4 replies to this topic

[Phukan]

I want to know the temperature of steam downstream of a control valve which is a at a pressure of 19.77 kg (I assume it to be saturated) upstream of the control valve. The pressure shown downstream of the control valve is 6 kg per sq cm. what equation of state should I use. Is it reasonable to assume it to be saturated and refer to steam table which gives it a value of around 158 deg cel. However, using the ideal equation of state gives it a value of 147 deg cel(assuming V to be constant).

[abhi_agrawa]

Phukan,

Use steam tables for accurate results. Also the steam upstream of the control valve would in all probability be superheated and not saturated.

abhishek

[kkala]

Adding a bit to previous post, one can consider isenthalpic expansion of steam from 19.77 kg/cm2 a to 6 kg/cm2 a. In the specific case steam temperature falls from 211 oC upstream to about 176 oC downstream of valve, versus a saturation temperature of about 158 oc.
One can use a simulator for the above results, instead of steam tables.

[katmar]

Assuming that the valve is well insulated, and because no work is being done, you can regard it as an isenthalpic process for all practical purposes. I would use steam tables to see that the enthalpy of saturated steam at 19.77 kgf/cm² absolute is 2795.1 kJ/kg. In order for steam at 6 kgf/cm² absolute to have this same enthalpy it would have to be at 175.3 °C, i.e. slightly superheated.

If the initial steam quality was not 100% then this outlet temperature would be a bit lower.

The temperature can certainly not be below 158 °C because that would imply that it is all liquid, and we know that does not happen in a valve.

[EyMiller245]

Steam and water vapor should never be treated as ideal gas if at all possible. Always use tables or more rigorous equations of state.

Edited by EyMiller245, 23 June 2011 - 10:42 AM.