1. tapped and not tapped flange holes in valve bodies. Generally, lug version of the body has tapped holes, which allowes direct bolting to pipe flanges without using nuts. On rare occasions, customers try to avoid tapped holes, because stripped threads are difficult to repair, so they may ask for lugs to be drilled through for bolt clearance, and use longer bolts or studs plus nuts to install the valves.
Flanges are always drilled through.
PN 40 (40 bar) is not considered high pressure for lug-bodied valves.
2. distinguishing valve construction by bodies, flanged versus lug, for example... this is done by visual inspection, of course. You can see whether the valve is flanged, or lugged, or a wafer etc. just by looking at them.
Whether a butterfly valve is zero offset, or single offset, or double or triple offset requires more expertise, but here are some rules of thumb:
Zero offset valves are typically fully lined rubber or polymer seated valves, with symmetrical disc and the shaft in the middle of the disc.
Single offset butterfly valves have shaft on one side of the disc, i.e. the disc is not symmetrical, and are almost always soft seated, i.e. rubber, or PFA clad rubber seats. Larger valves may have adjustable seats. These valves are mostly used in waterworks applications.
Double offset valves look like single offset valves, except they typically use a thin polymer not-adjustable seat.
Triple offset butterfly valves are almost exclusively metal seated, have a funny looking skewed disc edge and corresponding funny looking seat geometry. Typically, there is a lot of cap screws holding down the disc seal and the seat inside the valve, that right away signals a metal seated triple offset valve.
To my knowledge, size has nothing to do with the body type, there are very small flanged valves say smaller than 1", and there are very large lug-bodied valves, say 72". Wafer bodies tend to vanish past 48" size, because the wafer body becomes too flimsy to hold precisely the internal dimensions.
3. Manufacturer has to produce a certificate that the valve has been tested to a recognized fire safe standard, such as API 6FA mentioned above by Anegri. API 607 firesafe spec is for soft seats, and therefore theoretically does not apply to triple offset valves which have hard seats. However, manufacturers of triple offset valves sometimes test to API 607 and produce certificates to this spec, claiming that their metal seal is to a certain degree flexible, and hence the seat is "soft". For butterfly valves, it is easier to pass API 607 than API 6FA.
4) Bubble tight means what is says: no bubbles across the closed valve when tested with soap or under water with a gaseous test media, i.e. completely leak tight.
hi anegri,
Thanks for your suggestion and the website you had provide for me, I had browse their website, most of them have the fire-safe certified, as manufacturer we also can provide according to customer's request and test conform to API607, API 607 Incl. fire-safe test API 607 4th version and triple offset test with zero leakage,does it is the similar with API 598 for test with zero leakage? I am not familiar with API 6FA and ISO 10497 it is the similar with API 607 and API 598? all of them is test with zero leakage? what's different with them? how can I choice when I have to test triple offset butterfly valve and double offset butterfly valve?
Thanks.
hi hmmmmm,
Thanks for your patient explanation for my question.
1. the wafer type is the same like Lug has the tapped hole not drilled? How much pressure as usual can be withstanded for wafer and lug type valve?
2.Beside of distinguishing valves from construction by bodies,cost and applicable medium? can I use double eccentric to instead of centre butterfly valve?
Hi hmmmmm,
3. Can you tell me more about the different between Single offset with double offset? does it only distinguish from adjustable seat or non-adjustable seat?
4.most triple offset is matel seated does it can test conform to API 607? is there any other standard is suitalbe for triple offset matel seated?
1)Wafer bodies are smaller on OD than lug bodies, (the of a wafer OD would be about like the inside of the flange bolt circle), the flange bolts straddle the outside of the body. You can not bolt a wafer to a single flange - wafer is squeezed between two pipe flanges.
Lug body has either a large OD (the OD would be about the same like the OD of the flange), or it has protruding lugs to facilitate flange holes. Lug bodies can be bolted to a single flange at the end of the pipe.
2) Yes if compatible with the application conditions...
3) Single offset places the valve stem on one side of the disc, thereby ensuring uninterrupted sealing surface in contact with the seat. The problem is that the disc touches the seat all the time. Even when the valve is open and the disc is out of the seat, its edge touches the seat at two locations. The disc must be bigger than the seat to seal, hence in a single offset valve that has been open for a long time, the disc digs two impressions into the seat where it was in contact with the seat. Rubber seated valves can tolerate this condition, because rubber will readily flow into the depressed areas and seal the valve upon closing.
However, polymer seats and other harder seats can not react so elastically as rubber. Hence, a single offset valve with polymer seat would leak at two locations after it has been closed following a longer period of being open.
