Inherent Control Valve Characteristics 6

[cornejoi]

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

 

[shobi]

The relationship between the valve stem position and the flow is defined as the valve characteristic. This relationship with constant (design value) pressure drop is termed the inherent characteristic, and the relationship in a specific process in which the pressure drop may vary with flow is termed the installed characteristic. Two related units are used for the characteristic; one is flow in gallons of water per minute per stem percent that is used for sizing control valves. The other is percent maximum flow per stem percent which is used to plot typical 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.

 

[cornejoi]

shobi, thanks for your reply.

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

 

[Scotsinst]

You want a valve to control with a constant gain or linear installed characteristic (so you don't need to get too fancy with tuning).
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.

 

[shobi]

Igor,

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.

 

[Guidoo]

Source: Chemical Engineering Progress, "Ease Control Valve Selection", Nov. 2002, page 55, see

http://www.cepmagazine.org

:

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.

 

[Guidoo]

Same recommendations appeared on following link:

http://www.cheresources.com/valvezz.shtml

 

[japsy]

Don't make the mistake of trying to match a valve with your calculated Cv value. The Cv value should be used as a guide in the valve selection, not a hard and fast rule. Some other considerations are:
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 (

 

[Scorpio7]

There is some good advice here, but the bottomline is there is no simple answer to a complex question. Guidoo's general recommendations are a good start, but you must still look at the process characteristics as well as the fluid characteristics. ISA has some very good standards on control valve sizing and flow prediction, and noise prediction and cavitation are among the other isuues to be considered. Those standards should be used as a guideline to alert you as to when to involve a control valve specialist. If you do not understand everything you are reading and how the information provided about the process fits together with that material that should be a "RED FLAG" to involve the specialist.

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.

 

[1969grad]

If you can find a old hardcopy of Fisher's Catalog 10, which was their valve sizing catalog, it has some recommendations in the back regarding selection of the valve characteristic.