35 replies to this topic

[pasquale]

Hi all,

I need to know all about centrifugal pump minimum flow and about the way of sizing the minimum flow bypass valve (on the return minimum flow line)

[Nirav]

hello pasquale,

I suggest you to read the attached article which will help you to understand how centrifugal pumps are controlled. This was published in "Hydrocarbon Processing", July 1995.

The minimum flow of centrifugal pump depends very specific to pump model and selected impeller of the same. These are, in general, decided by pump vendors based on process data specified by Chem.Engr.

At any point of time during operation, pump should not be run below this flow. It will damage the pump & its internals. Therefore, there are various ways to ensure that pump will always have atleast the flow equal to minimum required. I'm listing some common ways.
[1] Minimum flow by-pass control valve
[2] Minimum flow by-pass restriction orifice (continuous flow)
[3] Minimum flow by-pass auto recirculation valve.

The configuration to be used depends entirely on your system design & philosophy. For example, the option [2] above will always have minimum flow from pump going back to its suction vessel. Which means this is addtional flow over and above process requirement flowing continuously. Which means you are consuming more pump power all the time.

On the other hand, option [1] would provide you flow through minimum flow line only when the main process flow is going below the minimum pump flow. I hope you understand the difference from option [2].

Now in brief, to size the control valve on min flow line, you must consider following.
# Total max flow through control valve = Pump min flow
# Pressure drop of control valve = Approximately (pump discharge pressure - operating pressure of suction vessel - pressure drop through minimum flow line).
You need to carry out detailed hydraulic calculations of pump system to find out pressure drop accurately.

I hope it helps.

[pasquale]

Thanks Nirav,

I know the attached article and I fully agree with you about the way of designign a centrifugal pump bypass line.

actually my problemi is a little bit different because I'm working on a basic engineering project so I don't know really the pump and I have no contact with the pump vendor.


So, as a rule of thumb, how I can estimate the pump minimum flow ?

I have estimated as minimum flow value 40% of the pump design flowrate and for the pump differential pressure at this flow I've used 110% of the design discharge pressure.

These values sounds to you?

Thank in advance

Pasquale

Process Engineer

Rome Italy

[Nirav]

Hi pasquale,

I think 40% should be enough for estimation purpose of minimum flow (in case if you are going ahead with control valve on your by-pass line). You can check with similar kind of pump in your past data, if you have.
You need to be more careful if you are implementing restriction orifice concept since higher estimation may lead to unnecessary higher operating and probably capital cost of pump.

In any case, I would suggest you to put a "note" on process datasheet of pump as well as min.flow control valve which says that the min.flow should be confirmed/updated during detailed engineering based on vendor's input. This is what I have observed in my experience.

Thank you,

[pasquale]

Hi Nirav

Thanks for your reply.

I never have done a detailed engeneering project untill now, so my datasheet are full of this kind of note....!!

Have you a pump vendor datasheet with all the data for a centrifugal pump?

Can you attach it to a reply?

What do you think of my estimation of pump's differential pressure at minimum flow (110% of design discharge? is good).

Thanks in advance.

Pasquale

[pleckner]

Pasquale,

You indeed do have contact with a pump manufacturer...the INTERNET. If you are in a plant, just look around to see the brand of pump the facility typically uses. If you are in an E&C, then the project must have a prefered bidders list. Choose a pump brand, contact the vendor via their website or even call them and discuss. They will be very happy to give you FREE help.

[palusa]

Pasquale,

at a very early stage, without specific info from Vendors, for 'normal' non critical pumps, a minflow 20-25% of rated flow can be considered as a ballpark figure.

For larger pumps (say >150-180kW) or in critical service (e.g. bfw) a larger amount 30-40% are advisable...in this case however info from OEM are vital.

For non critical pumps, the minflow could be carried out through a continuos bleed and a restriction orifice (routed to the suction tank); considering a 'typical' non-flat pump curve, a 110% head at 25% seems ok.

In case of larger amount, minflow is normally ensured through special recirculation valves on pump discharge, opening at specified pump head.

Hope this can help.

_Lf

[pasquale]

Hi Palusa,

Thanks for the informations, the data that you told me sounds good to me.

I've senn that you post from Bologna, so 'sei italiano'?

I work in Rome as process engineering of a enginnering company.

I hope to see other your post in this forum.

Ciao

Pasquale, rome

[Nirav]

Have you a pump vendor datasheet with all the data for a centrifugal pump?

