Controlling sequential VFD pump and control valve system

[rahxephon]

Hi there,

We have a system at a water treatment plant where large raw water tanks feed into the plant that's all on the same level. At high tank level water can gravity flow through with additional throttling and at about 50% tank level it will need to be pumped. I was hoping to get some advice on the controller setup for this system.

The delivery setup is storage tanks - pump - flow control valve - plant

The intent would be that the valve tends open, and once opened, pump starts up with the valve throttling back to give a smooth transition at pump minimum speed.

In other applications have done this setup off a single PI controller with 0-50% on the valve and 50-100% output on the pump. This gave a step change in output when the pump engages, although wasnt important in those cases like it is here.

Had considered trying it like a duty assist pump setup with valve on the controller and the pump open loop to startup, then position/speed match the two to the controller output. Although I think this would be feasible here, it'd mean a lot of the time the pump will be going against a throttled valve which is wasting power.

I wanted to see what suggestions others may have for controlling this system?

Only option I've come up with at the moment is using two sequential controllers.
Valve (PIv & SPv)
Pump (PIp & SPp)

Increasing
SPv > 85% start pump at 45% and lock speed PIp (SPv will drop as valve throttles)
SPv > 100% lock valve controller PIv, and unlock pump controller PIp

Decreasing
SPp < 40% lock pump controller PIp, and unlock valve controller PIv
SPv < 50% turn off pump

This setup feels a tad complicated and not too robust

Main risk I see with this is when I turn off the pump, the valve setpoint will jump up. Can calculate how far it'll probably go but still feels like may incidentally go past 85% sometimes and cause fluttering around the pump start up sequence.

If there's been a past Q&A on this in the forums I missed, apologies

Cheers,

 

[JTCat]

What type of pump? This is important. You need to be on curve or you 'WILL' cause some damage. Do you have experience with pumping applications? Depending on the type of pump you might not want to start it with the outlet valve open or you risk cavitation.

 

[JTCat]

Also, pumping against a throttled valve does not increase power usage unless it's a positive displacement pump, which is very very rare in water treatment applications.

Power is a function of work done. If less water is moving, less work is being done, which means lower amperage and a happy pump because it's operating at a lower wattage and staying nice and cool.

 

[rahxephon]

Hi JT,

They're regular end suction centrifugals, and am pretty familiar with them so am happy with the region that I expect it to be tracking about its BEP.

I've got some set flow rates I'm wanting to maintain. So then trying to put the same flow rate through a valve that's throttled more than it needs to be will need higher speeds.

Cheers,

 

[drbitboy]

What is the minimum and maximum speed references you want to send to the VFDs?

How much of a step in flow would it be between not running the pump and running the pump at the minimum speed if the throttling valve is left at whatever max open you choose?

 

[rahxephon]

Hi dr,

Wanting to set between 20 - 50Hz, its in NZ hence not ~60. But to give some deadband was thinking to start at ~22.5Hz

Its a small plant, doing between 10 - 20 m3/h. If I turned on the pump at 20Hz and with the valve fully open, it'll do ~22m3/h. So thereabouts on the top end of my range, but double on the bottom.

I'm expecting the control valve would need to be around 45% open with the pump at minimum speed to bring it to 10m3/h (~75% BEP).

Cheers,

 

[JTCat]

You need to do what the pump curve recommends. I would run the pump at full 50 if it’s still on curve and below fla and use the modulating valve to PID track the tank level and keep it more or less constant with a tie back to turn it off if you don’t intend it to run. That’s just what I’d do. I specialized in petroleum fracking before I moved to water treatment operations, so water isn’t my primary specialty but I’ve dealt with it a lot

 

[MaxK]

I'm dumb a not native speaker, so could you please explain a few things:

1. Do you maintain a given level in the tank, or a given flow rate?
2. Was I careless enough to miss out on pump ratings, valve flow versus level, etc.?
3. Do I understand correctly that you want to avoid a sudden change when the pump is turned on? Did I understand correctly that you want to start the pump immediately at 20 Hz? In my humble opinion, these conditions contradict each other.

 

[rahxephon]

Hi JT,

I think may be getting distracted by the pumping side, I'm confident enough in how I expect the pump to be running when its on. Its the transition from controlling the valve, to then controlling the pump I'm unsure on if that makes sense.

The raw water tanks provide attenuation from the source, and the plant treats a near constant range between 10 - 20m3/h.

The water tanks are 6m high, and the first treatment stage (clarifiers) are 4m high. Everythings on the same ground level. So when the tank level is high I will throttle it and when low, pump. But around that 4m mark will be a transition point between the two.

Hi Max,

No worries, I only speak English so you're better than me there.

1/ Maintaining a given flow rate. The raw water tanks level will change between 2m above the clarifier to 3.5m below. So sometimes throttling and sometimes pumping.

2/ Unsure what you mean sorry. But I think specific pump or valve ratings shouldnt be too important. Its more a general question on how best to approach controlling a pump and flow control valve situation.

3/ Correct. The pump has a minimum speed to be able to run continuously. When I start the pump it'll ramp up to 20Hz in about 5 seconds, for this context it is effectively immediately.

