How to calculate stoke time of an actuator?

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
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Martin G. wrote:

I think it has to be measured. It may also be affected by temperature in various ways.
Jerry
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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: <http://www.metsoautomation.com/automation/index.nsf/FR?ReadForm&ATL=/automation/info.nsf/WebWID/WT B-041109-2256F-09374>) 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: www.khace.com
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You could also look at the Valtek sizing and selection info ... here's a link to the sales literature if that helps http://fcd.flowserve.com/valves/literature/literature_result_brand.jsp ? BrandSelect##
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