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What is "turndown ratio" ?

What is "turndown ratio" ?

What is "turndown ratio" ?

(OP)
I understand that it's the range of flow ? for example max flowrate (100%open) = 25 ft3/min and has turndown ratio = 150:1 .It means the accurate minimum flow of this valve = 25/150 = 0.167 ft3/min, right ?  
Thanks
Nong

RE: What is "turndown ratio" ?

Another, more quantitative term for "turndown" is "rangeability". This is described in ANSI/ISA-S75.11-1985(1997)

RE: What is "turndown ratio" ?

Had Perry out for another posting. Found this description:
                   maximum controllable flow
Rangeability R=-------------------------------
                   minimum controllable flow

                    normal maximum flow
Turndown ratio T=----------------------------------
                   minimum controllable flow

Genrellay valves are designed so that T=0.7*R where R is between 20 and 50.

Best Regards

Morten

RE: What is "turndown ratio" ?

A comment to the formulas above:

You can actually not say much based on your numbers above, because the flow when the valve is 100% open is NOT controllable and you can therefore not calculate your minimum controllable flowrate.

Best Regards

Morten

RE: What is "turndown ratio" ?

(OP)
Morten,
Thank you, but I'm quite not clear from your answer, you mean T = 14 to 35 or 2.5:1 ? (because I always see that turndown = Numbers : Numbers

Rdgs,
nong

RE: What is "turndown ratio" ?

As you can see from my first posting then the turndown is the ratio between max. normal flow and min controllable flow. The number:number then simply tells you that you should not select this valve ir your normal max. flow is above 2.5 times the min. controllable flow (that you will have to calculate).

My second positing simply tells you a connection between R and T. But this connection is emperical and not true for all valves.

Best Regards

Morten

RE: What is "turndown ratio" ?

Turn Down Ratio is more of a process Parameter which is critical to ensure that the process parameters are also met at turn down conditions.
Mostly turn down ratio will indicate the flow parameters . but it is not a neccesity sometime pressure could be a critical turn dowm parameter for which we would be asked to give the gaurantee .
regards
kumar
kum@epc.ltindia.com

RE: What is "turndown ratio" ?

Turn down describes the ratio between the Minimum and maximum flow conditions in a system:
If Max flow is 100 GPM and Min FLow is 10 GPM
Turndown is 10:1  (100/10)
Some times turndown is incorrectly applied to valves, which is actually rangeability.

Rangeability is the range overwhich the valve can control - Ratio between the maximum and minimum flow coefficients. So if a valve provides good control between 2% and 98% of the flow capacity the rangeability is 49:1 (98/2).

Naturally this is a simple explaination and there can be other factors that may affect the outcomes, such as pressures.
But in general, these definitions should get you started.

RE: What is "turndown ratio" ?

Valvetech:

Perry's disagree with you. What you have stated is what they term "rangeability" whereas turndown (as i previously posted) is defined as:
             Normal max. flow
T=   ---------------------------
             min. controllable flow
and

             Max. controllable flow
R=   _____________________________
             min. controllable flow

Note that "normal max flow" is not the same as "max. controllable flow". Anyway since this post is more than 1 month old we better hope Nong already got it sorted out

Best Regards

Morten

RE: What is "turndown ratio" ?

Morten,
I have gone by those definitions for years, although I know that they were often used interchangably. But I checked the ISA standards this AM and it seems that I was wrong, "turndown"is obsoleted and rangability is stated as you say.

Thanks for the heads up.

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