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Design and Application Factors Affecting NPSH Margin

Maintenance & Reliability
NPSH Margin Ratio

Design and Application Factors Affecting NPSH Margin

A direct correlation exists between pump reliability and Net Positive Suction Head (NPSH) margin. To achieve acceptable pump performance, reliability and service life, while covering for uncertainties associated with available NPSH, an adequate NPSH margin must be established, especially for today’s higher speed, higher energy density pumps. Inadequate NPSH margin may affect pump head, noise, and vibration – all leading to reduced pump service life and higher costs.

To determine an acceptable NPSH margin, three factors must be considered:

  • NPSHR: the minimum NPSH given by the pump manufacturer
  • NPSHA (Available): the amount of absolute suction available to suppress boiling at the impeller eye. It is measured as hatm + hs – hvp, where hatm = atmospheric pressure head, hs = total suction head and hvp = liquid vapor pressure head.
  • NPSH3: the net positive suction head available based on a 3% head drop

NPSH margin ratio is determined by dividing NPSHA by NPSH3. The reliability of pump operations may be affected if an appropriate NPSH margin is not provided above the published NPSHR for the pump. And if the NPSHA matches the NPSH3, the first-stage head will be 3% less than the fully developed value due to flow blockage from cavitation vapor in the impeller inlet.

 

NPSH Margin Ratio

Most pumps can operate satisfactorily with minimal margin above the NPSH3 when operating near best efficiency point (BEP). Specifying an excessive NPSH margin, while not harmful to the pump, can result in a non-optimum pump selection, adding to costs.

NPSH Margin Considerations

When specifying adequate NPSH margin, the following pump characteristics and applications factors should be considered:

  • Inlet tip speed: values below ≈ 15 m/s are considered low, while values exceeding ≈ 30 m/s are considered high and should be evaluated for adequate NPSH margin
  • Larger impeller eye: occasionally used to lower NPSH3 values by lowering the liquid axial velocity at the impeller eye. However, a large impeller eye also increases impeller inlet tip speed, intensifies effects of suction recirculation, and increases the NPSH margin requirements to suppress cavitation at lower flow rates.
  • Suction specific speed: while low suction speed pumps can typically operate at or near their NPSH3 with little or no consequence from cavitation, high suction speed pumps are likely to be noisy and vibrate if sufficient NPSH margin is not provided
  • Pumpage effects: abrasives, corrosive properties and additives can increase cavitation effects while small amounts of entrained gas can actually cushion the forces from collapsing cavitation bubbles
  • Operating Range: pumps performing in the preferred operating region require less NPSH margin than those operating within allowable operating range
  • Impeller Materials: metals, composites, and plastics possess a different resistance to cavitation damage
  • Pump Size: larger pump diameters are more prone to cavitation damage
  • Duty Cycle: the longer a pump runs under cavitation conditions, the greater extent of potential damage

Application Considerations

  • Petroleum Pumps: because of the relatively low vapor volume when pumping hydrocarbons, these pumps can operate with relatively small NPSH margins
  • Chemical Process Pumps: operating most frequently at a wide variety of flow rates and using stainless steel impellers that offer greater resistance to cavitation, chemical process pumps can operate at relatively low NPSHA.
  • Electric Power Plant (non-nuclear) Pumps: cooling tower applications require special attention to NPSH margin due to the aggressive nature of pumpage. NPSH margin requirements based on minimum required impeller life may be specified for certain applications.
  • Water/Wastewater Pumps: municipal water and wastewater applications use a variety of pump sizes and types, operating with different fluid service, from raw water, treated potable water, raw sewage to other fluids. Experience indicates that an NPSH margin of 1.0 m (3.3 ft) is adequate to resist cavitation damage for most municipal and wastewater applications when operating within the Preferred Operating Region. Pumps equipped with impellers constructed of cavitation-resistant materials may opt for a lower NPSH margin. Pump speeds, heads, number of operating pumps and other variables may affect the recommended NPSH margin.
  • Pulp and Paper Stock Pumps: add sufficient NPSH margin to account for uncertainties in the NPSH3 and NPSHA from poor suction piping and entrained air
  • Building Services: while NPSH is not a concern for closed-pumping systems, it is a very important consideration for open pumping systems.
  • Slurry Pump: as cavitation-resistant hard metals or elastomers are used in pump construction with extremely rugged designs to operate in erosive slurries. These pumps often require a NPSH margin ratio (within the allowable operating region) of 1.1 or a margin of 0.6m, whichever is greater.  Other slurry pump applications require NSPH margin in excess of 1.5. (See ANSI/HI 12.1 – 12.6 for additional guidance.)

Recommended NPSH margin can vary by pump type, physical operating conditions as well as application. The factors outlined above should be considered in its calculation. The Hydraulic Institute offers detailed guidelines, drawings, calculations and charts on NPSH in its Standard ANSI/HI 9.6.1 Rotodynamic Pumps Guideline for NPSH Margin that can be purchased at http://estore.pumps.org/ .

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