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.


See our vertical turbine pump offerings and submersible pumps, including the new EX Series made of 316 stainless steel investment cast, our selection of jet pumps, bowl assemblies and stainless impellers at NGWA Groundwater Week, December 6 – 7 at Music City Center in Nashville.


WEFTEC was a great success! We had a great turnout at our booth and lots of people came by to see our vertical turbine pump offerings,  stainless steel impellers, EX Series Pump, and our NSF pump design. If we missed you, we hope we will see you at WEFTEC 2018, Sept. 6 – 8 in New Orleans. Plan now to attend!




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
Model Material
M6HC 304SS
M8MC 316SS
K10LC 316SS
K10MC 316SS
K10HC 316SS
J11LC 316SS
J11MC 304SS
J11HC 304SS
K12HC 304SS
M14MC 316SS
M14HC 316SS
H14LC 316SS
H14MC 316SS




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).

The 2017 SAWD Jubilee which was held in Myrtle Beach June 10th – 12th always draws many pump professionals and their families. This year’s theme was “A County Fair with Ocean Air.” In keeping with the theme, National Pump gave away red bandanas with the company logo for those who stopped by our booth. On display were National Pump’s selection of submersible and booster pumps. The Jubilee Show is always a good time to meet up with new and existing customers to discuss their pump needs, learn about new products and attend some technical seminars.

Pictured below is National Pump Territory Sales Manager Joel Rackley and National Pump Georgia Branch Manager Mark Boutwell with show attendees.