In the spirit of continuous improvement we like to occasionally remind our readers about important factors that are essential for a successful pump installation. Liquid properties such as viscosity has relatively little effect on positive displacement pump performance and in most cases makes them even more efficient, but viscosity is pure kryptonite for centrifugal (rotodynamic) pumps.


What is Viscosity?

Viscosity is that property of a fluid which tends to resist a shearing force. Simply stated; does it pour fast or slow …or how quick will a ball bearing fall through a vertical column of the liquid? You may also think of viscosity as the internal friction resulting when one layer of the fluid is made to move in relation to another layer. The viscosity of liquids will decrease as the temperature increases which is the opposite characteristic of gases.




Curve Corrections

Centrifugal pump published performance curves are based on water and so when moving viscous fluids the performance must be corrected. Technically the pump performance curve should be corrected for viscosities above 5 centipoise (cps) which most people do not do. Please be advised that at viscosities greater than 40 centipoise it is no longer an option and the curve must be corrected. For viscosity corrections we use centipoise units. See Hydraulic Institute/ANSI standard 9.6.7 for more details.



As viscosity increases:

  • Pump efficiency will decrease significantly
  • Flowrate will decrease
  • Developed head will decrease
  • Power (bhp) required will increase
  • NPSHR will increase and the NPSHA will decrease
  • System curve will become more restrictive
  • Affinity laws will become inaccurate


Centipoise vs. Centistoke

A simple way to explain the difference between these two terms is that…

Centipoise: Dynamic viscosity is a measurement of the force required to overcome fluid resistance to flow through a tube (or capillary).


Centistoke: kinematic viscosities are timed flow rates through orifices where the driving force is typically gravity.



The relationship between Centipoise and Centistokes is proportional via the specific gravity.
Centistoke = Centipoise ÷ Specific Gravity


Pumps Have boundaries

All pumps have limits as to how much horsepower can be utilized. This boundary is based on power frame (shaft and bearing) speed versus torque limits and is normally expressed in BHP per 100 rpm terms.



Note: These stated limits are based on direct drive and must be reduced for belt or engine drive. See engineering for assistance.

Maximum Limit: Should you need to pump liquids at higher viscosities please contact us to review the application. Viscosities above 2000 – 2500 centipoise must be handled with positive displacement pumps.

Caution: Do not apply the Affinity Laws directly to viscosity performance corrections. You must first correct the viscous performance to its equivalent water performance before the Affinity Laws are applied. Once you have determined the new water performance then the viscous correction factor can be applied to estimate the new viscous performance.

Good news: All of these required viscosity calculations can be easily accomplished in a few keystrokes on Summit Select.





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The first modern bearing isolator was patented in 1977. They were initially used in the food processing and chemical industries on pumps and mixers. Since the 1980’s they have been installed on most anything that rotates. Isolators protect the bearing housing from the introduction of liquids and debris (contamination ingress). By keeping the lube oil clean the bearing life is exponentially improved. Just as important the bearing isolators are also designed to keep the oil in the bearing housing of the pump.

We occasionally receive complaints from the field regarding bearing isolator leakage. 99% of these issues are easy to solve. We have complied a list of the most common remedies and some added tips.


Causes of Bearing Isolator Leakage:

    • 1.) Overfill of the bearing housing is the most common reason for oil leakage. The oil level should be at and no higher than the middle of the sight glass. This “sighted” level should correspond with the middle of the lowest ball in the ball bearing set, either radial or thrust.

      Field Tip: It is important to understand that once the housing is overfilled it will take some time for the excess oil to be dissipated from the saturated isolator. In the interim it is important to continue cleanup of the expelled oil for several days or even weeks of running post overfill incident.

 

    • 2.) Pump is not level. I recommend the use of a machinist level to check the pump and note the pump must be level in both directions (stem to stern and athwart-ships to describe it in nautical terms). Additionally, please make the distinction that “flat” is different from “level”.

 

    • 3.) Wrong isolator “Case A”. For example, the isolator for the inboard end of a reverse vane impeller pump has a different style isolator than other style pumps. Field Tip: Reverse vane pumps adjust the axial clearance of the impeller to the seal chamber (stuffing box) by rotating the threaded bearing carrier on the inboard end. The bearing carrier contains the isolator and so it could be in any radial position as a result of setting the impeller clearance. For this discussion the inboard is defined as being closest to the coupling and outboard being farthest from the coupling.

 

    • 4.) Wrong isolator “Case B”. For reasons other than #3 above, such as the wrong size or material. Please appreciate there are also different isolators for high/low temperature and or special applications such as a harsh chemical environment.

 

    • 5.) Installed improperly or damaged during installation. See Instruction and Operating Manual (IOM).

 

    • 6.) Expulsion port is in the wrong location. It must be at the 6 o’clock position.

 

 

 

 

 

 

 

 

 

    • 7.) Fit and tolerance. The housing bore dimension (ID) and / or the outside diameter (OD) of the stator is incorrect. Refer to the IOM and/or factory for proper dimensions and tolerances.

 

    • 8.) The isolator is installed properly but it is improperly moved at a later time. When adjusting the impeller clearance settings, or general maintenance and handling, care must be exercised so as not to damage the isolator. Sometimes the isolator stator separates from the rotor portion; in many cases you can simply push them back together.

