Best way to contol flow

[ortnvol]

In a paper mill I am working in, they are using recirc lines to control pressure and thus flow to a process. They use process line pressure to open and close a recirc valve, diverting flow back to a holding tank which provides for a pump's suction. The end result is that the pump is always at a constant flow, with only a portion of the flow going to the process. These recirc lines typically divert up to 20% of the flow through the pump! The TDH for these applications is high (100 psig), therefore, VFDs on the pump do not appear to be feasible. Will putting a control valve on the pump discharge to control flow be more energy efficient than current practice? What suggestions do you hve?

 

[TD2K]

These are centrifugal type pumps? If they are, they should save energy, you can easily determine how much and the $$ by looking at your pump curve. Some pump Hp curves are pretty flat.

The other thing to consider is the fluid, is there some reason they wanted to keep a constant flow in the pipe, keep solids entrained, etc? 20% recirc doesn't seem enough for this to me but I would consider it. Are there any other issues with this process that they would want to have designed the recycle this way?

100 psi dP is not that high of a differential pressure, VFDs might be an option but would be more expensive than a valve.

 

[CB2]

The VFD will save more energy than a valve! Much more on a centrifugal than a PD pump for this example.

With a VFD you can rough estimate the savings with "10% reduction on pump speed will give 20%reduction in power consumption" (centrifugal pumps).

As TD2K warned about solids suspention or other hydraulic considerations, if this is a standard pumping application, this is a VFD no brainer.

 

[electricpete]

No doubt a vfd is always the most energy efficient solution, but cost-benefit would need evaluation.

If you restrict yourself to comparing throttling of a recirc path to throttling of the main flow, I don't think the comparison is as simple as you suggest. For one thing, you may not save energy by throttling on main discharge... there is still an energy loss, and depending on where you are on the pump curve you may need more input power as you close the main flow discharge valve (if to the right of the peak of BHP vs flow curve).

Another thing to consider is that centrifugal pumps don't like to be operated under too restricted flow (far to the left of BEP). That can cause vibration and problems which will decrease pump life.

Just a few thoughts without much specifics. Hopefully others can add more.

 

[ortnvol]

I have looked at a VFD application. The system pressure is such that the minimum pump speed must be 70 to 80% just to overcome the minimum head requirements. That is why I am looking at a control valve application.

 

[electricpete]

Bottom line is you need to look closely at that BHP vs flow curve to determine which option (recirc control or main-discharge throttling) consumes less energy. Possibly fairly flat as TD2K says in which case you don't gain much energy either way. Typically there is a peak somewhere in the operating range, so the answer to which strategy saves you energy depends partly on which side of the peak you're on.

My inclination would be to leave it alone unless you have identified clear-cut/substantial energy savings AND have looked carefully at possible implications (vibration at lower flows, range of expected operating conditions etc).

 

[CB2]

Consider the following three example situations:

1) Typical horizontal split case pump, 2500 GPM at 100 PSI, 81%Eff, recirculating 20% flow, BHP (brake HP) will be about 185 HP.

2) Same pump, 2000 GPM (80%), 79% Eff, discharge control valve set for 100 PSI to system, BHP will be about 153 HP.

3) Same pump on a VFD, 2000 to 2500 GPM, 100 PSI, operates 24/7, 100%flow/5%time, 90%flow/50%time, 80%flow/45%time, $0.10 kWh. Should save about 198,000 kWh / $19,000 per year over the control valve application. In this example the installed cost of a 200 HP VFD would be paid for in about 14 months.

Example of "no brainer".

 

[BobPE]

Correct me if I am worng, but isn't picking the correct pump the most efficient choice? VFD's are in themselves verry inefficient devices when compared to across the line operation of a motor.

BobPE

 

[electricpete]

Bob - It is true that lowering the pump capacity can be a more efficient solution IF the pump has capacity beyond the maximum demand ever anticipated. (not always true if demand is varying).

One way of reducing pump capacity/power consumption in the case where pumps are oversized compared to the maximum demand is to trim the impeller. Also sometimes a stage can be removed. There was some discussion in another thread about overfiling and underfiling... one increases capacity and one decreases... I forget which.

