Inherent Control Valve Characteristics
Inherent Control Valve Characteristics
(OP)
Can anyone provide me with a a web link, book refrence, or any experience in how to choose an inherent valve characteristic?
I've looked everywhere for a praticle approach for choosing valve characteristics and so far I've only found "guidelines" that valve vendors provide. Moreover, most valve vendors tell me to stick to =% since you can now characterize in the positioner. My question is then "Why have different valve cages if characterization can be done in the positioner?"
Anyway, characterization in the positioner - based on my experience - is not too popular of an option.
So, what I'm looking for is a praticle method of determining which valve characteristic is best suited for a specific application and why.
If anyone is a guru in this area or knows where I can find good, regarded information on this topic I'd greatly appreciate it.
I'm gonna post this in the ISA forum as well.
Thanks guys.
--Igor
I've looked everywhere for a praticle approach for choosing valve characteristics and so far I've only found "guidelines" that valve vendors provide. Moreover, most valve vendors tell me to stick to =% since you can now characterize in the positioner. My question is then "Why have different valve cages if characterization can be done in the positioner?"
Anyway, characterization in the positioner - based on my experience - is not too popular of an option.
So, what I'm looking for is a praticle method of determining which valve characteristic is best suited for a specific application and why.
If anyone is a guru in this area or knows where I can find good, regarded information on this topic I'd greatly appreciate it.
I'm gonna post this in the ISA forum as well.
Thanks guys.
--Igor





RE: Inherent Control Valve Characteristics
The charecterstic selection is explained in a reference book called "Control Valves" by Guy Borden and Paul G. Friedmann and is available through ISA.
RE: Inherent Control Valve Characteristics
I understand the relationship between valve opening, CV, and flow.
My question is how do you determine which valve characteristic: 1) Linear
2) Equal Percent
3) Quick Opening
Is best suited in an application basis.
I'm not looking for a "Be all end all answer" as I know one does not exist.
I'm more looking for peoples experience, in particular plant start-up and commissioning, so as to choose the best characteristic and not cause trim changes during said times because the wrong characteristic was specified.
I will look at the book you mentioned. Thanks man!
--Igor
RE: Inherent Control Valve Characteristics
Calculate your normal, maximum and minimum Cv's. With a approximately constant pressure drop (typically backpressure control on a vessel) your range of Cv's should normally correspond to a linear trim valve opening where the minimum is above 15% of travel and the maximum below 90% of travel.
With varying pressure drop vs flow (typically in a pumped circuit) your Cv range will be wider. An =% trim is selected which will give a "linear" installed characteristic because pressure drop falls as flow increases (due to increase in piping friction and decrease in pump discharge pressure). If the pump curve is quite flat then default to linear trim. Your valve is typically two pipe sizes smaller than the line.
Generally quick-opening trim is only used for old-fashioned self-operating regulators (with 1/4" of travel typical) and is not used much nowadays.
RE: Inherent Control Valve Characteristics
Here is the guideline.
First, check your process and find out what the "delta P" across the valve is at the minimum required flow rate and what the "delta P" is at the maximum flow rate. Establish a ratio between the two. If the ratio is less than 2:1, choose a linear inherent valve characterstic. If the ratio is above 2:1, chose an equal% plug.
I hope this the answer you are looking for.
RE: Inherent Control Valve Characteristics
Guidelines that are helpful in deciding which type of flow characteristic is best-suited for a particular application. These are guidelines only, and should not be taken as absolute recommendations:
Equal Percentage:
- when the major portion of the control-system pressure drop is not through the valve
- for temperature- and pressure-control loops
Linear:
- in liquid-level or flow-control service
- where the pressure drop across the valve is expected to remain fairly constant
- where the major portion of the control system's pressure drop is through the valve
Quick Opening:
- for frequent on/off service, such as in batch or semi-continuous processes, or where "instanly" large flow is required, i.e., for safety or deluge systems.
RE: Inherent Control Valve Characteristics
http://www.cheresources.com/valvezz.shtml
RE: Inherent Control Valve Characteristics
a. Never use a valve that is less than half the pipe size
b. Avoid using the lower 10% and upper 20% of the valve stroke. The valve is much easier to control in the 10-80% stroke range.
Before a valve can be selected, you have to decide what type of valve will be used (
RE: Inherent Control Valve Characteristics
Most of the "control valve problems" that occur after startup are a result of misapplication of the valve style or inadaquate data during the design phase; most often both. Both of these potential problems can be significantly reduced by the involvement of a control valve specialist during the early stages of the process design when the most number of design options still exist. Fixing control valve problems during operation of a facility are often many times more expensive than the cost of doing it right the first time.
Today we are able to predict life cycle costs that include energy costs, M&O costs, opportunity costs, and assign values to other factors that impact the bottomline. The life cycle costs of design alternatives can now be better compared and often used to justify higher initial cost systems.
RE: Inherent Control Valve Characteristics