Primary Factors Affecting Vibration:

  1. Separation margin to structural natural frequency
    Operating on the structural natural frequency of the discharge head motor assembly will result in unacceptable vibration levels. A field margin of +/-10% is recommended to ensure low vibration levels.
    A structural natural frequency calculation or analysis (sometimes referred to as a reed critical frequency, RCF) will ensure proper design. Calculation or analysis margin should be larger than required field margin to account for uncertainties, consult the manufacturer for recommendations.

  2. Balance
    The higher the balance grade (lower permissible unbalance) the lower the vibration readings. On a vertical turbine pump, the motor balance primarily drives this based on the location vibration measurements are taken (top of discharge head).
    Ordering a motor with a precision balance grade or having the motor field balanced will result in the lowest possible vibration levels.
  3. Alignment
    Misalignment will result in higher vibration levels. Misalignment is typically seen between the top shaft and motor shaft.
    Vertical Hollow Shaft (VHS) Motors:
    Purchase motor with a steady bushing to help ensure the head shaft is aligned within the motor.
    Vertical Solid Shaft (VSS) Motors:
    Order precision couplings and or motors with refined based flange and runout tolerances (API). Have optional motor positing jackscrews on the discharge head to allow the motor to be dialed in. Have the pump manufacturer assemble and document runout.

Vibration Limits – Acceptance Testing

The flowing limits are for Vertically Suspended (VS1 & VS6) pumps operating at 600 rpm or greater pumping fluids free of solids. Limits vary depending on if the pump is operating within the POR or AOR.

  1. Hydraulic Institute – ANSI/HI 9.6.4-2016 “ Rotodynamic Pumps – Guideline for Vibration Measurements and Allowable Values”
    Location for testing: Top of the discharge head, reference figure
    Allowable field limits: Reference figure, overall values

    Below 268 hp (200 kW)0.13 in/s (3.3 mm/s) rms0.17 in/s (4.3 mm/s) rms
    268 hp (200 kW) & above0.17 in/s (4.3 mm/s) rms0.22 in/s (5.6 mm/s) rms

    POR = Preferred Operating Region

    AOR = Allowable Operating Region

    Figure – VS1 & VS6

  2. American Petroleum Institute – API 610 11th edition “Centrigugal Pumps for Petroleum, Petrochemical and Natural Gas Industries”
    Location for testing: Reference figure 32

    Allowable field limits: Reference table 9, all power ratings

    Overall5 mm/s (0.20 in/s) rms6.5 mm/s (0.26 in/s) rms
    Discrete frequencies3.4 mm/s (0.13 in/s) rms4.4 mm/s (0.17 in/s) rms

    POR = Preferred Operating Region

    AOR = Allowable Operating Region

    Figure 32 – Locations for taking vibration readings on vertically suspended (VS) pumps

Top of Motor Recommended Vibration Limit

A guideline for vibration limits at the top of the motor has been added as to ANSI/HI 9.6.4-2016 under Appendix C. The recommended limit (not intended to be used as an acceptance criteria) is 1.5 time the limit at the top of the discharge head.

Location for testing: The measurement should be taken near the upper motor bearing. Reference figure C.1.

Allowable field limits: Reference figure C.1, overall values

Below 268 hp (200 kW)0.20 in/s (5.0 mm/s) rms0.26 in/s (6.5 mm/s) rms
268 hp (200 kW) & above0.26 in/s (6.5 mm/s) rms0.34 in/s (8.5 mm/s) rms

POR = Preferred Operating Region

AOR = Allowable Operating Region

Figure C.1 – Top of motor locations

Alarm / Shutdown Recommendations

The Hydraulic Institute ANSI/HI 9.6.5-2016 “Rotodynamic Pumps – Guideline for Condition Monitoring” provides a table for recommended alert, alarm and shut-down limits. These limits are based on baseline or acceptance level depending on the type. Reference table

 Limit Recommendation
Alert130% of baseline
Alarm150% of acceptance limit
Shutdown200% of acceptance limit

Alert = Limit to identify pump vibration level which the pump may need service and begin the repair planning process (ordering of spare parts). This limit can be much lower than the industry standard equipment acceptance. Continued operation at this level is acceptable provided it is below the Alarm and Shutdown limits.

Alarm = Limit at which the maintenance/repair process should begin. Continued operation at this level might be warranted if below Shutdown limits.

Shutdown = Limit at which the pump should be shutdown to prevent severe damage which could be unrepairable. Applying an automatic shutdown should be carefully reviewed to ensure other system equipment, processes and safety are not adversely affected by an automatic shutdown.

Factory Vibration Testing

National Pump Company can provide basic vibration testing in the factory provided that the pump is being tested as a complete unit with the job motor. This test will provide rms velocity vibration measurements in three plane at the top of the discharge head.

