This section is divided into 3 parts: Definitions, Applicable Standards, and Formulae/Conversion Factors. This section deals primarily with centrifugal, diffuser volute pumps as manfactured by Fisher Industries.
DEFINITIONS
CENTRIFUGAL PUMP: A velocity-based work machine designed to move low viscosity liquid through enclosed conduits. It qualifies as centrifugal because it responds to the Affinity Laws, which predict the behavior of centrifugal machines. Machines which use impellers, as most centrifugal pumps do, should actually be called "impeller" pumps, since they do not use centrifugal force in the strict sense but in fact use vanes to impel (throw) fluid.
AFFINITY LAWS: These laws basically state that the theoretical performance of a centrifugal pump can be predicted by application of a change in either its rotating speed or its impeller diameter. Flow varies directly with the change and head potential varies with the change raised to the 2nd power. Since required driving power is related as the exponential sum of the change in flow and head, it follows that power varies with the change raised to 3rd power. In practical application, it should be noted that a slight variation with calculated results should be expected due to flow phenomena resulting from things such as casting variations and departure from optimized design conditions. Caution is especially advised when using the Affinity Laws to predict NPSHr.
FLOW: Volumetric throughput expressed in units of volume per unit of time. The most common are United States gallons per minute (USGPM) and cubic meters per hour (CMH).
HEAD: Linear expression quantifying the height of a column of liquid. Since a centrifugal pump is a velocity machine, head is used as the measurement of its pressure generating potential. The most common units for expression are "feet" and "meters".
TOTAL DYNAMIC HEAD: The total pressure potential generated by a centrifugal pump at a datum flow. This can be resolved to units of pressure (PSI) by consideration of the density (SG) of the liquid. The most common units for expression are "feet" and "meters".
NET POSITIVE SUCTION HEAD (NPSH): The absolute pressure, in excess of vapor pressure, acting on a liquid. This is resolved to head for use with centrifugal pumps and requires specification of a datum point to be meaningful. It requires quantifying that which is available (NPSHa) versus that which is required (NPSHr) at a common datum point in the suction conduit to be useful in application. The common datum point is normally the centerline of the pump suction nozzle. NPSHa is a function of the flow through the inlet piping, viscosity and vapor pressure of the liquid, and pressure or static suction head on the liquid. For complete discussion of NPSHa, refer to the Cameron Hydraulic Data book and/or the Hydraulic Institute Standards. NPSHr is a pump requirement unique to each design and can be generally predicted by design but is accurately established only by testing. It is a function of the location in the pump where the impeller begins to apply energy to the flow stream. In application, NPSHa must exceed NPSHr or cavitation will occur in the pump. The characteristic NPSHr curve of the pump under consideration should be evaluated to assure that ample NPSHa exists at all anticipated flows and that a satisfactory margin for safety exists.
CAVITATION: The result of a condition in which insufficient pressure acts on the surface of a liquid to maintain a liquid state (NPSHr exceeds NPSHa). Vapor "bubbles" occur in the low pressure regions of the pump impeller and are imploded in the high-pressure regions of the impeller where energy is applied to the flow stream. This implosion can result in localized pitting of the impeller material and in severe cases in the volute material. These pits appear as cavities, yielding the name "cavitation" which is commonly used to describe the condition.
EFFICIENCY (E) (e): The amount of power required at the input shaft relative to the amount of work being done by the pump at a defined set of conditions (Flow vs. Head). It is commonly expressed as a percentage on characteristic curves (ex. 75%) and as a decimal equivalent in power calculations (ex. 0.75). Inefficiency is attributable to several factors; the primary ones being lack of flow stream tangentiency to impeller vane geometry, internal recirculation and internal hydraulic friction losses. Parasitic losses from mechanical seal/packing and bearing friction are lesser causes of inefficiency.
RPM (N): Rotating speed expressed in revolutions per minute.
POWER (P): The power required to rotate the pump shaft at rated speed and load as determined by conditions of flow vs. head. Commonly expressed as Brake HorsePower (BHP) or Kilowatts (KW). Non-overloading power is that which is required to drive the pump at rated speed under any load determined by any conditions of flow vs. head allowed by the pump's characteristic curve.
THRUST (F): The axial force generated by the rotor of a pump. Hydraulic thrust is a function of the radial areas on the rotor exposed to different pressures. This force can be "balanced" or reduced by isolating and matching the areas exposed to different pressures. In the case of a vertical pump, the weight of the rotor is additive to the hydraulic thrust to obtain total thrust.
SPECIFIC GRAVITY (S.G.): The density of a liquid relative to the density of pure water. Commonly expressed as a decimal equivalent with pure water as 1.0 (ex. propane at 0.50 S.G.).
VAPOR PRESSURE: The absolute pressure on the surface of a liquid required to prevent flashing into vapor.
APPLICABLE STANDARDS
API-610: Standard #610 of the American Petroleum Institute defining the design, production, and testing requirements of heavy duty centrifugal pumps for service in the petroleum and related industries. This is the most complete and specific existing standard applicable to centrifugal pumps.
ANSI B73.1: Standard #B73.1 of the American National Standards Institute defining the design, production, and testing requirements of medium duty centrifugal pumps primarily for service in the chemical industry. This standard is less stringent and less specific than API-610.
ANSI E101-88: Standard # E101-88 of the American National Standards Institute defining the design, production, and testing of line shaft and submerged motor pumps for water well service. This is a poorly written and incomplete standard with the exception of the standard shaft/coupling calculations and sizing criteria contained is Sec.A-4. These criteria are accepted as the industry standard for vertical line shaft pumps; although the tensile strength requirements are unnecessarily severe and are therefore often ignored in favor of conformance to the yield strength requirements.
HYDRAULIC INSTITUTE STANDARDS: This is actually a set of standards embracing all common types of pumps and is more informative than specific. The testing sections for centrifugal pumps are complete and comprehensible and are therefore commonly specified by other standards.
NFPA #20: Pamphlet No. 20 of the National Fire Protection Association specifies not only the construction and performance characteristics of centrifugal pumps for fire protection, but also the ancillary equipment and installation requirements. This standard, along with certain sections of ANSI E101-88 and Hydraulic Institute Standards, form the basis of requirements for Underwriters Laboratories Listed and Factory Mutual Approved fire pumps.
FORMULAE/CONVERSION FACTORS
POWER:
| BHP = | GPM X TDH X SG |
| 3960 X E |
GPM/CMH:
FEET/METERS:
FEET = METERS x 3.281 METERS = FEET x 0.311
POUNDS (LBS)/KILOGRAMS (KGS):
LBS = KGS x 2.2 KGS = LBS x 0.455
PRESSURE (PSI)/HEAD (FEET):
PSI = FEET x 0.433 x SG FEET = PSI x 2.31 SG
PRESSURE:
PSI = KG/CM2 x 14.22 KG/CM2 = PSI x 0.07 PSI = BARS x 14.5 BARS = PSI x 0.69
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