Here is where the second offset comes in. The second offset is designed such that it places the valve stem not only on one side of the disc, but also a bit sideways, thereby ensuring that after the valve has been open, the disc edge clears the valve seat, and thus does not produce damaging impressions. The second offset is minimal, only enough to clear the seat. Dimensionally, for example, in a 12" valve, the first offset would be say 1.5 inches, second offset would be 60 thou. Second offset introduces offset torque, which tends to open or close the valve, depending from which side the pressure is applied. This torque may be very significant in larger valves.
Adjustable seats are used only for larger rubber seated single offset butterfly valves. I have not seen a double offset valve with adjustable seat.
4) Hard seated valves like triple offset should really be fire tested to API 6FA. Like I wrote before, API 607 is for soft seated valves, i.e. zero, single, and double offset valves. But almost all manufacturers of triple offset valves claim to have certified the valve to API 607. It is easier to pass API 607 because it allows leakage and is limited to only 30 psi, and that is probably why this test is more popular with manufacturers.
By the way, API 598 has nothing to do with testing for firesafe capability. It deals with hydrostatic tests of valves and with leakage tests across the seat.
Hi hmmmmmm,
Thanks for your more answers to my question. I think as more experiences as I have I will clear what you said, it will be very useful for me.
if possible I would like to ask you other question about butterfly valves.
1. Would you tell me what's different between the centre butterfly valve with pin and without pin? does it only come from centre butterfly valve or also appeared in single and double offset butterfly valve?
2. would you tell me what's different for double offset butterfly valve that the rubber seal is setting in the body with setting in the disc? what's different request for the applicable medium?
3.which one have longer application between double offset and triple offset.
Thanks a lot.
1) I am not sure what you mean by the "pin". Pins are often used to secure the disc to the valve stem. This feature is used on all types of butterfly valves. No pins would mean that the drive between the disc and the stem is internal to the disc, such as an internal square, DD flats, splines etc. Internal drive is considered better than pins, especially in the zero offset resilient seated fully lined valve category, because the disc to stem connection is protected from the flow media.
2) I don't understand the question...
3) Life of a valve always depends on the severity of the service. Triple offset valves are usually used in different applications than double offset valves, and vice versa, so the comparison between the length of usefull service valve is not realistic.
However, the triple offset metal seated valve would have wider rande of applications and longer life than double offset valve in many high temperature non-corrosive applications.
Double offset valve would be less expensive and possibly work better than triple offset in applications involving mildly corrosive and erosive service in lower temperatures.
Metal seated triple offset valves are becoming more and more popular as the price comes down with improved manufacturing methods and stiff competition.
hi hmmmmm,
1.Yes, You have good understand for my question, pins are often used in secure the disc to the valve stem. I just notice that it is on the centre butterfly valve disc. I will observe other butterfly valve how the disc to stem connect is protected from the flow media.
What's meaning of DD flats?
2. I mean what different between the seat is mounted on body with on disc?(double eccentric rubber seated butterfly valve, the rubber seat is mounted on the body or on the disc?) which one is better? does it distinguish from the media?
3. Refer to drinking water(I mean general temperater and general pressure condition) which one is better double offset rubber seated butterfly valve and double offset metal seated butterfly valve?
Thanks a lot
1) Double D flats are just two opposing flats machined to the round shaft. I guess the material removed would have a geometry of two Ds back to back, hence the double D name.
2) Seat is always in the body... Disc may have a separate sealing element attached to the disc edge, and sometimes, such a seal element is called a seat. Not exactly the best way to say it...
If the seat or the disc seal are resilient and soft, like rubber, then the seat in the body is considered better design than seal on the disc, because the seal on the disc is exposed to the full velocity of flow, and hence it can get eroded by the flow relatively easily.
The seat in the body is at the boundary of the flow way, and hence the erosion is not as severe.
3) In drinking water, with normal temperatures and pressure, you should look for a zero offset fully lined rubber seated butterfly valve, or possibly a single offset rubber seated valve. Anything else is in my opinion an overkill... Certainly not triple offset valve...
Hi hmmmmm,
Thanks a lot. I will choose rubber seated attached on the body in the future, I think.
Refer to seat is on the body, would you tell me what's a separate sealing element attached to the disc edge?
what's the advantage and disadvantage in character between double offset and triple offset butterfly valve? i.e. the torque, friction, etc.
Thanks in advance.
In certain type of butterfly valves, there may be both replacable seat in the body, and a replaceable seal ring on the disc edge. Triple offset metal seated valves are a good example. Typically, both the seat and the disc seal ring are replaceable (exception is Vanessa who weld stellite seat into the body and it is not field replacable).
smoothly until now.
Refer to my question I think I am clearly, but I wish to keep continue discussing this topic when the problem I meet in my work in the future.
Thanks for all the people who joined our discussing.
Thanks anegri, tsenthil, RXH, Ims1964...
