Vacuum Breaker Valve vs Air Release Valve

[ChemE584]

I am working on a water filtration plant that using pumps to vacuum filtered/clean water from a tank and then transfer it to a clearwell. I'm not sure if I should be using an air release valve or a vacuum breaker valve. Here's the setup.

The pump vacuums water (elevation not important as I have my suction figured out), then the discharge piping from the pump immediately goes up 12 feet, where a backpressure loop is then located (another 3 feet up). From there the piping goes down (below the pump centerline) to the clearwell. The high level in the clearwell is higher than the pump centerline, but lower than the high point in the piping. I've been told to use vacuum breakers when the clearwell is lower than the discharge piping, but I'm not exactly sure why as I don't believe contamination is of concern (water is clean).

I am calculating the TDH of the pump and the use an air release valve or vacuum breaker will affect what is used for static discharge. If I use an air release valve the static discharge will be based off the high level in the clearwell (as it's a submerged discharge) and if I use a vacuum breaker valve the static discharge will be based off the high point in the piping (since it's introducing atmospheric air to the system). For some reason I have a difficult time grasping this concept, no matter how much I read up on vacuum breaker and air release valves!

 

[BigInch]

Can you please post a diagram with elevations and distances from pump to all features noted? Nobody can tell you what you really need, until somebody can see the hydraulic grade line.

It would seem that all you might need is a check valve.
In any case, valve or not, your pump must have a discharge head equal to AT LEAST the highest fluid level found anywhere in the discharge system, which would appear to be the high level in the piping, but depending on rate of head loss and where the pump is in relation to that high point and the rate of head loss in the piping itself, even more head than that could be necessary.



"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[ChemE584]

Thanks for the reply. I've attached the hydraulic profile.

 

[BigInch]

As there is no means to hold sufficient backpressure at the clearwell (a control valve holding a minimum of about 12 psig backpressure, your down slope pipe will empty when the pump is off. If there is no air entering the system, the empty space will be filled with water vapor, as the exit will be covered with water, there also appears to be no way for air to ingress through the outlet. Water vapor will not present a problem, because when the pump is turned on again, pressure will rise over water's vapor pressure and the water vapor pocket will collapse. Assuming your pump has a minimum of 20-30 feet or so of discharge head. The vapor collapse, which could cause some transient pressures in the process, but that's all. The pipe will fill with liquid and begin flowing normally.

If there is entrained or coluable air in the water being pumped, that might tend to form air pockets in the pipeline. That in itself not a problem, if the pump has enough capacity to force the air down the pipe and into the clearwell. To be able to do that the pump must be able to pump a column of water up the first 15' slope and the riser height in the clearwell tank at the same time, while assuming that there is air trapped between 225 ft and 214 ft elevations in the downslope. When the pump turns on and the trapped air reaches the lowest point, 199', there will be 214-199 feet of head there. The total of both heads is 15' + 14' = 29 feet, so that's what your pump will need to overcome the total combined static head. It will also need some head to move the fluid through the pipe, friction head to move your desired flowrate through some 400 odd feet of pipe of some unknown (to me) diameter.

The pipe diameter should be sufficiently small to ensure that you have a velocity sufficient to push any free air out. Generally, if you have turbulent flow, which you most likely will, that will be enough to do so. Free air will be pushed to the outlet. However if the pump volumetric flow capacity is small in relation to pipe diameter, it may not supply enough velocity in the pipe to do so and you may need an air release valve. I'd look towards designing-in a 3 to 5 fps velocity in the pipe when the pump is running and eliminating any need for an air release valve.

If your pump has the capacity to supply say 40-50 feet of head (25 psig, or so), probably no valve of any kind is needed, unless the pipe has extremely thin walls and would collapse, if there is only the vapor pressure of water inside (say 0.5 psia) and atmospheric pressure outside (14.7 psia), a compressive hoop stress on the pipe of 14.2 psi * pipe diameter / 2 / pipe's wall thickness. If the pipe will collapse, then you must have a vacuum breaking valve to allow air to enter and keep the internal pressure at atmospheric pressure instead of allowing the water vapor pocket to form when the pump is shut off. If the pump will not collapse, then no.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[BigInch]

The only "danger" I can see is that once air is clear and flow is assured, you may have too much residual head at the outlet that could make a bit of a fountain. In that case, you may want to consider some manner of flow control into the clearwell.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[ChemE584]

The velocity is around 5 ft/s. I'm a little confused by your second paragraph, however. If the system is submerged (aka no atmospheric air is entering the system), why would I need to design the head based off the highest point in the piping? Shouldn't it be the highest liquid level in the clearwell

http://www.pumpfundamentals.com/head and pressure formulas.pdf.)?

Also, your mention of the 14' (199' to 214'), isn't whatever level that's below the original water level (218' on the suction side) just a negative static head (figure 13

http://www.pumpfundamentals.com/tutorial2.htm)?

This is what had me worried about a siphon (and hence the question of a vacuum breaker).

Thanks.

 

[Pumpsonly]

Do not know what air release valve you are having. ARV operating on a float mechanism will allow air into the pipe when pressure in the pipe is below atmospheric which will happen in your case when the pump stop.
Without breaking the vacuum, siphoning will take place.

Looking at your profile, the system will have negative static head of -4 ft when the pipe line is full. Assuming the water level remains constant in both tanks. You should based your pump TDH on this.
During starting, the max.static is only 7 ft (225-218). This is what the pump has to over come before getting water into the down ward leg of the discharge pipe line.
The friction loss of the pipe line will be definitely more than 7 ft and the pump shut off head will be higher.
You will have no problem during the re-start of the pump.

See attached file on air valve for pipe line.

 

[BigInch]

It is doubtful that siphoning will take place, as I assume, rightly or wrongly, that the stopped pump will stop most of that flow outside of minimal seal leakage and then the vacuum valve opens.

If you do have a vacuum valve, air will enter the system when the pump stops as I have just said, therefore you must displace that air when the pump starts and that requires that you at least pump it up to the 225' elevation, 15' before it will flow over the top and start flowing down the other side again. If you have no air release valve in the right place to empty that air, then any trapped air will be forced down the slope when the pump is turned on, requiring that the water ahead be pushed up the column in the clearing well to just short of 214' or so. That's why the two static heads should be added plus friction head to get total required pump head.

I don't see why you want the headaches of maintaining all these valves, unless your pipe is thin enough to collapse under vacuum. Otherwise, I don't see where you need them. We certainly don't let air into our gasoline & diesel lines with vacuum valves, even though we pump over the tops of a couple of mountain ranges. We don't need air release valves, since we have no soluable or entrained air, but I wouldn't have them anyway, just a fast enough velocity to clear it if it did get in. We deal with the negative pressure when the pumps stop using a backpressure valve and with pipe that won't collapse under vacuum.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[ChemE584]

Thank-you both for your responses. I'm going to sit on this and think about it some more...may have more questions tomorrow!