Was looking at a reply on 19 March 2010 - 2:23 pm using a butterfly valve as an example of a control valve and at the same time highlighting the problems it causes. Can anyone please explain to me why any design engineer would opted to use a butterfly valve at all as a control valve. Are they in some applications more suitable than globe valves, or is this all about cost saving and low priority control applications?
I find it difficult to accept that anyone that knows the flow profile of a butterfly valve will still use and try and do control with it. (10% valve opening will already give about 90% of max flow)
They are fine for many applications. Like a lot of things, there is an interplay between performance, cost, reliability and many other factors that comes into play.
Butterfly valves are usually preferred for applications with low pressure drop. From a control stand-point, globe valves are better. In cases where the pressure drop is very low, the Cv required becomes very high and typically a globe valve would not have enough capacity to cover the required Cv. A butterfly valve (since it has a stream-lined flow unlike a tortuous flow of a globe valve) has much higher capacity than a globe valve for the same size. So a butterfly valve would be a good choice in terms of cost for low pressure drop applications.
The statement that 90% of max. flow is covered at 10% opening is not true. The flow characteristic of a butterfly valve is determined by the disk profile and most of the butterfly valves have a linear flow characteristic. The Fisher Control-Disk model Butterfly valve is a latest advancement from Emerson. This model has a inherent Equal percentage characteristic due to the specially designed disk and is designed for control applications. From a control stand-point, this design is considered to be better than most conventional-type Butterfly valves.
Hope this helps you.
jronalddeepak [at] gmail.com
Good info, Ronald. I also use butterfly valves in many control valve applications have had much success.
And thanks for pointing out the Fisher Control Disk. I just read over their data sheets, and the product looks intriguing.
Considering a butterfly valve for a control application, one should limit the open to 60°? That is, one must choose a Cv corresponding to that opening?
I don't use many but for low pressure drop they work very well for instance I have one 26" on gas flow, basically a damper, you wouldn't use a globe there would you?
In mining application I have also used them on water flow at fairly high pressure drop 60#. They cavitate like crazy but if you look at the valve years down the road there's no sign of damage, can't say the same for pipe though.
Even Stelite coated plugs and seats gets damaged due to cavitation in the trim of a globe valve. Why do you think this butterfly disk of yours is not picking up any damage in this high cavitation application?
Anyway thanks to all that have responded to my question I have learned quite a bit and will reconsider my negative attitude towards butterfly valves. It make sense that they will have applications where they are more suitable to use than globe valves as long as they are not used on critical control applications just to save on cost.
From a process control standpoint (not process condition), the actuator determines the quality of control. A butterfly valve with a high performance actuator (not pneumatic!). Will control better than a globe valve with a standard actuator.
High performance butterfly valves exist because of the inherent control limitations and bad combination with pneumatics on standard butterfly valves. A high performance actuator eliminates the need for "high performance" butterfly valves. But, the industry generally considers the actuator to be an insignificant accessory. In fact, it is "control" in process control.
I've just read your comment and I'd like to ensure the following
Allow me to introduce myself to you – I am Eng. Ahmed Kamel from Egypt and we are manufacturing hydraulically operated globe valves for control process.
And I am responsible for the technical marketing issues
I always try to convince our customers to use our globe valves better than the BF valves in control for some reasons.
The main reason is that the BF has an inherent equal percentage curve which is kind of can be used in control. but I think that the main problem in the BG is it's geometry. because when the BF is opened at 10% or 15%, there will be a turbulence in the flow leading to a slaughtering in the BF disk and seat which will cause a quick failure to the BF valve.
knowing also that the self actuated Globe valves are much better than the hustle of the electric actuator, and the cables and using a controller and a sensor and a plc system. plus it doesn't need electricity to operates and it won't be affected if the electricity was cut off or something.