Pumpbiz

Frequently Asked Questions
Click a question below to jump to the answer:
1. How do pumps work?

Pumps move fluid in a variety of ways:

The accepted industry standard, as published by the Hydraulic Institute www.pumps.org, defines pumps according to the method energy is imparted to the liquid: kinetic energy pump, or positive displacement (PD) pump.

Kinetic energy type - A centrifugal pump imparts energy to a liquid by means of centrifugal force produced by a rotating impeller, disk or other blade form. Centrifugal pumps are made in many shapes and sizes, and differ from one another both internally and externally to an appreciable degree. In spite of appearance, all centrifugal pumps use the same mechanical principle. Pumping action is obtained from an impeller driven by a shaft or magnetic coupling, which is connected, to a motor or some other driving device. The impeller rotates (cw or ccw direction of rotation) at a high rate of speed (usually 1725 or 3450 rpm), and the liquid being pumped flows from the eye (center) of the impeller to the outside (periphery) of the impeller by centrifugal action. As the liquid flows from the periphery of the impeller, it is guided to the discharge port of the pump by a volute shaped passage. All centrifugal pumps bring liquid in at the center of the impeller, and move it outward between the blades.

Positive Displacement Pumps - Bellows, double-diaphragm, flexible impeller, gear, oscillating, piston, progressing cavity, rotary lobe, rotary vane, and peristaltic pumps have a fixed cavity that the fluid is pushed through by rollers, gears, or impeller. As the fluid is pushed through, it leaves a void or vacuum which pulls in more fluid. Metering Pumps - Bellows, diaphragm, peristaltic, piston, and syringe pumps are all metering pumps that pull the fluid through the inlet valve into a chamber, close the inlet valve, and then push the fluid through the outlet valve.

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2. Are centrifugal pumps variable speed?

Most centrifugal pumps do not have variable speed motors. However, you can control flow rate on the discharge using a valve.

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3. What exactly is a positive displacement pump?

A positive displacement pump emits a given volume of fluid for each revolution of the motor. Bellows, double-diaphragm, flexible impeller, gear, oscillating, piston, progressing cavity, rotary lobe, rotary vane, and peristaltic pumps are all positive displacement pumps.

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4. Which pumps can I run dry?

Peristaltic, piston pumps with ceramic heads, bellows pumps, and diaphragm pumps can be run dry for any length of time. Centrifugal, rotary vane, and gear pumps should not be run dry; exceptions are if the gear or impeller is made of a self-lubricating material such as RYTON in which case the pump can be run for a few minutes while priming.

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5. What is the maximum viscosity rating for pumps?

This depends on the type of pump and the specific pump. Diaphragm pumps (especially double diaphragm pumps) and gear pumps are usually the best for viscous fluids.

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6. What pumps do you carry that will handle particulates?

Diaphragm pumps, bellows pumps and peristaltic pumps will work well. When choosing materials, consider chemical compatibility and resistance to wear. Use a pump with larger fittings so they don't clog as easily.

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7. I need gentle pumping action, what do you recommend?

A peristaltic pump, used at low speeds. You can also use a diaphragm pump, again at low speed. Centrifugal and gear pumps, which work at high speeds and have high shear rates, should be avoided.

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8. When do you need to perform maintenance on pumps?

This depends on the pump and the application. In general, diaphragms on metering pumps last about 6 to 12 months; gears on gear pumps last about 3 to 6 months; and motors usually last for years. DC motors require periodic brush replacement. It is important to monitor brush wear; normally brushes should be replaced every 6 months.

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9. What is different about centrifugal pump vs. gear pump?

A centrifugal pump is of kinetic energy type - it imparts energy to a liquid by means of centrifugal force produced by a rotating impeller, disk or other blade form. A positive displacement pump imparts energy by mechanical displacement. Piston, diaphragm, plunger, screw, vane, and gear pumps are some examples.

Centrifugal pumps are essentially high liquid volume-low pressure. A large amount of liquid can be carried between the blades of the impeller, but as this is not a positive displacement pump, the volume of liquid drops off in proportion to the back pressure (head in feet) applied. A PD pump large enough to match the volume of delivery of a centrifugal pump would have enormous gears or diaphragms, and be impractical. On the other hand, to obtain the pressures of a PD pump, the impeller diameter of a centrifugal pump would have to be increased to an enormous size, and this would also be impractical. However, multistage centrifugal pumps can be used in place of PD pumps in many applications. A multistage pump passes the liquid from one impeller to the next, at each stage it imparts more head (pressure) to the liquid. Therefore creating higher pressures.