National Pump Company does not recommend factory vibration testing for the following reasons:

  1. The pump installation in the test lab does not represent the installation in the field. This results in the structural natural frequency not matching that of the field, which results in values which may not correlate to the field.
  2. A valve is used to throttle the pump in order to vary the system curve during the test. This valve can produce large vibration levels due to cavitation, which would not be representative of the pump in the actual system.

National Pump Company guarantees that pumps will meet the applicable vibration limits when installed regardless if factory vibration testing is performed.

Structural natural frequency, also referred to as reed critical frequency (RCF), is the single greatest factor which prevents excessive vibration in a vertically suspended pump. Operating close to or on a structural natural frequency will result in high vibration levels. This high vibration level can result in premature or catastrophic failure of the equipment.

Key Terms:

Motor RCF Data is information provided by the motor manufacturer. This data includes the center of gravity, deflection at the center of gravity, the unit weight and the reed critical frequency.

The Operating Speed Range is the speed range at which the pump will operate. A constant speed application will have a single operating speed range for the pump. A pump operating on a VFD will have a speed range typically from 70 -100% of full load speed. If sufficient system data is provided, the speed range will be determined based on that system data and the minimum flow of the pump.

Figure: System Data w/ Pump Data – Speed Range 69% to 100%

Separation Margin is the percentage of the operating speed which the natural frequencis are above or below. The analysis margin is greater than the field margin. This increase is to cover uncertainties in the analysis and data. Acceptance field margins are +/-10%. Acceptable analysis margins vary based on the analysis method.

Figure: Natural Frequency Plot w/ +/-25% Separation Margin & 70 – 100% Speed Range   Analysis on new equipment is the easiest and most cost effective way to prevent this type of issue. The Hydraulic Institute (HI) has developed a Dynamics of Pumping Machinery (9.6.8) standard which provides guidance on this type of analysis with several levels of analysis.

A level 1 analysis is a basic handbook calculation. National Pump Company performs a level 1 analysis at a minimum on all fabricated discharge heads. The calculation is based on a cantilever spring supporting a mass for the first mode natural frequencies. The discharge head acts as the spring which the stiffness is calculated for while the motor acts at the mass. The second mode natural frequencies are estimated as being four times the first modes. An analysis margin of +35%/-25% is used. This margin and the ratio of the 2nd modes to the 1st modes was determined from experience using this calculation method in conjunction with more complicated analysis and field data to ensure no issues result.

Recommended text from HI 9.6.8 Appendix E

To determine the potential for a critical structural natural frequency occurring within the normal operating speed range of the pump, a level 1 structural dynamic analysis shall be performed of the vertical pump structure in accordance with ANSI/HI 9.6.8 Rotodynamic Pumps Guideline for Dynamics of Pumping Machinery, Table The structural reed critical frequency (RCF) calculation shall be performed in accordance with Section and Appendix C. The minimum frequency separation margin obtained by analysis shall be ± _____ (to be completed by specifier).


National Pump Company would recommend a margin no less than +/-25% for a level 1 analysis.

Figure: Basics of a Level 1 Analysis

A level 2 analysis is a finite element analysis (FEA). National Pump Company performs a level 2 analysis anytime specified or when a level 1 analysis does not find a suitable solution. This analysis uses a 3D model of the discharge head along with a simplified motor which matches the manufactures specified RCF data. An analysis margin of +/-20% is used. This margin is due to the high accuracy of the analysis results compared to field data.


Recommended text from HI 9.6.8 Appendix E


To determine the potential for a critical structural natural frequency occurring within the normal operating speed range of the pump, a level 2 structural dynamic analysis shall be performed in accordance with ANSI/HI 9.6.8 Rotodynamic Pumps Guideline for Dynamics of Pumping Machinery, Table The pump structure shall be subject to a natural frequency analysis (modal FEA) in accordance with Section The minimum frequency separation margin obtained by analysis shall be ± _____ (to be completed by specifier).

National Pump Company would recommend a margin no less than +/-20% for a level 2 analysis and does not recommend a margin greater than +/-25%.

Figure: Mode Shapes from Level 2 Analysis (left to right: 1st Mode Inline, 1st Mode Transverse, 2nd Mode Inline, 2nd Mode Transverse)

A level 3 analysis is a more complicated finite element analysis (FEA) which requires consideration of the foundation and be subjected to a forced response analysis. This analysis is outside the analysis capabilities of National Pump Company and would require outsourcing if required. Consideration of the foundation is important when the foundation does not meet the as definition of rigid per HI 9.6.8. Simple foundation considerations can be applied to a level 2 analysis.

