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Mimum Flowrate centrifugal pumps

Mimum Flowrate centrifugal pumps

Mimum Flowrate centrifugal pumps

Hi everybody,

We use three centrifugal pumps to transfer refined product through a 12" and 258 km pipeline. For a softer start-up, we want to install a VSD driver.
I wonder if there is an empiric formula to determine minimal flowrate in the function of pump rotational speed (like the law of similarities). I didn't hear about that.


RE: Mimum Flowrate centrifugal pumps

Don't take on that liability. Get the minimum continuous stable flow (MCSF) from the vendor. If they just call it "minimum flow" but don't specify what kind, push back on them. I've gotten burned in the past being told a certain minimum flow was continuous but really it was thermal minimum flow. Whoops...

RE: Mimum Flowrate centrifugal pumps

What is the reason for the softer start?
Power side or piping side?
Are you starting against closed valves that you gradually open as the pumps reach speed?
Have you looked into other soft start options?
Full VFD just for soft start is overkill.
And whenever you are running you will have power conversion losses, higher motor losses, and maybe reduced motor life.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Mimum Flowrate centrifugal pumps

Softer start up of what?

Flow? Motor Amps?

I don't understand why you want to do this or what the question actually is.

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

RE: Mimum Flowrate centrifugal pumps

Vfd controlled pumps do not really have minimum flow requirements. Power consumption at low flow is so low, no significant amount of heat is generated.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

RE: Mimum Flowrate centrifugal pumps

So much depends on the profile in the line.

If you've got a high spot then until you generate that head you will have no flow...

Not enough detail to work out your issue.

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

RE: Mimum Flowrate centrifugal pumps

Right, in which case vfd does nothing for you.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

RE: Mimum Flowrate centrifugal pumps

Hi everybody,

Thanks for your answers.

RVAmeche, thanks. I will contact vendor

Edstainless, yes we start pumps one after another using a closed valve being open gradually in a common discharge line. Each pump starts in a stars-triangle mode. This usually cause very high pressure because of the line design. The Line was designed for two pumps and now because of the need to increase pipeline capacity, we are running three pumps. But i wonder if each pumps will have the same minimum flow running on differents speed?

Littleinch, the soft starter is for motors. For more understanding, read the answer to Edstainless.

Littleinch and 1503-44 the problem is only
about the rapid increase in pressure in the discharge line.

RE: Mimum Flowrate centrifugal pumps

Sorry but still makes no sense.

Can you describe a bit more your current design, operation and what your problem is with pressure, flows etc.

I really don't think there is an issue about minimum flow if you're trying to pump more flow. But your third pump if its in parallel might not increase flow very much if at all.

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

RE: Mimum Flowrate centrifugal pumps

Why do you see the need to calculate this, pump vendor ought to provide.
Pump startup current concern may not be the only reason for VSD, power savings at part load should be the main reason for VSD.
If startup current concern is the only reason, there are other ways of enabling slow ramp up on speed - speak to your electrical engineer.

RE: Mimum Flowrate centrifugal pumps

Gilles, High discharge pressure required. Exactly. Why have a vfd start against a closed valve. It is constant speed pumps that must start against closed valves. When starting against a closed valve, a vfd does nothing for you. It would only increase the time to reach discharge pressure and start flow into the system.

You have electric motor, so no warmup/speedup cycle for a diesel engine is needed.
Why would you otherwise want to delay your pump from reaching max pressure as soon as possible.
Electric motors have high output torque on startup. Do you have a very small, underpowered motor?

What is the new pipeline pressure?

I think you would be creating a need for vfd by installing a vfd.

As others have said, a soft starter might be what you need, and only to minimise inrush current draw.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

RE: Mimum Flowrate centrifugal pumps

Probably good to review your start up procedure! Valve being closed @ discharge you are reaching the shut off pressure. You may want to start one pump with discharge closed or partially closed then open the discharge and start the other pumps one by one. I guess you have Check valve on each line to prevent back flow, before the discharge valve.
As suggested supply a PID of your unit to better visualize your system and some info about pump (head, flow rate and pump curve).
I don't see the need for a VFD.
You may want to review the operation of pumps running //.

My view.

RE: Mimum Flowrate centrifugal pumps

If I understand your question correctly, I think there is not a simple emperic function. Your main problem is to accellerate the 258 km of liquid in the pipeline without getting too high pressure for too long time during start up with a filled pipeline. You can get an impression of the situation by simulating the situation. The total mass in the pipeline has to accellerate, which can be seen as a differential pressure. The pipeline also has system curve forming function of pressure drop caused by flowrate.

Those 2 pressure drops together with a possible static pressure drop in your system has to be delivered by the pumps.

The pump curve is determined by the the rotational speed of the pump. The pressure of the parallel pumps at the pipeline entrance is equal. If the pump discharge valve is not (yet) fully opened, this gives additional resistance (less pump pressure).

This combined should allow making curves in a spreadsheet showing situation and determining a safe start up acceleration of the pump rotational speed.

I hope this helps.