Can you attach it to a reply?

Hello pasquale,

There is a very good detailed reply by Palusa on thumb rules to determine minimum flow.

Most Vendors fill relevant data in the datasheet as described in API-610.
You may have it with you. But I'm attaching 1st page of this typical format for your reference.
1st page mainly contains all process related data. You can find pump minimum flow under the headnig "performance" on this page.

Thank you,

[palusa]

Good discussion and data on this subject can be found in

I.Taylor "Pump Bypasses now more important", Chemical Engineering, May 11, 1987, pg. 53.

_Lf

[pasquale]

Hi Palusa,

Do you have a copy af this article?

Pasquale

[palusa]

Pasquale,
attached the article.
Hope it helps.

_Lf

[pasquale]

Hi Palusa

Thanks for the article it is very interesting and it is what I need!!

Merry Christmas

Pasquale

[JLMONTREAL]

Hi, Palusa

do you mind to forward me a copy to JLmontreal@gmail.com?

Thanks.
Joe

[JLMONTREAL]

hello pasquale,

I suggest you to read the attached article which will help you to understand how centrifugal pumps are controlled. This was published in "Hydrocarbon Processing", July 1995.

Hi,

I am wondering if there are different opinons on the necessity of BYPASS LINE ACROSS A PUMP. I have depicted my understanding below for this issue. Please show your opinions so that it may help me to better understan this application.

The articles quoted above and that recommended by Palusa have discussed this topic mainly in the point of process control and minor discussion for equipment protection. These articles told the readers that a recycle loop is not so necessary.

To my understanding, a bypass is necessary for a centrifuge pump (there is no doubt for positive displacement pump) in majority of applications, particularly in continuous plant.

Besides some reasons cited in above articles, I would talk from the point of operation and plant management.

1. Pump test-run. Pump test-run has to be performed after installation before put into service. The pump needs to be started (normally with water) and kept running at least for several hours, even several days, so that technicians may find if there are some problems caused by pump fabrication, installation or even piping design by inspecting the vibration of pump and piping, noise, temperature and etc.. This practice is necessary to prove the reliability of a pump INSTALLED. Manufacturer warranty won't cover installation problems and associated piping problems. However, at this moment the equipments and piping are not cleaned enough, there may be dust, welding debris, rags in the pipe, so water cannot be discharged to downstream vessels because there may be tender instrument components or other sensitive components in the downstream pipes or equipments. This water cannot be discharged into gully directly because of water cost and waste water cost. The best way is to recirulate water via a bypass pipe.

Is there better solution to test run a new installed pump without a bypass pipe? One solution may be to establish water recirculation in entire system. But if any component has problem, the recirculation in entire system has to be stopped completely, so test-run can't be performed for other rotating equipments. With a bypass pipe, different jobs may be performed in parallel. This can save lots of time, particularly in a big plant.

2. Pump switch during normal operation. If a pump needs maintenance, it has to be taken offline without disturbing normal process status. The operator needs to start standby pump and keep it running for certain time under recirculation via bypass line. If the pump is believed running correctly, then operator switch it in service.

Once the repaired pump is installed, it has to be started up and kept running for some time under recirculation to check if it is well repaired.

In terms of management, even in some plant, the standby pump needs to be started and kept recirculation for certain while periodically to confirm its good status.

Without bypass line, I don't know how to perform these activities.

3. Safety. As above mentions reasons plus the reasons in the quoted articles, if a bypass installed for a pump, it will minimize the on-off frequency to a pump, increase the pump's working life,increase plant operation flexibility and minimize the shutdown of the plant, increase plant stability, and concequently reduce plant cost. And more important, the stability of a plant can minimize safety problems in chemical plant. The more fluctuation, the more possibility of safety problems.


That is for sure that installing a bypass will increase capital cost. However, comparing the benefits with the extra cost, there is more better-off. And also, it can save cost from point of management as mentioned above.

Please input your opinions, I would appreciate all input.

[Art Montemayor]

Joe:

I‘ve started to track this thread and I immediately spotted what I believe is troubling you (and probably others) when you try to understand the need for centrifugal pump protection regarding its inherent need to have a minimum, positive flow established through its casing.

The problem is that some engineers are using the wrong terms when they mean something else. I think you also are falling into the same trap. I also have had trouble with these types of communications in the past – especially when discussing piping arrangements and design with pipers and construction people. The fundamental problem is the use of the generic term “Bypass”.