 

    • 9.) Re/installed without the “O”-ring on the OD of stator. (Not applicable all styles)

 

    • 10.) Constant level oiler is set incorrectly (Cross nut adjustment is set to the wrong height… in this case too high) or it is installed in the wrong bearing housing penetration… or on the wrong side of the pump.Field Tip: Note constant level oilers are rotationally sensitive for proper operation. There is a correct and incorrect side of the pump to install them that depends on the direction of rotation of the pump.

 

    • 11.) Over-pressurization of the bearing housing. This issue can easily get complicated, but a simple explanation is that the pressure of contained air will rise and fall with temperature. Think “ideal gas” laws. Field Tip: With higher pressure in the bearing housing and the pump operating the resulting oil level can appear to be too low in the sight glass, which will erroneously invite the addition of oil and consequential overfill. Another issue is that the higher pressure can force oil level below the minimum level and the bearing and oil will overheat. The oil will turn black.

 

    • 12.) Excessive pipe strain. The resultant forces and moments of suction and discharge piping induced stress will cause the bearing bore and consequently the bore for the isolator device to be out of round, nonconcentric or noncongruent. The condition will likely create an offset centerline with the sister bearing and isolator. Field Tip: Check for pipe strain by placing dial indicators (or lasers) in two planes on the coupling hub. Release the bolts on one of the pump flanges…. if the dial indicator moves more than 0.002 inches then there is excessive pipe strain. Repeat for the other flange. Correct the cause of the stress before proceeding.

 

    • 13.) Excessive vibration. The vibration can be due to cavitation, air binding, impeller imbalance, surging and or operating outside of the allowable operating area.

 

    • 14.) Misalignment of the driver to the pump.

 

    • 15.) Drive train component such as a rotating coupling (spinning in the semi-enclosed space of an enclosed OHSA guard) is too close to the external face of the isolator. In rare cases and mostly on larger frame (higher horsepower) pumps, a low-pressure area can be created (by the air velocity also know in this case as windage). The low-pressure area creates a differential pressure outside of the bearing housing near the face of the bearing isolator. Consequently the oil can be picked up and moved with the air flow “windage”. To correct this situation you can distance the coupling and OSHA guard from each other and or from the isolator face. An alternate solution in many cases is to just use a bigger OSHA guard or incorporate perforated mesh metal on the ends of the guard.

 

    • 16.) Other miscellaneous reasons. Occasionally oil is leaking out of the pump, but it is not due to the bearing isolator. During the repair and maintenance cycles sometimes the bearing housing gaskets are omitted, manufactured in the incorrect material and/or of wrong dimension. In rare instances the bolts may not be properly torqued.

 

    • 17.) Oil sight glass is located at the incorrect height (manufacturing or repair defect). While not a leak issue, but of interest on this subject. We have witnessed the end user installing the (ANSI) pump next to a wall with the oil sight gage glass (indicator) on the wall side of the pump. To achieve direct line of sight to the indicator, the operator removed the sight gage from the port side of the pump and relocated it on the other side (starboard) of the bearing housing.On most ANSI pumps there is a normally plugged penetration where the finned oil cooler would be located, if used. Both the oil cooler and the sight glass penetrations are sized at 1″ NPT. However, the centerline of the cooler penetration is at a slightly lower level (in this case 2″) than the sight glass penetration to allow for shaft clearance. The result is that the new corresponding oil level will now be too low and consequently incorrect. The resulting bearing life will be severely compromised due to inadequate lubrication.


      Note that some pump manufacturers have penetrations on both sides of the pump bearing housing for the sight glass installation on either or both sides.

 

See my Pumps & System’s article for more detailed information on this subject.





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Dylan Thomas is a young pump technician (operator-mechanic) at the local chemical plant. One day he noticed that the batch process on unit C was taking longer than normal. He suspected something was wrong with the main pump. The pump was a horizontal ANSI type driven by an induction motor. Dillion thought back to his recent pump fundamentals training and remembered something the instructor had stated numerous times… “The pump will operate on the published curve at the intersection of the system curve if it is able to do so”. If the pump is not operating properly, it could be due to one or several reasons.

Dylan pulled out his notes from the pump school and reviewed the list of possible causes for reduced pump performance (more on that later). After reviewing the list, Dylan remembered that he would need to measure the differential pressure across the pump and convert that result from pressure to head. Additionally he would need to check the maintenance records and pump nameplate for the correct pump size and impeller trim and download the pump curve from the manufacturer’s website.


Evaluation:

Dylan checked the current differential pressure across the pump as 77 psig (178 feet). The shop maintenance records documented the pump at 85 psig (196 feet) differential when it was first installed 3 years ago.



After converting the differential pressure to head, he looked over the pump curve for the hydraulic point where the pump would operate for the stated head based on the known diameter and speed. The head of 178 feet was further out and to the right on the operating curve than when the pump was new at 196 feet. The system’s flowmeter indication did not correspond to the flowrate per the pump curve. The process was taking longer so a larger flowrate didn’t make sense unless the pump was not operating on its published curve.




