Pump phenomena and minimum flows

The accompanying chart Pump phenomena and minimum flows shows the relationships among the various off-design pump phenomena and minimum flow conditions.  The head versus rate of flow curve with indicated phenomena is a variation of S. Gopalakrishnan’s from his well-cited paper titled, “A New Method for Computing Minimum Flow,” Proceedings of the Fifth International Pump Users Symposium; Texas A&M University, May 1988, pp. 41-47.  As an aside, I recall Gopal (everyone knew him by that name) had made a local technical presentation using the now well-known chart, before it was published.  Evidently the chart was copied from a handout of the overhead slides and was quickly pirated by another, and then others.  Copies or variants of this chart are now found widely in papers and presentations on pumps.

The quoted minimum flow for continuous operation is usually called “Minimum Continuous Stable Flow” or its more common abbreviation “MCSF.”  It is the flow below which the pump should not be operated continuously.  The usual purpose of MCSF is to achieve satisfactory bearing and seal life; however MCSF may be based on other considerations.  Any of the following factors may be considered in establishing the MCSF:

• manufacturer’s experience
• rule of thumb
• calculated onset of suction recirculation or discharge recirculation
• radial thrust
• temperature rise
• cavitation erosion intensity
• maximum permissible pressure rise (for system purposes)
• maximum permissible power rise (high specific speed and axial flow pumps)
• a combination of the above factors or others not listed

For hydrocarbon process industry API 610 specification pumps, the value of MCSF is normally coincident with the lower flow limit of the “Acceptable Operating Range” (refer to chart titled “Vibration limits for Allowable Operating Range and Preferred Operating Range”) where a specified vibration limit is not to be exceeded.

Vibration limits for Allowable Operating Range and Preferred Operating Range

MCSF is a value that can range from roughly 10% to 80% of Best Efficiency Point flow depending on pump size and type, operating speed, impeller suction geometry, liquid density, and other factors.  A size 2” (50mm) discharge single-stage process pump may have an MCSF as low as 10% of BEP flow.  MCSF is often in the range of 30% to 60% of BEP flow for process pumps with discharge sizes 3” (75 mm) and larger.  Large mixed flow vertical pumps and very high head-per-stage centrifugal pumps may have an MCSF greater than 60% of BEP flow.  Axial flow pumps have a power curve that rises toward shutoff and minimum flow may be limited by the power rating of its driver.

On certain high energy pumps the minimum flow is governed by cavitation erosion damage.  Minimum continuous flow for 40,000-hour impeller erosion life is where the system NPSH Available curve intersects the pump’s NPSH Required curve, at lower-than-BEP flow.

Intermittent minimum flow, when specified, is usually given as a percentage of MCSF.  On some applications the governing value may be based on temperature rise.  On large high energy pumps the value of intermittent minimum flow could be, for example, “70% of MCSF and not to exceed 100 hours per year.”

For some applications a thermal minimum flow or “Minimum Continuous Thermal Flow” is specified based on permissible liquid temperature rise.  MCTF is usually, but not necessarily, lower than MCSF.  While a pump thermal minimum flow is not always specified, the end user can readily calculate its value based on input mechanical power heating up the liquid.  The limiting temperature rise is based on a safe margin to prevent flashing of the pumped liquid to vapor, potentially causing pump seizure.

Thermal minimum flow is not normally a concern at pump start-up as long as the closed discharge valve is set to begin opening right away.  If the margin of system NPSHA above pump NPSHR is minimal, then the temperature rise conditions at pump start-up should be checked carefully.

A few pump applications, such as a vertical turbine jockey pump for maintaining pressure in a large fire sprinkler system, can potentially operate continuously at shutoff while pump suction recirculation mixes with the water in the sump in which it operates.  The sump acts as a heat sink and a minimal water temperature rise is not a problem.  This example is a rare exception to an almost invariable stricture on operating the pump continuously at shutoff.

The purpose of minimum flow is generally to prevent undue wear and tear or damage to the pump.  In the real environment of a process or utility plant, a pump is operated at just about any condition demanded by the situation at hand.  Thus there are different pump minimum flows for different purposes.

For an independent evaluation of a pump minimum flow issue, contact an experienced consulting engineer who can help with your specific application.  Please take a look at our services to see our areas of expertise.

Hydrodynamics of Pumps, 2011 edition

Hydrodynamics of Pumps 1994 edition was made available online in 2003, for free, and the author solicited readers’ comments.  The 1994 online edition is still available, and with the photographs in color.  In my opinion the online manuscript strategy is a brilliant one for collecting ideas to improve the next edition.

The overall purpose of Hydrodynamics of Pumps is to serve as a reference for pump experts and advanced students of pump turbomachinery.  The book could just as well be titled “Cavitation, Unsteady Flows and Forces,” given that these are its main topics.  The first of these phenomena, cavitation, is associated with vapor bubble formation and collapse, mechanical damage, degradation of performance, vibration and noise, while the issues related to unsteady flows and forces are associated with the relatively high density of liquids and resultant dynamic effects on the machine and system.