 

[electricpete]

I should also mention that even if pump is oversized compared to maximum demand, a vfd may still save energy (compared to throttling or recirculating) if demand is varying.

 

[quark]

With VFD if we reduce the speed(by 20%) to get 20% less flow the head raising capacity of pump reduces by 36% at that flow. So you have to go for higher speeds rather. What is the design head of the pump? As Bob correctly suggested, istead of going for a VFD better look at pump if you have to go for constant flow rates. Trimming of the impeller is an alternative. Underfiling increases flow by 1 to 2% but I never knew anything about overfiling.

Have you checked Cycle Stop Valves?

You can control the discharge valve and there is no harm for 20% reduction in flow.

Regards,

 

[kbander]

Using a VFD is not typically a "no brainer". Often the installation is much more expensive that a valve...yielding a poor return on your investment (yes, after including the energy savings). You have to look at everything, including all the "goodies" that your Operations department may insist on. Do you have cabinet space in your substation? Do you need to be able to control the speed from the sub as well as from the control center? What do you do if the VFD fails (as far as control strategy goes)?

Also, as pointed out, the process is what is important here. If you have a flow to speed cascade, yet the VFD doesn't allow the pump to develop enough head at the reduced rate, then it simply won't work.

By the way, I believe that we don't use the term Variable Frequency Drive (VFD) anymore. They are now designated as Adjustable Speed Drives (ASD's).

I really like ASD's (especially for replacing adjustable pitch control on fin-fans)...but you do have to do your homework and apply them properly.


Good luck!


Regards,

Bob

 

[ortnvol]

Thanks to everyone who responded. I will check the pump BHP to flow curve prior to proceeding with a recommendation. I don't think we can change impeller or motor size since the recirc flow is typically ~20% of process flow, however, but the recirc valve varies from 0 - 100 % open. We need the full range of flow. Again, thanks for your suggestions.

 

[d23]

ORTNVOL

One thing I've been through that sounds similar was circulating fluid at a metal plant. They wanted flow on demand not seconds later. In the case they had I calculated ramp time going from 40 to 60 hertz and that was over 35 seconds. A VFD is a limited bus. Both pump and motor inertia as well as fluid viscosity need to be considered. If you require instant fluid I would recommend that you avoid VFD's.

David

 

[d23]

ORTNVOL

A couple other thoughts:

If you restrict (choke) the discharge you may need to consider water hammer when the valve opens.

When you choke the flow below the pumps BEP the pumps efficiency will go down. Most of this inefficiency will be converted to heat. The added heat may cause problems for your pump seals, but more important it may cause bugs or bacteria to form in your pipe system. The system could operate for months or years before it becomes an operational problem, but it could be very expensive to repair when it does. We did this in the metal plant I sited in a previous reply.

In a responce from "electricpete" he stated that at low flow the pump could vibrate. For some pumps this is a fact, but you should check with the pump manufacture about that. If the pump has adequate thrust protection you should not have a problem. My pumps don't have this problem, but we design for such contingencies.

You said at times you need to go to 0 flow. Your pump won't be able to do that. You need to check with the pump manufacture and find out the absolute min flow for that pump.

This may not answer your question, but I hope it helps anyway.

David

 

[sancat]

My opinion is that most advices here are correct. I am myself a VFD person, however they do have some limitations.
If you need an instant correction of pressure, the inertia of your control system will be lowest with the recirculation valve.
If you need to save energy, at the expense of some ripples, and delay at pressure response, a VFD might work. I think that if your VFD has a PID control input, which you feed with a downstream pressure measure, the VFD will know the speed needed to get the get the required pressure.
Pressure changes with the second power with speed, and
Power curves are flat for the same speed, but I think they change with the third power of speed.
For the same capacity flow, pressure will be directly proportional to power, assuming a constant efficiency.
I have 3 80HP VFD working on an aggressive environment (Hcl vapors), 24h/7d for the last 8 years, with no problems so far. For security purposes they need a backup at the warehouse, as any other valve or device, however to replace it could be faster than changing a damaged control valve (no spills too). I have already installed more than 60 units, and I had to replace only 2, due to mechanical misuse (they broke the cases).
sancat