Diagnosis:

Utilizing the troubleshooting list he checked the following items one at a time:

  1. Improper valve position… He checked the system valve lineup as correct.
  2. Incorrect speed… Using a strobe tachometer he confirmed the motor driver was at the correct speed.
  3. Net Positive Suction Head Available (Insufficient)… Dylan wasn’t sure how to calculate NPSHA but rechecked the tank level was at its normal height and he was confident that the other factors such as temperature (vapor pressure) and friction had not changed.
  4. Clogged suction… because of the materials, the unchanged pump suction pressure and the overall system design Dylan was fairly confident that the suction line was not clogged. He conceded it may be a remote possibility but not likely.
  5. Improper clearance setting… The pump clearance had been properly set years ago by the distributor’s technician. The pump had operated fine for several years.
  6. Direction of rotation was checked as correct.
  7. Change in the system curve… Based on the system materials, valve position and process cleanliness this factor was not totally eliminated but was considered a remote possibility.
  8. Change in the liquid properties such as viscosity or specific gravity. The company lab tech confirmed through sampling that the liquid properties had not changed.
  9. Pump wear… Dylan suspected this could be the most likely reason for the reduced performance and requested a pump shutdown.


Action:

At the earliest opportunity Dylan scheduled the pump shut down and tagged out. Without disassembling the pump he was able to check the total axial travel of the rotating element and noted it was 0.028 inches more than when the pump was new. Dylan reset the impeller clearance in accordance with the IOM (Instruction and Operating Manual) specifications for the size/model and product temperature.



The pump was returned to service and the pressure differential returned to 85 psig. The process time for unit C returned to normal with the improved flowrate.


Conclusion:

As the pump operated over the last few years the clearance between the impeller and the casing had worn by 0.028 inches. The wear occurred slowly over a long period of time and so the operators didn’t notice until the change was significant.

The wear (clearance) was enough to make the pump perform as if it was operating at a lower speed or the impeller had been trimmed. The pump was not operating on its original published curve due to the wear, however still intersected the system curve at a lower head.





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In high school, I received a water-resistant watch for my birthday. During the same time, I was also taking SCUBA lessons at the local YMCA. I soon discovered that water resistant does not mean waterproof.

Fast forward 55 years and I witness pump owners applying standard pumps into harsh slurry services and expecting great results. These owner-operators are then both surprised and disappointed when the pump fails prematurely, because the pump was perhaps wear resistant but it was not wear proof.


Know Your Slurry:

In essence there are four classes of slurries covered in Hydraulic Institute/ANSI Standard 12.1-12.6 (2016). The ANSI standard explains how the classes are progressively defined by solid size, shape, type, population, and specific gravity. The determination and ranking of the slurry class is the important first step to help you pick the proper pump, speed, and materials of construction for the service. Slurries that are corrosive and viscous, especially when coupled with higher temperatures will add another factor in the decision process.

Normally for a slurry application, beside the flowrate and total head, we also need to know the following information about the liquid / slurry properties:




Slow Your Roll:

Pump wear is directly related to the pump speed and duty cycle. The commonly accepted and best practice thumb rule is that pump wear is proportional to the pump speed cubed. We will always advise to keep the speed as low as possible. The mantra of engineers for slurry applications are pumps that are “big and slow”. We recommend that it is often better to have two (or more) properly spaced lower speed pumps in series than to let one pump generate all of the required head.

Impeller tip speed is another important factor and often misunderstood. You may look at an application and prefer the faster 12-inch impeller that can do the same hydraulics as a slower 24-inch impeller. The impeller tip speeds may be the same for each pump, but over time industry studies have shown that pump wear is more closely related to the actual pump speed (rpm) than it is to the impeller tip speed.



It is still important to consider the peripheral speed and the recommendations for the maximum impeller tip speed are as follows:




Maintain a Healthy Pump Life:

It is imperative to operate the pump as close to the best efficiency point as possible to avoid recirculation wear issues. Slurry recirculation is, in essence, “an effective and efficient sand blasting method for pump internals”.



Slurry pump impellers wear faster than those in other services; It is important to have them balanced both initially and often. There are many good reasons why slurry pumps have beefy bearings and unbalanced impellers is one of them.

Application data sheets are available on our webpage to assist in selection. Don’t let your next slurry pump application be a failure. Put the right pump in service.





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Why a Vertical Pump?

Vertical pumps offer a smaller footprint than a horizontal pump of the same size making them an ideal solution for applications with limited space. If you could tour the engineering spaces of a nuclear submarine, you would notice that all of the major pumps are vertical. There is consideration for headroom (overhead) because the compartments must be accessible for operations and maintenance.

Using a vertical pump will greatly simplify the normally complicated piping arrangements associated with barge loading/unloading, tanker/terminal farms, computer cooling systems, HVAC and Hydronic applications, etc. Additionally, for those unplanned applications where a customer needs to add a pump in an already congested area such as the pump room in a high-rise and/or institutional building, the vertical inline pump can be the perfect solution.

One of the most common types of single stage vertical pumps is the inline vertical pump that conforms with the ANSI/Hydraulic Institute standard B73.2 for type OH-3 (VB) pumps. These pumps are designated inline because both the suction and discharge flanges have the same centerline. Similar to installing a common industrial valve, this type of pump will fit directly in line with the piping.



The vertical inline pump does not require a tedious precision alignment of the driver because it is initially positioned, self-aligned and maintained in place by the register (rabbet lock) machine fits to assure positive and permanent alignment. Further, there is no need for a foundation, baseplate, or grout which saves initial installation cost in both materials and labor.

Due to the robust design of vertical pump casings, the maximum allowable flange/nozzle load limits are higher than on a corresponding horizontal pump of similar class and size.

Consistent with a horizontal ANSI B73.1 pump, the vertical design is also a back pull-out design. This means the mechanical seal, pump shaft and impeller can be maintained without disturbing the piping or the driver.