If the valve position is unchanged this will cause a step change in flow. So the valve would need to close a little so that the flow rate would decrease (it is a centrifugal pump so it would pull the duty left along the pump curve).

Cheers,

 

[JTCat]

I'm dumb a not native speaker, so could you please explain a few things:

1. Do you maintain a given level in the tank, or a given flow rate?
2. Was I careless enough to miss out on pump ratings, valve flow versus level, etc.?
3. Do I understand correctly that you want to avoid a sudden change when the pump is turned on? Did I understand correctly that you want to start the pump immediately at 20 Hz? In my humble opinion, these conditions contradict each other.

They do contradict. I think this is OPs first time here I’m going to give him the benefit of the doubt and call it a miscommunication.

If the centrifugal pump is flooded suction then it can be started lower. Dry sump however needs to come up to full speed until prime is achieved. We’re just assuming in our answers we don’t have a complete picture.

 

[GaryS]

It looks to me like you need to control the pressure as the discharge flow changes
And you have modulating valve to supply the flow when the tank is above 50%
That because the head pressure in the tank is greater then you need so the valve can flow / pressure
Below 50% you need the pump to maintain the pressure in the system so when you are running the pump you need the valve to open at 100% so the pump can control the and the pump controls the pressure
Something to keep in mind below as pump doesn’t produce much flow below 30% speed so the vfd should have a low limit of about 30%
Simple use 2 PID controls one to control the valve and the other to control the pump both using the same pressure feedback.
Use the valve position say 80% to start the pump once the pump is running set the manual output command for the valve to 100% so there is adequate supply to the pump and let the vfd do its job. You will have to play with the values a bit to bring everything together.
One big problem you may have is if the flow passes through the pump it may spin the pump in reverse
You should use a vfd the is able to brake the motor at start or catch a spinning load, if you don’t you could damage the pump. I have seen that years ago with a 200 hp pump would snap a shabby about once s month because the pump was spinning in reverse when the pump was engaged
Take your time and think before you make changes but it will work

 

[drbitboy]

How is this piped? I.e. both the pump and a line from the tank appear to be upstream of the control valve? But are they in series (so there is flow through the pump even when it is not running)? Or is there a direct line from the tank to the control valve, perhaps with a check valve, in parallel with the piping from the tank through the pump to the control valve?

You said the VFD/motor/pump will ramp up from stopped to 22.5Hz in about 5s. How long for the valve to compensate?

There is a target flow of 10-20m³/h; is that measured with a flowmeter?

Is there any way to control the inflow into the raw water tanks? It was stated that they are for attenuation (of flow changes to the plant?), so I suspect the answer is no.

What sort of short-term (10s-1minute) flow variation to the plant is acceptable?

What sort of long-term flow variation to the plant is acceptable? Because long term I would expect neither overflowing the raw water tanks nor running the tank empty are acceptable. So the long-term average outflow to the plant must match the inflow to the tanks, and the inflow is not controlled, and although it appears inflow will usually be steady for extended periods, there will be (step?) load changes.

Expanding on that (i.e. that outflow must match inflow over the long term): if the raw tank level is dropping toward the point (some delta above the clarifier? or clarifier level?) where the valve is wide open to maintain a constant outflow rate setpoint, then the inflow must be less than that outflow setpoint. If that inflow remains at that rate for an extended period, then either the outflow setpoint must be decreased or the tank will run dry. To avoid the latter, the outflow setpoint must be decreased eventually any time that the uncontrolled inflow decreases; the only free "parameter" is the timeframe over which that control change is made. That being the case, why not use only the capacitance (for attenuation) of the raw water tank, from slightly above to 2m above, the clarifier level, and control the flow based on the tank level (20m³ at some high level; 10m³/h at some low level but still above the clarifier) without ever using the pump? One possible answer to that "why not?" is that the tank gain (reciprocal of cross-sectional area) is too small, so there is too little capacitance in that limited level range to adequately attenuate the flow.

 

[MaxK]

1. It's preferable to see the curves of the system operation parameters at the current moment. (Perhaps this will answer some questions)

SPv > 85% start pump at 45% and lock speed PIp (SPv will drop as valve throttles)
SPv > 100% lock valve controller PIv, and unlock pump controller PIp

2. What gives you reason to be confident that when the valve PI reaches 100%, the pump PI will be greater than 40%?
3. What do you suppose to do with integral term "tails"?

 

[JTCat]

1. It's preferable to see the curves of the system operation parameters at the current moment. (Perhaps this will answer some questions)



2. What gives you reason to be confident that when the valve PI reaches 100%, the pump PI will be greater than 40%?
3. What do you suppose to do with integral term "tails"?

Right. We need to see the P&ID diagram or have an idea of how it’s structured. I would do absolutely nothing knowing what I know about this system, which is very little.

 

[MaxK]

Is there any way to control the inflow into the raw water tanks? ...

This is certainly interesting information, but since the control system is functioning, I believe these issues have been resolved in one way or another

But here are the questions I'm intrigued by the current system
1. Why does the valve <-> pump changeover occur at 50% of the PI controller, taking into account “The raw water tanks level will change between 2m above the clarifier to 3.5m below”
2. How exactly does the switching valve 100% <-> pump 40% take place? I got a trembling