Not exactly. The cracking pressure is the pressure that the check valve needs to open (to permit flow) the flow rate through the vale is related to the pressure drop. If you haven't reached the required cracking pressure you won't have anything flowing through the valve.
There's a good article on check valves at the spirax website.
so once the pressure reaches the cracking pressure and remains above it as the fluid flows- then the only pressure drop that there is - is just normal valve pressure drops?
So I just need the Cv factor? the 5 or 10 psi cracking pressure really doesnt have much to do with anything but getting the valve open...
That's right; once the valve has opened you only care about the normal pressure drop calculated from the Cv (so long as it's maintained open).
It's my understanding that if you relate this to a mechanical system, such as a cart, the cracking pressure would be the same as the static friction. They resist up until the point where the system starts moving. The pressure drop across the valve, once it has opened, would be the equivalent to the kinetic friction of the cart.
You may want to read up on sizing check valves. Check valves that are improperly sized can do all kinds of bad things to themselves. For instance a check valve that is too large will "chatter" and can become damaged.
An additional note (forgive me if I'm stating the obvious) is that if for some reason, flow drops off such that the pressure drop through the valve falls below the cracking pressure, the valve will close.
Robster,
I don't think that you are correct. The way I learned it, dP opens the valve and fluid momentum keeps it open. A 2-inch check valve takes about 0.5 psid to open and I've never measured anything close to 0.5 psi pressure drop across one that was open.
kacarrol's analogy to static and sliding friction is exactly the way that I've always thought of it.
It really depends on the design of the particular valve, but for most check valves the pressure drop across the valve will always be greater than the cracking pressure. So the most probable answer to the original question is yes, the cracking pressure is added to the piping pressure losses.
"Most valves"? I have never seen a gravity-loaded check valve that had a measurable (with field gauges) dP across it while flowing. Never. Not once. Not in any size. I have to say that my experience is at odds with that "most valves" statement.
The cracking pressure is an important parameter of every check valve. It is easily measured or calculated. It is the mass or spring force divided by the cross-sectional area of the valves. In large valves the cracking pressure may be less than one psi. In a 1/4" spring-loaded hydraulic valve it may be 50 psi. But, whatever the cracking pressure, it is a part of the piping pressure drop. My qualifier about "most valves" was simply to cover the fact that it is possible to design a check-valve that has less pressure drop when open than the initial cracking pressure. With most check valves the pressure drop due to flow friction will add to the initial cracking pressure.
Fluid momentum keeping the valve open results in a pressure drop.
For a lot of valves the pressure drop will be higher than the cracking pressure but this is not always the case. Again, back to the static friction case, it takes more force to get something moving than it does to keep it moving. It's a little more complex with valves and you will have to ultimately rely on the manufacturers data sheets to tell you how the valve should behave.
You can, theoretically, sink below the cracking pressure and still have the valve leaking a little bit of fluid. That's where the reseal pressure comes into play.
But for pressure drop calculations, I stick to my original statement, use the Cv of the valve to calculate that and make sure that your check valve is sized properly to stay open when needed and reseal when needed.
Absolutely, nothing is ever really free. That momentum does result in a pressure drop, and upstream pressure is always greater than downstream pressure or you wouldn't have flow.
But. I have digital pressure gauges that are advertised to have a repeatable uncertainty of 0.01 psig and in the 2 dozen times I've put them before and after a check valve in a flowing stream they always show a 0.00 dP. So I'm thinking that the pressure lost in the momentum transfer is in the microbar range. Not zero, but not measurable with field instruments.
On the other hand the purpose of the exercise has been to quantify cracking pressure (trying to understand performance of wellbore deliquification techniques), and those numbers are very much measurable on a field gauge (and no, simple calculation of dP times gate area is not very close to actual cracking pressure, it seems like there is always a non-zero adhesive force holding the valve closed, not sure why, but it is always there).
zdas, you need both a differential pressure gage and a pitot tube to measure the whole of what's going on in there. Cracking pressure is a misnomer for what should be termed "cracking head", which would actually be the sum of static head and velocity head, so the cracking pressure concept is only valid at zero flow, after which it becomes the sum of static differential head plus developing velocity head.
These swing-checks don't show an opening, or cracking pressure, probably because in most systems the downstream head would be many magnitudes greater than that anyway and it really wouldn't matter.
In a counterbalanced swing check, initially the differential pressure x the Sin(swing_angle)- the counterbalance weight x cos of the counterbalance angle would have to be greater than zero to open. As soon as flow began, the initial static differential pressure would decrease from the Bernoulli effect, as that static pressure converts to velocity head. Its not the static pressure vs the velocity head, or one or the other, its the sum of both that works to keep the swing plate open. Then the valve would remain open when flowing only when the sum of upstream pressure plus velocity head impacting the angled surface (from the vertical) of the swing-plate, V^2/2/g * Sin([α]) plus any frictional drag effects on the plate remains greater than the counterbalance force x the Cosine of its angle (measured from horizontal) - any downstream backpressure.
