Finding Water Deceleration in Pipeline - Check Valve Slam Analysis

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

 

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.

This was very useful (also 7 years after it was written), but I still have a doubt about the relationship between deceleration and reverse velocity. Some manufactures also add the deceleration-closing time graph (see examples below). Apparently for shorter closing times the reverse velocity at the closure (and therefore the surge) is higher. How can it be explained? Or did I misunderstand the closing time value on those graphs?

Manfredi,

I consulted with my colleagues on this. Credit for the below answer goes to Mal Sharkey.

This is a good reference: www.valmatic.com/.../DynamicCharacteristicsCheckValves11-2-11.pdf

It’s not the closing time that causes high reverse velocities, it’s the deceleration. The deceleration also impacts the closing time.

So, for higher system decelerations, the closure time is fast (because the reversing water helps push the valve closed). But, because deceleration is so quick, the reverse velocity is still high (even with a short closing time) and therefore likely to cause surge issues.

Jesse,

Thank you very much to you and Mal, you really helped me in getting a better understanding of the matter.

As future reference for other people interested in the subject, I paste here an extract of an email exchange I had with a valve manufacturer. He basically confirms what you wrote above...

"What causes confusion (for everyone!) is the concept of closure time. As valve manufacturers we are often asked to specify this value, but check valves are operated by the system they are in (i.e. reverse velocity) and this controls their closure time. I don’t view closure time as an effective modelling variable for check valves, really for the point you are highlighting. [...]
As we increase the system deceleration the closure time reduces and reverse velocity increases. You rightly comment that this is counter-intuitive, however it is due to the reasons above. [...]
The valve closure will begin when the flow is positive. As mentioned above a typical spring to use in an axial valve would have a critical velocity of 1.5 m/s. When the pump stops, the flow would slow down and the valve would begin to close when the line velocity drops below +1.5 m/s. When the line velocity reaches 0.0 m/s the valve would be closed IF the valve was perfect. However no valve is ever perfect and there will always be a slight lag between the valve and the liquid. When the fluid reaches 0 m/s the valve will be slightly open. The fluid will then begin to decelerate back down the line. Hopefully the valve will close quickly before too much reverse velocity develops . From the point of maximum reverse velocity to final valve closure the time is very short as there will be a reasonable back-pressure to reseat the disc".

Jesse,

Thank you very much to you and Mal, you really helped me in getting a better understanding of the matter.

As future reference for other people interested in the subject, I paste here an extract of an email exchange I had with a valve manufacturer. He basically confirms what you wrote above...

"What causes confusion (for everyone!) is the concept of closure time. As valve manufacturers we are often asked to specify this value, but check valves are operated by the system they are in (i.e. reverse velocity) and this controls their closure time. I don’t view closure time as an effective modelling variable for check valves, really for the point you are highlighting. [...]
As we increase the system deceleration the closure time reduces and reverse velocity increases. You rightly comment that this is counter-intuitive, however it is due to the reasons above. [...]
The valve closure will begin when the flow is positive. As mentioned above a typical spring to use in an axial valve would have a critical velocity of 1.5 m/s. When the pump stops, the flow would slow down and the valve would begin to close when the line velocity drops below +1.5 m/s. When the line velocity reaches 0.0 m/s the valve would be closed IF the valve was perfect. However no valve is ever perfect and there will always be a slight lag between the valve and the liquid. When the fluid reaches 0 m/s the valve will be slightly open. The fluid will then begin to decelerate back down the line. Hopefully the valve will close quickly before too much reverse velocity develops . From the point of maximum reverse velocity to final valve closure the time is very short as there will be a reasonable back-pressure to reseat the disc".