“Wishin’ and Hopin’”—both Dionne Warwick and Dusty Springfield had hit versions of this pop song in the early ’60s. The lyrical message is that you will not get want you want if you just sit around wishing and hoping; instead you need to take action to achieve the desired outcome.
The same advice holds true for pump performance. I witness an alarming number of people who unwittingly wish and hope their pump would perform in the proper manner, but they are wishing and hoping with total disregard of the system curve, pump capabilities, the laws of physics and the fluid properties. Time to take action.
I start my pump training courses with the simple personification that “pumps are stupid.” Put a centrifugal pump in any system, and it does not know where to operate. It is the system, not the pump, that dictates where the pump will operate on its performance curve—if the pump is even capable of operating at that point.
Further, and intended only as a comedic anecdote to help my students learn, I refer to the pump as the “husband” in this marriage with the system curve, aka the “wife.” Perhaps, and with ostensible apologies to the “PC Police,” I suggest that the marriage works best if the husband (pump) listens and obeys the wife (system). If there is a mismatch in the two, then divorce (pump and system failure) is imminent.
The pump will operate where its performance curve intersects the system curve, but we don’t always know where that point is—and just to complicate matters, it can change quickly because of many variables. Two roadblocks that make this determination difficult are:
- We frequently have no idea of the system curve geometry
- The pump is often forced off its published curve by outside factors
We will address the system curve calculation in a future article. For now, the system curve is the absolute summation of the system’s static head, pressure head, velocity head and friction head. The geometry of the system curve is directly related to the flow rate, pipe size, elevation changes and losses due to friction of all the components in the system. Note the system curve is dynamic and will change with tank elevation and system pressure changes. It will also change with valve position, system age, fouling and corrosion. This month’s article will address No. 2: How pumps can operate off their published curves, and we will look at a few common examples.
Causes for Operating Off Curve
Here are some of the common issues:
- worn clearances
- different or incorrect size impeller
- different or incorrect speed
- viscosity not corrected or accurate
- net positive suction head (NPSH) margin insufficient
- air entrainment and/or dual phase liquids beyond 3 percent
- inadequate submergence (also see air entrainment)
- partial or restricted blockage of the suction line
- operating the pump in the wrong direction
First Things First
To determine where your pump is really operating, you will need to calculate the pump performance curve in the field “as is.”
First, obtain a copy of your pump performance curve as published or purchased. Then, using the pump’s discharge valve position, create a series of several different flow rates (recommend at least six points including shutoff head), determine and record the suction and discharge pressures for each flow condition, convert these pressures to differential head and plot them on your curve. Be careful to correct for gauge elevations, temperatures and specific gravities.
Does your curve match the published curve? If it does within 5 to 10 percent, then there is likely no problem.
Let’s look at a few cases where the curve does not agree with the published curve. Each case tells a critical story to help understand what is happening with your pump and the system.
As a pump wears, the hydraulic performance deteriorates. Most people understand the pump efficiency will drop due to wear, causing the power to increase, but not all users realize that the head and flow will also deteriorate.
Note the revised pump curve still intersects the system curve, but the meeting point is at a lower flow rate and a lower head. See my Pumps & Systems articles from January 2016 and July 2017 for more details on this subject (Image 1).
Image 1. Effect on pump performance with wear, speed, impeller size (Images courtesy of author)
It is important to understand if you are also plotting power on the curve: if the wear is simply the impeller or casing wear rings, the power will increase noticeably. But if the wear is in other areas such as internal passages, cutwater or the impeller vanes, the power will only increase a small amount.
A special note for ANSI pump designs that use the critical clearances between the impeller and the casing or stuffing box: the effect on power will be as the aforementioned pump with wear rings—that is, the power will increase noticeably.
Different Impeller Size or Speed
Often, one of the reasons for poor pump performance is that the wrong size impeller is in the pump. This could be caused by several different reasons, mostly human error. Another common reason is that the speed is different than perceived. This is common with systems that are controlled with variable frequency drives (VFDs) as part of the system control process. I mention both of these issues (speed and diameter) here because the performance manifests almost exactly the same as worn clearances.
See Image 1 to see the performance of smaller impellers or lower speeds.
Viscosity—the Kryptonite of Centrifugal Pumps
Viscosity has a direct and negative effect on centrifugal pump performance. The pump efficiency is affected the most, but head and flow are not far behind. Of course, all of these negative factors will contribute to increase the power required to pump the fluid at the same flow rate and head as if based on water performance.
Note that viscosity is directly proportional to temperature, so be cognizant of the fluid temperature. Also, smaller pumps are more affected by viscosity due to the smaller passage size.
See Image 2 for how viscosity will affect the pump performance, and also refer to my article on this subject from the November 2017 issue.
Image 2. Performance effect with increased viscosity
Insufficient NPSH Margin
Pumps require a certain amount of NPSH to operate satisfactorily at a given point of head and flow on the curve to prevent cavitation. The points are empirically determined by the manufacturer and are denoted as net positive suction head required (NPSHr).
