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API 610 – WHY “BEP” SHOULD BE BETWEEN

“NORMAL” AND “RATED” 24 MAR 2015

By: Muhammad Imran – Mechanical Rotating Engineer – LUKOIL Overseas Services B.V Dubai

PI 610 is the International Standard which specifies requirements for centrifugal pumps for petroleum, petrochemical and gas industry process services. The Standard is based on feedback from the decades’ long experience of the designers and the end users.

The Standard contains requirements with varying degrees of importance and guide words are used to illustrate the relative importance of each requirement. For example, the requirements which are specified with guide word "shall" denote a minimum requirement to be fulfilled in order to conform to the specification. The requirements which are marked with guide word "should" denote a recommendation which is advised but not required in order to conform to the specification. Similar to this, some requirements are stated as “preferably” which means not mandatory but a preference should be given to such requirements. This API Standard has been serving the industry from several years and has equally benefited both the designers and the end users including plant operators. Despite being used from several years and with a continual update and revision practice, apparently the Standard has not proved itself as a perfect solution to help avert all the problems faced by the industry. Several Engineering companies and plant operators have developed their own version of specifications based on the API 610 Standard which makes further additions or modifications to the requirements in the original Standard. However, still a good number of end users suffer from numerous troubles associated with equipment design/selection and operation. Though there are areas of improvement in the Standard, a correct application and strict compliance to the existing Standard has brought real benefits to many of the users. Now looking on the other side of the picture, there are frequent cases when the Contractors’ Engineers responsible for the application and ensuring compliance of this API Standard are not well versed with the true understanding of the Standard requirements. Most often, the pumps are selected with hidden and/or expressly approved deviations to the critical requirements, or no adherence is paid to the preferable design features in the Standard. The project finally ends up with a pump selection which is not in real compliance with the Standard and hence the main goal of applying this API Standard remains as a futile effort despite investing precious resources in terms of time and money. In order to ensure a true compliance, it is highly imperative to understand the practical implications of each requirement in the Standard, especially those which are stated with guide words “should” or “preferably”. Without comprehending underlying concept of each requirement, the Engineer responsible for reviewing and approval of Supplier/Vendor proposals is not in a position to take wise decisions while accepting a “Deviation” or “Exception” or “Alternative” to a certain requirement or to ensure a best selection by making use of the “Preferable” requirements. In the context of above preamble, this paper explains the underlying concept of selecting a Centrifugal Pump with a “preferable” design feature and finally draws a conclusion as to why this particular design feature is important and should preferably be considered in the pump selection. The paper also provides a brief description of some fundamental concepts and definitions to serve as a quick refresher for the reader to understand the subject matter.

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A BRIEF REVIEW OF FUNDMENTALS:

Total Differential Head (TDH) is a term used to represent the energy (kg.m/kg=m) required to maintain a certain flow rate (m3/h) in a pumping system. A Pumping system is a combination of a source of suction and a destination for discharge with interconnecting piping, valves, fittings and equipment(s) etc. Based on the system layout, TDH may be a combination of two components namely Static Head (mostly fixed) and Friction Head (variable). Static Head may be zero in some systems (e.g. closed loop recirculation systems); however Friction Head (or Head Loss due to Friction) is always present in the TDH calculation. TDH (total differential Head) = Static Head + Friction Head Static Head mostly remains constant for a particular pumping system once the system layout design is fixed. However, the Friction Head can be varied by applying discharge throttling (introducing additional head loss) or may vary at its own due to increase in the piping friction or equipment ∆P over time.

Fig-1: Typical Pumping System “Static Head” and “Friction Head” Fig-1 shows a system with constant Static Head as we increase flow rate from zero to any higher value. The Friction Head is a function of the flow rate squared (Hfriction ~ Q2) and always shows a parabolic curve. Friction head is zero when flow rate is zero, and it starts increasing in direct proportion to Q2 with an increase in the flow rate. Hence, to maintain a certain flow rate (Q) in a system, we need to supply a certain TDH (H = h1 + h2) and this is done by applying a properly sized pump in the system.

Fig-2: Typical Pumping System demand curve

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Once system design is fixed and pump hydraulics are done arriving at the TDH (H - m) required to maintain a certain follow rate (Q – m3/h), an appropriate pump is selected such that the pump exactly meets the system demand requirements (see Fig-3 below). When the properly selected pump is employed in the system, it produces the required TDH (m) and maintains the required flow rate (m3/h) meeting the system demand curve requirements.

