Understand Specific Speed & Disc Diaphragm Pump Coupling

Q. What are some differences between specific speed and suction specific speed for a rotodynamic pump? A. The Hydraulic Institute defines specific speed as an index of pump performance (developed total head). It is determined at the pump’s best efficiency point (BEP) rate of flow, with the maximum diameter impeller and at a given rotative speed. Specific speed is expressed by the following equation:

Where: n_s = specific speed n = rotative speed measured in revolutions per minute Q = total pump flow rate measured in U.S. gallons per minute (cubic meters per second) H = head per stage measured in feet (meters) It should be noted that when calculating specific speed using units of cubic meters per second for flow rate and meters for head per stage, 51.6 is the conversion factor for specific speed in U.S. gallons per minute and feet (metric × 51.6 = U.S. customary units). The usual symbol for specific speed in U.S. customary units is Ns. Suction specific speed is an index of pump suction operating characteristics. It is determined at the BEP rate of flow with the maximum diameter impeller. (Suction specific speed is an indicator of the net positive suction head required [NPSH3] for given values of capacity and also provides an assessment of a pump’s susceptibility to internal recirculation.) Suction specific speed is expressed by the following equation: Where: S = suction specific speed n = rotative speed, in revolutions per minute Q = flow rate per impeller eye measured in U.S. gallons per minute (cubic meters per second) = total flow rate for single suction impellers = one half of total flow rate for double suction impellers NPSH3 = net positive suction head required in feet (meters) that will cause the total head (or first-stage head of multistage pumps) to be reduced by 3 percent
 When suction specific speed is calculated using cubic meters per second and meters, the conversion factor to suction specific speed in U.S. gallons per minute and feet is 51.6. The U.S. customary symbol N_ss is sometimes used to designate suction specific speed. For more information about specific speed and suction specific speed, see ANSI/HI 1.1-1.2 Rotodynamic (Centrifugal) Pumps for Nomenclature and Definitions. Q. How do mechanically coupled and hydraulic coupled disc diaphragm pumps differ? A. A mechanically coupled disc diaphragm liquid end contains a flexible, round diaphragm, which is clamped at the periphery and in direct contact with the process liquid being displaced (see Figure 1). This type of design is inherently leak free. The diaphragm material is typically a fluoropolymer, elastomer or fluoropolymer-elastomer composite. A connecting rod is connected directly to the diaphragm. The diaphragm is not pressure balanced because the process pressure is acting on one side of the diaphragm and atmospheric pressure is acting on the other side. This results in higher stress levels in the diaphragm. Therefore, these pumps are typically used for lower pressure applications. In operation, the process liquid is admitted through the suction check valve as the diaphragm/connecting rod assembly moves away from the wet end. As this happens, the suction check valve closes and the discharge check valve opens, discharging liquid. A hydraulic coupled disc diaphragm liquid end contains a flexible, single or double configuration diaphragm, clamped at the periphery and in direct contact with the process liquid being displaced (see Figure 2). This liquid end design is also inherently leak free. Liquid end designs featuring flexible metallic diaphragms are available. These diaphragms are used in applications where severe operating conditions prohibit the use of fluoropolymer or other elastomers. In operation, the diaphragm is moved by a hydraulic fluid, which is displaced by a reciprocating plunger or piston. The stresses in the diaphragm are minimal because the process pressure acting on one side of the diaphragm is balanced by the hydraulic pressure acting on the opposite side. The process liquid is admitted through the suction check valves as the diaphragm moves rearward. As the diaphragm moves toward the wet end, the suction check valve closes, and the discharge check valve opens and discharges liquid. Liquid end designs may include provisions such as contour plates, springs or diaphragm positioning hydraulic control valves to ensure the diaphragm does not move beyond its elastic limits (see Figure 3). For additional information regarding various controlled-volume metering pumps, see ANSI/HI 7.1-7.5 Controlled-Volume Metering Pumps for Nomenclature, Definitions, Application, and Operation.