National Pump Company does not perform any analysis on cast discharge heads. These discharge heads have extensive operation history, showing suitability of various models and motor combinations operating at constant speeds. These heads can be used in variable frequency applications, however, National Pump Company only guarantees the performance at full load speed. If a structural natural frequency is identified at a lower speed, a lock out range must be used. If a lock out range is not acceptable, a fabricated discharge head is required. The fabricated discharge head is then analyzed and designed specifically to meet the required speed range.

In times of need, ASR wells are designed to inject and recover reclaimed water into shallow aquifers for underground storage purposes. National Pump Company (NPC) has worked on various ASR projects to provide well pumps with the testing, designs, and capabilities needed to inject and/or recover water as needed. Below are various Arizona projects that NPC has been proud to participate in, working with our customer partners.

NPC has performed extensive FLOW and REVERSE FLOW testing in its Lubbock, Texas facility to optimize recharge flows into the wells without developing air entrainment into the well, damaging the formation.

City of Phoenix, Wells 299, 300, Cave Creek Water Reclamation Plant (CCWRP)

– NPC supplied Qty. 3 – American Turbine Pumps for the following projects:

Each ASR pump was designed to accommodate a defined recharge capacity by back flowing water directly through the pump. To determine the correct pressure drop requirements, NPC engineers worked with the client to perform pump performance and back flow regression testing to determine the best hydraulic fit for both pumping and reverse flow testing.

City of Chandler, Ocotillo Recharge Facility – ASR Wells #7, #8, #9, #10

NPC supplied Qty. 4 – Model M14MC – 2 stage 100 HP well pumps set at 255′, to provide 1500 GPM of pumping capacity. Each ASR well is also designed to accommodate a recharge capacity of 1,500 GPM through a Baski Flow Control Valve (FCV) which allows the flexibility to test and monitor the recharge capacity under various flow scenarios.

For more information about Chandler’s ASR facilities, refer to the  AZ Water Association Kachina News articles below:

AZWaterSpring2016Kachina p12_13


We offer pump training courses at our various branch locations throughout the year. For those new to the pump industry or those who just want to sharpen their skills and knowledge, our pump instructors are full of valuable information.

Topics include, but are not limited to:

Contact your local branch office or sales representative for more information.


National Pump Company has developed new impeller patterns specifically to pour 300 series stainless steel impellers, which now provide the same high performance and efficiency as our bronze designs. With fourteen (14) models to choose from, they are available from inventory for a quick turnaround. Flow ranges are from 200 GPM to 3,200 GPM at 1800 rpm. Larger models are available on request.

To select a pump, go to our PumpFLO selector, and choose the “304SS.IMP_MATL” pump type.


Available Now




The location that the pump operates on the pump curve can play a critical role in the design and evaluation of the pump. Hydraulic Institute (HI) standards recommend or define values based on which region the pump is operating in. The Rotodynamic (Centrifugal and Vertical) Pumps – Guideline for Allowable Operating Region (ANSI/HI 9.6.3-2012) defines two operating regions, the Preferred (POR) and the Allowable (AOR). The POR is a narrow region within the broader AOR where the pump exhibits the best efficiency and reliability. Operating beyond the AOR is not recommended.

The POR on National Pump Company curves is denoted by the selection window (yellow shaded region) in PumpFLO. Pump models which come up as options when using the Design Point Search are always within the POR. When operating within the POR, the pump requires a smaller NPSH margin (reference ANSI/HI 9.6.1-2012) and will have the lowest vibration readings. This region is typically between 70 and 120% of the best efficiency point (BEP).

The broader AOR on National Pump Company curves extends from the minimum flow to halfway between the end of the POR and end of the curve. Pump models must be manually selected for operation in this region. Along with requiring greater NPSH margin, the allowable vibration limits specified by HI are increased by 30% (reference ANSI/HI 9.6.4-2009a).



Note: The dashed line showing the upper range of the AOR can be added to curves upon request.

National Pump has what you’re looking for!  When NSF /ANSI 61-G certified product is required on your project, we will proudly display the NSF emblem on the pump bowl assembly nameplate. This NSF nameplate is confirmation that through testing, your National Pump meets and exceeds the safety standards set forth by the U.S. Safe Drinking Water Act and the National Sanitation Federation International (NSF). NSF is an independent, non-governmental, non-profit public health and environmental organization that serves the public globally, by testing products for health and safety.

NSF / ANSI 61-G means that your National Pump, manufactured with our standard materials, meets the strict industry standards without the risk of contaminants leaching into your drinking water. Our pumps have also been tested to NSF 372 to insure that they are certified to meet the 0.25% lead-free requirements.

