Calculators for Friction Loss, Imperial & Metric Conversions, Affinity Laws & NPSH-A
General Pumps Pty Ltd provides these calculators and guides to assist with general queries and investigations and recommends working with qualified persons to ensure suitability and verification of information for applications and systems that this information is being used for. General Pumps Pty Ltd will not be responsible or liable for any errors, omissions, misuse of the contents of this site or consequences occurring from the use of the information provided from this website. These online tools do not constitute professional advice. For assistance, please contact us at General Pumps Pty Ltd or a qualified engineer or consultant.
Friction loss calculators
Friction Loss Calculator
Fitting Pressure Loss Calculator
Conversion Calculators
Conversion table between imperial and metric flow and pressure measurements
Affinity Laws
What are the Affinity Laws?
The affinity laws are a set of forumlas that predict the impact of a change in rotational speed or impeller diameter on the head and flow produced by a pump and power demanded by a pump.
If you know the shape of a pump performance curve at a certain speed or with a certain impeller diameter, you can use the affinity laws to predict the performance of the same pump at a different speed or with a different diameter impeller with a high degree of accuracy.
There are three affinity laws:
Law 1: Flow is Proportional to Shaft Speed or Impeller Diameter
Where Q is equal to flow, N is equal to shaft speed, and D is equal to impeller diameter.
Law 2: Pressure is Proportional to the Square of Shaft Speed or Impeller Diameter
Where H is equal to head, N is equal to shaft speed, and D is equal to impeller diameter.
Law 3: Power is Proportional to the Cube of Shaft Speed or Impeller Diameter
Where P is equal to power, N is equal to shaft speed, and D is equal to impeller diameter.
Use of the Affinity Laws
Application of the affinity laws to predict the impact of changes in speed can produce highly accurate results. However, as the diameter of an impeller changes the efficiency of the impeller also changes. Therefore, application of the affinity laws to calculate the impact on pump performance of a change in impeller diameter are helpful but not always highly accurate.
What is NPSH-A?
NPSH-A is a system property. Net Positive Suction Head Available is calculated from the suction-side system configuration. It is essentially the suction-side pressure less the vapour pressure of the pumped fluid at that point. NPSH-A must exceed the pump’s NPSH-R rating for the chosen operating conditions to ensure that cavitation is avoided. Normally, a safety margin of 0.5 to 1m is required to take account of this and other factors such as:
- The pump’s operating environment – is the temperature constant?
- Changes in the weather (temperature and atmospheric pressure).
- Any increases in friction losses that may occur occasionally or gradually during the lifetime of the system.
A greater margin (see What is a safe NPSH margin?) may be necessary with some systems. For example, sealless pumps can become unbalanced even with minor cavitation and this may cause bearings to fail. In these cases, a higher margin is necessary.
How is NPSH-A calculated?
NPSH-A is calculated from the suction side configuration taking into account friction losses and the vapour pressure of the pumped fluid:
NPSH-A = (((Pe–Pv)/ρ)x10.2)+Hz–Hf+(V²/2g) where:
- Pe= Absolute pressure in pumped vessel (bar)
- Pv= Vapour pressure of fluid (bar)
- ρe= Density of fluid (kg/dm³)
- Hz= Minimum fluid level above pump (m)
(negative term if below pump) - Hf = Friction losses in suction side pipework (m)
- Ve= Fluid velocity in pump flange (m/s)
- g = Acceleration due to gravity (9.81m/s²)
NPSH-A can also be determined experimentally by measurements on the suction-side system.
