August 9, 2012
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№ 4 (April 2012)

PumpCalcs.com Guru Ponders the Ins and Outs of Centrifugal Pumps

   He has more than 30 years of rotating equipment experience in the petrochemical industry and has numerous machinery reliability articles to his credit. Robert Perez holds a BSME degree from Texas A&M University at College Station, a MSME degree from the University of Texas at Austin, and a Texas PE license.

By Chikezie Nwaoha

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   The author of a new book entitled, “Operator’s Guide to Centrifugal Pumps” and co-creator of the PumpCalcs.com website on Centrifugal Pumps in a Nutshell.

OGE guest correspondent Chikezie Nwaoha (AMIMechE, MOSHAN) interviews Robert Perez:

Oil&Gas Eurasia: How has centrifugal pump technology evolved over the past decade?
Robert Perez: In the past decade I’ve seen several major trends.  The first is a trend to lower and lower fugitive emission requirement for mechanical seals.  This has been a real challenge to machinery engineers.  Luckily mechanical seal manufacturers have come to rescue with design improvements, such as reduced face loading designs and dry gas seals.  Another solution to this problem has been mag-drive and canned motor pumps, which address the issue in a different fashion. The hydraulic range of sealless pumps has expanded significantly in the last decade.  The second trend has been the offering of low-cost pump monitoring solutions, which include wireless systems that can monitor pump vibration and temperatures remotely. The prices of these systems continue to drop while their capabilities are expanding.  Some of these systems are now viable options for monitoring critical process pumps.

OGE: How is the centrifugal pump technology of today more effective/efficient than the technology of previous generations?
Perez: In the 1970s and 1980s, 3,600 rpm (or 3,000 rpm in 50 Hz countries) pumps became the norm in process pump applications due to pricing pressures.  This led to more vibration problems that were caused by rotordynamic and hydraulic stability issues. Thanks to a lot of smart people most of the common problems were solved by the end of the 1980s.  
Many of hydraulic stability issues were solved by limiting suction specific speeds (Nss) to less than 11,000 in English units (9,460 in metric units, i.e. cubic meters per hour) and limiting operation to the 80 percent to 110 percent of the best efficiency flow.  Aftermarket companies also began to offer “hydraulic rerates” for poorly applied pumps.  (Rerating a pump means redesigning the pump internals and fitting them to an existing pump casing.)   
A lot of the rotordynamic issues were resolved by 1) limiting shaft flexibility, 2) improving balancing best practices, and 3) discouraging the use of certain overhung rotor designs. For overhung impellers, the term L3/D4, where L is the overhung length and D is the shaft diameter are both given in inches, is often used to define shaft flexibility.  An upper limit of 60 for English units (2 in metric units) is recommended for L3/D4 to limit shaft deflection.  
In the 1980s, multistage and double suction overhung designs were outlawed in the API 610 Standard.  These overhung designs were found to be highly unreliable and even dangerous in flammable services.
Another huge improvement in the 1980s  was the development and acceptance of the mechanical cartridge seal.  This dramatically improved seal reliability by reducing early failures and allowing seal testing before their installation.

OGE: When selecting centrifugal pump, what are some key considerations an end-user should make to ensure success?
Perez: Here are 10 tips to help you select efficient and reliable centrifugal pumps:
Only select pumps with suction specific speeds (Nss) less than 11,000 – an Nss of less than 9,000 is even better.
Never select a pump that will have to operate below 70 percent to 80 percent of its best efficiency point.
Remember that 1,800 rpm and slower pumps are usually more reliable that 3,600 rpm pumps
Hydraulic efficiency peaks at specific speeds (Ns) between 2,000 and 3,000 and drops dramatically below 500.  If efficiency is important, try to select pumps in the Ns range.
Use double suction impellers sparingly.  They are less stable at off-design conditions than single suction impellers.
Never select pumps with a maximum diameter impeller.  You may need to increase the impeller diameter in the future for more flow or head.
Always provide expected normal, minimum, and maximum pumping rates and temperatures in the bid specifications.  This will allow bidders to make pump and seal recommendations that will meet the true process needs.
Use hydraulic stability, not temperature rise as criteria for setting the minimum acceptable pump flow.
Incorporate a healthy NPSH (net positive suction head) margin or ratio, i.e.  NPSHa/NPSHr, into your selection. This ratio should be anywhere from 1.1 to 2.0 depending on the liquid, criticality, and suction energy level.  A larger NSPH margin is always better.
Consider liquid volatility when making your pump selection. Be more conservative in your pump selection when the liquid has a single boiling point (more volatile); as opposed to a liquid with a wide boiling point range (less volatile).

