PUMP MINIMUM CONTINUOUS STABLE FLOW

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What is MCSF and how is it determined?

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By E. M. Araza

CENTRIFUGAL PUMPS – Modern Design and Practices

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Minimum continuous stable flow?(MCSF) is a widely discussed topic on centrifugal pumps but is still not well understood. If some engineers were shown a pump curve and data sheet, and asked for its MCSF, it is likely there would be as many different answers as there were people in the group because there are several factors that affect MCSF but there is no consistent industry practice for its calculation.

Minimum continuous stable flow refers to the lowest flow a pump can run continuously without exceeding the vibration limits specified by an industry standard such as ANSI, ISO, HI, API, etc., or by the user’s specification that may be a variation of an industry standard. In most instances, the MCSF corresponds to the lowest flow in the pump’s allowable operating region (AOR).?

MCSF is a standard-based figure – the figure can change depending on the applicable standard for which the pump is sold. And it may also change depending on the applied edition of that standard. Case in point: an older API 610 7th edition pump will have a lower MCSF than a current edition API pump because the newer edition has a more stringent vibration acceptance criteria.

Although the main purpose of specifying MCSF is to identify the lowest flow the pump can run smoothly and reliably, it also has the beneficial effect of reducing environmental noise. A pump operating below its MCSF may produce loud surging, or cavitation-like, noise inherent in low flow operation. In some jurisdictions, it is required that companies comply with environmental noise abatement rules. In one instance, the homeowners in a locality sued a pipeline company to force it to mitigate the excessive noise coming from its pump station when the pumps operate below their MCSF.

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What is minimum flow? Contrary to what others assume, MCSF is not a fixed number but is just a recommended value that pump vendors commonly round to a higher figure to be conservative. Example, if the estimate were 276 GPM the vendor might advise 300 GPM as MCSF. The vendor has no incentive to advise the lowest value if the user’s system minimum flow is higher. Example, if the data sheet showed 300 GPM as the system minimum flow, and the pump was good for 200 GPM, the vendor might advise 250 GPM as the MCSF to maintain a safety margin. There have been many instances of pumps failing their performance tests at MCSF because the recommended values were pushed to their minimum - ?well below what the end users require. The pumps would have passed the tests had the MCSF values were given higher at just the right numbers needed by the end users.

Understandably, pump vendors would not push the limit to the absolute minimum if they could get away with higher values of MCSF that the end user would accept. Thus, it may still be acceptable to run a pump slightly below its stated MCSF if needed.

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What is meant by continuous operation in MCSF? There are people who contend that MCSF does not apply to pumps that are in intermittent or short-event service because they are not in continuous operation. But what is meant by continuous operation?in the context of MCSF? Pump standards do not define this, hence there are different interpretations of what constitutes a continuous service. Some people consider continuous as a 24/7 operation, others an 8-hour run, etc.

Operating a pump above its MCSF is intended to prevent the occurrence of high vibration ?that can damage the pump mechanical seals and thrust bearings, cause wear parts to seize or gall, damage the shaft, or otherwise result in pump failure. The damage or failure can occur even if the pump ran for just a short period of time, such as when it is being tested at a pump facility. In the context of protecting the pump, continuous operation should be defined as any operation other than at pump start-up or transient condition such as when a pump is getting online, or switching over between main and standby service.

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Basis for determining MCSF There are different ways manufacturers determine the recommended MCSF for a pump depending on the availability of information. It may be based on:

§? Historical vibration test results?specific to the pump type and size

§? Pump hydraulic design

§? Mechanical design and construction

§? Liquid characteristics and?energy density.

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Historical test-based MCSF

One common, and probably more reliable, method of determining MCSF is to base it on the historical vibration test results of pumps having the same type, size, and hydraulic design as the current one, whose recommended MCSF meet the vibration limits specified for those pumps. But this method can be used only if adjustments are made to the MCSF values to account for the differences in vibration acceptance criteria between historical and current standards. Expectedly, this will result in upward revision of historical MCSF values in the manufacturer’s database.

Case in point: when API 610 8th edition came out, its vibration limit was so stringent that many pumps failed to pass the vibration limit at MCSF.? Many manufacturers scrambled to revise upward their published recommended pump MCSF. There was a strong push back to the 8th edition vibration limit such that in the 9th edition the limit was revised to a more realistic value.

Adjustment should also be made to compensate for differences in volute lip clearance, or B-gap that has a significant effect on the pump vibration magnitude. For example, API 610 requires that high energy pumps should have a minimum of 6% volute B-gap to reduce their vibration magnitude. Experience has also shown that high energy pumps handling liquids with high gas content should have a minimum of 10% B-gap. Generally, the bigger B-gap will result in lower vibration amplitude.

The downside to this test-based method is that it rely on hydraulic design information and internal test records that only the pump manufacturers can provide with very limited third party validation.

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Empirical-based MCSF

An early method of estimating MCSF is based on calculating the onset of impeller internal suction and discharge flow recirculation, and whichever is the higher value is used for recommending MCSF by applying an empirical factors. This method gained ?early adoption for lack of an alternative method but eventually fell out of favor for being tedious and needing proprietary impeller hydraulic information.

Another empirical-based method is based on the maximum allowable incidence angle between the flow angle at MCSF and the angle at suction shockless entrance flow (Qse). This method has the advantage of not relying on the BEP, which can be a moving target depending on the volute used. The disadvantage is the absence of a clear consensus as to what the maximum incidence angle should be.

