Pump Siphoning and Backpressure Control Valves

[wrd2032]

I have what I imagine is a straightforward question for someone who's done any serious amount of pipe design or has reference materials handy. Alas, not me right now.

We're pumping treated wastewater from a tank to a drain with a pump on a VFD. Depending on operation, there's anywhere from 20 feet to 30 feet of difference in height, with the tank liquid level being higher. We're getting a fair amount of siphoning / pump overspeed. A small (3/4") vacuum breaker at the high point of the (4") line downstream of the pump has reduced but not eliminated this. The high point of the line is above the liquid level.

Is a larger vacuum breaker or a backpressure control valve somewhere in the discharge line the right answer? We'd prefer the vacuum breaker for maintenance reasons, but I'm worried bigger won't solve what the smaller one hasn't yet.

Thanks in advance.

 

[bimr]

A vacuum breaker will break the siphon, but will not work in your application. A vacuum breaker may break the siphon and cause your pump to run dry.

If the flow is continuously operating, you can install a pressure sustaining valve on the outlet.

Or you can install a flow meter and control valve to control the flow.

If the problem is only occurring when you shut off the pump, then install an on-off valve that will close when the pump shuts off.

If the drop in elevation is occurring after the pump, then you may break the siphon after the pump.

 

[LittleInch]

Please draw this in section so we can see what the system is. The profile needs to be included.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[wrd2032]

Thanks for the help. I've included a basic profile. It's obviously not to scale, but the elevations are good. Total pipe length from pump to discharge is about 350 feet.

We've got a flowmeter on the discharge side that I neglected to show, and a VFD on the pump. The complaint I'm getting from the project manager is that they're pumping the same volume at say 20% on the VFD as what would take 40% if we were pumping to an adjacent (identical) tank instead of to that discharge.

The vacuum breaker is 3/4", and the flow is around 200 gpm. I'm wondering if a larger vacuum breaker in the same spot would eliminate the siphoning effect, or if we'd need to install a backpressure valve in the system.

 

[Compositepro]

It seems you hardly even need a pump for this.

Replace your vacuum breaker valve with a 3-4" open standpipe. This arrangement, however, will cause the pump to always use the maximum power needed to pump over the hump in your piping, which seems to be what you want to do.

 

[bimr]

At 200 gpm, the liquid velocity will be 5 ft/sec. In general, that is too high for a gravity flow system. It is also not a good idea to mix full flow piping systems with two phase flow systems. Weird things start to occur with the air in the piping.

If you install a vacuum breaker or standpipe, you will entrain air into the discharge pipe which will decrease the pipeline capacity. With the reduced capacity, you may not obtain the desired flow. A vacuum breaker or standpipe would work if you had a larger discharge pipe.

If you are interested in better accuracy, the best option is to install a 3-Inch flow control valve somewhere on the discharge of the pump. With the controller, you can also control the VFD.

Another solution with less accuracy is to add an orifice plate on the discharge point. The orifice should be sized to counter the 20 feet of additional head gained on the discharge pipe. You will have to play around with the orifice to obtain the correct size.

You can also add a backpressure valve, but the preceding options are better.

I would recommend that you remove the vacuum breaker.

 

[wrd2032]

bimr,

My controls engineer has expressed doubt about a VFD and control valve both trying to set flow rate. I haven't done any controls, so I'm not in a position to argue. Would you typically let both the VFD speed and valve open percentage change simultaneously to get to the desired flow rate? Or would you use one for low range and the other for high range? I've never seen a control valve used downstream of anything other than a single speed pump.

Thanks for the help.

 

[georgeverghese]

Dont see the need for a pump here unless you have very high flows and high frictional press drop. A low dp control valve may be used to control flows as required.

But this entire run should be fully liquid primed before startup - to do that place this control valve at the end of this line and have the liquid exit completely immersed in water. Fill up the line manually with water from the high point till all air is displaced.

 

[bimr]

The answer to this question could be yes or no depending on the process. You have not stated what you are trying to achieve with the process.

One strategy would be to run the VFD at 100% and use the control valve to control the flow. With the system that you have, there is minimal to no energy saving with the VFD although the VFD can be used to set the flow rate. If a pump operates against constant (static, or mostly static) head, there is not much potential for energy savings with a VFD.

Another strategy is a split range control. You can use a controller to set the output from say 0 - 50 % where the valve will control the pressure, and from 50 - 100% the VFD shall control the pressure through the pump speed. The split is calculated by the size of the control valve as compared to the pump power.