Right up front in the book’s foreword a complete five-page nomenclature listing is provided.  The subscript listing uses a sample variable “Q” to show what each subscripted form looks like.
Here is an example: “Q_T1              Value at the inlet tip”.  In my opinion, this method of showing the entire form is superior to simply listing the subscripts independently.

Chapter 1 Introduction provides the high level overview of the two major subjects of the book, cavitation and unsteady flows.  Dimensional relationships in hydraulic turbomachinery are also explained.

Chapter 2 Basic Principles provides an elegantly clear presentation of pump geometry, velocity vectors, blade notation, energy transfer and pump performance.  In the early years of my career at Byron Jackson Pump circa 1980 I recall helping to prepare the submittal of Impeller X and Volute A drawings for Cal Tech.  Various experimental results using these pump elements are found throughout the book.

Chapter 3 Two-Dimensional Performance Analysis presents both axial and radial cascade analyses, flow deviation angle, vane solidity, slip factors, and viscous effects.

In Chapter 4 Other Flow Features, three-dimensional flow effects, radial equilibrium, prerotation and other secondary flows are presented.  Also included is a section on “discharge flow management” covering volutes, diffusers and collectors.

Chapter 5 Cavitation Parameters and Inception covers the parameters used to describe cavitation, cavitation inception, net positive suction head (NPSH), types of impeller cavitation and cavitation inception experimental results.  Coverage of pump cavitation technology essentials in this chapter is at least as good as I have seen anywhere else.   Moreover, I could stop here and make a case for buying this book just based on the material up through Chapter 5.  But there is more.

In the remaining chapters 6 through 10, cavitation bubble dynamics, cavitation damage, cavitation noise, cavitation and pump performance, inducers, cavitation thermal effects, pump vibration, rotating stall, rotating cavitation, surge, auto-oscillation, acoustic resonances, unsteady flow, time domain methods, frequency domain methods, transfer matrices, radial forces, rotordynamic forces, hydrodynamic bearings and seals, damping, cross-coupling and other advanced topics are covered.

It is a big leap from the basic pump concepts and formulas to cutting edge cavitation technologies, analysis of unsteady flows and rotordynamic analytical theory but Brennen deftly manages this transition.  The author begins each topic with a concise but clear presentation of basic concepts and principles and continues on to cover the subject matter in depth, in a well-organized volume.

I highly recommend this book to the newcomer as well as to the expert who wants to better his understanding of pumps. Hydrodynamics of Pumps is an especially valuable and authoritative reference for engineers involved in pump cavitation issues, unsteady flow-related problems and pump rotordynamics.

volute

Most of the time the specifying engineer or end user does not decide on the type of stator to be used. By default it is determined by what the pump manufacturers’ offer. The principal deciding issues are manufacturability and cost, suitability for the application, modularity of design and efficiency.

For high pressure between bearings multi-stage pumps, diffuser designs are more compact compared to volute designs. Compactness generally translates into a smaller pump casing size and lower cost of materials and manufacturing.

Diffusers are normally designed as a one-piece or a two-piece ring assembly secured into the pump casing. Diffusers are modular components. For a given pump casing, variations of the diffuser passages can be designed to cover a range of operating conditions.

Diffuser

For a single stage centrifugal pump, a diffuser type design is usually more costly to produce because the diffuser ring is an extra part plus some incremental added machining for the casing. The casing must still function as a collector to convey the flow from the diffuser to the discharge nozzle. No matter how this is done, the diffuser can offer little comparative advantage in the size of a single-stage pump.

Diffuser designs are often more efficient at the best efficiency rate of flow, compared to that of a volute. Also, a custom diffuser can be made for each application in order to maximize the efficiency for a specific duty point.

A volute proponent might argue that the diffuser is less efficient at off-peak flow rates where the pump will operate a good portion of the time. The efficiency differences may not be significant and unless large amounts of power are involved, these debates seldom carry much weight in relation to the competing prices of the pumps offered, or user preference for either volute or diffuser.

Radial thrust acting on the impeller develops from a non-uniform circumferential pressure distribution. The stator design plays an important role in this. For some applications, especially with a single-stage overhung impeller type pump that will operate continuously at flows substantially away from its Best Efficiency Point flow, a diffuser/collector arrangement can provide a lower magnitude of radial thrust.

One of the two basic stator types may be particularly suited for specific applications. For instance, most API users of axial split case type BB3 between bearings pumps prefer opposed impellers in a double volute casing, which offers some assembly and maintainability advantages. Volute type casings are the norm for solids handling pumps that require wide open passageways. A few specialty high pressure single casing pumps utilize the structural support that the vaned diffuser can provide for the collector scroll. Vertical turbine and vertical bowl type casings are mostly of the vaned diffuser type.

Manufacturers have generally rationalized the choice of pump stator based on market needs, application requirements and production costs. Any evaluation regarding the selection of a diffuser or a volute should be considered in the context of specific pump types, specific applications and manufacturers’ product offerings.

For an independent evaluation of function and performance, contact an experienced consulting engineer who can help with your specific applications.  See the complete list of Ekwestrel’s engineering services.