RE: Mimum Flowrate centrifugal pumps


Thanks to all,

FMJalink I got it and will do as you said and come back.

Pierrick, your links are very helpful

RE: Mimum Flowrate centrifugal pumps

"This usually cause very high pressure because of the line design."

If you explain the line design and how the units start with pressure / flows etc then we might be able to help more, but you're not giving us any details.

I really can't see how starting a main line pump on a pipeline is going to give you very high pressure. That's a long pipeline for only one set of pumps and there will be a need to maintain the pipeline in a pressurised condition when you stop it otherwise it will be very difficult to start without generating large surge pressures. Is this what is happening?

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

RE: Mimum Flowrate centrifugal pumps

Hi everybody,

For more détail please note that:

- pumps are 100 bars/ 100 m3
- pipeline spec : Api 5L Max P : 100 bars
- pumps are started-up directly with a
control valve closed being open
gradually. The valve is operated

- with two pumps running with the valve fully opened the pressure in the Line is about 90 bars. So when starting-up the third pumps without soft starter there will be a surge of pressure.

RE: Mimum Flowrate centrifugal pumps

What is the max source pressure for these pumps ? And max density of the pumped fluid.
Can we also see a Q-h curve for these pumps at the speed these pumps run at the moment.

RE: Mimum Flowrate centrifugal pumps

What reason is there to start with a closed control valve?
Starting with vfd and closed control valve makes no sense. It just takes longer to get to the point when the control valve can open.
Probably better to start in parallel with a check valve in parallel with your other pumps.
API 5L is not a design specification/code. It is a pipe spec only.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

RE: Mimum Flowrate centrifugal pumps

Please provide the documents requested to get meaningful answers (PID, Pump curve, Operating point, start-up procedure). Every day we are discovering something new!
Please tell us about your role within your organization for better support.

RE: Mimum Flowrate centrifugal pumps


With respect, the answers you are giving tell me you really don't have a lot of experience in this field and you seem to be expecting miracles. What is your design code, what is the DESIGN PRESSURE of the pipeline, wall thickness, pipe grade etc (API 5L comes in many grades)

If your current two pumps are running at 90 bar inlet and your design pressure? is 100 bar, there is virtually no increase in flow available to you.

You need in practice to have a bit of a gap between max operating pressure and design pressure to allow for alarms, trips and relief valve settings. Typically this is 8-10% below design pressure.

So basically you are already operating flat out.

Pumps - I'm assuming these are centrifugal pumps - do not come in units of flow so 3 x "100m3/hr pump" does NOT mean you can flow at 300m3/hr. It depends on what the system curve of your pipeline looks like. Centrifugal pumps are essentially a constant pressure machine with variable flow.

What you propose - VFD - will not work in this instance as until the speed increases to the point where the head of the pump (proportional to speed^2) is equal to or a bit higher than the other two running pumps there will be no flow through this third pump.

All that will happen is that your three pumps will more or less share the flow between them, but this flow in total will not be more than 5% higher than what you are getting with two pumps. So there is no point in starting a third pump. However if you really really want to to prove the point then simply start it against a closed control valve (globe valve or similar) and then SLOWLY open it. Starting against a gate valve or a ball valve won't give you the control you need. Even then your three pumps might exceed you max operating pressure / pump high pressure trip.

Bottom line - give us ALL the information we've asked and then we can tell you for certain. But you can't re-invent the laws of fluid flow.

If you want more flow then you need to build a booster station at the hydraulic middle point. This might not be the middle of the pipeline physically. Depend son the topography / profile of the pipeline.

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

RE: Mimum Flowrate centrifugal pumps

Hi EveryBody,

Thanks @LittleInch,

You are right! see below information asked

- Design Pressure 104 bars/ Test pressure 127 bars
- wall thickness: 12.7 mm
- Pipeline grade API 5L grade B
- MAOP: 100 bars

&Concerning Pumps there are centrifugal and identical and parallel layouts, there are designed to deliver 90 bars at 100 m3/h. you'll find attached the pump's curve and station PID.

the pipeline is 258 km long with a 194 m altitude between the inlet and delivery stations. the highest point is located 249.7km from the inlet station and is 243 m high.

Yes, we are discussing the creation of a boosting station 151 km from the inlet station.

RE: Mimum Flowrate centrifugal pumps

And what is max suction pressure to these pumps ? If it from a tank, what is max elevation diff between high level and pump centre line?
Dead head dp = 95bar at density of 860kg/m3.
And what is actual density of this "DDO" at min operating temp?

RE: Mimum Flowrate centrifugal pumps

C'est quoi le veritable probleme/crainte? rupture de canalisation ou de partie(s) d'equipements ou autre?
The pump data sheet was issued in 2008 , meaning that the line has been in operation for 15 years. Tell us what the real issue is?
Very flat curve (Delta P vs Flow rate), wheels already machined, 2950 RPM. Did you get other proposals, from other vendors?
Can you share the specification sheet (scope of this project), where we can see the variability of your process? (Min, Max, Normal values).
Can you share the altimetry, both suction and discharge lines to start to build a model? What about the pressure at the suction of the pump and discharge of the line?