First, lets all agree on the definition of “bypass” before employing it. The definition of a bypass line or piping circuit is a section of pipe run that is used to literally go around a piece of downstream equipment – eliminating that portion of flow from going through the equipment. In other words, the flow is eliminated from going through the in-line equipment and is allowed to continue on downstream, to its designed ultimate target. When we do this, we have bypassed the downstream equipment.

That is NOT what one does when one needs to ensure a minimum flow through a centrifugal pump. I have been on top of this very application for the last 6 months on several, very huge centrifugal water injection pumps equipped with Variable Frequency Drives. These pumps are for a major international oil company and they are multi-staged, 3,000 psig discharge, 2,000 hp and 7,000 hp in size. These are serious and important issues for these pumps and are not to be taken lightly. The usual method to ensure that a centrifugal pump has a minimum flow through it is to divert its discharge to one of two targets in the event the designed target is throttled beyond the official minimum flow rate:

1. Recycle the flow back to the source (in some cases this might have to be the pump’s own suction line;
2. Cascade the flow to a lower pressure system that can take the flow;

The above two actions do not constitute a bypass. The flow is either recycled or it is sent to a different system. The flow does not go around a system or equipment in its normal downstream process path. By using the term bypass a lot of confusion is being created, in my opinion. It is not intentional; it is just mis-used. It does not accurately describe what is happening and it leaves the wrong mental imagery and interpretation.

Your comments have nothing to do - nor are they related to – with the subject of this thread: Centrifugal Pump Minimum Flow. I’m not trying to be critical or negative. You raise some very good questions and comments, but I think they belong in a thread of their own and I would title that thread “Centrifugal Pump Flow Controls Design”.

I would love to participate in Centrifugal Pump Minimum Flow discussions and also on Centrifugal Pump Flow Controls, but I don’t think both can be handled within the same thread with a minimum level of confusion.

Could you start a new thread on your subject matter? I think your points are important and they often come out – especially in operating plant environments.

[JLMONTREAL]

Thank you Art.

I fully agree your definition for BYPASS. I thought it could be understood and kept using the same words through my post. In fact, I used to use PUMP RECIRCULATION LOOP.

I would create a new thread as FUNCTION OF CENTRIFUGE PUMP RECIRCULATION LOOP, so that we can concentrate on its function from the point of flow control, operation and/or other standpoints.

Please make your correction if you have better title for the new thread.

Thanks.
joe

[Minti]

Hi,
i want to know about centrifugal pump minimum bypass flow, can you please send me the files to study which were mentioned in the discussions above at my email amit.us.pande@gmail.com

1) published in "Hydrocarbon Processing", July 1995.
2) I.Taylor "Pump Bypasses now more important", Chemical Engineering, May 11, 1987, pg. 53.

Thanks & Regards
Amit

Hi Palusa

Thanks for the article it is very interesting and it is what I need!!

Merry Christmas

Pasquale

[krvishwa]

Can you please me the article to my mail ID krvishwa@gmail.com

Regards
Vishwanath

Hello!

If someone could forward me the two articles it would be much appreciated!! :-)

jo_leblanc@hotmail.com

thanks!

Jo

[TheEdge]

Hi guys,

Thank God I found this forum! This is a great place to learn and there is just so much to information shared here. I am very new in pump and piping know-how.

I would just like to find out what is the common practice for pump recirculation line. It is just that my current company practices one and my previous company practices another. The former recirculate back to the vessel supply while the latter recirculate it back to the pump suction. I am quite confused about this.

Thank you in advance for your guidance here.

[prengr]

Dear all,

In my case, canned motor pump have been used.
This pump has a large amount of minimum flow. The largest minimum flow is almost same of pump rated capacity(appox. 90%), and the smallest minimum flow is appox. 5% of pump rated capacity. And, in some case, those have a reverse circulation line to prevent pump cavitation.

Just being there to see.

[mishra.anand72@gmail.com]

Does not minimum flow to be maintained through pump depends upon specific heat value of process fluid being pumped?

[deltaChe]

In Bechtel Design practice, the minimum flow can vary form 10 to 50 percent of the rated flow.
If a continuous minimum flow is provided by means of a continuous flow recirculation line with a restriction orifice, the recirculating lfow should be added to the rated flow. Hope this help.

[breizh]

Hi ,
Take a look at this resource.

Breizh