The suction system itself must in return provide a certain amount of NPSH, and that is referred to as net positive suction head available (NPSHa). There must be more NPSHa than NPSHr for the pump to operate satisfactorily. The difference is referred to as the margin or ratio.
NPSHa ÷ NPSHr = margin ratio
NPSHa – NPSHr = margin
Refer to Hydraulic Institute 9.6.1-2017 for details.
A frequent field issue is that the margin calculations were not done, not done correctly or there were changes in the pump and/or system that have not been accounted for. A common issue in the field is that the pump has sufficient margin at lower flows, but as the flow rate increases the pump will begin to cavitate. See Image 3 to see how insufficient NPSH margins affects pump performance.
Image 3. Insufficient NPSH—pump will cavitate at higher flow rates (compare to Image 5 restricted flow)
Inadequate Submergence & Air Entrainment
The pumped fluid could have air entrainment from different sources for many reasons, but a common cause is insufficient submergence.
No matter the source of the air, standard centrifugal pumps cannot handle it well. At about 3 to 4 percent air entrainment, the pump performance will drop off as if the impeller was trimmed to a smaller diameter or the pump is being operated at a lower speed.
Please refer to my April 2016 and December 2017 articles for details on air entrainment and pump performance.
Also note that at low flow rates the fluid velocity is insufficient to “sweep away” the air that collects in the impeller eye. Consequently, the pump will become blocked (air bound) and will stop pumping or it will surge in a destructive and alternating cycle of air blockage and subsequent flow passage.
See Image 4 to see how air entrainment de-rates the pump performance.
Image 4. Effect on pump performance with increased air entrainment
Restricted Pump Suction
This issue typically manifests on new construction, post repair startups and when there is equipment installed in the suction line like a filter press, foot valve or strainer. As flow increases the head will drop off. This is similar, but different, than an NPSH margin issue.
See Image 5 to see how a suction restriction affects the pump curve.
Image 5. (left to right). Effects of suction line restriction. Actual performance will vary with the magnitude of the restriction.
I have written about pumps operating in the wrong direction many times because it is unfortunately an all too common issue. In the case of ANSI pumps (with impellers that thread on the shaft), the pump will, 99 percent of the time, pronounce in a millisecond that the direction is incorrect as the impeller grinds into the casing creating expensive and extensive damage and yet hopefully tripping out the motor.
For pumps that have impellers keyed to the shaft it may or may not be obvious. Pumps running backwards will typically be a little noisier and exhibit higher vibrations than a similar pump operating in the correct direction. But this is not always easy to determine in an operating plant due to background noises and other field interference.
Performance will depend on the pump design and is mostly, but not always, a function of the impellers’ specific speed. As a general rule, the flow rate in a reverse operating pump will be about 50 percent, and the head will be somewhere near 60 percent. The higher the impeller specific speed, the lower the head will be. Concentric casing designs will yield different results.
Please see Image 6 to see how reverse rotation may affect pump performance.
Image 6. Effects of running the pump in the wrong direction. Assumes low- to mid-range specific speed. Do not confuse with pumps running as turbines.
Pumps will operate where the system curve dictates, but the pump curve is not always where you think it is and neither is the system curve. You will not get the performance you need and want by just wishing and hoping—you need to measure the parameters and manage the system.
MYTH: Factory Supplied Pumps are “Plug and Play”
Pumps shipped from the factory are NOT ready to be started when and as received in the field.
As an annual ritual 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.
- There is NO OIL in the pump.
- The impeller may or may NOT be set to the proper clearance.
- The driver is NOT aligned to the pump.
- The direction of rotation on the motor has NOT been determined.
- 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.
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.
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.
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:
Does Your Pump Have an Alignment Problem?
19 Tips and Common Alignment Mistakes
Driver Direction of Rotation
A pump 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.
☑ 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
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.
Open or Closed Tanks and the Effect on NPSHA Calculations
When working through NPSHA calculations for pump applications we need to know if the suction supply tank is open to the atmosphere or not. If it is an open tank the calculation is easy; as we just convert the ambient pressure to head for the first factor in the NPSHA calculation. Don’t forget to convert to absolute values and compensate atmospheric pressure for the local altitude above sea level. If the tank is closed, then we need to do a little more work converting the pressure or vacuum to absolute head for the calculation.
Often the customer will tell us the tank is closed to atmosphere, but it really isn’t; consequently the NPSHA calculations will be incorrect. The resulting NPSHA error will lead to a noncompetitive pump selection.
Sometimes the suction supply tank appears to be closed to atmospheric pressure, but if you look closer at the tank you will see it has breather valves installed. If there is a breather valve installed the tank pressure will always be very close to atmospheric pressure. It is very common, especially in the oil and gas world and also in the chemical and petro-chem arenas to use breather valves on the big bulk tanks. You may actually witness these breather valves on any size tank because the owner needs to protect the investment. Please realize these scenarios may also include rail tank cars, but do not confuse these examples with tank cars that are specifically designed to be pressurized or placed under vacuum for unloading purposes.