Our NSF / ANSI 61-G standard materials of construction includes Cast Iron or Ductile Iron Bowls, with Silicon Bronze C876 Impellers. On projects where alternative impeller materials are specified such as Aluminum Bronze or Nickel Aluminum Bronze, consideration should be given to offer Stainless Steel Impellers. Stainless is deemed an acceptable material substitution under the standard, making the pump NSF compliant. National Pump offers 304 stainless steel & 316 stainless steel impeller options for 6″ – 30″ pumps. Inventory is available for quick shipments on selected models. Please call your factory branch for further information.

On display at our booth for AWWA ACE17 was the new EX-Series submersible pump. The show, which was held in Philadelphia, June 12th  – 14th drew a large crowd and many attendees stopped by our booth to look at and get first-hand information about this heavy duty submersible pump, designed for aggressive water applications.

Features are:

Pictured is Regional Sales Manager, John Hubbard (right) discussing the EX-Series with interested AWWA attendees Tyler Nolan (left) and Newton White (center).

When selecting a pump, it is extremely important to ensure that the net positive suction head available (NPSHA) is greater than the net positive suction head required (NPSHR) of the pump to prevent cavitation.

NPSHA is the amount of absolute suction head available in feet of liquid at the first stage impeller datum minus the absolute vapor pressure of the liquid.

NPSHA = h,atm + h,s – h,vp = ft of liquid


h,atm = atmospheric pressure head = ft

h,s = total suction head = h,gs + h,vs + z,s = ft

h,gs = suction gauge head = ft

h,vs = suction velocity head = ft

z,s = elevation from the gauge centerline to first stage impeller datum* = ft

(*The “J” dimension shown in the National Pump Company Engineering Catalog for bowl assembly dimensions shows the location of the first stage impeller datum)

h,vp = liquid vapor pressure

NPSHR is a minimum NPSH given by the pump manufacture for the pump to achieve the specified performance. This value may be equal to the NPSH3 of the pump or many contain a margin above the NPSH3 of the pump depending on the manufacture.

NPSH3 is the net positive suction head required resulting in a 3% loss of total head at the first stage impeller due to cavitation (Figure 1). This is the NPSH value determined during testing. National Pump Company publishes a NPSH3 curve, which is labeled as NPSHr in PumpFLO.

If a pump is design with an insufficient margin between the NPSH3 of the pump and the NPSHA, cavitation will occur. Cavitation is due to the pressure within pump dropping below the vapor pressure of the liquid which results in the liquid becoming a vapor. This vapor reduces the area where the liquid can flow causing the loss in performance. As the vapor travels though the pump, the pressure increases causing the vapor bubbles to collapse returning to a liquid. This collapse results in noise, vibration and material loss reducing the pump life.

The Hydraulic Institute has a guideline for NPSH margin, ANSI/HI 9.6.1. This guideline discusses a number of items which influence the needed margin along with recommended margins. The recommended margin is based on either the:

While the recommendations vary based on application, size, materials and operating region, a 3.3-foot (1.0-meter) margin or 1.1 ratio, whichever is greater, is typically acceptable (Figure 1).

Figure 1 – Curve Showing Head Performance and NPSH3 at a Specified NPSHA

Figure 1 – Curve Showing Head Performance and NPSH3 at a Specified NPSHA

Ensuring an adequate NPSH Margin between the NPSH3 of the pump and the NPSHA will ensure the pump performances properly.

Figure 2 – CFD Analysis Showing Pressure Gradient Thru the Impeller (left) and Cavitation Bubble Sheet (right)

Figure 2 – CFD Analysis Showing Pressure Gradient Thru the Impeller (left) and Cavitation Bubble Sheet (right)

Bruce Ticknor

Bruce Ticknor

Director of Engineering

National Pump Company – Glendale, Arizona

National Pump Company is proud to announce our ISO 9001 standard certification renewal from ISO 9001:2008 to the new ISO 9001:2015 standard.

As part of a continued focus to improve its quality management system in efforts to exceed customer expectations, National Pump Company is proud to announce that it obtained the ISO 9001:2015 certification. This certification transitioned NPC from the previous ISO 9001:2008 quality system standard to the new globally recognized quality system that all ISO 9001 certified companies will be required to comply with by September 2018.

ISO 9001 is the international standard for quality management systems that is recognized and respected in more than 170 countries with over one million companies certified. This standard is based on a number of quality management principles including; a strong customer focus, the motivation and engagement of top management, the process approach, and continual improvement. Using ISO 9001 helps ensure that customers consistently get quality products and services, which in turn brings many shared business benefits.

By achieving this level of certification, National Pump Company continues to demonstrate our ability and desire to effectively and efficiently provide a quality management system while continuously improving products and services to valued customers.

Martin S. Anthony
Martin S. Anthony
Director of Quality and Continual Improvement
National Pump Company – Glendale, Arizona