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Reliability tips for centrifugal process pumps

05.01.2014  |  Bloch, H. P.,  Hydrocarbon Processing Staff, 

The pump’s behavior can be likened to that of a manual transmission in an automobile. While a car can be driven 80 mph in first gear, it would be a mistake to do this for an hour or so.

Keywords: [pumps] [centrifugal] [reliability] [maintenance]

Without pumps, there would be no hydrocarbon processing industry.1 Pumps are simple machines that lift, transfer, or otherwise move fluid from one place to another. Process pumps are usually configured to use the rotational (kinetic) energy from an impeller to impart motion to a fluid. The impeller is located on a shaft; together, shaft and impeller(s) make up the rotor. This rotor is surrounded by a casing. Located in this casing (or “pump case”) are one or more stationary passageways that direct the fluid to a discharge nozzle. Impeller and casing are the main components of the hydraulic assembly; the region or envelope containing bearings and seals is called the mechanical assembly, or power end (Fig. 1).2

  Fig. 1. Principal components of an elementary
  process pump.


However, many process pumps are designed and constructed to facilitate field repair. On these so-called back pullout pumps (Fig. 2),3 shop maintenance can be performed while the casing and its associated suction and discharge piping are left undisturbed. Although operating in the hydraulic end, the impeller remains with the power end during removal from the field. The rotating impeller (Fig. 3)4 is usually constructed with swept-back vanes, and the fluid is accelerated from the rotating impeller to stationary passages in the surrounding casing.

  Fig. 2. Typical process pump with suction flow entering
  horizontally, and vertically oriented discharge pipe
  leaving the casing tangentially.


  Fig. 3. A semi-open impeller with five vanes.
  As shown, the impeller is configured for
  counter-clockwise rotation about the centerline A.




Reliability tip: Greater spacer lengths reduce angles of misalignment in case of the unavoidable differential temperature-related parallel offset between the centerlines of pump and driver. Greater-length spacers may cost a bit more, but, by reducing bearing loads and vibration severity, they beneficially affect the likely cost of future maintenance.

In this manner, kinetic energy is converted to potential energy, and the fluid (often called pumpage) moves from the suction (lower) pressure side to the discharge (higher) pressure side of a pumping system. As the fluid leaves the impeller through the pump discharge, more fluid is drawn into the pump suction, where, except for the region immediately adjacent, the pressure is lowest.4

 
Reliability tip: Pipe elbows located too close to the pump inlet nozzle may save money initially, but they often create flow disturbances, which tend to reduce pump life.

Pump performance expressed as head and flow

Pump performance is always described in terms of head, H, produced at a given flow capability, Q, and hydraulic efficiency, h, attained at any particular intersection of H and Q. Head is usually plotted on the vertical scale or vertical axis (the left of the two y axes) of Fig. 4; it is expressed in feet or meters. Hydraulic efficiency is often plotted on another vertical scale, the right of the two vertical scales; i.e., the y axis in this generalized plot.

  Fig. 4. Typical H-Q performance curves are sloped
  as shown. The BEP is marked with a small triangle;
  power and other parameters are often displayed
  on the same plot.

Head is related to the difference between discharge pressure and suction pressure at the respective pump nozzles. Head is a simple concept, but this is where consideration of the impeller tip speed is important. The higher the shaft rotations per minute (rpm) and the larger the impeller diameter, the higher the impeller tip speed will be—i.e., its peripheral velocity.

The concept of head can be visualized by thinking of a vertical pipe bolted to the outlet (the discharge nozzle) of a pump. In this imaginary pipe, a column of fluid would rise to a height H. If the vertical pipe would be attached to the discharge nozzle of a pump with higher impeller tip speed, the fluid would rise to a greater height (H+). Note: The height of a column of liquid, H or H+, is a function of only the impeller tip speed. The specific gravity of the liquid affects power demand, but it does not influence either H or H+. However, the resulting discharge pressure does depend on the liquid density (specific gravity). For water (with a specific gravity of 1), an H of 2.31 ft equals 1 psi, while, for alcohol, which might have a specific gravity of 0.5, a column height or head H of 4.62 ft will equal 1 psi. Therefore, if a certain fluid has a specific gravity of 1.28, a column height (head H) of 2.31/1.28 = 1.8 ft will equal a pressure of 1 psi.