You have to set a high enough fixed minimum speed at the VFD that will assure the required pressure to be reached for this to work and allow the control valve some fixed percentage of control.

Another control scheme is a cascaded loop (two PIDs running, one feeds a setpoint to the other).

Run the flow PID as your primary loop with its output directly to the drive speed. The setpoint for this loop will be based on the minimum flow setpoint but will also be affected by the pressure loop. You could multiply the flow setpoint by (1 + Pressure Loop Output) or have the pressure loop output as an adder to the flow setpoint.

The pressure loop will use the minimum pressure setpoint. If the minimum flow is met and the pressure is above the minimum then this loop will naturally ramp down to 0%. If the minimum flow is not meeting the pressure requirement then this loop will begin ramping up. As it ramps up it will then increase the flow setpoint, which ramps up the flow PID and the drive speed.

You need to start with a functional description of what you are trying to achieve with the control.

For example:

Are you trying to maintain the tank level?
Why is flow control important?
What accuracy is required?
What are the shut off/ start requirements?
Is there a min/max?

By the way, what does the peak height represent? Are you pumping over a dike or something?

 

[bimr]

The question of whether the pump is needed or a siphon can be used will depend on what flow rate is possible and the accuracy of the flow rate that you are trying to achieve.

You are most likely not going to get more than 200 gpm through this pipe. It is already at 5 ft/sec.

You seem to be focused on accuracy of the flow setpoint. With a siphon, the flow rate will depend on the piping configuration.

A siphon will be problematic

1. You have to manually start and stop the siphon.
2. The flow rate will depend on whatever headloss the piping has and the head that is available.
3. You have little to no control over the flow rate. It will change with the flow and tank head.
4. As the tank level drops, the flow will peter out.
5. You will need some type of vacuum pump to start the siphon.

These are some of the reasons that siphons are mostly used in agriculture applications. I have never seen one in an industrial setting.

 

[LittleInch]

wrd 2032,

The problem you face as far as I can see is that your design is actually a mixed system. At low flows (I'm not sure if your 200 gpm is 100% of flow or what?) it would seem that your system is essentially acting as a syphon / gravity system with your pump doing virtually nothing other than wheeling around, hence the loss of flow control. However when you increase flow to the point where the hydraulic gradient in your discharge line exceeds the hydrostatic head ( i.e. more than 24' head loss over the distance from high point to end point) then you move into a standard pipe situation.

IN circumstances like this I normally put a back pressure control valve at the end of the down leg set to maintain a small positive head at the high point (25 feet in this case) (there are many pilot operated valves that do this without needing power) as this gives you better control and something for the pump to work on.

You are correct that you wouldn't want both VFD and a discharge control valve to both try and control flow, but you would set your control valve (next to the pump) to control flow rate and the VFD to some percent guaranteed to meet whatever you dial in, say 20% more.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

This problem isn't making sense.

You really need to control flow when you're going into a sewer? Really?
You really need to control flow, when you have no flow control valve? Really?
You want to control flow with a VFD, but you have no flow meter and a possible siphon? Really?
Aren't you really trying to keep the tank between some low level and some high level?

What's the problem with air in the down spout piping to the sewer?
Why not use a level switch control.

Alternatively I'd use a simple suction pressure signal to VFD, or to trigger an on/off switch to a constant speed pump.

With VFD:
Highest Tank Level will give highest suction pressure. If high tank level is 122' that's 8 psig suction pressure. If tank low level is 105, 2' over outlet is about 1 psig suction pressure. So rig the VFD to pump min or 0 RPM/sleep at 1 psig and to be linearly increasing to max RPM at 8 psig. Additionally when the pump sleeps, send a signal to GVsuction to close, or a pressure switch set to 1 psig should close it if you prefer. Be sure your pump can generate goosneck head+ at lowest rpm.

With level controls:
Turn pump on at the 122 ft level, trip it off at 105' and close GVsuction.

Your vertical column will remain more or less full to the gooseneck 120ft elevation. Downstream of the gooseneck will empty. What's the problem with that?

 

[georgeverghese]

At 200gpm with 20ft of static head, 350ft straight length, 100ft for fittings, we still have 28kpa surplus on this line without the pump, so there is ample room here for a low dp backpressure valve or a level control valve, whichever suits you. It is evident that this line has no provisions included for priming. It seems this is a copy and paste from the adjacent tank system, and a hydraulics check was not done for the actual elevation profile at this tank.