How to Measure Control Valve Stroke Time
Stroke time is the simplest dynamic measurement a valve offers: how long it takes to travel from one end to the other after the command. It matters twice, once for control valves whose speed shapes loop response, and once for shutdown valves whose closure time is a safety property. The measurement is easy to take and easy to take wrong, because the number depends entirely on what you define as the start and the end. This page covers making it repeatable enough to compare across months and technicians.
Measure Valve Stroke Time in one line: To measure control valve stroke time, define the start event, the command being issued, and the end event, the valve arriving at its end position, and time between them for a full stroke in each direction using the same method every time. Expect the two directions to differ, since filling an actuator and venting it are different processes. Record the times along with supply pressure and how the stroke was commanded, because a stroke time without its conditions cannot be compared to anything.
Define the Measurement Before You Time Anything
Two people with two stopwatches can time the same stroke and disagree, not because either was sloppy but because they chose different events. Decide, and write down: the clock starts when the command is issued, at the control system output or the trip signal, and stops when the valve reaches its end position as shown by a firm event, the stem hitting the stop, a limit switch making, or the position feedback settling at its final value. Any definition works; mixing definitions does not.
Where the number will be judged against a requirement, as with a shutdown valve whose closure time the safety design assumed, use the events the requirement uses. A stroke timed from solenoid de-energization to seat contact is a different number than one timed from stem movement to seat contact, and only one of them is the number the safety case cares about.
Time Both Directions From the Same Trigger
Run a full stroke in each direction and time them separately, because they are genuinely different events: one direction fills the actuator through the positioner or solenoid port, the other vents it through an exhaust path, and the slower of the two paths sets that direction's time. A large asymmetry is normal on many assemblies, and a change in the asymmetry over time is diagnostic, an exhaust path slowing down while the fill stays constant points at the vent, not the supply.
Repeat each direction at least twice. Strokes on a healthy valve repeat closely; times that scatter between repeats mean an unstable air supply, a sticking stem, or a loose definition of your events, and each of those is worth resolving before you record anything. If smart positioner diagnostics are available they can timestamp the stroke internally and remove the human reaction time from the measurement, which matters most on fast strokes where a hand-held stopwatch is a meaningful fraction of the total.
Record the Conditions With the Number
A stroke time is only comparable to another stroke time taken the same way, so the record needs the conditions, not just the seconds.
| Field | Why it matters |
|---|---|
| Direction of stroke | Fill and vent paths differ; the two times are separate data |
| Start and end events used | Different event pairs give different numbers for the same stroke |
| How the stroke was commanded | Positioner ramp, step command, and solenoid trip behave differently |
| Supply pressure at the airset | A sagging supply lengthens strokes and explains drift |
| Process state | Flowing conditions load the plug differently than a depressured line |
Five fields turn a stopwatch reading into a baseline.
The baseline is the real product of this exercise. A single stroke time means little on its own, because what counts as fast enough is set by the loop or the safety requirement, both site-specific. What the measurement gives you is change: the same valve, timed the same way, drifting slower across quarters is telling you about its air path, its packing, or its lubrication long before it fails a requirement. Trending stroke times in a maintenance system or historian is what converts this cheap test into early warning.
Verifying the Result
Sanity-check the numbers against the valve's own history first, then against its peers: similar actuators on similar service should live in the same neighborhood, and one outlier in a family of siblings is a finding even when no formal requirement exists. If this test is part of a proof or performance program, compare against the acceptance value from the design documents, and treat a failure as an engineering finding rather than something to fix by timing again more generously.
If a stroke has slowed since the last baseline, split the problem before writing it up: rerun the measurement with the supply gauge watched, since a starved supply masquerades as a lazy valve, and note whether one direction or both changed. One direction slowing points along that direction's air path; both slowing together points at friction in the valve and packing, the same territory a partial stroke test exercises on safety valves without a full closure.
Common Mistakes
The commonest is the undefined measurement: a number in a log with no direction, no events, no supply pressure, and therefore no way for the next person to reproduce it. The next is timing only one direction, usually the convenient one, and missing that the other direction, often the one the safety function depends on, has been slowing for a year.
Beware also the reaction-time trap on fast valves, where the human on the stopwatch is a large share of the reading; use switch contacts, feedback trends, or positioner timestamps for anything quick. And do not time a safety valve's trip stroke by commanding it from the positioner when the real event is a solenoid dump, the two paths can differ substantially, and only the trip path's number belongs in the safety records, a distinction the guide to a full stroke test treats properly.
Frequently Asked Questions
What is a normal stroke time for a control valve?
There is no universal number: stroke time depends on actuator volume, supply capacity, positioner and accessory sizing, and the exhaust path, all of which vary by assembly. What matters is whether the time meets this loop's or this safety function's requirement, and whether it is stable against the valve's own baseline. A valve drifting slower against its history is a finding regardless of what any table of typical values says.
Why is my valve faster in one direction than the other?
Because the two directions use different air paths: one fills the actuator through the positioner, solenoid, or booster, and the other vents through an exhaust port or the positioner's relay. The slower path sets that direction's time, and asymmetry is normal. What deserves attention is change, if the vent direction slows while the fill direction holds steady, inspect the exhaust path, silencer, and any speed controls on that port.
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