When to use butterfly valves instead of gate valves.
Butterfly valve are chosen more often recently for the following reasons:
- Cheaper than gate valves.
- Easy to operate.
- Smaller lay-length and more compact top-works.
The size and pressure rating of butterfly valves has increased significantly over the last 20 years or so. They are now available in sizes up to 3m (and above) and in pressure ratings up to 150 bar.
So why are more users not installing butterfly valves ?
The reason is that there are limitations and disadvantages as follows: Higher pressure drop through the valve. Increasing awareness of energy efficiency means that the higher pressure drop (compared with gate valves) is not tolerable. The pressure drop worsens as the pressure rating increases because the components have to be bigger to be strong enough.
Higher wear rates leading to shorter life span than gate valves. Butterfly valve seals are more exposed to flow velocity, particularly during opening and closing under high differential pressures. This leads to higher wear rates.
Caution is still required even though butterfly valve seal design and operation has improved to allow for higher differential pressures.
It is generally accepted that velocities through a butterfly valve should not exceed 6m/s. Lower velocities would normally be recommended.
Improved designs have made higher velocities possible:
- Double eccentric designs reduce local high velocity flow by opening the whole circumference of the blade immediately the valve is cracked open.
- Metal seated butterfly valve with stainless steel seats can withstand higher velocities better.
But, in spite of these improvements, the seals are always in the flow path, and thus more vulnerable than gate valves.
High flow velocities arise when a valve is opened at high differential pressure. In most cases, a differential pressure higher than 3 bar will produce a velocity greater than 6 m/s through a butterfly valve.
Designers should consider these limits carefully before specifying butterfly valves.
I saw a spectacular failure of a butterfly valve on a recent project.
Butterfly valves were installed on a reservoir. The upstream pressure was as high as 18 bar. The pressure was reduced through pressure reducing valves and flow control valves. The design was sound. All the valves would operate comfortably within their capacity under normal conditions.
The contractor was running late. He filled the reservoir by opening a butterfly valve. The control valves which would have reduced the pressure hadn’t been commissioned yet and were locked in the open position.
Thus the butterfly valve was opened under a differential pressure of 18 bar. You could have kept the lights on in a small city if you’d had a hydroelectric plant handy!
Later we calculated that the velocity through the butterfly valve was greater than 50 m/s. It caused severe damage and eventually total failure as the valve broke into several pieces.
A high pressure gate valve would have been a far better choice as no velocity and turbulence forces are transmitted onto a blade which sits directly in the flow path.
It was an expensive reminder that design engineers have to think beyond normal use cases. We have to allow for unskilled operators, poor maintenance and people doing things we hadn’t intended.