Butterfly Valves are used for isolating and regulating flow. The closing mechanism is a disc that allows for quick shut off, much like a ball valve. Butterfly valves are light in weight so they require less support. Unlike the ball valve, the disc is always present within the flow, therefore a pressure drop is always induced during flow, regardless of valve position. The disc is typically rotated a quarter turn when the valve is fully opened, but the valve may also be opened incrementally in order to throttle flow.
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Butterfly valve consist of a disk which forms a seal against an elastomer or metallic seal. The disc rotates inside the body thereby restricting or allowing the flow. Butterfly valves are very versatile. It can be used for manual or process control, throttling on/off of liquids or gases. They are often used because of their shot take-outs and lower costs.
Butterfly valve turn 1/4-turn at a time. Lever operators are often equipped with a squeeze handle that disengages a pin from a round rack of teeth as shown in the picture.. The pin can be used to set the position of of disk inside and create a desired throttle. The valve can also be locked in position with thumb screw. This makes them ideal for use on mud pumps used in O&G rigs. There are two types of butterfly valves;
Wafer Type Butterfly Valve
Wafer type ball valves fit between two flanges, with studs passing from one flange through other. The valve is held in place and sealed with gasket by the tension of suds. These are considered low cost butter fly valves.
Lug Type Butterfly Valve
Lug type butter fly valve is usually used where the valve is at the end of a pipe since there will be no 2nd flange to secure the studs. Instead, lugs are cast on the valve with tapped holes which matches the bolt pattern for the size and pressure classification of the flange. Bolts are passed through the flange holes and are threaded into tapped holes of the lug, as shown in the picture.
In most cases the pressure rating of a butter fly valve, especially at the end of line service is derated by a safety factor of two since there is no companion flange to secure the assembly together. Some manufacturers offer full pressure rating so it is best to verify the rating with the particular valve manufacturer.
Most common butterfly valve types come with elastomeric seal. These valves are good only for infrequent positioning such on and off service or where valve is set in the position for longer period of time. Applications where the butterfly valve is used frequently to throttle the fluid, require metallic seal to avoid erosion of the elastomer.
Butter fly valves offer several benefits over other types of valve, especially in larger sizes. These valves save weight, space and cost less. Maintenance cost is also low because there are fewer moving parts. Cavitation is also less since there no pockets to trap fluids.
Butterfly valve design offer suitability for large flow of liquids at low pressure and for handling slurries with large amount of suspended solids and particles. This makes them an ideal choice for mud pumps which often has lost circulation material, friction reducers and other additives that aid during drilling process. Larger opening and fewer moving parts helps in avoiding entrapment of suspended solids.
Think of wafer style of lug style butter fly valve as a pipe damper. The flow control is achieved with a disk which roughly matches the ID of the pipe where the valve is installed. The disk rotates on horizontal or vertical axis allowing fluid to pass right through it.
Disk and Stem Design - Butterfly Valve
Key component of a butterfly valve is disk and stem. They are separate pieces in both type of butterfly valves. Disk is machines to mate with the stem. Disk can be secured to stem in one of the two ways. In one design, the disk is bored and attached to stem with bolt or pins. The 2nd butterfly valve design uses disk that is bored first and stem is matched float inside the disk. The self centering of disk is advantageous when disk is covered or clad for use in corrosive application.
Q&A with a Valve Distributor
We recently sat down with a valve distributor which specializes in smaller size valves (up to 4") mainly for O&G customers. Below is the transcript of the session;
Speaker 1: Ok so these are butterfly valves, we offer them in a ductile iron body, we will cover it in a wafer style, a lug style, as well as a wafer with alignment holes. Basically, these valves are ductile iron body, like I said wafer lug. Now we do offer it in a ductile iron nickel plated disc, with a Buna-N seat. We also offer it in a 316 stainless steel with a Viton seat, also with a EPDM seat, what other seats available?
Speaker 2: Teflon, essentially that's it. Teflon, EPDM, Buna-n, and Viton and the reason we offer different seats for these butterfly valves depending on what the application these customers are running through. Here recently we've been having a lot of customers run through a lot of acid and acid will tear these valves up. So were working on something now to get an epoxy coated disc and to be able to withstand the effects of acid, and also you would want to use a Viton seat or a Teflon seat depending on what the percentage of acid is. So essential that's it, we got the wafer style butterfly valve, this is going to be your ductile iron disc, with a Buna-N seat. Buna-N is rated a 180 degrees. This here is going to be your lug style, and you can see the difference here, you got the wafer with the notches, and then you got the body that's aligned with the bolts for the lugs. This one will be the stainless steel with the Viton, Viton being rated at 400 degrees Fahrenheit, and then you've got your alignment hole butterfly valve as you can see it's wafer style alignment holes. This really helps the operator out on the field if he's working by himself to be able to stab this, or align this hole up to a bolt that's already put on the source side flange. To be able to just let it hang there while he gets everything situated, while he gets his tools ready to be able to put this thing in line.
Speaker 1: We do offer these also- forgot to mention, sorry to cut you off. We do offer these in a two-inch all the way up to a twelve-inch. On the alignment holes right now what we do offer is only a four-inch, but we do have other sizes coming in a three, four, six and possibly an eight-inch coming soon.
Speaker 2: And we can get anything made to your specs. For example, we had some 48-inch butterfly valves, four pieces made for a company over in Russia. So we have the capabilities to ship international all over the United States and we can make anything built to spec.
Speaker 3: What is the pressure rating of these?
Speaker 2: 200 class, 200 pounds, or a 150 pounds.
Speaker 3: Water oil gas?
Speaker 2: Yes, well water along gas depending on the application. Now your ductile wafer ductile Buna, ductile iron disc, nickel coat plating, Buna-c is what you would want for water oil gas. When you get into gas pockets, crude oil stuff like that; that's got H2S in it.
Speaker 1: Any acid you are going to have a different seat, different disc on there as well. So depending on the application we'll make some changes there.
Speaker 2: From our factory, these come tested, MTR's, anything.