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BEP-based MCSF

A popular method of determining MCSF is to base it on a percentage of flow at the best efficiency point (BEP) in conjunction with other parameters such as suction specific speed and rated horsepower. The popularity of this method is its simplicity – it does not need to consider some hydraulic information that may not be readily available, or that pump manufacturers are unwilling to disclose for being proprietary.

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However, there are many weaknesses to this method.

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One weakness is the inconsistent ways manufacturers determine MCSF. Some based it on ?BEP at maximum impeller diameter only (see Fig. 1), but others based it on BEP at the actual trim diameter (see Fig. 2). The inconsistent ways shown in Fig. 1 and Fig. 2 came from the same manufacturer but is also practiced by many others. Other manufacturers, or even end users, have developed their in-house empirical charts showing the MCSF on this basis. (Examples are shown in Fig. 3 and Fig. 4.)

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Another weakness is that the BEP is not necessarily an indicator of a good hydraulic match - an impeller can have different BEP values depending on the volute in which it is used. To expand the coverage of a hydraulic performance map and reduce the number of patterns and its associated costs, it is not unusual for manufacturers to use an impeller pattern with different volute designs – low flow, medium flow, high flow, or something in between.

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For example, consider an impeller designed for 1000 GPM; it would have a BEP of 1000 GPM if optimally matched with the right sized volute. But if that impeller was used in a low flow (undersized) volute its BEP would move to 900 GPM, and if used in a high flow? (oversized) volute its BEP would move out to 1100 GPM. If one assumed 40% of BEP as its MCSF, then the MCSF would vary between 360 GPM and 440 GPM. So which one would be a more accurate value?

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The vibration magnitude, and thus the MCSF, is also affected by many factors. To a large extent it is affected by the volute design – its radial clearance or B-gap, if it was of single or double volute design, etc. – things that will not be known by just looking at the BEP.

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Because of the weakness of the basic BEP-based MCSF, the author developed a modified method that takes into consideration other known parameters that affect MCSF. Based on the Araza method, the percentage of BEP is adjusted based on other known parameters.?

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The recommended baseline MCSF based on a percentage of BEP are:

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30% - for low energy pumps

40% - for medium energy

50% - for high energy

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Additions:

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+ 5% if single volute

+ 5% if overhang pump

+ 5% if Nss is 11,000 or more

+ 5% if two-stage or multistage pump with different first stage impeller

+ 20% liquid has high gas content, such as Benfield solution, carbonates, dense CO2, etc.

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Subtractions:

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-5% if radial volute clearance (B-gap) is in the range of 7% to 15%

-5% if NPSH margin is 50%, or more

-5% if pumped liquid is hydrocarbon

-5% for impeller enhancements (V or angle cut, underfile, OF, staggered vanes, etc.)

-10% for mechanical upgrades that reduces the pump L^3/D^4 (shaft flexibility factor)

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Provided that, with all the included additions and subtractions, the minimum MCSF shall not be less than 20%, and the maximum shall not be more than 70%, of the flow at BEP.

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In the full version of this article the author discusses the rationale for the additions and subtractions mentioned above. It is important to consider that each situation is different and unique - the above may not be applicable to a user’s specific needs.

Instrument-based MCSF

Most recommended MCSF is given in a conservative value that is rounded upwards. This conservative value is fine provided the pump will not run below MCSF. What if a lower value was needed but the vendor was not agreeable to go lower and the end user does not want an automatic recirculation valve (ARV) because of its energy loss? One option is to provide the pump with monitoring device for vibration alarm and shutdown settings. This will allow the end user to run the pump well below the vendor’s recommended MCSF, if necessary, for as long as the alarm and shutdown limits are not breached.

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Operating below MCSF If a pump had to run below its MCSF and shutting it down to avoid damage and failure is not an option, a potential solution is to provide a low flow by-pass line to recirculate part of the flow that is equal to the difference between its MCSF and the lowest flow rate the ?pump would run. The downside to this option it that it wastes energy and is costly; it may also result in unwanted temperature rise of the pumped liquid. A better solution may be to re-rate the pump hydraulically with new lower flow impeller design to allow the pump to operate at the lower flow rate.

Effect of impeller cut diameter on MCSF The two most common factors affecting MCSF are suction and discharge flow internal recirculation. Suction recirculation is the more dominant factor found in most centrifugal pumps. On the other hand, discharge recirculation are found in pumps with high specific speed (NS), with oversized impeller vane width (BA), with large eye diameter to impeller diameter ratio (D1/D2 ratio), and with excessive impeller cut diameter.

If suction recirculation was the dominant factor, then cutting the impeller diameter would not affect its MCSF because it would not alter the pump suction hydraulics. The MCSF is not reduced by reducing the impeller diameter. On the other hand, if discharge recirculation was the dominant factor, then trimming the impeller diameter would most likely increase its MCSF.

Effect of viscosity on MCSF This author has not found any test-based information, or empirical data, that points to a definitive correlation between viscosity and MCSF. But because viscosity is a property that resist flow such that the flow of viscous liquid is reduced by the factor Cq, then it can be assumed that viscosity also resists internal flow recirculation to the extent that MCSF is also reduced by the viscous flow factor Cq.

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Other factors

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In the full version of this article the author discusses two other factors that affect MCSF. The values obtained from these factors, if the figures were higher, should prevail over the figures obtained by other means of estimated MCSF.

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Conclusion

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MCSF is a standard-based parameter and its value may differ depending on the specific standard used in the pump specification and procurement.

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There are many ways of estimating MCSF depending on the availability of data used in the calculation.

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MCSF is just an estimated and recommended value, it is not a hard number. Running a pump slightly below its MCSF may be permissible. Consult with the pump vendor if a substantial reduction in MCSF were required due to a change in process condition.

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