why a ball valve has less resistance than gate valve when wide open?

It is a sample problem from 2001 T&F.

#511

My thought is gate valve will have less resistance.

Because the gate valve's seat tends to slow down the flow. While ball valve's ball conforms to the exact inside diameter of the piping therefore less resistance.

http://www.tpub.com/fireman/69.htm

Look at the cross section and cutaway views.

I wish the the questions on the test were more from memory, but when I took the exam it asked a lot of non-calculation word problem questions about things I had no idea about, like very specific material processes. I found that most of these sort of questions, whether on practice exams or the real exam could be found in the MERM, when I got to the sorta question like which device or which method is best for, etc. I would look up each answer in MERM index and write the associated section next to the answer, then go find the definitions of each.

Thank you.

I am a licensed engineer and worked on valves all my life. As per my knowledge, when gate valve opens or closes under fluid pressure more resistance is offered by valve disc. Compared to ball which is already inline with fluid flow. A butterfly valve has even less fluid resistance. I bet it is based on geometry of valve and how aerodynamic it is to fluid flow.

gaurav0323 said:

A butterfly valve has even less fluid resistance. I bet it is based on geometry of valve and how aerodynamic it is to fluid flow.

Click to expand...

If we're talking full open valves, a ball valve will have the least resistance to flow (or a higher flow coefficient, Cv). A full-port ball valve has the same ID as the pipe it's connected to. This has the highest Cv and most flow through it. A reduced-port ball valve has a slightly smaller ID than the pipe it's connected to, but it's a smooth transition. It has a somewhat lower Cv.

The problem with a butterfly valve is that the disc is still in the flow stream when it's fully open. Its Cv may be comparable to a reduced-port ball valve, but definitely not a full-port.