8 replies to this topic

[JLMONTREAL]

Hi, all

As suggested by Mr.Art Montemayor, I create this new thread for the new topic.

I am wondering if there are different opinions on the necessity of CENTRIFUGAL PUMP RECIRCULATION LOOP, i.e. the pipe from pump discharge to its source vessel or suction pipe. I have depicted my understanding below for this issue. I would appreciate that you could share your experience with me. It will help me a lot.

The articles, ' Pump Bypasses Now More Important' in Engineering (May 11, 1987) and 'Controlling Centrifugal Pumps' in 'Hydrocarbon Processing'( July 1995) have discussed this topic mainly in the point of process control and minor discussion for equipment protection. The conclusion of these articles told the readers that a recirculation loop was not so necessary.

To my understanding, a recirculation loop 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 recirculation loop.

Is there better solution to test run a new installed pump without a recirculation loop? 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 recirculation loop. 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 recirculation loop, I don't know how to perform these activities.

3. Safety. As above mentions reasons plus the reasons in the quoted articles, if a recirculation loop 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.


Without recirculation loop, a plant may be started and run OK, if everything is close to perfect. Because the plant won't be flexible enough, a minor problem may easily become major one.

That is for sure that installing a recirculation loop 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.

Joe

[johnsonn239]

JLMONTREAL,

One reason that I've seen recirculation lines for centrifugal pumps is simply for agitation. Having a recirculation loop that is(for arguments sake) 10X the flow of your process line, it may provide sufficient agitation and no agitator is needed for the vessel.
Of course you would have to look at the cost of running a larger pump for the larger circulation as opposed to a smaller pump and running an agitator.

Another reason may be to have more flexibility in pumping capacity. For situations where only small flows are needed through the process line, the circulation would be high(i.e the control valve on the recirc line would be 100% open). For situations where larger flows are needed through the process line, the cirulation would be lower(i.e. the control valve might only be 10% open). This will create the backpressure on the discharge line enough to get the larger desired flow in the process line.

johnsonn239

[JLMONTREAL]

Yes, Johnsonn239, I met this application too. Pump recirculation may be used for rough agitation or avoid blockage by solids.

Do you imply that all other centrifuge pump applications are without recirculation loop? Then how are test-run and pump switchover performed? Production team feels good?

Thanks.
Joe

[fallah]

I think it could'nt be performed a classic test run with recirculation loop, especially in long time,because pump works in point other than working point and with very low head and high capacity.It is expected a lot of noise and vibration due to high velocity (especially if the size of recirculation line is not equal to discharge line and being smaller).Therfore there is no gain for actual working of the pump.
Regards

[johnsonn239]

Joe,

I think that recirculation loops, in my experience thus far, are initially installed in the majority of pumping systems for many different reasons if for no other reason to have it just in case. Always design for worst case senarios. Another reason could be for testing of the pump initially as you mentioned. For applications where there doesn't need to be a recirc loop per se, there might still be one for safety reasons. They might have a safety relief valve on this reirc line so it wouldn't be used at all in normal operation but it could be if necessary.
Let me know if this helps,

Nic

[Andrei]

I would characterize some of the reasons previously mentioned here as "waste of energy". I will not pinpoint them.
There is a clear distinction between reasons of installing a recycle loop in centrifugal pumps and positive displacement pumps.
A manufacturer requirement for a centrifugal pump is called Minimum Operating Flow. The pump has to be operated all the time above that point, and that is the main reason why the recycle is installed. At least this is the design case for all the centrifugal pumps I worked on. The minimum flow is a function of the specific fluid and operating pressure/temperature. Everybody knows that there is friction between fluid and pump's mechanical parts, and that friction for sure is converted into heat. It may not be a problem if your fluid is water at ambiant conditions, but think about petroleum products, and in general all volatile products. Think about a pump that extracts the product from the bottom of a tower. The temperature there is exactely at equilibrium, and the slightest heat can partially vaporise the product, and you know: vapor in the pump leads to cavitation, vibrations and possibly structural damage of the system.There are special valves that are installed on such recycle loops that are opening at a preset pressure, and usually there is a restriction orifice installed on the line to avoid energy waste caused by a to high flow.
The reasons of installing a recycle loop in positive displacement pumps is different and is Structural Integrity of the system. There is a pressure relief valve installed on the recycle that brings the liquid from the discharge to the suction. Some of the pumps have this recycle line built in, and the PSV is installed and set by manufacturer.
I reccomend a good look at API standards 610, 674, 675, 676,ant other related. These are not reading literature and, as I know, they are mandatory, at least in North America.

All the best,
Andrei

[Art Montemayor]

Andrei is precisely correct in his succinct interpretation of a centrifugal pump’s recycle stream. It is a true example of wasted work – and, as such, is energy employed inefficiently.

