Pressure Differential on Closed Butterfly Valves

If you had no pressure differential, why would you need a valve?

This sounds like homework to me!

We don't do homework.

And, if you are a "professional" asking this question, QUIT YOUR JOB NOW!!!!!!!!!!!!!!!! You're in the wrong line of work.

Lylynch tends to beat hard in not so well formulated questions. I'll try to review it and translate to engineer terms in a more fashionable way.

The question should be posted this way: "How does a closed butterfly valve reacts to variations in delta-pressure regarding sealing and closing characteristics?"

Answer: depends on the valve model. Some are pressure assisted to close via an actuator or an offset in butterfly shaft. Some have pressure fed disc seals that expand and contribute for sealing against the seal or sealing diameter. Some close themselves by simple reaction to the flowing fluid. Some behavior the opposite way, some will just leak more, some will observe no change at all. Add closing or opening spring assistance, a combination of spring and pressure fed actuators, remote piloted differential units, double bodied valves with phased discs... the sky is the limit.

If it's a homework, I recommend a google search or wikipedia and read about valve designs. If not, please, be more specific in your question.

"translate to engineer terms in a more fashionable way".

Nicely put. Very gracious of you.

You're welcome. Just a small amount of grace to the amusement of the audience...

Hi...

The disc is designed to hold the pressure. During my time, we used to conduct pressure leakage tests even on large diameter butterfly valves for hydro power plants. Valves of 4m diameter were common but the company even built 6m dia B/F valves which were leak tight. You can see them in operation at the Aswan Dam.

Anil Tiwari / New Delhi

WOW!! Great indeed.Who built these remarkable valves??

Hi...

It was Sulzer Hydro AG of Ravensburg Germany for the 6m dia's & their Indian subsidary made all the 4m dia valves. The high performance B/f valves used to weigh 45MT & up. This firm was earlier known as Escher Wyess who invented the Hydro turbine in 1842. They have made several inventions for the benifit of mankind including pitch propellers for ships after the Titanic disaster.

In fact in the late 90's in-situ repair of the 6m dia valve seats was carried out at Aswan Dam. A make shift VTL had to be assembled in the site and inside the valve. A unique feat of engineering in itself. I'll have to dig up some old photos.

Anil Tiwari / New Delhi

If you had selected and designed your valve upon the design conditions of the system (pressure, temperature and material), and you had decided the rating/class of the valve, don't worry about the differential pressure between upstream and downstream.

Otherwise, if you have a doubt, you can add an isolating valve downstream of the butterfly valve.

Hey, Steve in Vegas, where are you? Did you have to go to school?

Hello S in V,

I worked on a small compressor which had brass butterfly valves. These were spring loaded and closed as soon as there was negative pressure on the back of the valve, (positive pressure) from normal air pressure combined with the spring to close the valve. With the valve fixed on one edge. They closed flat against the air pressure cylinder 'head'.

Hope this is of help.

Dear Steve,

I have been closely involved in butterfly valves for hydro generating units for penstocks upto 6m dia. The BF valves are designed to withstand the differential pressure in closed condition. The valves are also tested (in the presence of the purchaser) for 1.5 times the working pressure to check for any deformities and leakages. In fact, the BF valves are designed to close during full load trip with the water flowing through the penstocks at the max. designed velocity. However, the valves of large diameters should not be opened with differential pressure. A by-pass valve is provided wherein water is admitted downstream of the BF valve, which equalizes the pressure on both sides of the BF valve. Only when the differential pressure is zero, will the BF valve open. There are inbuilt safety interlocks which prevents the BF valve to open when the differential pressure is not zero

Hope this clarifies your doubts. Please feel free to contact me for any additional information

Bye!

SAMUEL CHELLIAH