For reasons of material strength and reasonably priced metallurgy, the head per stage is usually limited to about 700 ft. This is an important rule-of-thumb limit to remember; however, when too many similar rule-of-thumb limits combine, pump reliability cannot be expected to be at its highest. As an example, a particular impeller-to-shaft fit is required to have a 0.0002-in. to 0.0015-in. clearance on average-sized impeller hubs. With a clearance fit of 0.0015 in., a somewhat greater failure risk can be anticipated if this upper limit is found on an impeller operating with maximum allowable diameter.


Reliability tip:
The higher the peripheral impeller velocity, the greater the rate of erosion in solids-containing pumps. High-tip velocity pumps manage with fewer impellers than would be needed for equivalent-head low-tip velocity pumps, but beware: lowest installed cost today usually equates with higher maintenance outlays tomorrow. 

In Fig. 4, the point of zero flow (where the curve intersects the y axis) is called the shutoff point. The point at which operating efficiency is at a peak is called the best efficiency point (BEP). Head rise from BEP to shutoff is often chosen around 10%–15% of differential head. This choice makes it easy to modulate pump flow by adjusting control valve open area based on monitoring pressure. Pumps “operate on their curves,” and knowledge of what pressure relates to what flow allows technicians to program control loops.


Reliability tip: Operation at locations too far from BEP comes at a price. Inefficient operation increases power consumption, or maintenance frequency, or both. 

The generalized depictions in Fig. 4 also contain a curve labeled NPSHr, which represents “net positive suction head required.” This is the head of liquid that must exist at the edge of the inlet vanes of an impeller to allow liquid transport without causing undue vaporization. NPSHr is a function of impeller geometry and size; it is determined by factory testing. The NPSHr of an impeller can range from a few feet to a three-digit number. At all times, the head of liquid available at the impeller inlet (NPSHa) must exceed the required head of liquid (NPSHr).


Reliability tip: Some non-hydrocarbon process fluids have properties that make it advisable to provide an available NPSH well in excess of the published required NPSHr. Investigation of the applicable process experience is advised.
 

Operation at zero flow.

The rate of flow through a pump, in gallons per minute (gpm), is labeled Q, and is plotted on the x axis. Note: For a given speed and for every value of head H read from the y axis, there is a corresponding value Q on the x axis. This plotted relationship is expressed as “the pump is running on its curve.” Pump H-Q curves are plotted to commence at zero flow and highest head. Process pumps need a continually rising curve inclination; a curve with a hump somewhere along its inclined line will not serve the reliability-focused user.


Reliability tip: Operation at zero flow is not allowed and, if over a minute’s duration, could cause temperature rise and internal recirculation effects that might destroy many pumps.
 

A published pump curve is valid only for this particular impeller pattern, geometry, size and operation at the speed indicated by the manufacturer or entity that produced the curve. Curve steepness or inclination has to do with the number of vanes in that impeller. Curve steepness is also affected by the angle each vane makes relative to the impeller hub.

In general, curve shape is verified by physical testing at the manufacturer’s facility. Once the entire pump is installed in the field, it can be retested periodically by the owner-purchaser for degradation and wear progression. Power draw may have been affected by seals and couplings that differ from the ones used on the manufacturer’s test stand.


Reliability tip:
Occasionally, high efficiencies are alluded to in the manufacturer’s literature when bearing, seal and coupling losses are not included in the vendor’s test reports. Make sure to install a large-enough motor. 

Impellers and rotors

Regardless of flow classification, centrifugal pumps range in size from tiny to very big pumps. The tiny ones might be used in medical or laboratory applications; the extremely large pumps may move many thousands of liters or even gallons per second from flooded lowlands to the open sea.

All six of the impellers in Fig. 5 are shown with a hub fastening the impeller to the shaft, and each of the first five impellers is shown as a hub-and-disc version with an impeller cover. The cover (or “shroud”) identifies the first five as “closed” impellers; recall that Fig. 3 had depicted a semi-open impeller. Semi-open impellers are designed and fabricated without the cover. Finally, open impellers come with freestanding vanes welded to, or integrally cast into, the hub. Since the latter incorporate neither disc nor cover, they are often used in viscous or fibrous paper stock applications.

  Fig. 5. General flow classifications
  of process pump impellers.


To properly function, a semi-open impeller must operate in close proximity to a casing internal surface, which is why axial adjustment features are needed with semi-open impellers. Axial location is a bit less critical with closed impellers. Except on axial flow pumps, fluid exits the impeller in the radial direction. Radial-flow and mixed-flow pumps are either single-suction or double-suction (“double-flow”) designs. Once the impellers are fastened to a shaft, the resulting assembly is called a rotor.

