flow characteristic rotary steam valve

[diapensiya]

Hello,

In our plant, we have a steam valve which controls MP steam flow to a reboiler of a distillation column. I investigate whether it is possible to increase flow through this steam valve by changing the valve (internals).
The type according to documents is Masoneilan Camflex. Exact details are not available. Checking the Masonailan handbook, I expect it to be a rotary valve (possibly butterfly) and to have linear flow characteristics or equal percentage. Quoting from the booklet: "The Camflex II valve has a modified linear flow characteristic, which is the same in either flow direction. It can be easily transformed to an equal percentage when equipping the valve with a positioner 4700 series, 8000 series or SVI Smart Valve Interface".
However based on plant data when I draw steam flow versus valve% opening, I see a flow characteristics of a fast opening valve. See drawing.

Why does it look like this ? Before finding info on the type of the valve, I thought that this was due to the valve being fast opening. Now I do not understand why the graph looks like this.
Is it possible to increase the flow through this valve more than 6000-7000 kg/h by changing the valve or is it limited by the process ? Why do I see more variation after 50% opening?

Thanks!

 

[btrueblood]

Because the valve is oversized for the application. Beyond 25% to 50% opening, the valve no longer controls steam flow (expect the pressure differential across the valve drops off to nearly nothing) and the flow instead is controlled by another orifice, or by the rate of condensation in the reboiler, or some other process.

 

[diapensiya]

Thanks for the answer. Let me check if I understand you correctly.
You mean that the valve flow characteristics is linear but the size is not suitable. Thus, I see a nice controlling valve till 25% opening and above that there is another limiting factor which causes the variation in the steam flow. I know that there is no other orifice. It must be related to process limitations.
How did you conclude that it is oversized? Is it because control is better at low flows ?
In that case if the valve is changed to a smaller one, do you expect better control above 50% opening as well?

 

[gerhardl]

I agree with btrueblood. Before you proceed you have to decide what you want. Do you wish to increase the flow or have a better regulation, or both?

Increased flow is limited by input to the pipeline, input pressure and restrictions before and in the valve and rest of pipeline.

Camflex can (for some versions) be equipped/supplied with smaller orifices. The curves you have measured indicates (ref btrueblood) that your your actual steam input to the system already is smaller than suitable for the selected valve orifice. All else equal a better regulation will then require a smaller orifice and hence a smaller possible total flow.

Conclusion: You can have better regulation with a smaller valve orifice. Or: You can have both better regulation and higher flow if you enlarge restricting pipeline/valve factors and/or increase input combined with a correct sized/dimensioned valve.