However, in the case of centrifugal, dynamic machines such as the centrifugal pump and the centrifugal compressor, this “comes with the bride selected”. It is an inherent negative feature of dynamic machines. Variable speed drives are one answer to combat this weakness, but they are limited in the ranges that can be applied and the solution is often very expensive – both in capital and operating costs. The application of a variable speed is subject to a detailed and accurate economic optimization study and report. The centrifugal pump is hampered by minimum flow and the centrifugal compressor is affected by surge flow – both are analogous to the type of machine and must be confronted to ensure a steady, reliable process.

Andrei mentions a recycle loop in positive displacement pumps as being different and I use this opportunity to alert all engineers – especially the younger ones – about the importance of describing this process correctly and understanding exactly what constitutes a “recycle flow” and a safety relief device’s discharge. The two flows mentioned are NOT EQUAL, NOR SIMILAR. They are totally different in scope and in process requirements. Please allow me to define this very important point in order to avoid any serious misinterpretation of what I write or what is meant.

The reason for incorporating a recycle line around a reciprocating pump is usually for the reason of controlling the capacity of the pump – primarily in those cases where the speed of the pump cannot be varied. The only other viable and practical method of controlling the capacity of a reciprocating pump is varying the stroke (in a reciprocating type).

The use of a pressure relief valve (PSV) (and this is VERY important!) on the discharge of a reciprocating pump is solely for the prevention of a hazardous over-pressure condition caused by the continued operation of the pump and a possible blocked (or severely reduced) discharge. IT IS NOT A FUNCTION OF THE PSV TO PROVIDE RECYCLE FLOW. Please excuse my “shouting" the previous statement; I consider it very important to make this point very clear. I (& many other experienced engineers) have successfully argued and fought over this subject at countless Hazops and PreStartup Safety Reviews (PSSRs). All process engineers should confront and conform to the fact that safety instruments – such as PSVs – have never been intended for the use of controlling a process. They are placed in the process in order to protect it – as a means of last resort.

Believe me when I discuss this point. Responsible and renowned process companies such as DuPont will back me up on this point. We have extensively researched and studied the subject of PSVs on the discharge of positive displacement pumps and the results have always been that the pump manufacturer places them there (even builds them into the pump casing, in some instances) in order to protect the pump – never to furnish recycle. Additionally, I have never found a reputable positive displacement pump manufacturer that has been able to furnish detailed PSV calculations with respect to his built-in PSV. Therefore, I ALWAYS furnish my own, calculated, and documented, specific PSV on the discharge of all positive displacement pumps that I design or apply --- with no exceptions.

The fact that you can't use the PSV to rely on "recycle" means that the process engineer has to furnish any required recycle flow stream in a separate, independent, and safe manner.

You don’t have to do what I emphasize; but you would be wise to heed what I say from experience.

[Mahesh@A&M]

Art, sorry to repeat what you commented. Yes, Andrei is right. I would like to present a live example I am facing in my company.

A centrifugal pump which serves as internal reflux between two columns is designed for 55 gpm/178 feet head. This line also has a flow control v/v and flow is set at 17 gpm while Minimum Continuous Stable Flow is 30 gpm. Couple of years ago, there were several seal failures, vibrations and shaft deflection problems. I forgot to mention that the fluid is mostly propanol, water and some percent of acetone at 150 deg F( close to equilibrium temp). MTBF for this equipment was 16

This problem was solved to some extent by adding a recirculation loop from pump discharge to suction and replace sleeve shaft with solid shaft.

I will post pump curve and specs on this pump as a new topic. Please give me your inputs to resolve this problem.

Thank You

Chemroopa@Aggie06

[JLMONTREAL]

Dear All

Sorry for my late response. I was busy with a process package and just finished it. I appreciate all the inputs from all of you.

I would summarize the posts as below:

1. From the design point, a recirculation loop is mainly to meet the minimum flow requirement of a centrifuge pump. And this is the typical design in majority of applications as per Andrei. On the other hand, for this reason, it may not be needed for those pumps without this minimum flow requirement.

2. For a complex chemical plant, the pump recirculation loop is important to ease the exectution of test-run for many pumps at the same time and save execution time.

3. In a complex chemical plant, a pump recirculation loop increases operation flexibility, which can minimize entire plant shutdown and minimize the resume time. It can also extend pump mechanical life by minimizing off/off frequency, particularly for those pumps with high flow rate and high horse power.

4. For special cases, like slurry duty, a recirculation loop is necessary to prevent solid from settlement no matter if there is a minimum flow requirement from pump manufacturer. The same for pumps with mixing functions as per Johnsonn239.

5. Although a recirculation loop can increase capital cost and energy waste, comparing the gain from it, for most of cases, the entire plant can better off more.

Am I right?

Thanks.

Joe