OGE: How does a centrifugal pump differ from other types of pumps?
Perez: Centrifugal pumps are designed to operate in a narrow operating range, i.e. near the best efficiency point (BEP). This means that if they are not properly applied, they will be highly inefficient and unreliable.  Once the proper pump is selected and installed, the proper controls are required to ensure the pump stays in its ideal flow zone.  If the pump control system is not able to maintain a safe flow, then a safeguard such as a low flow spillback controller or fixed spillback flow orifice is highly recommended.

OGE: What are some best practices you can offer end-users in the areas of specification, installation and maintenance of centrifugal pump technology to ensure long-term performance?
Perez: Always purchase and install heavy duty pump baseplates that are grouted in and minimized piping loads on the pump.
Monitor your pumps regularly.  Monitor flows, pressures, vibration, and either replace the lubricating oil regularly or have the oil tested in a lab for contamination and wear metals. When it‘s time to repair the pump, this data will help define the pump repair workscope and reduce repair costs.
Develop detailed repair standards that contain balance standards and acceptable limits for critical fits and runouts.  These standards will pay for themselves in reliability improvements.

OGE: What are some pitfalls you see end-users commonly encounter in centrifugal pump applications? How can end-users best avoid and/or respond to such application pitfalls?
Perez: Pump control issues are common.  If the process controls permit pump operation outside the ideal flow range, some type of flow protection should be implemented.
A pump does not like to be started up with inadequate liquid level or without sufficient backpressure.  This means that pump operators must always ensure that the proper suction liquid level and discharge backpressure conditions are met before starting up a pump.  Detailed start-up procedures will help provide low stress pump start-ups.
Operating centrifugal pumps in parallel with other pumps can lead to flow problems if their head-flow curves do not match.  Always check with a pump professional to see if it’s acceptable to operate pumps in parallel.
Most processing plants view centrifugal pumps as commodities and sometimes as throw away items.  This makes it difficult to make design improvements that cost any significant amount of money.  I recommend that you work to get a sponsor from upper management to champion pump reliability efforts.  This will ensure more visibility and clout to get critical upgrades approved and implemented.

OGE: How does the industry intend to expose the applications that did not meet expectations? And what is the procedure for achieving that?
Perez: Mean time between failures (MTBF) is a commonly used metric for assessing reliability.  MTBF is generally defined by the following equation:
where M is the total pump count, T is the reporting time, and R is the total number of repairs during the reporting time.  For example, if you have 200 pumps and 20 failures in a three-month period, then the MTBF is 200 x 3/20 = 30 months between repairs.  A MTBF of six years is considered above average for API pumps and a value of 3.5 years is considered average for ANSI pumps.  I have found that plotting monthly MTBF values versus time for critical process units is an easy way to indentify a developing reliability problem.
If the MTBF of your pump population falls below these benchmarks, you must determine what type of failures you are experiencing and what their root causes are.  This requires your engineers, technicians and mechanics to be trained to conduct root cause failures analyses (RCFA’s).  The purpose of an RCFA is to identify the initiating cause of your failures.  Once the root causes are identified and eliminated, the plants MTBF should rise to or above the levels mentioned above.  Keep in mind that this process is a team effort and requires buy-in by maintenance, plant engineering, and operations departments.

OGE: What is on the prospect in terms of centrifugal pump technology? How will the centrifugal pump technology of tomorrow be more effective/efficient than the centrifugal pump technology of today?
Perez: To get the most out of our process equipment, they need to be designed with efficiency and integration in mind.  For pumps, this requires highly efficient pumps, i.e. “smart pumps,” that can “think” and react to changes in the process in order to minimize power costs and maximize reliably.  An example of this would be a variable speed pump that has an integrated flow meter, pressure transmitters, spillback controller, vibration sensors, temperature sensors, emission sensors, and PLC.  The PLC would constantly be assessing current pump operating conditions and determine if changes are required.  The PLC is always looking for the lowest power demand condition that satisfies the process and doesn’t harm pump components.  Ideally the pump “brain” would learn from the past and eventually become an expert at optimizing pump operation.  This “smart pump” would also trend its performance and mechanical condition and alert plant personnel whenever a fault is detected.

