What is slip? How does it affect
Positive Displacement Pump performance?


Is Your Pump Slipping?

Fluid slip is a common term used to describe reverse fluid flow inside a pump or other turbomachinery. Slip is affected by internal clearances of the parts, temperature, pressure and viscosity.

In a positive displacement pump, slip can be easily calculated just by looking at the flow being produced while in operation and subtracted from the nominal flow rate of the pump per one revolution.

Another way to determine the amount of slip is looking at the pump curve. At 0 psi, the pump is producing its nominal flow at the RPM. Notice as the pressure increases running at the same RPM, the flow will decrease. The difference between the operating pressure and 0 psi is the amount of fluid slip the pump is experiencing. This method is more of an approximation whereas the above measured flow minus the nominal flow per revolution is more accurate.

 

Why Should we Care About Slip?

Most importantly, too much slip can increase wear and decrease pump life, especially if the fluid is abrasive or has solids. Abrasive fluid passing through the clearances of the internal parts has the same effect of sand blasting. This opens up the clearances even more, amplifying the issue.

Next, too much slip can increase the cost of operation and loss of efficiency. In a positive displacement pump, if you are not getting the desired flow rate the first thing you do is turn up the speed, but doing this also increases the required power needed to meet same flow and pressure.

 

Excessive amounts of slip will introduce heat into the pump. This is critical in both the Progressive Cavity and Internal Gear Pump lines. Elastomers in the Progressive Cavity’s stator, or any rubber, has a set life limit based on how much heat it can absorb.  The more heat it absorbs, the shorter its life.

The clearances in the Internal Gear Pump are extremely tight and with extra heat introduced can cause the rotor and idler to expand and potentially lock up against the head or casing.  As a side note, if the safety relief valve is being used as a flow throttling mechanism, it can also cause the rotor and idler to expand as well.

 

How do you Decrease Slip?

When sizing positive displacement pumps, our guidelines is to keep slip under 15% of the desired flow rate.  One option to achieve this is to choose a smaller pump, but keep in mind RPM restrictions. When sizing Progressive Cavity pumps, our guideline is nothing faster than 300 RPM, to minimize the amount of heat generated to maximize stator elastomer life.

 

“Summit Pump Man, 
I don’t want to change my existing pump size, what else can I do to minimize slip?”

Reduce the pressure of the system. Reduce the all unnecessary fittings, increase pipe diameter, operate at a lower flow rate, ensure all filters and pipe runs are clean of debris, shorten the distance the fluid has to run.

A convenient feature of the Progressive Cavity pump is the ability to simply change the stage of the pump. For example, increasing a 2-stage rotor and stator to a 4-stage will reduce the amount of pressure per stage the pump experiences. Ultimately, reducing the amount of slip by a factor of the stage change. As with everything, there is always a tradeoff, torque and power required will increase and resulting factors need to be examined, such as motor size, pump frame size and location space available.

Click to Enlarge

– The Summit Pump Team

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

 

The team at Summit Pump celebrates the first full day of spring with a March Madness Mexican Fiesta!

Providing the best value in the pump market works up quite an appetite!

 

Baseplates: Flat and Level

Pump Alignments
We all know that proper alignments between the pump and the driver are critical. Did you know that the bearing load increases in direct proportion with the misalignment? But even more importantly, a bearing load increase will decrease the bearing life by a cubic function. Simply stated, if the bearing load increases by a factor of two, due to misalignment, then the bearing life decreases by a factor of eight.

Warning: All pumps must be final aligned in the field. No matter how precise the alignment from the factory, it will be lost when the unit is shipped and installed.

Baseplates: Flat and Level
Baseplate flatness has become an issue in recent years as customers are looking to simplify installation and alignment and reduce MTBF (mean time between failures), while attempting to also reduce costs.

Most of us understand that the pump baseplate has to be level. The biggest reason for that criterion is the oil level in the bearing housing. It is possible to have too much oil on one bearing (for example the radial bearing) and not enough on the other bearing (thrust bearing). If the proper oil level is the middle of the bottom ball then it doesn’t take a whole lot of “being un-level” to miss this crucial mark.

What is flat?
A typical fabricated steel baseplate for an ANSI or standard industrial pump will be flat to within 0.005 inches (0.127 millimeters) per foot (0.3048 meter).