In radial- and mixed-flow pumps, the number of impellers following each other, typically called “stages,” can range from one to as many as will make such multistage pumps practical and economical to manufacture. Horizontal shaft pumps with up to 12 stages are not uncommon; using more than 12 stages on a horizontal shaft risks causing the rotor to resonate or vibrate at a critical speed. Vertical shaft pumps have been designed with 48 or more stages. In vertical pumps, shaft support bushings are relatively lightly loaded; they are spaced so as to minimize vibration risk.

The meaning of specific speed

Pump impeller flow classifications and the general meaning of specific speed deserve further discussion. Moving from left to right in Fig. 5, the various impeller geometries reflect selections that start with high differential pressure capabilities and end with progressively lower differential pressure capabilities. Differential pressure is simply discharge pressure minus suction pressure.

Specific speed calculations are a function of several impeller parameters; the mathematical expression includes exponents and is found later, in Fig. 6. Staying with Fig. 5 and again moving from left to right, we can reason that larger throughputs (flows) are more likely achieved by the configurations to the right of the illustration, whereas larger pressure ratios (discharge pressure divided by suction pressure) are usually achieved by the impeller geometries closer to the left.

  Fig. 6. A pump-specific speed nomogram
  allows for quick estimations.

Impellers toward the right are more efficient than those near the left, and pump designers use the parameter-specific speed, Ns, to bracket pump hydraulic efficiency attainment and other anticipated attributes of a particular impeller configuration and size. A similar-sounding parameter, pump suction-specific speed, Nss or Nsss, should not be confused with specific speed, Ns.

As an example, observe the customary use where, with N and Q as the typical given parameters that define centrifugal process pumps, a pivot point can be determined. Next, with pivot point and head H, Ns can be easily determined. In Fig. 5, Ns is somewhere between 500 and 15,000 on the US scale. In this range, a pump is known to exist, and the general impeller shape can even be anticipated from Fig. 5.

Remember that thousands of impeller combinations and geometries exist. Impellers with covers are the most prominent in hydrocarbon processes, and uneven numbers of impeller vanes are favored over even numbers of vanes, for reasons of vibration suppression.


Reliability tip: Pump manufacturers often use modular casing construction. A given casing size may, however, accommodate several different impeller sizes or geometries. Once an existing plant has determined actual operating flows and heads, it may be cost-effective to purchase custom-built, optimized “upgrade” impellers from knowledgeable manufacturers.

Pump-specific speed (Fig. 6) might be of primary interest to pump designers, but average users will also find specific speed helpful. On the lower right of Fig. 6 is shown the equation for Ns; it is easy to see how Ns is related to the shaft speed, N (rpm), flow, Q (gpm), and head, H (ft).

As an example, consider a particular flowrate Q and what could happen at some other speed. A straight line is drawn to establish the pivot point. Then, drawing a line from H through the pivot point and to Ns, those pumps with Ns outside of the rule-of-thumb range (500 to 15,000) can be identified; these pumps should not be used.

In another example, after establishing the pivot point, an impeller with a maximum head capability of 700 ft can be selected by drawing a line through the pivot point. If the resulting Ns is too low, a higher-speed N can be attempted.

While there are always fringe applications in terms of size and flowrate, this article considers centrifugal pumps in process plants. These pumps are related to the generic illustrations in Figs. 1 and 2, and others in this article. All of these pumps would typically—but by no means exclusively—range from 3 hp to perhaps 300 hp (2 kW to 225 kW).

Process pump types

The elementary process pumps illustrated in Figs. 1 and 2 likely incorporate one of the radial vane impellers, as shown in Fig. 5. If a certain differential pressure is to be achieved combined with higher flows, such a pump is often designed with a double-flow impeller (Fig. 7).5 One of the side benefits of double-flow impellers is very good axial thrust equalization (axial balance). A small thrust bearing will often suffice; it is shown in the left bearing housing in Fig. 7. Note that the two radial bearings are plain, or sleeve-type. Certain sleeve bearings have relatively high speed capability.

  Fig. 7. Double-flow impellers are used
  for higher flows and relatively equalized
  (balanced) axial thrust.

If elevated pressures are needed, several impellers can be lined up in series on the same pump rotor. Of course, this would then turn the pump into a multistage model, as shown in Fig. 8.

  Fig. 8. A six-stage centrifugal process pump.


