Definitions
| Capacity |
Flow rate required, i.e., GPM, BPH, etc. |
| Suction |
Pressure PSI or Head (ft.) |
| Discharge |
Pressure PSI or Head (ft.) |
| NPSH |
Available in absolute pressure |
| Product |
Type liquid & its conditions (temperatures, viscosity, specific gravity,
clean, solids, abrasive, corrosive) |
| Type |
Centrifugal, rotary, reciprocating--gear, lobe, screw--horizontal, vertical, in-line, submersible, self priming- etc. |
| Materials of Construction |
Desired materials and/or construction features, if any |
| Driver |
Electric/Air/Hydraulic motor - gas/diesel engine - gas/steam turbine |
Fomulas
- Head (ft.) = (psi x 2.31) / sp. gr. = (in. Hg.) / (sp. gr. x .88)
- PS1 = (head (ft.) x sp.gr) / 2.31 = (.49 x in. x Hg.)
- Velocity (ft./sec.) = (0.4085 x gpm) / [i.d. (in.) of pipe] ^ 2
- BHP (Centrifugal) = (GMP x head (ft.) x sp. gr.) / (3960 x pump eff)
- BHP Positive Displacement = (GMP X PSI) / (1715 x pump eff.)
Conversions
| Wt of Water |
8.34 lb./gal. or 62.37 lb./cu. ft @ 60 degrees |
| 1 lb./sq. in. |
2.31 ft. water (1.0 sp. gr.) |
| BPH x .7 |
GPM |
| BPD x .0292 |
GPM |
| 1 BBL. (petroleum) |
42 gallons (U.S.) |
| 1 ft. water |
.433 PSI |
| Atmospheric Press |
14.7 psi or 33.9 fr. fresh water @ sea level |
| 1 inch of mercury (Hg) vacuum |
1.133 ft. water |
| Dynamic Suctions Lift (ft.) |
Vacuum in inches of Hg. / (0.883 x sp. gr.) |
Net Positivie Suction Head (NPSH)
- NPSHr = head required @ the eye of the pump's impeller to prevent cavitation, a function of pump design.
- NPSHa = head available from the suction side of the system, must always equal or exceed the required (NPSHr) to prevent cavication.
NPSHa Factors Table
| Positive |
Negative |
| 1) Absolute pressure on liquid |
1) Vapor pressure of the liquid, absolute |
| 2) Static suction head |
2) Friction & entrance losses |
| |
3) Static suction lift |
Affinity Laws
Effects on centrifugal pumps with limited change of speed (RPM) or impeller diameter.
- Capacity (Q) varies directly as the speed (N) or impeller diameter (D).
- Head (H) varies as the square of the speed or impeller diameter.
- BHP varies as the cube of the speed or impeller diameter.
- Law 1 = Q^1/Q^2 = N^1/N^2 or D^1/D^2
- Law 2 = H^1/H^2 = (N^1/N^2)^2 or (D^1/D^2)^2
- Law 3 = P^1/P^2 = (N^1/N^2)^3 or (D^1/D^2)^3
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