Due to the centrifugal pumps design simplicity, high efficiency, wide range of flow and head, smooth flow rate, and ease of operation and maintenance. Positive displacement pumps are of lower flow range, have pulsating flow rate and are usually self-priming.

centrifugal pump   double diaphragm pump   gear pump
centrifugal pump   double diaphragm pump   gear pump
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10. What are the different types of centrifugal pumps?

Centrifugal pumps are made in many shapes and sizes, and differ from one another both internally and externally to an appreciable degree. In spite of appearance, all centrifugal pumps use the same mechanical principle. Pumping action is obtained from an impeller driven by a shaft or magnetic coupling which is connected to a motor or some other driving device. The impeller rotates (cw or ccw direction of rotation) at a high rate of speed (usually 1725 or 3450 rpm), and the liquid being pumped flows from the eye (center) of the impeller to the outside (periphery) of the impeller by centrifugal action. As the liquid flows from the periphery of the impeller, it is guided to the discharge port of the pump by a volute shaped passage. All centrifugal pumps bring liquid in at the center of the impeller, and move it outward between the blades.

They are grouped into several types using different criteria such as its design, construction, application, service, etc. Thus one specific pump can belong to different groups. Some of these groups are:

ANSI pump
single stage
disk or shear
sump
sanitary
back pullout
self-priming
direct-coupled
submersible
multi-stage
vortex
API
end suction
horizontal
mechanical sealed
non-metallic
magnetic drive or magnetic coupled or sealless
jet
well
trash
center line discharge
vertical
close-coupled
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11. What is a self-priming pump?

Self-priming pumps are inherently designed to allow the pump to re-prime itself typically under lift conditions. These pumps are very effective to the end user in that they will eliminate the need for foot valves. To prime a pump, you add liquid into the pump casing or in an accessory priming chamber to displace or evacuate the entrained air by expelling it to the discharge piping and create a liquid seal inside the casing. Straight centrifugal pumps are not able to develop suction and are, therefore, not self-priming. The volute / impeller must be immersed in the liquid for vertical pumps, or have a static positive head (be below the liquid level in the tank) for horizontal pumps.

Self-priming pump is one that develops a vacuum sufficiently enough for atmospheric pressure (14.7 psi at sea level) to force the liquid to flow through the suction pipe into the pump casing without priming the pump. Only positive displacement pumps are truly self-priming but the term has been loosely used to include self-priming centrifugal pumps. Thus it is always important to PRIME a self-priming centrifugal pump before initial operation. The static lift and suction piping should be minimized to keep priming time to a minimum. Excessive priming time can cause liquid in the priming chamber to vaporize before prime is achieved.

A self-priming centrifugal pump is especially designed with a large chamber at its discharge side that acts both as an air separator that separates the air from the liquid, and a reservoir that holds residual liquid used for priming or re-priming the pump. The pump has to be primed during the initial start-up but re-priming is done automatically without outside attention. The suction piping should be designed so that no high points are created where air can be trapped/accumulate, which can prevent priming.

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12. How does viscosity affect a pump?

Viscosity affects centrifugal pumps to a different extent than it does PD pumps. First, viscosity is a value relating the physical property of a fluid resistance to flow. Water having a very low viscosity and molasses has an extremely high value. The some liquids change the viscosity the more they are made to flow and under temperature changes. For the most part a centrifugal pump has a very low limit of how thick or viscid the liquid is it is pumping. On the other hand a PD pump by its naturally low volume operation, slow speed and fixed volume operation, tend to handle high viscosity fluids with ease. PD pumps are the preferred choice. In either case consideration needs to be made for the high power (BPH) required or reduced flow rates.

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13. What is specific speed (ns)?

It is the speed in RPM at which a pump, if sufficiently reduced in size, would deliver 1 GPM at a head of 1 FT. This definition is meaningless and has no practical application. In fact, because its equation has inconsistent units, ns is considered dimensionless. Note: ns is a dimensionless number or index that identifies the geometric similarity of pumps. Pumps of the same NS but of different size are considered to be geometrically similar, one pump being a size-factor of the other. See same in PumpU for technical continuation of this subject.

