4 replies to this topic

[sgkim]

Hello Members,

According to the inherent characteristic required for control valvs, various types of trims would be required, and the valve opening f(x) can be expressed as a function of valve position x (or controller output) as,

Valve opening, f(x)= x^n, where x is valve position.

The exponent "n" shall have typical value with respect to each valve characteristic like the fashion:
0<n<<1 for quick opening, n=1 for linear, 0<<n<1 or 1<n<(n)eq for parabolar, 1<<(n)eq for equal %, in which (n)eq is often defined by the equation f(x)=(1/R)*R^x, where R=rangeability.

Will someone please inform me of the following,
(1) each typical range of value n for each type of valve,
(2) what does the word "equal" really mean ?

sgkim

[proinwv]

Possibly the place for you to start would be the ISA who publish a recommended practice which defines valve trim characteristics.

Also, the larger valve manufacturers may publish a paper that would help you.

Are you just trying to specify a valve, or are you actually designing the trim to manufacture?

[sgkim]

Tanks proinwv,
I want to get more detail guides for specifying valves than those given generally in ISA 75.11.01. And I can hardly get more spcific data from vendor catalogs.

sgkim

[proinwv]

The "n' is the exponential and it defines the shape of the curve.


Quick opening, linear and equal percentage are the most commonly used trims. The first in regulators, the latter two in control valves.

In an attempt not to be rude, I don't understand what you mean by what does equal mean. Equal Percentage is defined in the literature.

[sgkim]

-snip-

In an attempt not to be rude, I don't understand what you mean by what does equal mean. Equal Percentage is defined in the literature.

Thanks proinwv,

Now I can see (df(x)/dx)/f(x)= ln® = r = constant, because

f(x)=(1/R)R^x, so df(x)/dx = d((1/R)R^x)/dx=(1/R)R^x ln® = ln®*f(x) = r f(x),
where x is valve position, f(x) valve opening, and R rangeability.

Which says, for the FULL range of stem position x, the INCREMENTAL valve opening df(x)/dx is just PROPORTIONAL to (i.e. EQUAL percent of) the valve opening f(x) at valve position x.

sgkim