Introduction to Centrifugal Pump Design (Part-2)
3. Pump Selection
Based on mechanical data sheet, vendor will submit performance curve for their proposed pump. Every pump has a specific curve relates to head, flow, power, NPSHR and efficiency. Within the bidding stage, engineer should be able to select a pump with a rated point as close as possible to the best efficiency point (BEP), as specified in performance curve.
Mechanical data sheet also stated material of construction and kinds of inspection & testing. Selection of material depends on fluid properties and chemical composition; it shall consider such as corrosiveness and presence of dissolved gases. Engineer shall confirm the material requirement and the type of testing that shall be done. Client usually request witness test for performance test and mechanical running test at manufacturer.
The following is general guidelines for proper selection of centrifugal pump:
- Select the pump based on rated conditions as per data sheet.
- Select the pump with suction specific speed less than 11000 (9500 is better). Except for high speed pump (type OH6) may be up to 24000
- The BEP should be between the rated point and the normal operating point.
- Please remember that hydraulic efficiency is high at specific speed 2000 and 3000 and drop below 500. Higher efficiency means less vibration and noise.
- Do not select pump with maximum diameter impeller. The pump should be capable of a head increase at rated conditions by installing a larger impeller The head/capacity characteristic-curve should continuously rise to shutoff.
- Minimum continuous flow shall be based on hydraulic stability not temperature rise. Furthermore, the pump should not be operated below minimum continuous flow rate.
- Select driver at the highest power required with margin (example 110% of maximum power required). It may also need to select driver that can operate to the end of curve in case specific requirement is defined, i.e., auto-start requirement.
- For safe operation, NPSHA shall exceed NPSHR more than 1 m at the rated condition. As the NPSHR vary depending on the head and flow, it is safer to select the margin at the end of curve.
Once the specific pump has been chosen, then detail design and vendor document review shall be done. This engineering phase is more detail and consists of some major engineering document approval to start the fabrication. The major documents for approval are usually general arrangement drawing, driver drawing, pump data sheet pump, performance curve and mechanical seal drawing. Inspection and Test Plan/Procedure shall be approved to ensure the quality of equipment.
Figure 2 Typical Pump Curve
4. Selection of Driver and Seal
The required driver power for the pump is depend on head, flow and specific gravity. The selection of driver depends upon many factors such as type of service, availability of steam or fuel, and also cost. The most common driver is the electric motor due to motors can vary in size depending upon power, speed, frame size, area classification, orientation and other considerations. A motor should be sized to meet the maximum specified operating conditions and shall be able to meet end-of-curve power requirements if specifically required (i.e., auto-start is required).
It is important to select the right mechanical seal or packing for rotating machinery. Packing is used primarily in non-hazardous services, where leaks of fluid are not a safety concern. The wrong selection of mechanical seal and their plan type could lead pump failure.
The first step in seal selection is to identify the fluid to be handled. The metal parts must be corrosion resistant, usually steel, bronze, stainless steel, or Hastelloy. The mating faces must also resist corrosion and wear. Carbon, ceramic, silicon carbide or tungsten carbide may be considered. Stationary sealing members of Buna, EPR, Vinton and Teflon are common. Abrasive liquids create excessive wear and short seal life. Double seals or clear liquid flushing from an external source allow the use of mechanical seals on these difficult liquids. On light hydrocarbons balanced seals are often used for longer seal life even though pressures are low. The seal type and arrangement selected must meet the desired reliability and emission standards for the pump application. Double seals and double gas barrier seals are becoming the seals of choice.
Finally, the selection of right mechanical seal and their plan type depends upon the type of pump, its operating conditions, the type of fluid (hydrocarbon, non-hydrocarbon, flashing hydrocarbon), the temperature, whether it is hazardous or non-hazardous and other specific requirement, if any. It is also important to consult to vendor seal for the correct seal selection, flush plan and fluid for a certain service.
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