These actuators do not use a spring to provide a definite fail mode, but rather use air pressure both to open and to close the valve. In the event of an air pressure loss, the actuator will neither be able to open nor close the valve, and so it will tend to remain in position. If the valve is of the globe design with unbalanced trim, forces exerted on the valve plug will move it in one direction (causing drift).


Reprinted from "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.

Upper Midwest Water Treatment Plant says "Out with the old, in with the new!"

Before project.

Project begins.

An Upper Midwest water treatment plant was experiencing high failure rates, long maintenance periods, and too many repair issues with their existing pneumatic scotch-yoke cylinder valve operators. 

Upon investigation, the problem boiled down to how the scotch-yoke cylinder's linear movement is converted to rotational movement. To do so requires gearing, yokes and linkage which are wear points. In medium to high cycle rate applications these wear points soon become failure points. 

Kinetrol rotary vane actuators utilize a single moving part - the one-piece vane and shaft.  There are no gears, yokes, or linkages and 100% of the movement is transferred to the actuator shaft. The one piece vane and shaft eliminates these wear points, and therefore eliminates the resulting failure points.

Out with the old.

In with the new.

After a planning review meeting, this particular water treatment plant clearly saw the advantages of the Kinetrol design. The argument was so strong and the case so clear,  they decided to replace all (60) scotch-yoke cylinder actuated valves with Kinetrol vane actuators in one fell swoop. 

The new actuators have been operating for 6 months now without problem, cycling approximately 15 times per day. 

Click on the images above to see a larger view. For more information, contact Kinetrol USA by visiting https://kinetrolusa.com or by calling 972-447-9443

The Kinetrol Actuator Vane Assembly Design

The Kinetrol rotary vane actuator utilizes an advanced design vane assembly that is engineered to provide millions of trouble-free cycles. This video highlights the advanced features that make it the most reliable 1/4 turn pneumatic actuator on the market today.

https://kinetrolusa.com
972-447-9443