## 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

Thanks

Nong

## RE: What is "turndown ratio" ?

## RE: What is "turndown ratio" ?

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" ?

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" ?

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" ?

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" ?

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" ?

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" ?

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" ?

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.