Cautionary Notes:

Because the pump is vertically orientated, the stuffing box/mechanical seal chamber is the liquid high point in the pump. Air and other non-condensable gases coming out of solution can be trapped in the seal chamber causing damage to the seal. Since the pump is not self-venting it is recommended to be vigilant when venting and priming the pump and to apply the proper seal pipe plan as applicable.

Due to the vertical orientation, flooded oil lubrication is not an option. The common solution is to elect grease lubricated bearings.



Because the liquid flow into the pump suction must navigate a 90 degree turn before reaching the impeller, the net positive suction head required (NPSHR) will be slightly higher than similar size horizontal pumps. Additionally, this pump is not recommended for paper stock applications because the suction elbow can potentially become a bottleneck and block the paper stock.

Simply because the pump and driver fit neatly “in-line” with the piping does not mean you can ignore the additional weight with regard to pipe stress. Smaller pumps/motors can normally just be installed in the piping but larger units may require some sort of support. Please consult with a piping expert on how to properly support the pump and associated piping system for your application.


Bottom Line

Given the proper application and careful consideration of precautionary measures you may confidently benefit from the small footprint and reduced total cost of ownership (TCO) associated with going vertical.





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Imagine if you will…

You are a conscientious pump technician/owner/operator. You have a strong passion to properly care for the pumps under your safekeeping… your goal is to attain the highest possible level of machine reliability. You do all of the right things to maximize and extend the run time between failures. You want to be the best in the industry.


What Killed my Pump?

Ironically, after completing a list of best practices for your pump it will die a short time after you commission the system. Why? Because what you didn’t plan for was the elimination of simple pipe strain.

Pipe strain is the misalignment of suction and discharge piping in relation to the pump. Resulting forces are transferred into the pump, stressing internal components, degrading alignment, and ultimately causing premature equipment failure. Ideally, no external force should be required when aligning pipe flanges.

However, while you were busy working on another project, the group that installed the pump used a 10-foot pry bar, three come-alongs, one hoist motor, two mules and some heavy chains to persuade the already installed piping* to matchup and bolt to the pump flanges. If you could remove the bound flange bolts at this point (that are wedged in due to strain) the pipe would likely swing wildly two feet away from the pump flange.

*Note: You should always pipe away from the pump, not to the pump.


Example 1: Incorrect Pipe Fitting

An 8-inch suction line of schedule 40 steel. The line is supported approximately 6 to 7 feet away from the pump. A simple quarter inch of parallel offset at the flange (0.250”) will result in well over a ton (? 2000 lbs.) of force at the pump flange.



 


Example 2: Thermal Expansion

A 100-foot run of 6-inch schedule 40 steel pipe will expand over 1.50 inches when heated from ambient to around 200 degrees F. The force exerted on the pump flange would be close to 190,000 lbs. if left unrestrained.




Sum and Substance

Pump internals are not designed to operate within the environmental forces associated with pipe strain. Leaving out the formulas and stress calculations for simplification… with excess pipe strain the centerline of each bearing is no longer congruent to the other so the shaft does not run true. Further, the shape of each bearing is no longer round, the stress has made it eccentric.

Remember, the most expensive pipe anchor you could ever purchase is a pump.





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Dispelling the Myth that Factory Supplied Pumps come “Plug and Play”.

As an annual ritual, like a reoccuring new years resolution, I am compelled to remind pump industry people that 99.35% (approx.) of industrial centrifugal pumps do not arrive ready to run and play – unfortunately this “Plug and Play” pump industry myth continues to persist.


Why Not?

  1. There is NO OIL in the pump.
  2. The impeller may or may NOT be set to the proper clearance.
  3. The driver is NOT aligned to the pump.
  4. The direction of rotation on the motor has NOT been determined.
  5. The mechanical seal is NOT set.

If you already know these 5 things and fully understand the significance, then you can stop here. If you don’t know or would like a refresher please read on.


Oil

A pump shipped from the factory will NOT have oil in the bearing housing. Someone at the site must add oil prior to startup.



 

Oil is considered a hazardous substance in the commercial shipping world, consequently it is a violation of several federal laws to ship oil in the pump… Yes, there are means and methods to overcome this issue, but it requires special shipping, more money and paperwork. Additionally, OEM pump manufacturers are not in the business of stocking the multitude of different oils that a customer may request.

 


Impeller Clearance

A pump shipped from the factory may or may not have the proper axial clearance when it arrives on site. The factory adjusts the clearance at a nominal setting for the pump type and size based on ambient temperature water specifications.



The factory does not know the liquid’s temperature or other properties for the operating system. Note: it is also very possible the settings could have been adjusted after it left the factory. Confirming the clearance in the field is both easy and a professional best practice. Why take the chance? Also, prior to running the pump is the perfect time to take the initial total axial movement readings for the maintenance records.


Alignment

The driver will NOT be aligned precisely to a pump shipped from the factory. The factory utilizes laser manufactured templates for layout and performs a series of nominal checks to ascertain that the motor can be precisely aligned to the pump. Even if the factory did align the driver to the pump in accordance with the highest standards… as soon as the skid is picked up/transported the precision alignment will morph to unacceptable levels.



To learn more about about pump alignment, please check with your regional sales manager or refer to my articles on this subject:


Driver Direction of Rotation

A pump assembly shipped from the factory will NOT have the coupling spacer installed because you must first complete the driver rotational check with the coupling (spacer) removed. Additionally, with the coupling removed the process to set the impeller and mechanical seal is simplified.

