For most control valve applications, valve travel time is of little significance. This is because most control valve movements are quite small. However, you do need to be sure that the valve actuator and/or positioner are appropriately sized to overcome friction and fluid forces, allowing the valve to make these small movements.
Valve travel time becomes important when the valve must make large control moves. Generally, this will only happen under 2 circumstances: 1. Large operator changes, such as large changes in setpoint, that require fast movement to new operating conditions. 2. Large upsets, load changes, or disturbances.
In most processes, the setpoint changes are incremental (except at start-up & shut-down), and the disturbances are not huge, so valve travel time does not become a big factor.
If valve travel time does become a factor, it will show us as a slower control response. Note: it is likely to be assymetrical, too...The valve open response may be faster or slower than the valve closing response. For purposes of controller tuning, make sure you do bump tests in both directions.
In addition to George's comments I would add a couple of things.
1) Significant setpoint changes are possible in batch processes and slow response may slow down batch production times.
2) For safety applications usually the block and vent valves have a specified performance time. They generally travel through their full stroke to perform the required function. If the valves don't perform their required function within the performance time there is a possibility that plant safety may be compromised. During plant shutdowns we have manually tested certain critical valves to verify that the valve is still performing. I have also put in some reporting functions so the operator can take "credit" for accidental tests such as unplanned shutdowns. Verification of performance times and other safety valve characteristics is, in Europe, becoming something that Safety Authorities are paying more attention to. Monitoring of trends in stroke times could assist in condition based maintenance programmes. AS said previously make sure actuators are adequate for their duty.
Just want to confirm. Do you mean that to change the travel time by just change the integral (Ti)?
> 1) Significant setpoint changes are possible in batch processes and slow response may slow down batch production times.
> 2) For safety applications usually the block and vent valves have a specified performance time. They generally travel
> through their full stroke to perform the required function. If the valves don't perform their required function within
> the performance time there is a possibility that plant safety may be compromised. During plant shutdowns we have
> manually tested certain critical valves to verify that the valve is still performing. I have also put in some reporting
> functions so the operator can take "credit" for accidental tests such as unplanned shutdowns. Verification of
> performance times and other safety valve characteristics is, in Europe, becoming something that Safety Authorities are
> paying more attention to. Monitoring of trends in stroke times could assist in condition based maintenance
> programmes. AS said previously make sure actuators are adequate for their duty.