Process pump response to flow changes

Finally, it is useful to examine changes in flowrate and how these changes can affect the mechanical response of centrifugal process pumps. After the pumpage leaves at the impeller tip, it must be channeled into a stationary passageway that merges into the discharge nozzle. Many different types of passageway designs (single or multiple volutes, vaned diffusers, etc.) are available. Their respective geometry interacts with the flow and creates radial force action of different magnitude around the periphery of an impeller (Fig. 9).6 These forces tend to deflect the pump shaft; they are greater at partial flow than at full flow.

  Fig. 9. Direction and magnitude of fluid
  forces change at different flows.



A so-called recirculation phenomenon often aggravates the problem at low flows. Recirculation is a flow reversal near either the inlet or the discharge of a centrifugal pump. This flow reversal can produce cavitation-erosion damage that starts on the high-pressure side of an impeller vane and proceeds through the metal to the low-pressure side.6


Reliability tip: Operation outside the design range will have some repercussions. There are no exceptions to this immutable rule.
 

Pump internal recirculation can cause surging and cavitation, even when the NPSHa exceeds the manufacturer’s published NPSHr by considerable margins.7–9 Also, extensive damage to the pressure side of the impeller vanes has been observed in pumps operating at reduced flowrates. These are the obvious results of recirculation; however, more subtle symptoms and operational difficulties have been identified in pumps operating in the recirculation zone for extended periods.

The pump’s behavior can be likened to that of a manual transmission in an automobile. While a car can be driven 80 mph in first gear, it would be a mistake to do this for an hour or so. Machines are built for operation within a finite design range. As a final analogy, a small Piper Cub aircraft can be flown 50 mph for as long as it has fuel; however, that speed is not possible with a superbly designed Boeing 787—at least not at cruising altitudes. HP

LITERATURE CITED

1 This article is synthesized, with permission, from: Bloch, H. P., Pump Wisdom: Problem Solving for Operators and Specialists, John Wiley and Sons, Hoboken, New Jersey, 2011.
2 SKF USA Inc., “Bearings in centrifugal pumps,” Version 4, Kulpsville, Pennsylvania, 2008. Excerpted or adapted by permission of the copyright holder.
3 Emile Egger & Cie., Salt Lake City, Utah, and Cressier, Switzerland.
4 ITT/Goulds Pump Corp., Installation and Maintenance Manual for Model 3196 ANSI Pump, Seneca Falls, New York, 1990.
5 Mitsubishi Heavy Industries Ltd., Publication HD30-04060, Tokyo, Japan, and New York, New York.
6 World Pumps, February 2010, p. 19.
7 Fraser, W. H., “Avoiding recirculation in centrifugal pumps,” Machine Design, June 10, 1982.
8 Ingram, J. H., “Pump reliability—Where do you start?” ASME Petroleum Mechanical Engineering Workshop and Conference, September 1981, Dallas, Texas.
9 Bloch, H. P. and A. Budris, Pump User’s Handbook, 3rd Ed., Fairmont Press, Lilburn, Georgia, 2010.

The author

Heinz P. Bloch is HP’s reliability/equipment editor. A practicing consulting engineer with 52 years of applicable experience, he advises process plants worldwide on failure analysis, reliability improvement and maintenance cost avoidance. He has authored or co-authored 18 textbooks on machinery reliability improvement and over 550 papers or articles dealing with related subjects. More up-front reviews of process compressors can be found in the most recent Bloch-Geitner book, Compressors: How to achieve high reliability and availability, McGraw-Hill, New York, New York, 2012.



Have your say
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Praveen Orakkan
06.05.2014

Verey useful article.
You say in the article centrifucal pumps should not be run with
zero flow for more than one minute. Is this applicable only to pumps handling volatile fluids?
Some times we run our effluent pumps ( water with around 100 ppm oil)with discharge valve closed for about 10 minutes without any problem..
Thanks

Hemant Joshi
05.25.2014

Very useful and well illustrated article...

The figure 4 and figure 5 are identical ...I think figure 5 is different impeller with various GPM and head for various impeller sizes....change figure 5 with appropriate figure.

Thanks.

Hemant Joshi

V.S.Kumar
05.21.2014

Dear Mr.K.P.Bloch,
Thanks for a nice article. Fig.4 & 5 are same. Basically Fig.5 is missing. Please send the Fig.5.
I require your permission to use this article, in my presentations and lecture notes, to train my young engineers.
regards

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