The factory has a 50/50 chance of guessing the correct electrical phase rotation at your local site. If the rotation is wrong, the pump quickly converts to an expensive pile of useless scrap metal shortly after startup.




Mechanical Seal Setting

Factory installed mechanical seals will NOT be set. The pump comes with the seal clips in place (sleep position) to ostensibly preclude damage to the seal during shipping and handling. Plus prior to setting the seal the impeller clearance will need to be checked/set and the alignment completed.




Summary

Pumps shipped from the factory are NOT ready to be started when and as received in the field.

• Read the instructions
• Add the oil
• Set the impeller clearance
• Complete the alignment and rotational checks … then set the seal
• Install the coupling spacer and the OHSA guard

Need some assurance when commissioning your pump? Give your RSM a call and/or perhaps review this article on the subject: The Basics of Pump Startup.


Finally

A warning tag is attached to each pump to communicate these 5 key steps to the end user/installer. Of course these steps have always been stated in the Instruction and Operations Manual (IOM). The IOM is included with every pump and if misplaced can also be downloaded from our website.





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As we near the end of 2021, we “regift” some stocking stuffer pump tips
to equip you for a reliable New Year.

1.
The root cause of almost every pump problem will be on the suction side (85%). Look there first.

 

2.
In theory, a self-priming pump (at sea level) can lift water almost 34 feet, but in reality, that will never happen due to friction, vapor pressure and imperfect systems. It may be better to think of the maximum lift as 25 feet and even that is for pump applications with ideal conditions.

 



 

3.
Speaking of self-priming pumps… note that pumps on a lift condition will have a negative pressure on the suction side of the system. Consequently, if there are leaks in the suction piping the liquid does not leak out, but the ambient air will leak into the pipe.

 

4.
ANSI pumps: Check the oil level in the pump sight glass to be at or slightly below the midpoint. The maxim of “if a little bit is good… than more must surely be better” does not apply to oil level. High oil levels (above the middle of the bottom ball bearing) will increase the oil temperature and air entrainment. Both of these factors accelerate the oil’s oxidation rate and reduce both the life of the oil and the bearing. When was the last time you changed the oil in your pump(s)?

 



 

5.
New pump applications: Always calculate the Net Positive Suction Head Available (NPSHA) for the system and realize that suction pressure is not NPSHA. Failure to do so is a very risky and expensive mistake.

 

6.
Always calculate critical submergence and realize that just because you do not see vortices on the surface of the liquid does not mean it is not pulling air into the pump. A good thumb rule is one foot of submergence for every one foot per second of suction line velocity.

 



 

7.
As little as 4% air entrainment in the liquid will create poor pump performance.

 

8.
Pump wear is proportional to speed and is especially true if there are solids present in the liquid stream. The pump wear to speed ratio is a cube function which you can also think of as a factor of 8.

 



 

9.
All pumps have physical boundaries. The main ones are speed, allowable horsepower / torque as a function of speed (bhp per 100 rpm), working pressure and temperature. You should know what these parameters are for every pump that you apply. You don’t want to get arrested by the pump police.

 

10.
Avoid damage to equipment. Never start/operate your progressive cavity pump(s) dry.

 



 

11.
Never operate a centrifugal pump with the suction valve closed and minimize the amount of time that it operates away from the best efficiency point. If you have gauges installed you will know where that operating point is.

 

12.
The pump will always operate on its curve at the intersection of the system curve if it is capable of operating at that point. Note that the “pump curve” may not be the manufacturer’s published curve due to wear (open clearances), air entrainment, speed changes, cavitation, specific gravity, or viscosity.

 



 







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When it comes to system pipe sizing, we confront a ‘pay me now or pay me later’ scenario.


Quest for the Optimal Pipe Size

You want to pump 600 gpm of liquid from point A to point B. What is the correct pipe size for your new system? The simple answer is it depends on what you are pumping, how fast you want the process to occur, how far you are pumping and the duty cycle. The other key question is how much money do you have?

 

Many people will select a pump, look at the discharge size and assume the pipe size should be the same as the pump. This course of action is typically an expensive mistake because the pipe size will be too small. Initially it appears to make sense because the optimum pipe size would be the smallest diameter size simply based on the cost of the pipe per foot. However, if you are pumping sulphuric acid or suspended solids (slurries) the resultant high velocity would quickly erode the pipe. The system would have a short, unreliable, and costly life.

 

The key factor to consider is what it will cost to pump the liquid over some period of time. The smaller the pipe size, the more energy it will cost to pump the liquid due to the higher friction losses. The larger the pipe size, the lower the friction factor and the corresponding cost to pump the liquid. Of course, the larger pipe size carries a higher initial cost for the project. There will be a just right “Goldilocks” choice of not too big or not too small.



 

We can’t drill down into the selection details in this forum, but if you need assistance, the US Government Department of Energy (DOE) will be a good source of information. Also see my February 2021 “Pumps and Systems” article on this subject.


What’s the Value?

For today’s exercise let’s assume we are pumping ambient temperature water with little to no suspended solids. Let’s also assume the difference in static head (liquid elevation) between the two points is fairly small. From an energy efficiency aspect, we can’t do much about the difference in static head in any system, but we can address the other main factor which is friction.

