Avoid a Bad Turn

Centrifugal pump selection demands attention to details.

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A surprise awaited me in Korea. Management installed magnetic drive pumps. It was a poor choice and made the start-up a nightmare.

Choosing a centrifugal pump is complicated. After sizing the pump you face challenges in selecting the pump, impeller, seals, protection plan and motor. And you must finish the piping details, e.g., the check valve.

Centrifugal pumps come in two basic types: axial and radial. In axial designs, flow goes through middle of an impeller centered in a pipe. In radial designs, liquid approaches the impeller at a tangent. Most pumps are radial. Axial pumps provide higher efficiency and lower required net-positive suction head (NPSHR). Radial pumps offer higher head. But the choice is more complicated. To achieve high head there're multiple-stage pumps: liquid is pumped from chamber to chamber at higher and higher pressures. To reduce thrust load on bearings from flow on one side, sometimes suction flows to both sides, 180° apart. This is called double suction; doubling the suction means doubling the seals. A pump also can be close-coupled, i.e., on the same shaft as its motor. Magnetic drive, "seal-less" and canned pumps fall into this category. In a magnetic drive pump the shaft is merely a guide; magnets turn the impeller in the same way a motor is driven by a magnetic field. In a canned pump the motor is cooled in the pumped fluid. Although the least expensive pump would be a radial close-coupled single-stage pump, process requirements may point to another choice.

Three classes of impeller exist: open, semi-open and closed. Open is an impeller with vanes (blades) and no shroud; with semi-open, vanes are attached to a baseboard; closed impellers contain vanes between a front and back plate. Open impellers create the highest shear but can manage clogging materials best. Closed impellers suit clean fluids. Most impellers are semi-open. Open impellers require adjustment of impeller-volute clearance for wear and thermal expansion but they are cheaper and lighter (less shaft deflection) with a higher efficiency and easily can be cut for adjustments; however, erosion on an exposed vane is worst. Closed impellers require wear rings to divert flow to the vanes, preventing bypassing behind the impeller. Closed impellers can't be trimmed economically and efficiency decreases as rings wear. Semi-open impellers offer characteristics in-between the other classes.

Modern pumps must meet a tight standard.



When it comes to seals, modern pumps must meet a tight standard: the American Petroleum Institute (API) seal plans. Hazardous chemicals require double or tandem seals. The simplest seal plan is API 54 using a central seal solution and a double mechanical seal. API-52, 53A, 53B, 53C involve a barrier fluid, like glycerin or water; water is best because of its high heat capacity and conductance.

In a typical plant, the seal plan must be approved through the Management of Change (MOC) process. The MOC committee also will need to okay the safety integrity level (SIL) protecting the pump and pump seal. (For more on SIL determination, see "Do You Really Need SIL 3?" www.ChemicalProcessing.com/articles/2010/015.html.) At a minimum, you can expect to require a rotameter for monitoring flow and pressure switches on the seal water and pump discharge. With proper design, you can trim instrument costs.

With pump details refined, you now can select the electric motor. Once the power draw (P) is defined, the next step is to select a motor safety factor. This isn't the same thing as the insulation safety factor you see on induction motor nameplates, e.g., "SF = 1.15." The API created a reliable safety standard for motors: P ≤ 25 hp, use 1.25; P > 30 hp, < 70 hp, use 1.15; and P > 100 hp, use 1.10. So, if a pump required 18.4 hp, the motor should be 18.4 ×1.25 = 23 hp (or 25 hp, the next available size). As a standard, motors are totally enclosed, fan (air) cooled (TEFC) induction motors up to 250 hp. Beyond that point, they are water cooled or TEWC.


Ordering is the next step. Use a one-page data sheet. Once you've selected a pump, establish a delivery schedule with checks along the way for: seals, pump internals, motor and documentation. You need one spare per pump for every part replaced annually. If practical, establish expectations for baseline data on vibration, clearances, motor performance, etc.

Prior to shipping the motor, check its insulation resistance, i.e., "meg" the motor insulation; 20 mega-ohm is the minimum acceptable value for a new 460-VAC motor ― 50 mega-ohm is better. Typically, testing is with 500V DC, corrected to 40°C, measured lead to frame or ground. Megging establishes the baseline. Most plants throw out a motor that megs below 2 mega-ohms.

With the details done, it's now time to look ahead to installation and commissioning.


Dirk Willard is a Chemical Processingcontributing editor. You can e-mail him at dwillard@putman.net.

 

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