Example: If the distance between the pump mounting pad and the motor mounting pad is 4 feet (1.2192 meters), then the motor pad can be 0.020 inches (0.508 millimeters) higher or lower than the pump pad. If a pump pad is one foot (0.3048 meter) long, it can be 0.005 inches (0.127 millimeters) lower or higher on one end.

Process Industries Practices (PIP) (and API610) bases for the same model can be 0.002 inches (0.0508 millimeters) per foot (0.3048 meter), or less than half of the standard baseplate.

Benefits
A flat surface will allow the installer to more effectively level the baseplate prior to grouting and then align the motor to the pump by shimming under the motor feet. When attempting to align the pump and motor shaft to within 0.002 inches (0.0508 millimeters) TIR (Total Indicator Run-out), it is easier to start out on a level playing field rather than have unequal shim stacks under each foot of the motor.Extra Credit:

Good millwrights and pump technicians/mechanics know to check for *soft-foot before they start the pump to driver alignment.

*“soft-foot” is where one or more feet are not in contact with the mounting pad when the unit is in the unbolted condition (this should be corrected by shimming, not by tightening the bolt which may distort the motor frame).

If you had a soft-foot of 0.016” and then bolted the motor down anyway; you have just introduced 0.008” of strain/stress into the driver. There is now potentially 0.008” of offset between the front and rear bearings, regardless of the motor size. If you do a similar offset to the pump, again you have introduced an internal stress on the bearings and shaft. Even without soft-foot on either pump or motor, if the baseplate is not flat, you have just done the same thing to the driver and or motor. Bases must be level and flat.

– The Summit Pump Team

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We are your Best Value by
“providing quality pumping
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Truth: Pumps Shipped From the Factory are NOT Ready
to be Started and Operated

Once a year I attempt to remind all Summit Pump distributors of the “Plug and Play” myths that unfortunately persist in the pump universe, like fake moon landings and that the earth is flat.

Please make sure you and others on your staff know these 5 key points:
1) OIL: A Pump shipped from the factory does NOT have oil in the bearing housing.

Someone at the pump system site must set the proper amount of the proper oil prior to startup. It is a violation of several federal laws to ship oil in the pump, as oil is considered a hazardous substance.

2) IMPELLER clearance: Final impeller clearance must be set prior to startup. The factory sets the clearance at a nominal setting for the pump type and size, based on ambient temperature liquids, as the factory does not know the specific fluid temperatures or properties.

3) MECHANICAL SEAL: A pump shipped from the factory does NOT have the mechanical seal set, in hopes of preventing damage to the sealing faces.
The seal should be set only after adjusting impeller clearance, pump alignment, and rotational checks have been completed

4) ROTATIONAL DIRECTION: A pump shipped from the factory will NOT have the coupling spacer installed because you must first complete the driver rotational check. Additionally having the coupling removed helps in the process to set the impeller and seal. We have a 50% chance of guessing your local electrical phase rotation. If we are wrong, the pump becomes scrap metal.

5) ALIGNMENT: A pump shipped from the factory will NOT be precisely aligned to the driver.  The factory conducts/logs a rough alignment check during Assembly.

Even if we precision laser aligned the driver to the pump in accordance with NASA and USA Space Force standards, the very Nano-second the skid is picked up by a forklift or other device that alignment will disappear.

Note, that industry best practices (*1) dictate that a driver to pump alignment be checked/adjusted at least 5 times prior to startup. If you don’t know or are unsure about these 5 alignment stages, please check with your Regional Sales Manager.

(*1 Source TAMU Pump Symposium and Heinz Bloch “Pump Users Handbook”) 

 

Summary
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 IOM. The IOM is included with every pump, and can also be downloaded from our website in at least 5 languages.

Retort / Conclusion
Several people have retorted that the competition does these 5 things and so their pumps are “plug and play”. I have checked with several knowledgeable and key sources at these competitive firms and that URBAN MYTH is simply NOT true.

As a matter of fact, the other OEMs state they have the same errors / issues with their end users not heeding the warnings on installation and startup.

Exceptions: I will venture to state that perhaps some distributors may offer these 5 key steps as part of their value package. If you do then you are best in class and get a gold star.