We typically suggest flow velocities to be around a nominal 10 feet per second on the discharge side of the pump. For a 6-inch pipe that would result in a flowrate of 850 gpm and for and 8-inch pipe it would be 1500 gpm.

Given a 70% efficient pump that is operated 8700 hours per year at an electrical cost of 5 cents per kilowatt hour. The DOE estimates that to pump 600 gpm a distance of 1000 feet would cost $1,690/year for a 6-inch pipe …and $425/year for an 8-inch pipe. Even for this simple example you can see the marked difference in cost by changing one pipe size.


Pay Me Now or Pay Me Later

For your system, look at the cost of the pipe based on size and compare to the energy cost to pump the liquid over time. Plot these two costs on a graph/chart. Normally these are exponential functions, so it will be a curve. Once plotted you will see the optimum pipe size versus the energy costs over the design life of the system will be where the two curves cross.

 



 

You can have an initially cheap installation that will cost you in energy and maintenance over time… or you can pay a few more dollars up front for larger pipe and reap the benefits of a reliable and energy efficient system for many years.





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Let’s examine a short list of common reasons why your pump may have lost its head.

Speed is the main factor in head development for a given impeller diameter. Check that the motor/driver is operating at the correct speed. Normally not an issue, but we have witnessed problems with frequency control (remote sites that generate their own power or rogue VFD/VSD controls). A three-phase motor can also lose a phase while in operation and develop an issue that is known as “single phasing”, which will cause a speed reduction. Other types of drives such as engines and turbines may be operating at the wrong speed due to governor issues. Belts and sheaves may also be improperly selected.

Impeller trim size is the second most common issue. A common scenario is for one party to install a small diameter impeller and later a different party sees the pump nameplate stating a larger diameter, consequently they question why the pump is not operating to the curve.



 

The intersection of the system curve with the pump curve will dictate where the pump will operate on its performance curve. A common problem is when a pump is used to initially fill a system and there will be no downstream friction or static head resistance, consequently the pump will operate far right on the curve with little or no head.



 

A subset of the issues with system curves could be as simple as a sump level that is too low (below the pump) for the pump to lift the liquid due to friction, vapor pressure or absolute pressure issues. This is why you should always check for sufficient Net Positive Suction Head.

Air entrainment even as little as 2 % will deteriorate pump head performance. Usually by 14% the pump performance will fail. Air entrainment will typically be caused by insufficient submergence on the suction side and/or air leaks in the suction piping and/or through the packing.



 

Viscosity of the liquid must be accounted for and properly corrected on the pump performance curve. An increase in viscosity has a direct negative effect on the developed head. Viscosity in also directly related to the temperature. A common field issue is a system designed for warmer temperatures is started cold, which reduces the head while at the same time overloads the driver.

Don’t forget blocked suction lines. Look for improperly positioned or failed valves and clogged suction strainers.

For most centrifugal pumps with impeller designs in the lower half of the specific speed range reverse rotation will reduce the head a nominal 50%. If the impeller is threaded to the shaft as it is on ANSI pumps it will, 99.9% of the time, back off the shaft shoulder and attempt to friction weld itself into the casing.



 

In this unsuccessful process of attempted self-removal, the process will damage the impeller, the casing, and the mechanical seal. Further it will bend the shaft and damage the bearings with all the makings of a real Halloween nightmare.





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Tips for optimizing pump efficiencies by setting proper impeller clearances.

In the ubiquitous world of horizontal ANSI pumps…all share some common dimensions and a back pull-out feature but note there are also two distinctly different styles.

Both styles utilize an external feature to adjust the impeller clearance. This adjustment device helps the owner to set and later re-establish pump performance and efficiency by axially compensating for wear…all without disassembly of the pump.


Two Choices

On one style; the open/semi-open vane impeller, operating clearance is set to the casing/volute and on the other style, the reverse vane impeller it is the opposite…that is, the clearance is set to the seal chamber/stuffing box. For either style the purpose of the adjustment mechanism is twofold. First to set the initial clearance for the pump size and product temperature and then later when wear inevitably occurs… you can re-adjust the clearance to regain performance and efficiency.



 

Both impeller designs have their pros and cons. For example, the reverse vane impeller can be accurately set without the casing in the safe and controlled environment of a shop. The open impeller design normally offers a larger wear area that may translate to longer periods of reliable and less costly operation. The reverse vane impeller presents a consistent lower seal chamber pressure (note: check for vapor pressure issues). The open impeller handles stringy and fibrous solids better.

Please note that Summit Pump offers both styles. For more details on the advantages and disadvantages of either style please contact your Regional Sales Manager.

Regardless of which type of pump you choose; the initial impeller clearance must be set prior to commissioning the pump in the field. The actual clearance specification varies as a function of the pump size and the product temperature. See your Instruction and Operating Manual (IOM) or contact our engineering department for details.

Even if the purchase order specified the factory to set the impeller clearance at a certain dimension you should always recheck the clearance in the field to verify the setting is correct. Unless there was an unbroken chain of custody with the pump, you cannot be positive and besides, it is easy to check.


Why Do You Care if the Impeller is Set Incorrectly?

As clearance increases pump efficiency decreases. You are effectively reducing the impeller’s size when the clearance increases. Just 0.015 to 0.020 inches off the correct clearance can reduce the performance of a 10-inch impeller to perform like 9.5 inches.

 



 

The general “rule of thumb” is that the pump will lose about 1% of its capacity for each 0.001 inches of impeller clearance for the first 0.005 to 0.008 inches of added clearance over the initial setting. The issue gets markedly worse with increasing clearance.