Specific speed is also used in designing a new pump by size-factoring a smaller pump of the same specific speed. The performance and construction of the smaller pump are used to predict the performance and model the construction of the new pump.

Rule-of-Thumb: For similar pumps with about the same capacity at BEP, the pump with higher specific speed will typically have a higher efficiency also.

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14. What is Best Efficiency Point (BEP)?

Best Efficiency Point (BEP) is the capacity at maximum impeller diameter at which the efficiency is highest. BEP is in that many calculations such as specific speed suction specific speed, hydrodynamic size, viscosity correction, headrise to shut-off, etc. are based on capacity at BEP. Many users prefer that pumps operate within 80% to 110% of BEP for optimum performance. You will see most pumps have their BEP close to the mid part of their performance curve.

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15. What are the Affinity Laws?

The Affinity Laws are mathematical expressions that define changes in pump capacity, head, and power required (BHP) when a change is made to pump speed, impeller diameter, or both. According to Affinity Laws:

Capacity Q changes in direct proportion to impeller diameter D ratio, or to speed N ratio:
   Q2 = Q1 x [D2/D1]
   Q2 = Q1 x [N2/N1]

Head H changes in direct proportion to the square of impeller diameter D ratio, or the square of speed N ratio:
   H2 = H1 x [D2/D1]^2
   H2 = H1 x [N2/N1]^2

HP changes in direct proportion to the cube of impeller diameter ratio, or the cube of speed ratio:
   HP2 = HP1 x [D2/D1]^3
   HP2 = HP1 x [N2/N1]^3
where the subscript: 1 refers to initial condition, 2 refers to new condition

If changes are made to both impeller diameter and pump speed the equations can be combined to:
   Q2 = Q1 x [(D2xN2)/(D1xN1)]
   H2 = H1 x [(D2xN2)/(D1xN1)]^2
   HP2 = HP1 x [(D2xN2)/(D1xN1)]^3

This equation is used to hand-calculate the impeller trim diameter from a given pump performance curve at a bigger diameter:
   H2 = H1 x [Q2/Q1]^2

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16. How does a magnetic coupled pump work?

Instead of a direct shaft (rigid or coupled) from the driver to the impeller, a virtual coupling created by the magnetic fields transmit the required torque. In this manner there is no DIRECT shaft or mechanical connection between the driver (motor) and the driven (impeller) elements.

graph

In order to contain the liquid in the pump and to eliminate any type of mechanical seal, therefore called seal-less, a barrier or cup is placed between the two rotating magnetic assemblies.

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17. What are explosion proof motors?

Under certain operating conditions, i.e. pumping a hazardous liquid or operating a pump in a hazardous environment, there exists the potential of a spark or heat to ignite the fluid / vapors. Therefore it is critical to safety that you bond and ground all pumps and containers in such a application to dissipate any static electricity and that you use a approved EXP motor, fittings and wiring procedure. The EXP motors themselves simply put are designed to contain any explosion within them and dissipate the flame before it exists the motor enclosure. They are also designed to operate with in a very defined surface temperature rating. For complete and current details check with your safety officer, Under Writers Laboratories, or the NFPA / NEC codes. Only a device carrying the UL EXP rating should be used. A typical totally enclosed motor (designated TENV, TE or TEFC) do NOT meet EXP classifications.

UL and CSA Hazardous Locations (EXP) – sample listing.

Class I Group D (C I Gp D) locations are atmospheres containing elements such as Gasoline, Hexane, Naphtha, Benzene, Butane, Propane, Alcohol, Acetone, Benzol, Lacquer Solvent Vapors or Natural Gas.

Class I Group C (C I Gp C) locations are atmospheres containing elements such as Ethyl-ether, Ethylene and Cyclopropane.

Class II Group F & G (C II Gp F/G) locations are atmospheres containing dust such as (F) Carbon Black, Coal or Coke Dust, (G) Flour, Starch or Grain Dusts.

CAUTION
Motors misapplied in hazardous environments can cause a fire or explosion resulting in destruction of property, serious injury or death. Only the end user or a qualified underwriter is to identify and select the proper class, group, division, and temperature code motor to meet the requirements of each installation. PumpBiz, Inc. personnel and or the pumpbiz.com web site can advise what listing the motors used with the pumps carry, but cannot evaluate nor recommend what motors may be suitable for use in hazardous environments.

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