More than a minute… Extra Credit
Post Script: On a recurring basis we have people rotate ANSI pumps backwards, consequently that trips the motor on overload. Why? Because the impeller will unscrew and “mate” with the casing. The operator subsequently corrects the directional issue (phase rotation), but does not disassemble the pump to check and correct the resultant damage.

Please note that if the impeller has “mated” with the casing there is a very high probability (99%) that the impeller will require replacement, repair and or rebalance, the casing will also require repair, and the shaft is now bent beyond specification, further the bearings and mechanical seal have been mechanically shocked.

Rotation in the wrong direction is a costly mistake.

From all of at Summit Pump…
we hope for all to have a prosperous 2019!

              Happy New Year // Feliz año nuevo // bonne année // Shana Tova //
????  (X?nnián kuàilè ) // ? ????? ????? (s novym godom)

 

– The Summit Pump Team

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

 

 

You have choices
In the horizontal pump world of ANSI, the (14) mid frame pump sizes offer you a choice of selecting an MTO (medium) or an LTO (large) frame size. See sidebar below for more info.

Most often your competitor will choose the MTO over the LTO version because it is less expensive. They often choose the least expensive version because they don’t know or aren’t sure how to sell reliability, durability and the lower total cost of ownership (TOC).

I will acquiesce that some people will never be convinced regarding the “upsell”. At times I agree, depending on the pump duty and the duty cycle, that sometimes going cheap is indeed acceptable.

What is the difference?
The main difference between an MTO and LTO pumps ARE the shaft diameters. The MTO uses a 1-3/4” mechanical seal, whereas the LTO uses a 2-1/8” mechanical seal. Of course, this also affects the bearing sizes, both inboard (radial) and outboard (thrust), coupling hub size, and consequently the stuffing box size.

The larger shaft diameter directly correlates to a lower Shaft Deflection Index (L3/D4) rating as well. The lower the number, the less likely the shaft will deflect. See below chart:

All these “bigger sizes” do add up to more cost and a higher price, but they can also translate directly to higher reliability; meaning longer intervals between maintenance and lower total cost overall.

When and How do you decide to offer the LTO?
Before you decide, first consider some of the conditions the pump will experience. Look at the application, the fluid, the curve, the design point(s) and the duty cycle. Discuss with your customer where and for how long they will operate on the curve. Also ask if the customer had previous issues due to operations away from BEP?

Does the pump operate 24/7 or 5 minutes a month?
What is the energy level involved? (More important on a 50 HP application than 5 HP.)
Is the duty critical? Is there a backup pump in place?

 

We always recommend LTO over MTO when the following conditions exist:

  • Engine driven (intermittent torque application from combustion engine)
  • Belt drive (high cyclic stresses)
  • Operations away from BEP and approaching minimum flows or end of curve.
  • Conditions (fluids) that push the pump into a sleeved shaft configuration (higher L3/D4).
  • Applications with fluids that are viscous, non-Newtonion or high Specific Gravity (approach shaft BHP limits)
  • Fluid conditions /properties that push for a more sophisticated mechanical seal application (bigger seal).
  • Extreme temperature applications; over 325 F or under minus 20 F.
  • Variable speed applications; especially if it will run at low speeds at or below 900 RPM or has abrupt changes in speed. (Torque varies inversely with speed)
  • Applications with very low suction pressures or abrupt fluctuating changes in suction pressure. (Axial thrust increases as suction pressure decreases on end suction pumps.)

– The Summit Pump Team

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We are your Best Value by
“providing quality pumping
products in a timely manner,
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NPSHR and Impeller Diameter

Okay, this may take longer than 60 seconds.

Prior to computerized software for pump selection, manufacturers simply drew one NPSHR curve for the pump on the head versus capacity curve. It was used regardless of the impeller diameter chosen. This is not a good idea for the smaller diameter selections.

NPSHR for a fixed flow rate is less for the larger diameter impeller than the smaller diameter impeller. Or stated in another way, at a given flow rate the NPSHR increases as the diameter of the impeller decreases. (Note the head changes but the flow rate remains at some fixed value.)

As an alternative method to thinking about the science behind this, open your Summit Select program and select a Summit Pump model as below:

  • 2196 in 60 hertz
  • Perform a Design Point Search for ambient temperature water
  • 250 GPM at 65 feet TDH
  • Select the pump size 2 x 3 – 6 at 3500 RPM
  • Open the curve

You will note that for 250 GPM the 6.00 inch impeller will require about 11 feet NPSHR, the 4.63 inch impeller will require 13.3 feet NPSHR and the minimum 4.00 inch impeller will be 18 feet NPSHR.

