Reparing Gate Valves without Dismantling Pipe Assembly and Butterfly Fast Shut-off 3

[IBechir]

For large chilled-water pipes, dia 200-350mm, what type of isolating valves would you use?

I am aware of the differences between butterfly and gate valves; namely the price, size and weight, but I am concerned that butterfly valves shut-off too quickly (I read that for large valves the last 20% of the quarter turn controls 80% of the flow) so there is a higher chance for a water hammer.

Also, I am aware that gate valves can be repaired without removing it from the pipeline but I am not sure if this means that some repairs can be done without draining the system. If the system still has to be drained, what advantage is there for being able to repair a gate valve with disconnecting from the pipe?

Thanks in advance to all that share information.

 

[willard3]

You can throttle with a butterfly and not with a gate, a strong advantage.

 

[SAK9]

Butterfly valves have a linear characteristic for about one third of valve travel. The characteristic is quick opening initially only.I have used them on 250mm lines at 3 bar without any problems.If you need access to the gate or seat ring of a gate valve ,you have to either drain or freeze the line.You will need a worm gear for large dia butterfly valve operation.

 

[ChasBean1]

Butterflies are better for your application. Gates are difficult to open against higher DP. The butterflies in your application (typ. 200-350 mm) have an offset, gear-driven actuator that prevents rapid opening or closing, so it takes many turns to stroke rather than a smaller butterfly with a latch handle that has a 90° stroke.

 

[IBechir]

Thank you everyone.

 

[rconner]

As a multi-turn valve, a gate valve is far less likely to be involved with water hammer. Quarter-turn valves including butterflies can close quicker, so this question should be considered. It is also true that you can repair a gate valve without removing the valve body from the line. This cannot be done, however, without removing line pressure and evacuating the line. The reason there is a possible disconnect on the advantage of this feature is likely because one may be considering only a flanged applications, and even then it is typically much easier to remove the seating / wedge assembly of a gate valve than it is to remove a flanged and valve body from an existing pipe network. In cases wh​ere the end configuration is anything other than a flange, such as in buried service conditions (e.g. mechanical joint, etc.), the butterfly valve would normally have to be cut from the line and the gate valve would not (in any normally open service a bfv disk may extend beyond the laying length of the valve, further complicating removal/service).
Other advantages of a modern gate valve over a butterfly valve include the inherent design reliability / durability characteristics of the valve, larger seating surface for greater forgiveness, as well as improved energy consumption. The pumping costs of a butterfly valve, dependent on the assigned flow coefficient of the valves, can be as much as 10X greater. As for throttling, the vast majority of valve applications are simple open and close service. So unless the application is for a modulating service there is no disadvantage ever for a gate valve.