Pump Tips

  • On average you can adjust the impeller for component wear three times before the pump should come apart for closer inspection and before the clearance has doubled.
  • When you adjust the impeller, you are also changing the setting on the mechanical seal. On the open impeller style, you will be reducing seal face pressure and on the reverse vane you will be increasing seal face pressure. Resetting the seal may also be required.
  • On the open impeller style, the axial thrust will increase each time the impeller is adjusted due to the impeller’s back pump out vanes moving farther from the seal chamber. The effect will increase the axial thrust and reduce bearing life.
  • Net Positive Suction Head Required (NPSHR) increases as clearances open.
  • Regardless of the wear location, the seal chamber/stuffing box or the casing/ volute, there is also some wear on the impeller. Post repair or new, we highly recommend that before the pump is commissioned to measure and record total axial travel of the rotating element. With this benchmark information you can measure total travel at later intervals and compare. You will not know exactly which component is wearing or by how much, but you will have an important metric data point to help in the maintenance decision process.


References for More Information





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This month, we review the reasoning for multiple pump alignments and why there may actually be more required than you think!


At the factory… the motor is positioned on the base plate and checked for proper alignment to the pump. As soon as the unit (base/motor/pump) is loaded on the truck the alignment integrity is lost. Later the unit is moved from the truck or warehouse/staging area to the foundation, consequently any semblance of an acceptable alignment tolerance is placed in jeopardy.

Industry best practices (experienced engineers, millwrights, and rotating equipment experts) will tell you that a pump requires a minimum of at least four (4) alignment checks prior to startup/commissioning. There may be an additional 4 or 5 checks to be conducted in the full commissioning process for a possible total of nine (9).

Align the Pump 9 Times

When I tell people that they need to conduct 9 alignment checks in the process of pump commissioning, I receive a wide range of interesting comments…please first let me explain my statement.

One… the first alignment
As mentioned above; prior to shipment the pump is positioned on the base and centered within its bolt tolerance. The motor is then placed on the base and a “rough alignment” is conducted to ensure that a precision alignment is possible and can be successfully conducted later in the field.

There can be exceptions to this example, and it depends on the type and size of driver. The initial factory alignment may only cover the side-to-side adjustments and not the aspects of the vertical or angular offsets, as those will be addressed in the field. No matter how precise the alignment conducted at the factory and how carefully the equipment is transported … the alignment will change in the process.

Second alignment check 
When the pump, motor and base are received at the site and initially set on the foundation the second alignment should be conducted to correct for changes in transit. It is critical that the second alignment be conducted prior to grouting or connecting the piping to the pump. Omitting this step is a common and expensive mistake.

Three… the third alignment
You may have thought you had the alignment completed before, but now that the base is grouted you will find that that the alignment has again moved out of specification.

The process of grouting the base will oftentimes warp or otherwise move the base and offset the alignment. During the grout curing process there is a large amount of heat and some expansion force involved that will potentially change the alignment.

Four… the fourth alignment
Now that the base is properly grouted and the driver is aligned to the pump, it is time to connect the piping to the pump. It is always better to “pipe away” from the pump and not from the system to the pump to avoid unwanted moments and forces. Done correctly, connecting the piping to the pump should not alter the alignment in any way. However, from my experience this is a common reason for the alignment to change out of specification and introduce unwanted stresses into the pump.

Alignment steps 5 thru 9 
Involve further preventative measures, double-checking and troubleshooting. If you are experiencing issues with alignment procedures and achieving acceptable results in the field, it could be from omitting key steps in the process.Still interested in correctly conducting alignments and commissioning procedures? Please review my articles on this subject for more information regarding the remaining steps by clicking the links below.

 

 

 

 

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Recently, we investigated numerous issues with mechanical seals failing in self-priming pumps on suction lift applications. In each instance the pump/system was not installed or operated properly. Root cause analysis suggested a misunderstanding of basic physics. We thought it would be beneficial to review a few fundamentals for pumps on suction lift installations.  


suction lift simply means the maximum level of the liquid to be pumped is physically below the centerline of the pump impeller. Most centrifugal pumps can operate with a suction lift if they are primed first. Primed means the suction line, pump casing and impeller are full of liquid and all of the air or non-condensable gases are removed.

 

A centrifugal pump cannot “suck” or ‘lift” the liquid into itself. Atmospheric pressure is the force pushing the liquid into the pump for open systems. From this information we can conclude; the maximum suction lift at sea level with a perfect pump, a perfect liquid and a frictionless leak free system can approach 34 feet (Atmospheric pressure at sea level is 14.7 psia X 2.31 ? 34).

 

NPSH Available

 

You should always calculate the NPSHA for any application, but this rule is especially true for suction lift applications. From the NPSHA calculation you can see the following effects that subtract from the max suction lift:
  • If you are at an elevation above sea level then the max suction lift is reduced accordingly because atmospheric pressure will decrease with elevation.
  • As the temperature of the liquid increases so will the vapor pressure. As vapor pressure increases the max suction lift decreases.
  • The higher the static lift the lower the NPSHA and corresponding max suction lift.
  • Friction loss in the system will reduce NPSHA and the max suction lift.

