What are some differences between specifi c speed and suction specifi c speed for a rotodynamic pump? h e Hydraulic Institute dei nes specii c speed as an index of pump performance (developed total head). It is determined at the pump’s best ei ciency point (BEP) rate of l ow, with the maximum diameter impeller and at a given rotative speed. Specii c speed is expressed by the following equation:
ns = n(Q)0.5 (H)0.75
Where:
ns = specii c speed
n = rotative speed measured in revolutions per minute
Q = total pump l ow 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 specii c speed using units of cubic meters per second for l ow rate and meters for head per stage, 51.6 is the conversion factor for specii c speed in U.S. gallons per minute and feet (metric × 51.6 = U.S. customary units).
h e usual symbol for specii c speed in U.S. customary units is Ns.
Suction specii c speed is an index of pump
suction operating characteristics. It is determined at the BEP rate of l ow with the maximum diameter impeller. (Suction specii c 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 specii c speed is expressed by the following equation:
S = n(Q)0.5 (NPSH3)0.75
Where:
S = suction specii c speed n = rotative speed, in revolutions per minute
Q = l ow rate per impeller eye measured in U.S. gallons per minute (cubic meters per second)
= total l ow rate for single suction impellers
= one half of total l ow rate for double suction impellers
NPSH3 = net positive suction head required in feet (meters) that will cause the total head (or i rst-stage head of multistage pumps) to be reduced by 3 percent
When suction specii c speed is calculated using cubic meters per second and meters, the conversion factor to suction specii c speed in U.S. gallons per minute and feet is 51.6. h e U.S. customary symbol Nss is
sometimes used to designate suction specii c speed.
For more information about specii c speed and suction specii c speed, see ANSI/HI 1.1-1.2 Rotodynamic (Centrifugal) Pumps for Nomenclature and Dei nitions.
Understand Specifi c Speed & Disc Diaphragm
Pump Coupling
By Hydraulic Institute
pu mp-z o ne .c o m | Au g us t 2 0 1 4
103
How do mechanically coupled and hydraulic coupled disc diaphragm pumps differ?
A mechanically coupled disc diaphragm liquid end contains a l exible, round diaphragm, which is clamped at the periphery and in direct contact with the process liquid being displaced (see Figure 1, page 102). h is type of design is inherently leak free.
h e diaphragm material is typically a l uoropolymer, elastomer or l uoropolymer-elastomer composite.
A connecting rod is connected directly to the diaphragm. h e 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. h is results in higher stress levels in the diaphragm. h erefore, 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 l exible, single or double coni guration diaphragm, clamped at the periphery and in direct contact with the process liquid being displaced (see Figure 2). h is liquid end design is also inherently leak free. Liquid end designs featuring l exible metallic diaphragms are available. h ese diaphragms are used in applications where severe operating conditions prohibit the use of l uoropolymer or other elastomers.
In operation, the diaphragm is moved by a hydraulic l uid, which is displaced by a reciprocating plunger or piston. h e 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. h e 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, Dei nitions, Application, and Operation.
HI Pump FAQs® is produced by the Hydraulic Institute as a service to pump users, contractors, distributors, reps and OEMs. For more information, visit pumps.org. Figure 2. Hydraulic disc with contour plates
S
ulzer has long been a leading service provider for rotating equipment due to a continued commitment to deliver advanced, customized service solutions. In an effort to expand on that commitment and provide customers with a full portfolio of services, Sulzer has integrated Sulzer Pumps CSS, Sulzer EMS, and Sulzer Turbo Services into a single Rotating Equipment Services division. The new division combines services for pumps, motors, generators, turbines, and compressors, and expands Sulzer’s geographical footprint to over 100 service centers worldwide.As a third-party service provider, Sulzer aims to offer customers an alternative approach to maintenance, repair, overhauls and upgrades. By leveraging combined capabilities and integrated service centers, Sulzer is able to provide customers with a single access point for service, eliminating the need to source separate vendors for varying types and brands of rotating equipment.
Pumps
No matter how simple or complex the machine, Sulzer is dedicated to improving customers’ pumping systems and operational reliability. At our expansive network of service centers, an expert team of engineers and repair staff respond to industry needs and focus on ensuring the highest performance for your equipment. Pumps services
LQFOXGH HQJLQHHUHG UHSDLUV UHWUR¿WV DQG ¿HOG VHUYLFHV Sulzer’s Pump Equipment factories are also available to provide support with spare parts programs, performance and system analysis, and operation and maintenance training.
Electromechanical Equipment
:LWK¿YHHOHFWURPHFKDQLFDOVKRSVLQ1RUWK$PHULFD6XO]HU specializes in a full range of electrical and mechanical