Control Valves 3

[kriger]

When looking at characteristics of various types of valves it seems to me that butterfly valves are superior to other types regarding control applications. The control range is wider (0.5 &lt; gain &lt; 2.0) than for both ball and ball sector types.<br>
<br>
Can anybody explain me why I shouldn't recommend butterfly valves for control purposes?

 

[svanels]

In general globe valves are used for control applications. This because the opening of the this valve is linear with the flow output. There are special types of ball valves, but the opening is in this case modified to have the same behaviour as the globe valve.<br>
<br>
I hope this info satisfy your question

 

[kriger]

Thank you for your reply, Svanels. Although the purpose of a control valve is to adjust resistance in a fluid cycle, I most often find that the resistance of a globe valve is to big. Ball valves and butterfly valves have higher kv-values. However, the dealers keep telling me that I should select ball sector valves with special cut to achieve optimum control characteristics. But when I draw up the kv-values against the valve opening, I see that the control characteristic of a butterfly valve is superior to those of ball valves and ball sector valves.

 

[TedT]

Resistance is necessary for good control! Typically 25% of the system pressure drop is necessary for good control. Otherwise you may end up with full or mostly open or mostly closed in actual operation. Make sure you understand the installed valve characteristic, because this changes depending on the size of the valve versus the overall sysem pressure. Linear characteristics may change to parabolic, i.e. on-off.

 

[DenverEngineer]

The question of why you would not want to use a butterfly for control is mostly related to the type of butterfly. If you have a simple plate type valve then it will not exhibit the full range you have indicated. You need a butterfly designed to be a control valve. These valves have several internal differences to make usage as a control valve reasonable.<br>
<br>
I have made usage of butterfly control valves for modulating service. Typically these have been greater than 4" where other types of valves are not economical. Any control valve must be applied within it's range. <br>
<br>
There are some ball valves with minor modifications that can serve as excellent control valves, and have a turndown range of 1:100. However the basic ball valve as you have indicated has a poor ability to modulate flow. <br>
<br>
Regards<br>
<br>
Dan Price

 

[ghillis]

The main problem with a butterfly valve is that the valve works poorly near the shut off position. A lot depends on whether you are going open or closed loop in your control system, the actual flow, media, etc. As to pressure drop, if you are looking for a wide range of flow then why are you concerned about pressure drop in the full open position? After all, you won't be operating in that mode much. Look at your pump curve, it will show you how much drop you must have to reduce your flow to a desired flow. You will probably be surprised at how high the drop is. If coarse control is sufficient and a small drop across the valve are required, then a butterfly is OK. An alternative if you really need minimal drop is split ranging two valves in parallel, the small valve opens first for fine control and the large valve opens once the small valve is fully open, for control near the high end of flow.

 

[kmuhsen]

Most of butterfly valves ahve a poor throttling performane comparing with globe valves, but what makes people specify butterfly because it is easy to fix a control actuator for automatic control which is hard to do in the case of using globe valves in addition to space saving.

 

[pintu]

can you tell me more about sonenoid valve.its need and importance

 

[svanels]

A Solenoid valve is a control valve actuated by an electrical control, moved by electro magnetic forces. It is typically used for rapid shut off purpose in combination with gate, knive or butterfly valves, not for controlling flows.
You would encounter them on gas lines of burners, when in case of a failure (flame, electrical, draft) it would cut off the fuel supply. Steven van Els
SAvanEls@cq-link.sr