Fig-3: Typical System Demand and Pump Selection

Normal Flow: is a term used to specify the flow rate which is “normally required” from the pump. Rated Flow: it is common practice to add some margin on pump duty (e.g. 10 – 20% or even more) to cover the future demand increase or to accommodate any unaccounted calculation errors or assumptions so that the pumps should not underperform in the future. Therefore, we have another flow term called “Rated Flow” Rated Flow = Normal Flow + Margin (x) % of Normal Flow Rated Flow > Normal Flow

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In some rare application when there is no margin added on the pump duty, the above equation reduces to “Rated Flow = Normal Flow”. Best Efficiency Point (BEP): it is the flow rate at which the pump achieves its highest efficiency.

PUMP OPERATING REGION AND LOCATION OF RATED FLOW:

API 610 11th Edition, provides guidelines for selecting a pump with rated flow point lying within a certain window of range as quoted below. “6.1.12: Pumps shall have a preferred operating region of 70 % to 120 % of best efficiency flow rate of the pump as furnished. Rated flow shall be within the region of 80 % to 110 % of best efficiency flow rate of the pump as furnished”

Fig-4: Preferred Operating Region and Rated Flow Point

In addition to the requirements in 6.1.12 quoted above, the API 610 further refines the recommended location for the Rated Flow point as quoted below. “6.1.13: The best efficiency point flow rate for the pump as furnished should preferably be between the rated point and the normal point”

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Fig-5: Preferred Location for BEP in Relation to Normal and Rated Point

Why it is Preferable to have BEP Flow Point between Rated and Normal Flow Point:

It is important to understand why API 610 has first restricted the operating region and then further specified additional preference for the BEP flow point to be located between the Rated Point and the Normal Point. A brief explanation about this requirement is provided as below. As stated earlier, Rated Flow > Normal Flow, so once we try to get the BEP flow point between the Normal flow and Rated flow, the Rated point falls on the right hand side of the BEP flow point (see Figure-5). Pumps are selected for Rated flow point, but are actually operated either at the Normal Flow for a greater or at least for an early part of the operational life. For this purpose, the pump flow is regulated and the operating point navigates between the Normal flow point and the rated point. In other words, the pumps are purposely oversized (Rated Flow) while selection and then operated at a lower flow rate (e.g. Normal flow). In order to run an oversized pump (selected at Rated Flow) at a lower flow rate (e.g. Normal Flow); one of the widely used methods is to modify the Friction Head curve by throttling a control/manual valve at the pump discharge. Discharge throttling actually kills a portion of the TDH (m) produced by the pump across the control/manual valve and consequently the pump is able to maintain a lower flow rate in the same pumping system operating at the same speed. This process is illustrated in the typical schematics shown in Figure-6.

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Fig-6: Pump Selection at Rated flow, BEP between Normal and Rated As seen from the Fig-6, Rated flow point is on right side of the BEP. When pump flow is regulated from Rated Point to a lower flow rate or the Normal flow point, the operating point either moves towards or remains in a close proximity to the BEP. On the other hand, when pump flow rate is regulated from Normal flow point towards Rated flow point, the operating point moves in closer proximity to the BEP (see indication by blue arrows in Figure-6 above). On the contrary, if we select a pump which does not have BEP between Normal flow point and Rated flow point, the flow regulation between Normal and Rated flow moves the operating point such that it never comes any further closer to the BEP (see indication by blue arrows in Figure-7 below).

Fig-7: Pump Selection at Rated flow, BEP is NOT between Normal and Rated

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It is established fact that pump operating at or closer to the BEP flow pint has less operational cost in terms of power consumption due to higher efficiency, far less operational & maintenance issues and a much longer life expectancy. Keeping this fact in mind and the discussion in the preceding text, it is evidently clear why it is “Preferable” to have a pump with BEP flow point between the Normal flow and Rated flow.

CONCLUDING REMARKS:

In order to reap full benefits of applying API 610, it is imperative to understand the true interpretation of the requirements stated in this standard. An in depth review of supplier/Vendor technical proposals should be done and a wise decision should be taken to select the pump which not only comply with the mandatory requirements of the Standard (e.g. Rated point to falls within 80% - 110% of BEP), but also have justifiable compliance to the preferable requirements (e.g. BEP should be between Rated flow point and Normal flow point). There are stated or implied implications of each requirement in the Standard. While it is straight forward to apply mandatory requirements; it requires deeper understanding while deciding upon the application of the preferable requirements. A well prepared Technical Bid Evaluation (TBE) report from the EPC Contractor should demonstrate the final decision upon all the “preferable” requirements in the API Standard. About the author

Muhammad Imran is Mechanical Rotating Equipment Engineer presently working with LUKOIL International Services B.V based in Dubai. He has 15 years of professional experience in Oil & Gas and Petrochemical projects including EPC detail engineering, PRE-FEED, FEED and hands-on experience in the Pre-commissioning, Commissioning, Startup and initial operations. Before joining LUKOIL, he worked for WorleyParsons Qatar as Lead Mechanical Design Engineer for Rotating Equipment and Packages. He can be reached at “muhundis@gmail.com”.