Tricentric butterfly valve 2

[flinana]

Hi,
Our EPC contractor installed atricentric butterfly valve as a control valve, after about less than a year it is showing signs of poor regulation. Sometimes the valve won't respond or responds late. We have changed the positioner but that seems to be ok.
I said from the beginning that this type of valve was no good for control purposes but no one agreed.

Find the Data Sheet attached. Please any suggestions as in what type would be more suitable are welcomed.

 

[gerhardl]

The control aspects of your valve is depending how one is to understand your data sheet.

The 'driving force' across any valve is depending on the pressure immidiately upstream, usually given as P1, and the pressure immidiately downstream, P2.

Question: how is the data sheet pressure data, lines 10,11 and and 12, to be interpreted? (Has engineering, supplier and valve producer the same opinion of this?) What is the connection between the figures? What are the actual figures before and after the valve (with fully open valve)?

If the answer is that the valve is expected to regulate a flow between minimum and maximum flow volume figure, at a reasonable pressure difference (1-4 bar?), the answer might not be that the valvetype and size is unreasonable.

Right enough will the valve have to be restricted to perhaps an opening of 15% or lower at increasing pressure difference in this range. (I have no calculation tools for this type of fluid).

At openings in range below 15%, and even more for below 5-10% for a BFL valve, regulation results become 'unsure' for a BFL valve, but depending on details in construction. Double and triple offset valves generally better than regular.

Reasonable pressure differences will keep the valve out of cavitation area, theoretically, and at larger openings.

Local cavitation or abrasion, material and sealing wear, might even then be a problem if the valve is down to small or cracking openings, and/or also acting as shutdown valve or earlier damaged. The high temperature does not help.

Another thing to check is if the regulation curve is sufficiently damped (no unnecessarily fast swingings and corrections, in-signals correctly measured at undisturbed flow location)?

There might be several causes for your problem, and maybe more than one in combination. One indication would of course be given by taking out and inspecting the valve.

I would anyway have rechecked that the different parties are agreeing on the valves suitability with a common understanding of operating parameters. Recheck also all lines marked with 1) in the data sheet. (Data according to manufacturers model).

If the parties don't agree it is not the first time a datasheet is giving everything you need, except the correct information for supplying a valve that will work.

A regulating valve should normally be selected after flow, not diameter. The valve Cv is the key factor, and a lesser Cv, eg constricted construction or smaller valve, should perhaps be considered as an alternative.

Or, even as you say, depending on task definition and importance: another valvetype.