The Breather Valves Protect the Supply Tanks from:
- Overpressure (rupture) and or vacuum (implosion) issues
- Fire protection
- Evaporation; loss of product
- Corrosion protection
Another purpose is to prevent excess air and or water (plus other bad stuff like general pollution, O3 and NOx) from destroying the product integrity while it is in the tank. The purpose is to protect the product from outside influence and or to protect the outside environment from the product.
These protection/breather valves are normally required by EPA and or OSHA …they are not just a good idea, they are often a legal requirement in many product applications. Most tank owners apply the same rules to all of their tanks regardless of the product, tank size or location. Note that both the EPA and OSHA will defer to API 2000 for the selection and sizing criteria for the breather valves.
So…I Just Want to do the NPSHA Calculation, What Now?:
If the tank has a breather valve, the answer is to simply use the local atmospheric pressure for the NPSHA calculations, because the actual tank pressure is going to be very close.
What if There is no Breather Valve and the Tank is Really Closed Off to Atmosphere?:
When you have a closed tank; I recommend you read my two Pumps and Systems articles on this subject from October and November of 2018, where the basics are covered.
If after reading you are still in doubt, call your RSM or engineering for assistance.
Given a pump system with a supply tank open to atmosphere: note that on the suction line to an operating pump it is not uncommon to have a pressure lower than ambient. You may only expect this situation on a pump that is involved in a suction lift, but even for a flooded suction condition the suction pressure at the pump inlet can be at a vacuum. You can accurately calculate the actual pressure (vacuum) anywhere along the line by using Bernoulli’s Equation. Open and shut case… Easy peasy – lemon squeezy.
OSHA 1910.106 July 1985
API 2000 Venting for Tanks 7th Edition 2014
API 12 (49 CFR 195.264)(b)(1) Specification for tanks
API 650 (CFR 132(b)(3) Specifications for large welded tanks
API Bulletin 2521 (Evaporation Reduction)
API Bulletin 2513 (Evaporation Losses)
EPA 40 CFR 112. Note: This regulation does not actually use the terms “aboveground storage tank.” Instead the term “bulk storage container”.
DOT (various/numerous with respect to rail cars)
How fast was I going officer?
Speed is a critical limit for any pump, but even more so for Positive Displacement (PD) pumps. The maximum allowable speed of a PD pump is determined by several factors including the viscosity and temperature of the pumpage. Other important factors are the level of abrasives in the product, acceleration head, and the Net Positive Suction Head Required (NPSHR).
Commercially available and cost effective electric induction motors nominally operate at speeds well above the optimum PD pump speeds, consequently some method must be used to reduce the drive output speed. Direct drive is just not all that common in most Internal Gear Pump (IGP) and Progressive Cavity (PC) applications for this reason.
The boundary for PD pump speed will typically be managed with either a gear reducer or a Variable Frequency Drive (VFD) and/or a combination of the two. For even more precise flow modifications a servo motor can be used in conjunction with both a gear reducer and a VFD.
As the product temperature and/or abrasive concentrations increase, the pump should be operated at even slower speeds to reduce the inevitable wear and increase reliability. This may also mean a bigger and slower pump is required. Pump wear is exponentially proportional to speed. Even for relatively small increases in speed the wear rate can increase by a factor of eight.
Prior to purchase, the allowable speed range for the pump should be reviewed so that the correct choice of materials and speed control are made to achieve the lowest Total Cost of Ownership (TCO) and Mean Time between Failures and Repairs (MTBF/R).
Gear reducers (aka “gear sets” or “gear boxes”) are both essential and common components in the drive train of many PD pumps. Unfortunately the fixed output of a gear box will lock the end user into one operating speed. Therefore, the pump’s hydraulic duty point (at some speed) and the maximum allowable speed must both be considered when making the selection.
One additional benefit of a gear reducer is the increase in the amount of torque delivered to the pump shaft. Gear sets are frequently referred to as “torque multipliers” for this reason. Adding a gear set may potentially reduce the required motor size when compared to direct drive.
VFDs are often used in applications where speed dependent flow requirements will/can vary over a range. A VFD in conjunction with a gear reducer will allow the pump to operate across an acceptable range of speeds, while simultaneously providing the required proportional flow rate.
Note: Pump speed limits must be applied when initially programming the VFD. The VFD operational parameters must be set within the pump’s speed limits to avoid the critical and common over speed mistake.
Don’t get Pulled Over by the Pump Police for Speeding:
Operating correctly saves time and money.
Teaching owners and end users about pump boundaries allows them to choose smart solutions for their application and ensure the equipment is effective, efficient and reliable… reducing the TCO and MTBF/R.
If you have any questions please contact your Regional Manager or Engineering in Green Bay.