Hi John,
There are a couple of steps to this, but it doesn't take too long.
First run the Initial Condition analysis and record the velocity in the pipe downstream of the check valve. Let's assume it is 4 ft/second.
Next, run the transient simulation and open the Transient Results Viewer. Plot a time history graph of flow in the pipe downstream of the check valve and measure the time from when the pump turns off until the flow curve crosses the x-axis (i.e. when flow reaches zero). Let's assume it is 2 seconds.
So the deceleration of the water column is 4 ft/second / 2 seconds = 2 ft/s^2.
This is an average deceleration rate, but from what I have seen that generally seems to be what the valve manufacturers quote in their literature.
Also, I will see if we can get an actual deceleration rate result field into HAMMER at some point in the future.
Regards,
Mal Sharkey Product Manager Bentley
Mal Sharkey: This is an average deceleration rate, but from what I have seen that generally seems to be what the valve manufacturers quote in their literature.
Hi Mal,
This is an interesting topic and one we have been struggling to answer using the transient results. Perhaps using examples from our case would help enrich the discussion. Please find below flow deceleration and dv/dt estimates for a single pipeline where water is pumped from about 50 m to 250-280m via a single pump station near the source. We have two flow cases to consider in our surge analysis and found dv/dt estimates changes dramatically if we literally look at the changeover point vs. a representative dv/dt before flow reversal occurs. One thing to note in both flow cases is there is a prolonged duration of low flows (ignoring the flow direction) before a sudden/sharp drop in flowrate occurs. I say sudden because the flow drop dv/dt=15 m/s2 estimate is based on occurs in one time step according to the results from Hammer.
Also, one manufacturer curve I have indicates, as I understand, the lower the dv/dt ratio the longer it takes for a check valve to close and therefore, the worse the transient pressures will get. Is that true? Although the manufacturer curves include Reverse Velocity as well, closing time is the only parameter with which we can test the sensitivity of results in Hammer. There seems to be a correlation between Reverse Velocity and dv/dt that I do not fully understand.
Regards
Sukru
Hi Sukru,
I think that what you see in your results comes back to the fact that water column deceleration in a real water system (as opposed to an ideal one) is often not constant or regular. That's why I mentioned in a previous post that the charts you see from valve manufacturers showing max. reverse velocity vs. deceleration are more like 'rules of thumb' than explicit valve properties, because I believe that those charts are mostly created in laboratories using simple, ideal systems. I expect that if you tested those same valves in a real system (like the one you are currently analyzing), the maximum reverse velocities could differ.
There is a good discussion about dynamic check valves in Wylie and Streeter's 'Fluid Transients in Systems' (1993, pg. 252). It shows a chart of maximum reverse velocity versus mean deceleration |dV / dt| for a number of valve types.
As I understand it, the relationship between the max. reverse velocity and mean deceleration is as follows:
- flow initially travels forwards through the checkvalve, generally up hill in the case of a pumped main; - if the pump stops, gravity causes the column of water in the pipe to decelerate; - if the pipe has a steep incline the flow will decelerate quickly. If the incline is not steep the flow will not decelerate as quickly. Either way though, the flow will soon reverse and start to come back down the hill; - the check valve begins to close as the flow reverses but this doesn't happen instantaneously (i.e. for swing type valves the disc must travel a certain distance to close off the valve and that takes time. There might also be a damping mechanism attached which slows the disc movement, etc.) - Once the flow reverses, the flow starts accelerating down the hill and it will reach a certain reverse velocity before the check valve has time to close and completely stop the flow. If the pipe has a steep incline, the reverse velocity will get relatively high (since the flow will accelerate quickly down the hill). If the incline is less steep, the reverse velocity will be lower.
This relationship is complex though because faster reverse velocities will impart more force on the check valve, influencing the time to close. However, in answer to one of your questions, it always seems to be the systems with the highest mean deceleration that result in the highest reverse velocities, which in turn result in the highest transient pressures as a result of check valve closure.
This discussion doesn't really help you find the right check valve 'time to close' parameter though, but I think that will come down to engineering judgement, as well as a few sensitivty studies covering a range of likely check valve closure times. Using the valve manufacturers 'rule of thumb' curves as a guide, the 'time to close' will be approximately equal to the max reverse velocity divided by the mean deceleration.
In the future we are considering one or more of the following enhancements in HAMMER:
1. Allow users to enter a max. reverse velocity instead of a time to close at a check valve. Once the max. reverse velocity is reached the valve will close instantly
2. Allow users to enter a mean deceleration vs. max. reverse velocity curve and possibly also a sampling period for computing the mean (i.e. compute mean deceleration over the last X timesteps, where X is user-defined)
3. Detailed physical modeling of the check valve (i.e. analyze the different torques acting on the valve and compute its opening / closing rate accordingly). The problem with this approach is that it is hard to generalize (because there are many different types of check valves), and also it is hard to get detailed information on things like spring constants. disc weights, etc.
Hope that helps.
Mal
Here is a little annecdote about swing check valves.
I worked on a pump station with a short, steep dishcarge line and the check valve was slamming when the pump turned off. The rule of thumb I had heard was that if a check valve was slamming, it was closing too fast so we weighted down the check valve to slow its closing. This made the problem worse.
What we realized was that the flow was reversing so quickly, the check valve did not have enough time to close before the flow reversed and it was the reverse flow that was slamming the valve shut. We then reduced the weight and it reduced the problem.
As Mal said, if you have a long pipe with not a great deal of slope, you shouldn't have too much of a problem with regard to slamming.
To get a handle on your situation, what is the length, average slope and flow in your force main? What kind of closure times are you looking at?
Tom
If we compare two flow/time graphs, we can see that the lower acceleration was achieved when the higher velocity (higher flow) took place. The decelarion of fluid is associated with the initial condition, meaning that greater initial flow will not create so sever deceleration and consequently, the check valve slam.
Greater initial flow will povide greater pressure reduction so the pressure dowstream of the check valve will be lower (even considering higher initial pressure required to convey more water) and will not create the environment of the reverse flow as serious as it is in the case when the flow is low.when the flow is low, only a part of the pressure is disipated on friction and pump overcomes the static head, so especially vulnerable system have great static/fristion head ratio.
in the case when the pressure vessel is installed, the governing pressure will be the pressure from the initial condition because check valve slams occurs in first second or two following the pumps' trip. The pressure downstream of the check valve is still high (the pressure vessel maintains it) and the check valve slam occurs.