Check valves at force main connection of 4" and 14" lines 1

[STrunk]

We are proposing to connect a 4" force main to an existing 14" force main. Based on old design plans, the 14" force main is operating at 1,600 GPM @ 216' TDH (two submersible 130 HP pumps). Our proposed lift station will operate at 100 GPM @ 50' TDH. We are proposing to add two check valves on either side of the MJ at the force mains connection. QUESTION: Assuming the p.stations can both operate at the same time, will the check valves actually function/open up against the pressure of the opposing force main flow? In other words, if the 14" force main is running at the aforementioned flow, will the 4" check valve open once the smaller proposed pump station is in operation?

 

[bimr]

You asking if it is possible for a pump operating at a lower pressure (50' TDH) to pump into a higher pressure (216' TDH) force main.

It should seem obvious that this will not work. The check valve will never open.

Not sure why the need for the higher pressure on the larger force main, but if you want to pump into it, you will have to have a greater pressure than the line that you are pumping into.

 

[cvg]

TDH (at the pumps) does not equal pressure at the point of connection. You have not provided the minimum info necessary to respond to this question. However, the 4 inch may open if pumping occurs while the pumps on the 14" main are not running

 

[redbridge]

What is the purpose of the check valve at the connection? Isn't there a check valve at the pump station. I would put a plug valve or gate valve at the connection so you can test the new force main.

 

[bimr]

You have posed what seems like a simple question, but in fact, it is a complicated question:

1. You will have to evaluate the system head curve. When both pumps are operating, the system head curve will increase because of the total increase in flow. Therefore the discharge pressure of each pump should increase. If the pumps are not designed to pump against the higher head (from the combined flow), then the flow will drop off accordingly. The magnitude of this effect depends on the system head curve.

2. The pump discharge pressure when both pumps are operating will be the common for both pumps. The pump operating point of each pump will move to the common discharge pressure point. Assuming your pumps are installed at the same elevation, the low pressure pump operating point will move over on the pump curve to the shut off head (and zero flow).

3. It is generally not good practice to rely on a check valve to hold back the pressure when you have multiple pumps. Check valves do fail and you will have a mess on your hands. Consider an automatic valve to close when the pump shuts off. There is little benefit to installing check valves in series either.

4. Consider using VFD's. The VFD's will decrease the peak flow and water hammer when the pumps start/stop.

5. The large increase in pressure when the high pressure pump operates may cause a significant water hammer and/or reverse flow that may damage a pump.

 

[STrunk]

The check valves will act, or check, to keep wastewater from entering the opposing force main. There is a check valve located near the pump station discharging w/ the 14" force main, but it's several miles downstream. I don't think I should assume the force main is completely full up to the 4" force main connection. If not, then there is no reason to believe the wastewater wouldn't flow in the opposition direction until the line is full (air release valves are located at all high points).

We are fairly green at things of this nature, but would we do the following calcs....See link attachment for force main profile. 1600 gpm in 14" = 3.1 cfs, length of 14" force main from pump station to 4" connection = 12,311 ft.

Left hand side of equation {z + p/62.4 + V^2/2g + hp}
= 816 + 0 + 0 + 216 = 1,032 ft.

Right hand side of equation {z + p/62.4 + V^2/2g + hl}
= 960 + p/62.4 + 3.1^2/62.4 + 34' = 994

Solving for p = 2,371 lb/ft^2 which equals 16 psi, or 37 ft.

So as long as my proposed pump can pump at least 37 ft. of head, then the check valve should open. Is this correct?

 

[coloeng]

"I don't think I should assume the force main is completely full up to the 4" force main connection."

It appears that your elevation at your discharge point is higher than any other point on your force main. If this is true, than not only should you assume that your force main will be full, it will be completely full at all times, unless you have a leak or are releasing it somewhere else. A pressure pipeline does not operate with a portion of it flowing partially full.

It is hard to tell from your profiles where the two force mains connect. It appears to be at point 3/G. Are points 2 & F also common points? If they are common points, why is the distance different?

In order for both pump stations to operate simultaneously, the pressure at the connection point in both force mains would need to be exactly the same. You then have to determine what the headloss is between the connection point and the discharge point for the total flow. Add to this any elevation difference and this will be the head required at the connection point. You would then need to determine your headlosses from your connection point and elevation difference back to your pump station to determine your total head required.

 

[bimr]

You have an error in your math. V stands for velocity, not flowrate. You also do not state the materials of construction either. The estimated velocity should be about 2.9 ft/sec.

You state 12,311 ft. to the injection point. That would be 133' past the summit on the downhill side.

One should assume that the air release valves are operating and the pipe is full.

Since you are pumping with 216' of head and your injection point is just after Point F, the static head at the injection point is 816' + 216' - 952.9 = 79.1'.

The pumping head loss across the entire pipeline is about 47'. The pumping head loss to the injection point is about 41'. The estimated headloss also depends on the materials of construction and diameter.

The pressure in the 14" line at the injection point should be 79.1' - 41' + 7.1' (static head recovered from summit) = 45.2'.

I think that you basically have the head calculation correct, the question is whether you need the 2nd check valve on each pipeline.

There is no valid reason to have redundant check valves. However, you should put block valves at the connection point to allow you to maintain each pipeline without have to shut the other pipeline down.

Specify a non-slam type of check valve.

 

[tkall]

Strunk:

Please calculate the pressure in both lines at the point of connection and you will have your answer.

Tom

 

[PEorl]

bmir - I am confused about the static head recovered from summit. Specifically the 7.1', because it seems like it is double counting. What am I missing.....

You state: The pressure in the 14" line at the injection point should be 79.1' - 41' + 7.1' (static head recovered from summit) = 45.2'.

But with the other equation, isn't this already counted:

The static head at the injection point is 816' + 216' - 952.9 = 79.1'.

Can you explain further? Because this implies to me that if the injection point was at say elevation 852, then static head recovered from the summit would be 108'.

Thanks!

 

[STrunk]

Thanks for the help. It seems we are heading in the correct direction by using the energy equation to solve for the pressure (& converted to head) to the 4" force main connection. So we then just need a pump that will pump greater than this amount of head to open the check valve. [To coloeng....sorry for the confusion. To make the problem easier, "F" and "2" are the same point on the attached profiles]
[To bimr....you were right. I meant to say fps not cfs with regard to the velocity].

One last question: In terms of using the energy equation to determine the pressure at the force main connection, shall we include the downhill sections of 14" force main to calculate the friction loss up to that point of analysis (so for this example it'd be 12,178 ft.) ??

 

[bimr]

To be correct you should. However for your project, the energy is not significant.

To PEorl. After looking back on my posts I see your are correct. 79.1' is incorrectly labeled as static head.