Characteristic Curves for butterfly valves-results are odd? 1

[stanfea]

Have done a lot of reading over the last couple weeks and am still at a bit of a loss on a couple items. I am in the preliminary design stages of a coke oven gas line and am working at determining the flow rates and pressure drops for the system. The system is relatively short at 70 feet. There is however some significant limitations on the project.

1) The pressure can only drop 0.75 psi (other posts have relieved some concern over this issue)
2) The flow must be regulated
3) There are many valves in the system (primarily open however)
4) The control valve target is 50% open.

The thing I am struggling with currently is that the valves intended to be used for flow control are butterfly valves. I have read in many posts that these are not the preferred choice for control of gas flow. I have run some analysis using both the Crane program (to get psid) and the Metso program to get flow characteristics of the control valve.

My issue is with Metso, when I put the upper flow rate and associated psid in, I get a certain "% valve opening" and "installed fully open flow rate". I then put in the lower flow rate and get a completely different "installed fully open flow rate" but the SAME % valve opening...? How could the same valve with the same inlet pressure have completely different max flow ratings at the exact same %opening?

What I was imagining would come out was that at the higher flow the valve would be say 70% open, and at the lower flow would be say 25% open. The valve in either case comes out to be about 46% open (46.1 & 45.7). The problem may be that I am using the psid values from Crane with the valves at 50% open for both cases. Am I misunderstanding the use of this Metso chart.

Thanks
Frustrated Stan

 

[profengmen]

http://www.engineeringtoolbox.com/45.html

 

[jsummerfield]

Butterfly stule valves have several properties of interest. These include a changing pressure recovery factor and an actuator torque reversal.

The pressure recovery constant varies based upon the percentage open. A nearly closed valve causes much permanent loss. As less of the valve interferes with the flow the more likely for the drop to recover downstream of the vena contracta. This is largely true of ball designs too.

The actuator torque changes as a conventional butterfly valve exceeds 60 degree open. This is not an issue with eccentric style butterfly control valves.

Consult the valve manufacturer for details on the valves. You need to look at the control valve manufacturers instead of conventional disk butterfly valves for most control applications. Domestically in the US, Fisher, aka Fisher-Rosemount or Emerson something or other is a leader with lots of data.

John

 

[stanfea]

The real issue I cannot get my head around is the fact that the valve manufacturers program tells me that at 50% open the "installed fully open flow rate" is either 290,000 SCFH or 100,000 SCFH. Same valve, same pipe, same inlet pressure, same amount open? Only thing different is the pressure drop thru the valve. The pressure drop is calculated using Crane methods and Cv of 50% open valve. What I really want to do is see the flow rate drop/increase at some pressure drop I input. As it stands right now the results don't make sense, or I am looking at the forest and can't see the trees...Not sure which.

Stan

 

[katmar]

Hi again Stan,

If what you say is true then it is extremely confusing and I would suggest calling the valve rep in with his sizing program. What I suspect is happening is that the valve selection program is selecting 2 *different* valves, and not displaying this clearly.

For example, at the higher flow rate you specify an opening of 50% with a fixed pressure drop, and the program says the nearest I can find is a 6" valve and it will be 46.1% open at the PSID you specify. Now you specify a lower flow, but same opening and PSID. The program says says the nearest is a 3" valve and it will be 45.7% open at the PSID you specify. Of course the installed fully open flow rate will be much higher for the 6" valve than for the 3" valve. This is the only way I can see you getting these results from the program.

To get back to your question on the suitability of a butterfly valve. A few years ago it was conventional wisdom that butterfly valves were not suitable for control. But things have changed. Not so much with the valves - the characteristics are much the same, but the speed, power and accuracy of the controllers and positioners have improved beyond recognition.

In the last year I have come across two butterfly valves that were completely wrongly specified and were operating at less than 10% open, but were controlling beautifully. Twenty years ago you could not have done this.

One problem I have experienced, with two different but "reputable" manufacturers, is backlash in the link to the positioner. There were plastic linkages between the valve stems and the positioners. These "chattered" and the positioner never knew where the valve was and it never got stable. We swapped them for stainless steel linkages and the control became rock steady.

BTW, I do not know the METSO sizing chart - is it available online?

regards
Katmar

 

[stanfea]

Katmar and others

Thanks for your responses.