Difficult to read the PID, missing the source. Can we get a PDF?
More info better answer.

RE: Mimum Flowrate centrifugal pumps

Also arrival pressure, trip pressure and what flow rate are you operating at with two pumps?

I think it might be about 220 to 230 m3/hr?

Not sure how you manage to start with one pump without going off the end of the curve? How does it start up?

When you shut down how do you do it? What is the settle out pressure at the pump end?

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

RE: Mimum Flowrate centrifugal pumps

Also, as said above, your pump curves are really quite flat and when running pumps in parallel you can easily find one pump doing >60% of the flow. With three pumps it will be even worse.

But my previous comment applies - basically your third pump is going to add about 10m3/hr to the total if that.

You need about 21 bar to get any flow past the high point.

Do you have a flow assurance dept or team to get them to develop a system curve for you. Or decide where to the best location is for your booster station?

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

RE: Mimum Flowrate centrifugal pumps


the max suction pressure is about 1 bar, and tanks connected to pumps are at atmospheric pressure
The max elevation is about 13 m
The actual density range of transported products is 720- 870 kg/m3


I am thinking about how to start the third pump because of modification of the process (boosting station, VFD stater chosen design by the project team) and I was wondering about the influence of motor actual speed on pump minimal admissible flow (pump security). in the literature, it said a pump might be operated between 80% and 120% of the Best Efficiency Point which is about 100 m3/h. I don't worry about the line because we set up a security level of up to 97 bars.
Pumps data are very close to actual data.

Suction line: 1 m

Discharge line: 2.5 m but the pipeline is buried after a few meters ( see pid)

as I told you, in normal operating conditions suction pressure varies from 0.2 to 1 bar and discharge pressure from 88 to 90 bars


Yes the flowrate is about 220 m3/h and goes up to 250 m3/hr ( when pumping gasoline)

the arrival pressure is 7 bars.

Start-up process :
- the control valve (PCV10 see PID) set to 20% or 25% opening
- Starting the first pump
- we set the control valve to 30% opening (to avoid the first pump being overloaded. At this stage, motor power reaches 495 Kw with a nominal of 450 kW) then we start the second pump and open gradually but quickly the valve so that we limit pressure surge on the pumps discharge side. so when reaching permanent operation conditions discharge pressure is 90 bars and the flow is 100 m3/hr. note that the discharge valve of each pump is automatically opened at the pump starting up ( 60 s to be fully opened).

Finally yes we have a project team, but a former team already chose the location I indicated and began the project on this basis. it is now very difficult to change plan.

RE: Mimum Flowrate centrifugal pumps


Your method of starting is not good. IMHO.

The first pump is going way off the end of the curve if you're hitting 495kW - you must be doing about 150m3/hr...

You might need to start with the control valve at 10% and control on flow for one pump to get the line moving then start the second unit. As you're so close to the MAOP limit you might benefit from a soft start, but the pump speed needs to get up to the first one within about 30 seconds otherwise it will just stall as it's not producing enough head.

For your booster station I would start all three first then start the booster station.

SO yes, a soft starter might be needed or fit a proper control valve to the discharge of each pump so that you can control the initial surge of pressure until they all even out. As said before though, with that flat a pump curve it will be a miracle if all three units operate within 5% of the same power draw.

Your concerns about pump minimum speed and operating within 20% of the BEP are different things and I don't think you really understand what the issues are here. They are fixable, but might need a different mode of operation.

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

RE: Mimum Flowrate centrifugal pumps

Thanks LittleInch,

RE: Mimum Flowrate centrifugal pumps

So max dead head pressure would be 1+(95*.87/.86) = 96.1barg, which is some 95% of MOP of 100barg - looks ok to me.
The process sketch attached shows no min flow recycle control loop for each of these pumps - this should help with startup woes. At this pump power, a dedicated safety low flow trip for each of these pumps would be good.

RE: Mimum Flowrate centrifugal pumps


Gilles has said "security level of up to 97 bars". Now I think that's lost in translation, but suspect it means trip or at least alarm.

Also I missed the information that the arrival pressure is 7 bar.

I hate min flow recycle loops, this only needs a control valve on each pump and you can play tunes on that.

gilles - Thanks for the response but a little more feedback would be appreciated. Did it answer all your questions?
Are you going to do anything differently?

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

RE: Mimum Flowrate centrifugal pumps

There is an FS 11 / 12 / 13 on these pumps - dont know what these are.
So what is the nameplate rated power on each of these motors ? - it isnt on the pump datasheet.

You will need a flow recycle loop during flow turndown, at least one loop on the common discharge header. The VFD on each of these pumps will probably get a speed reset from the pipeline PIC.

RE: Mimum Flowrate centrifugal pumps

They are flow switches.
Detailed information available on the right side of the PID. Complete set in French, well documented.
Note: Big money if you intend to purchase VFD for each pump.


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