The 3 condition points for the 2 x 3 – 6 (3500 RPM).

  • 250 GPM at 150 feet 6.00 inch and NPSHR 11.2 feet.
  • 250 GPM at 65 feet 4.625 inch and NPSHR 13.4 feet.
  • 250 GPM at 30 feet 4.00 inch and NPSHR 18 feet

Note your results may vary by a small fraction. Also be aware if you adjust the impeller trim from the original pump search, the flow and head will change due to the Summit Select’s predicted system curve at that trim.

– The Summit Pump Team

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Jim Elsey's Pumps and Systems Articles

 

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

 

 

API versus ANSI Pumps

API pumps are utilized in applications where pressures and/or temperatures are higher than ANSI standards allow. API pumps may handle fluids that are flammable, toxic, hazardous, and my favorite…explosive. API pumps should be used when safety is especially important. An example would be the release of toxic liquids or vapors to the atmosphere, which would compromise safety and/or some environmental restrictions.  API pumps can easily operate at temperatures over 1000oF, but are limited to 750 PSIG if the temperature is approaching 500oF.

 

There was a relaxation of API 610 9th edition about ten years ago. Consequently, some customers are rightfully trying to save money on applications where API compliance was really not justified. Other people are carelessly obsessed with trying to use an ANSI pump, as an API pump. We witness more and more inexperienced engineers attempting this substitution. As grandma used to say… “You should not try to teach a pig to sing…it just upsets the pig, and is a waste of time.”

Plenty of ANSI pumps are used in hazardous and high temperature applications, so when and how do you know which to use?

Consider an API pump if:

  • Liquid is flammable, toxic, hazardous or explosive
  • Head (TDH) is over 375 Ft.
  • Temperatures are over 350 degrees F
  • Driver horsepower is over 125 BHP
  • Suction pressure is over 75 PSIG
  • Driver speed is over 3600 RPM
  • Impeller diameter is over 13 inches

The following expression used by one company has worked well:   kW x RPM
If the result is greater than 675,000, they will likely use an API pump.

An ANSI B73.1 pump, by specification, is not as restrictive or detailed as an API pump, but it is also significantly less expensive, with shorter lead times.

Please work with your customers to know which one is required. Do not use an ANSI pump when it is obvious you should be using an API pump. Also, do not be afraid to use an ANSI pump on a tough application that is not specifying API pumps.

 

– The Summit Pump Team

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Jim Elsey's Pumps and Systems Articles

 

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

 

NPSHR versus NPSHA margins

From several sources I have recently studied: The minimum accepted margin between NPSHR and NPSHA should be a factor of 1.35 times the NPSHRor a minimum of 5 (five) feet (1.524 meters), whichever is greater. To eliminate the risk of cavitation to an acceptable level, the margin really needs to be a factor from 2 to 5 depending on the pump design and the fluid properties.

Remember, when the pump manufacturer publishes that the NPSHR is some value “X”, that really means the corresponding head for that pump has already dropped 3% at that flowrate.

Simply stated, the pump is already cavitating for the published NPSHR at that point. Want the specific background and data? Please refer to spec ANSI/HI 9.6.1 – 2017.

NPSH and Specific Gravity

Should you correct for the Specific Gravity when calculating NPSHA?

Technically, the answer is yes, but when calculating Net Positive Suction Head Available (NPSHA) for a given installation, Specific Gravity may or may not be ignored. If the suction source is below the centerline of the pump (“Lift” situation) and the source is open to atmospheric pressure, then you should correct for the Specific Gravity, but only if the value is greater than one (1.0).

If the source is above the centerline (“Flooded” situation) and the liquid’s Specific Gravity is less than one (1.0) then it can usually be ignored.

– The Summit Pump Team

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

 

 

Is Your Impeller Balanced?

The purpose of balancing an impeller (rotor) is to ensure a safe and reliable machine. Unbalance refers to the impeller’s (rotor) center-of-gravity (mass) being out of alignment with its center-of-rotation (eccentricity). If left unbalanced, the “centrifugal forces” will generate heat, vibration and noise during rotation. All of these losses also show up as inefficiency.