Katmar

The site is metsoautomation.com, the valves I am using are Jamesbury high performance butterfly valves. Your comments as to the control end of things makes a lot of sense.

The possible different valve situation is not happening however as I am selecting the valve size manually.

The pressure drop across the valve however is different, as due to the lower flow the pressure drop is less. It seems like I am entering the pressure drop values for 50% open at a certain flow, the program then tells me that yes at 50% open that flow is attainable.

Then I enter a different flow (say higher) with the pressure drop which is larger and the program says yes with the valve at 50% that flow is attainable? Not only that it is telling me that the "installed fully open flow rate" is different? I have a call into the valve manufacturer to figure out what the heck is going on here. In the meantime i may check out some other manufacturers and their software.

Thanks Again Everyone.
Stan

 

[MikeHalloran]

Stan, the words you use to describe the metso program's behavior are interesting. When it says, "yes that flow is attainable" with the given conditions, does that also mean that some higher flow might also be attainable? I.e., the program seems designed for checking whether the operating point lies somewhere within the constraints given, but a yes/no answer does not tell you what the actual operating point might be, or by what margin you are satisfying the constraints. Maybe you do need a different program.

Also, and I don't mean to insult you by asking this, but you do realize that the Cv changes with %open, right?



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[katmar]

Aha, now it makes sense.

The basic sizing equation for a valve is

Q = Cv x Sqrt( PSID / SG )

You are changing both sides of the equation - lower flow (Q) and lower pressure drop (PSID). By coincidence you have chosen numbers that result in the same Cv (more or less) to keep the equation in balance, and the valve position stays the same. I suppose it is not really coincidense - you got there by sizing the valve via the Crane method I think?

When you specify the the higher flow you also specify a higher pressure drop. When the valve is fully open the Cv is the maximum for the valve, and does not vary with flow. But because you have a high PSID with the same Cv the flow increases (see equation above).

Does this explain the anomaly?

regards
Katmar

 

[stanfea]

Katmar

Yes, I suppose I am changing both sides of the equation at the same time... I did in fact calculate the psid using crane and used the Cv of a 50% open valve to get it. Looks like I am in an endless loop, putting in psid at 50% open and the program is telling me set the valve at 50% open. :-(

I guess my real issue here is that I am trying to find out if this valve will actually control the flow and all I keep getting is that the valve will handle the flow. If I were to select a valve for flow control would this situation not occur all the time with everyone? At different flow rates the psid is always different? But the valve position should be different as well, no?

If I was starting over how would I know if the valve could actually control the flow. ie 25% open flows X, and 75% open flows Y.

Thanks Again
Stan

 

[katmar]

First off - I am not a control engineer, so perhaps some of my comments need checking.

Anyway - the rule of thumb I use is that the valve should consume about 25 to 33% of the total line pressure drop at the design flowrate. This makes it sensitive enough for decent control. That "fixes" your PSID for the valve. For a butterfly valve you want the valve to be at about 50% open at the design rate. For a globe valve use 60 to 70% open.

This allows you to size the valve. i.e. from flow and PSID calculate Cv, and then look up in a catalog (or program) what valve gives this Cv at the required opening.

As the controller works it will open or close the valve to balance the pressure drop across the valve and the rest of the line so that the total pressure drop is always (almost) the same. As the flowrate gets lower the pressure drop through the line gets less, so you need to close the valve to consume more of the pressure drop. And vice versa.

To know what valve opening you will need for a given flowrate you need to construct the system curve, and plot it against your pump (or other supply) curve. Any book on pump sizing will explain this.

regards
Katmar

 

[stanfea]

Katmar

Thanks first off for all your help.

Second I have made some good progess today in both understanding and this project. The problem I had was indeed that the inputs I was using were not created using variable positions of the valve.

I now have the max flow set with the valve at 70% open, and the min flow with the valve set at 30% open. I input the appropriate flows and pressure drops and it works. I definitely did not have the psid high enough for the lower flow. In other words I was trying to keep the valve open too much and still cut the flow down,( as I was primarily thinking of pressure drop only ). I can see now that I needed to give up some pressure to get that flow to reduce, it seems more obvious now.

It now appears that I have two options for valves (8" and 10"), and now I am investigating Gain and a couple other parameters. So far so good. Anyway I can at least see some of the trees in the forest now, it was a struggle however...

Thanks Again
Stan