OGE: What are the plans for the future, from the industry stand point?
Perez: Process plants continually have to do more with less.  This requires more efficient equipment designs, more efficient process designs, better process controls, and fewer people.  I see training, controls, and monitoring as the keys to a safer and more profitable future.  To get the most out of our people, we must provide the best training available and provide state of the art monitoring that allows them to keep an eye on their machines.  
It goes without saying that nothing happens without effective process controls.  Controls are required to optimize process yields.  Optimizing requires balancing production yields with energy and maintenance costs.  Future control technology will incorporate all these factors in their designs in order to get the most out of processes.

OGE: From your viewpoint, what are the high-level best practices you typically propose?
Perez: Every processing facility should have:
Detailed pump selection and installation best practices for all classes of pumps and drivers to ensure the right pumps are selected and properly installed.
Written procedures for all normal and abnormal pump start-ups and shutdowns to ensure pumps are properly operated under all process conditions.
Written procedures for all pump repairs to minimize early failures and maximize run intervals.  Also, the new trend in pump maintenance is the establishment of maintenance agreements with OEM’s or third parties.  I have seen these agreements work well for mechanical seals.  There is no reason that similar agreements cannot be made for total pump maintenance.

Centrifugal Pumps in a Nutshell

Centrifugal pumps are one of the simplest of all the pump designs.  They have one moving part, called the rotor.  The rotor has an impeller attached to it that accelerates liquid from its suction eye, or inlet (see Fig. 1), to a maximum speed at its outer diameter.
The liquid is then gradually decelerated to a much lower velocity in the stationary casing, called a volute casing.  As the liquid slows down, due to the increasing cross sectional area of the casing, pressure is developed, until full pressure is developed at the pump’s discharge.  This simplicity of design and operation is what makes centrifugal pumps one of the most reliable of pump designs, assuming they are applied properly.
This process of converting velocity to pressure is similar to holding your hand outside of a moving automobile.  As the high velocity air hits you, it slows down and pushes your hand back due to the pressure developed.  Similarly, if you could insert your hand into the pump casing at the impeller exit and “catch” the liquid, you would feel the pressure produced by dynamic action of the impeller.  When any high velocity stream slows down pressure is created.  (This effect is called Bernoulli’s Principle, which simply states that energy is always conserved in a fluid stream.)  The greater the impeller diameter or rpm the greater the exit velocity and therefore the higher pressure developed at the pump’s discharge.
Another benefit of centrifugal pumps is that they can cover a wide range of hydraulic requirements, meaning they can be used in a wide range of flow and pressure applications. They can easily provide flows from less than 10 gallons per minute to well over 10,000 gallons per minute (gpm).  Centrifugal pump impellers can easily be staged, that is arranged so that one impeller’s output is directed to a subsequent impeller, so that over 4,500 pounds per square inch (psi) of pressure can be generated.
One key disadvantage of centrifugal pumps is that their efficiencies are usually less than positive displacement pumps.  While positive displacement pumps can deliver efficiencies greater than 90 percent range, centrifugal pump efficiencies can range from less than 30 percent to over 80 percent depending on the type and size. Here are few samples of centrifugal pump efficiencies:
10,000 gpm centrifugal pump – 75 to 89 percent
500 gpm centrifugal pump – 60 to 75 percent
100 gpm magnetic drive pump – 40 percent
100 gpm canned motor pump – 35 percent
Basic Centrifugal Pump Construction:
All centrifugal pumps have the following common elements (see Fig. 2):
1. Impeller, which adds energy to the liquid by accelerating it
2. Pump casing and volute, which contains the liquid being pumped and decelerates the liquid expelled by the impeller
3. Shaft seal, which allows rotation of the rotor while preventing product leakage around the shaft
4. Shaft and bearings, which maintain the position of the rotor with respect to the pump casing
Reliability and Efficiency:
The key to exemplary centrifugal pump reliability and high overall system efficiency is selecting the right pump for the required application.  This means that the pump’s best efficiency flow (BEP) and head should always closely match process’ hydraulic requirements.   A poor hydraulic match will lead to high energy bills and maintenance costs.
Centrifugal pumps like to operate close to their best efficiency point (BEP) where:
Hydraulic efficiency is greatest.
Vibration and pressure pulsations are lowest.
Shaft deflection is minimized due to the improved pressure distribution of internal pressure around the impeller. This has a major positive effect on mechanical seal life. At the best efficiency point, the pressures around the impeller are nearly balanced so that radial forces on the impeller and shaft are minimized.
Overall mechanical reliability is highest.
BEP is the flow point where centrifugal pump designers would like to see their pumps to be operated.  Conscientious pump operators should always strive to comply with the designer’s intent by operating all centrifugal pumps near their best efficiency point.
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