 

Compressor vs. Pump

During the priming process the displaced air has to go somewhere. Even a great centrifugal pump is a really poor compressor due to the difference in density between air and water (? 800). If there is a check valve on the pump discharge, a parallel pump in operation and or a residual vertical liquid column, the pump will not prime. The air has to be vented somewhere, usually back to the suction source.

Submergence

The critical submergence must also be calculated so the pump does not create vortices and pull air into the pump. Even a self-primer has limits for air entrainment.

 

Final Note

You can have sufficient NPSHA and not enough submergence …and you can also have adequate submergence and not enough NPSHA.

If you are ever in doubt regarding a pump application, please contact your Regional Sales Manager and/or our engineering group for assistance.

For more information see my related articles on the topic:
• Calculate NPSHa for a Suction Lift Condition 
• 10 Common Self Priming Pump Issues 
• Guidelines for Submergence & Air Entrainment

 

 

If time and money were no object, the pump OEM would be very happy to design a pump specifically for the customer’s unique operating point;
it happens, but not very often.


In essence, all single stage centrifugal pumps are designed for one operating point on the curve. This point is commonly referred to as the Best Efficiency Point (BEP). All other operating points are a degree of compromise with efficiency, cavitation, radial thrust, and recirculation issues. Ignoring these stressors will shorten the life of the bearings and mechanical seals, thereby making the pump less reliable and more costly to operate.

Of course, most end users don’t have just one operating point and normally they want to operate in a wide area that is commonly referred to as a safe or allowable operating region (AOR). Therefore, most pump applications will require operation away from the area of BEP.

 

Reliability Management Methods

Assuming the pump selection was the best compromise (choice) for the application there are methods to mitigate the potential for the consequential negative effects. All the methods come standard with the added cost of reduced efficiency, but that cost can often be an acceptable tradeoff for reliability and reduced maintenance costs.

Some common mitigation methods are as follows:

      • Variable Speed Drive or Variable Frequency Drive (VSD/ VFD)
      • Bypass loop (manual or auto-process controlled) if operating too far left on the curve
      • Throttling control (manual or auto) if running to the right of BEP
      • Operating a different number of pumps in either series or parallel
      • Using different size pumps in the same system to accommodate system outliers

There are also common methods to manage the symptoms versus the problem:

      • Balance the impeller to a stricter standard
      • Ensure a proper alignment is conducted
      • Ensure the shaft runout and the radial/axial allowable movements are within OEM tolerance
      • Establish and maintain operating clearances to specification
      • Reduce or eliminate pipe stress at the pump flanges
      • If possible, choose a pump with dual volute or a diffusor style casing
      • Upgrade the pump shaft to a more robust design to reduce deflection (Reduces the L over D ratio)

 

Another Possibility – Low Flow 

In cases where the pump is operating on the left side of the curve it may be feasible to replace the pump with a low flow version. Standard single stage centrifugal pumps are typically of an expanding volute design. Expanding volute designs have the benefit of being more efficient, but the downside is increased radial thrust if you operate the pump away from the design point.

Low flow pumps utilize a circular casing where the casing is concentric to the impeller. By utilizing this design the radial thrust component can be appreciably reduced by amounts, normally in the range of 65 to 85%. In many cases using a low flow pump will reduce the need and cost for the more robust shaft (lower L over D ratio) and bearing system.

These are just some of the more common methods to reduce stress on the pump. If you are faced with an application that seems to have a “no right fit pump solution” Summit Pump can offer assistance.

We are your Best Value by
“providing quality pumping
products in a timely manner,
at a fair market price.”

Last month we reviewed basic vacuum principles.
The reason was to better understand how to correctly measure the differential pressure across an operating pump.


Understanding your suction pressure and knowing how to read a vacuum is critical in identifying performance issues. In the field, instruments measure in units of PSI …whether gauge, absolute or vacuum. When diagnosing issues, PSI must be converted to head in order to calculate Total Dynamic Head (TDH).

 

Total Dynamic Head

Consider Total Dynamic Head as the amount of energy the pump converts from suction to discharge. This information is critical in identifying where the pump operates on the performance curve.

Suction Pressure

To illustrate reading vacuum or pressure on the suction side of the pump, we will examine both a flooded suction and a suction lift application. Each example will assume the same pump, the same liquid and the same operation speed. The differential pressure of the pump will be the same for each example.

Note: For simplicity, we will not convert pressure readings to head in these examples, but understand the conversion mentioned above must be done in order to calculate TDH. For a more in-depth explanation, see my full article on the topic.


Flooded Suction
In flooded suction, the liquid level is above the pump datum and has energy to provide to the pump. Meaning the pump does not need to convert this energy and will use this “given” energy elsewhere. In this case, a higher discharge pressure.

 


Suction Lift

In Suction Lift, the pump must generate a low enough pressure at the eye of the impeller to have the fluid pushed into the pump by atmospheric pressure. Energy is used to make this happen and results in less energy available for pressure at the discharge.

Notice the overall pressure readings are lower but the differential pressure (75 PSI) remains about the same in both flooded and suction lift applications (using the assumptions we made earlier).

 

Take-a-ways

Pressure gauges neglect velocity energy and read only one type of energy, which is pressure.Understanding your suction pressure and knowing how to read a vacuum is critical in calculating the pump’s TDH (Total Dynamic Head). Knowing the TDH can lead you to resolving several issues including operation cost, safety, pump life, inaccuracy or just nuisance issues.

 

 

We are your Best Value by
“providing quality pumping
products in a timely manner,
at a fair market price.”