BigInch,
My only problem with your equation was that I took it to imply that whatever dP was required to open the check valve would be present in the flowing system. My experience has been that dP builds up to initiate flow and drops off to a vanishingly small number. Adding a multi-psi cracking pressure to "less than measurable" dP would overstate system dP.
Robster1us,
"Check valves" are one of those ubiquitous pieces of equipment that we don't think about much. In my mind a "check valve" is a "swing check valve" and all my knowledge that says that is only one of many types doesn't stop me from thinking that a check is a check. I've seen the same blind spot about ball valves. We still don't know what the OP was asking about or if he has answers to his questions.
I've been kind of argumentative in this thread, primarily because the concept of "magnitude" keeps getting lost in fluids engineering. When I go through the methods in Crane 410 and add up the additional pipe lengths for fittings and valves, I get a number. If I take that pipe length into a gas flow equation like the AGA Fully Turbulent equation, I get about the same number as if I hadn't bothered to account for the fittings. It is different, but I've never had it lead me to a different decision. And that is what is important any any mathematical modeling. The magnitude of the effects just doesn't change my decision. It is useful to be as "rigorous" as possible, but once you've reached that point you get a number. Then you have to look at that number and see how it effects your decision. A microbar dP across a gravity check valve just doesn't push me to a different pipe size.
I have been gone most of the weekend and am really looking forward to reading through all the posts tomorrow at work... seems like a lot of good discussion... my old job dealt nothing with piping... but my new job has a bit of it... so this is good stuff.
I was told that the valve has to keep oil from leaking into a boiler, and there is about 25' of 1/2"ID vertical pipe above where the valve will be and then after the valve there is an oil burner... so I guess I will have to get a valve with a spring to give me a higher cracking pressure to overcome the static head...
I guess they want a valve to keep the lines primed with oil, but without having to install any actuators or anything else but a check valve in each line
zdas, check this ref again. The only one of the cases where the diff pressure almost disappears is for "NO SPRING". For check valves with springs, it remains.
Understanding the full impacts of check valves in the line is proabably a whole bunch easier if you've had to fix an undersized pump suction line where somebody forgot to include 1 km of length.
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I don't understand why this has gotten so complicated. The mechanics of a check valve is very basic. For a spring check or a gravity operated swing check a certain minimum force is required to open the valve (cracking pressure). In both cases the force increases as the valve opens. It does not decrease. Cracking pressure is always added to system pressure drop. In this case the the OP wants a significant cracking pressure to prevent a fuel line from draining into a boiler when it shuts down. Cracking pressure must not be insignificant for the valve to perform its function.
CompositePro,
The OP's use of the valve is completely different than its designed use.
Correct me if I'm wrong but a 'spring' in a spring loaded check valve is to assist in the reseating, not to be used as a regulating device.
The arguement between BigInch and David is semantical at the sizes we're talking about. Forest for the trees.
Whether its a loss in static head or velocity head is immaterial....the spring requires a constant force to be overcome therefore it needs to be accounted for.
I have to say using a spring check as an actuating device is not very appropriate in my opinion.
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Its funny you mention that... I have been thinking along those same lines recently.... about how a check valve in this situation just seems like it would be used for something it wasnt designed for... or was that 5 or 10 psi cracking pressure designed for an application like this? I dont know...
The problem is that the customer does not want to run any pneumatic or electric and manual is out of the question.... so the check valve was all I could think of...
Many spring loaded check valves are designed just for this type of service that the OP has. The valve is designed to be closed tightly by the spring only, and not rely on fluid back-pressure to operate. That is why the cracking pressure is often 10 psi or higher.
What is a poppet check valve? It is a spring check valve similar to what you are describing right? the pipe is only a 1/2" oil line so there might be limited options...
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