So, the "cracking pressure" is really the static head equivalent of the energy required to open the swing with no flow, which then becomes the sum, if you will, of all forces, pressure, drag and velocity impact forces acting on the plate, I think more appropriately should be called what, "the total differential head across the valve".
"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit
I don't know about a pitot tube, but two time-synchronized, recording electronic test gauges with a 60 hz recording frequency seems to be almost as good as a dP gauge.
We've had the discussion about the difference between head and pressure over and over again, and it is just too subtle for my tiny brain. I can measure pressure. Most descriptions of measuring "head" read just like what I do to measure pressure. OK, call it "cracking head", but that sounds like something a Chicago cop does on a Saturday night. But yes, what seems to be called "cracking pressure" is at zero flow.
My contention is that (just like static friction vs. sliding friction) some non-trivial amount of dP is required to open the conduit to allow flow to begin, then the required dP to keep the conduit open becomes vanishingly small. Not zero, but very small. In my mind your equation that adds the cracking pressure to a momentum term is nonsense. It should be:
dP[sub]total[/sub]=P[sub]post-cracking[/sub] + SG*(Q/C[sub]v[/sub])[sup]2[/sup]
Even with reasonably high velocities (the highest I've done this on is Mach 0.25 in a 4-inch line), the dP across a check valve and 12-inches of pipe has been less than I could measure. As I said, on the order of micro-bars. There's no doubt that it is there, it just isn't a very large number.
The original question was "On a check valve... is cracking pressure something to take into consideration when adding up pressure drops?". I still say no.
My equation is the same as your's ... and I was first.
The other "word equation" is a Bernoulli type,
z[sub]1[/sub] + [ρ][sub]1[/sub]/[γ][sub]1[/sub] + v[sub]1[/sub][sup]2[/sup]/2/g - Hl =
z[sub]2[/sub] + [ρ][sub]2[/sub]/[γ][sub]2[/sub] + v[sub]2[/sub][sup]2[/sup]/2/g
It works for any two points within or across the valve.
Some of the velocity head is converted to pressure as it stagnates when it impacts the plate, and some of the available pressure is used to reaccelerate it so that gpm in = gpm out; net result is pressure drop.
Evidence abounds for pressure drops for check valves. Hard to believe its all wrong.
The Wheatley data was really interesting. On Page 15 there is a chart of flow vs. dP for water flow. The lowest water flow is 100 gpm and that gives you 0.11 psid in 6-inch. That is the same mass flow rate as 26 MMSCF/day. At 900 gpm, the dP in 6-inch is only 1 psid. 900 gpm is the same mass flow rate as 236 BSCF/day. If I put 236 BSCF/d at any starting pressure through a 6-inch I would expect a dP.
If you extrapolate the graph to the 2-inch, 3-inch, and 4-inch valves that I did my tests on, and extend the lines down to the 200-600 MSCF that was normal flow for most of the tests that I did, you get a vanishingly small dP. Not zero, but pretty low.
Nothing I've said contridicts the references that you've provided. I've not said that past research is wrong. I have said that at the flows that I normally experience, I've never had instruments sensitive enough to measure the flowing dP. I never said that they weren't there, I just said they were too small for field instruments to see.
The difference between the equation I took exception to and my minor rewrite is that I took your P[sub]c[/sub] to be cracking pressure. My (clarification? refinement? meddling?) was to clarify that there is a dP required to initiate flow (cracking pressure or head) and a different, lower dP once flow starts. The OP is asking if you have to add cracking pressure to the system head, and I still contend that you don't.
Flowserve's chart on page 5 shows the difference for a spring and a no-spring valve, where static pressure is converted to momentum, the cracking pressure decreases, as the valve opens, then increases again once reaching full open,
Note that it doesn't go away, the pressure drop is Pc + SG * (Q/Cv)^2, however Pc is variable due to the conversion of static pressure to kinetic flow as the valve opens.
You've motivated me to try to do a CFD for an opening swing check valve to see if it matches the Flowserve chart showing that variable Pc reduces, as pressure drops when the fluid accelerates (according to a Bernoulli venturi type prediction), opens the disc and dP begins increasing again. I have this very strong feeling that it will.
"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit
That is a lot of good informaiton. Nothing that I've said above applies to the spring-loaded check valves (which I have always thought of as non-adjustable minimum dP regulators). For the gravity valves, the information in the documents shows that dP for a flowing valve is a very small number. That has been my point from the beginning. I don't know how else to say it.
Not that it really matters at this point, zdas, but I was talking about spring checks, since that's mostly what I use. I would agree that other designs may require no dP to keep them open. I suppose I should be specific. I guess we should all try harder at that.
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