Hi,
Is there any hand ready way to calculate the stroke time of a given
spring-return actuator volume? Exhaust will be through a orifice with a
known Cv.
It seem that this calculation is non-linear because of the choked flow.
Thanks
Martin

I've not come across a "hand ready way to calculate" this. However, in principle
the solution is
similar to the sort of examples you get in undergraduate thermodynamics text
books: flow of
compressible fluid out of a cylinder in response to the movement of a piston.
The math involves the
solution of differential equations for the mass and energy, and for the flow
through the orifice
and the forcing function - where there are non-linearities. In practice there
may be other,
difficult to predict, influences, such as actuator stem-friction, out-of-balance
forces on the
actuator due to process pressure, the influence of pneumatic piping, etc. In
summary: a little
dynamic simulation (numerical solution of the non-linear differential equations)
is required, using
your favourite dynamic simulation software package.
A bench test on the completed valve & actuator is probably the most practical
approach for real
kit.
I recall that the Neles-Jamesbury (now Metso Automation see:
) research centre in Finland did a lot of work on actuator
dynamic modelling,
and had a sophisticated model and simulation software that could be used to
predict the behaviour
of their valve-actuator combinations.
Kelvin B. Hales
Kelvin Hales Associates Limited
Consulting Process Control Engineers
Web:

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