Why do we care?

When there is imbalance in the impeller (rotor) during operation, stresses are created in the shaft, bearings and seals. The mechanical seal is where the issue will normally manifest first. During pump operation an unbalanced impeller will create a shaft phenomenon known as “whip”. The imbalance creates a dynamic bending force on the shaft similar to deflection (Deflection is a dynamic bending of the shaft due to unbalanced radial hydraulic forces like operating back on a curve). In both cases of whip and deflection, the shaft is not actually permanently bent, and would test as straight if the pump was stopped and the shaft runout was checked with a dial indicator.

In summary: Imbalance creates Whip; the shaft is not actually bent, but will act as if it is while running.

Notes:

  • Both whip and deflection can occur at the same time.
  • At Summit Pump we dynamically balance 100% of our Impellers to ISO standards 1940/1941.
  • Summit Pump dynamically balances all impellers including maximum diameter impellers, which some OEMs do not. They assume the impeller will be trimmed prior to assembly and consequently balanced, which is not always the case in real life.
  • You should always rebalance an impeller if any work or trim was completed on the piece.
  • We recommend all impellers be dynamically balanced regardless of size, speed or service.
  • Please note we are only balancing the impeller and not the assembled rotor.
  • For pumps of low to medium energy and speeds below 3600 RPM; normal industry standards for impeller balancing are typically single-plane balanced if the ratio of diameter to width D/b is 6.0 or greater. The width b is measured between the outside of the shrouds at the impeller OD. For and open impeller it is in essence the vane height at the OD. Two-plane (or dynamic) balancing is typically performed otherwise.
  • The lower the number for the quality grade the higher the balance tolerance… (The higher the tolerance the better the balance. Also thought as there is less unbalance).
  • Industry best practices and Summit Pump standard default balance quality grade for centrifugal pumps is ISO G 6.3.
  • For impellers balanced to a higher tolerance than ISO G 6.3, industry studies show that there is little to zero gain in reliability or reduction in vibration levels to be achieved.
  • Summit Pump will balance to quality grade ISO G1.0 or ISO G 2.5 for a small fee, determined by size and shop schedule.  Please see the inside sales group for the actual costs. Note that slight delays are possible, and your requirements must be outlined as a separate line item on the purchase order.
  • Specifications such as Mil Spec 167-1, API 610 // 617, ANSI //HI 9.6.4 // B73.1 and ANSI S2.19 will have more information and different acceptance tolerances.
  • The International Standards Organization, ISO, published Standard 1940/1 “Balance Quality Requirements of Rigid Rotors,”… it was adopted by the American National Standards Institute (ANSI) in 1975 , and is also known as S2.19-1975, and is titled “Balance Quality Requirements of Rotating Rigid Bodies.”

– The Summit Pump Team

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We are your Best Value by
“providing quality pumping
products in a timely manner,
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Pressure Relief Valves: Never Throttle your Safety

Each of Summit Pump’s Internal Gear Pumps come standard with a pressure relief valve. These valves are designed and positioned to protect the pump by SAFETY means only; they are not meant for throttling the flow or pressure of the system.

When the system pressure is increased and the pressure relief valve is opened, possibly due from a blocked pipe or closed valve, it recirculates the fluid within the pump via the valve. Even though this avoids building pressure in the system.

The heat generated by the moving parts of the pump has nowhere to go, except into the pump materials and the recirculating fluid. This can become problematic for a few reasons:

  • The pump materials expand, closing the specially designed clearances for the application, pump, model and size potentially causing the pump to lockup.
  • The vapor pressure of the fluid increases, lowering the NPSHa past NPSHr causing the pump to cavitate, causing damage to the casing, rotor and/or idler.
  • The fluid flashes in the valve. This means the fluid rapidly expands potentially exploding the pump and/or valve. This is even more critical with fluids with low boiling points (saturation temperatures), such as propane or ammonia, as flashing will happen at lower temperatures, depending on suction pressure.

There should always be other means of pressure relief in the system. Switches and alarms should also be installed if the pressure relief valve to were ever open. Measuring flow in the discharge is a good way of doing this.

The valves are set to a standard pressure based on the pump model and size, unless otherwise specified on your purchase order.

 

– The Summit Pump Team

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Jim Elsey's Pumps and Systems Articles

 

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