Refineries and chemical plants use pumps to move liquids. Pumps are used in a variety of applications and processes, including refrigeration, automobiles, home heating systems, and water wells. The liquids moved by a pump vary from liquid sodium and liquid potassium for cooling nuclear reactors to domestic drinking water systems.
Cavitation—the formation and collapse of gas pockets around the impellers during pump operation; results from insufficient suction head (or height) at the inlet to the pump.
Discharge head—the resistance or pressure on the outlet side of a pump.
Dynamic—class of equipment such as pumps and compressors that convert kinetic energy to pressure; can be axial or centrifugal.
Head—is described as Pressure (at suction) 3 2.31 4 Specific gravity; 1 psi is equal to 2.31 feet of head.
Impeller—a device attached to the shaft of a centrifugal pump that imparts velocity and pressure to a liquid.
Mechanical seal—provides a leak-tight seal on a pump; consists of one stationary sealing element, usually made of carbon, and one that rotates with the shaft.
Net positive suction head (NPSH)—the head (pressure) in feet of liquid necessary to push the required amount of liquid into the impeller of a dynamic pump without causing cavitation.
Net positive suction head available (NPSHa)— a measure of how close the fluid at a given point is to flashing, and so to cavitation.
Net positive suction head required (NPSHr)— the head value at a specific point (e.g. the inlet of a pump) required to keep the fluid from cavitating.
Positive displacement—class of equipment such as pumps and compressors that move specific amounts of fluid from one place to another; can be rotary or reciprocating.
In general, pumps can be classified as dynamic or positive displacement. Both classes are designed to transfer liquids, but the way the transfer is accomplished is different. Dynamic pumps accelerate liquids axially (in a straight line) or centrifugally (in circles). They are operated at high speeds to generate large flow rates at low discharge pressures. Pressure moves the liquid through the piping and equipment system.
Positive displacement (PD) pumps transfer liquids by using a rotary or reciprocating motion that displaces liquid on each rotation or stroke. They are used in processes that require specific amounts of fluid to be delivered. The operation of positive displacement pumps is significantly different from that of dynamic pumps. Positive displacement pumps transfer specific amounts of fluid no matter what the discharge pressure is, whereas the amount of fluid transferred by dynamic pumps is greatly affected by discharge pressure.
1. Centrifugal Pumps
Centrifugal pumps are used widely in chemical processing plants and refineries. The primary principle used by centrifugal pumps is centrifugal force. As liquid enters the suction eye of a centrifugal pump, it encounters the spinning impeller. The liquid is propelled in a circular rotation that forces it outward and into the volute. Centrifugal force and volute design convert velocity energy to pressure. As the liquid leaves the volute, it slows down, building pressure. Diffuser plates also can be added to the impeller and volute area to slow down or change the velocity of the liquid.
2. Axial Pumps
Another way to accelerate and transfer fluid is to push it axially, or in a straight line. Axial pumps are designed to provide this special feature. A common example of this principle is a boat motor. The motor turns a set of blades, forcing water to accelerate along a straight line. An axial pump operates using this same principle. Most axial pumps are located in an elbow on a piping run. The driveshaft extends through the elbow and into the process flow. A propeller is located on the process end of the driveshaft. The propeller is sized to fit the inside diameter of the pipe. The blading is engineered to pull fluid axially down the shaft. A mechanical seal prevents leakage where the shaft penetrates the pipe elbow. A specially designed thrust bearing prevents axial movement of the shaft. Heavy duty radial bearings support the pump shaft and prevent radial movement. Some axial pumps have thrust-bearing oil coolers. An optional safety seal oil system or thrust-bearing lube system is available for some models. The motor is mounted just outside the elbow on a pad that allows for exact driveshaft lineup. A coupling securely connects the motor to the pump. Axial pumps can be mounted vertically or horizontally. Axial pumps are frequently found in pipeline service and as the primary transfer device on loop reactors.
3. Screw Rotary Pumps
A progressive cavity, or PC, pump consists of only one moving part, the rotor. The rotor turns inside an elastomer-lined stator. When the self-priming rotor turns, cavities, or voids, are formed between the rotor and the stator. These voids progress axially from the suction casing to the discharge outlet. During operation, the cavities fill with fluid. Progressive cavity pumps provide high suction, extremely low shear, and smooth pulsation-free operation. These features are important where turbulence affects fluid composition. The PC pump is ideally suited for metering operations. Typically, this type of pump is used for heavy or viscous fluid service. The solids content of the process fluid does not affect the effectiveness of a PC pump. Progressive cavity pumps can be found in a variety of applications.
4. External Gear Pumps
External gear pumps have two interesting gears that rotate parallel to each other, allowing fluid to be picked up by the gears and transferred out of the pump. One of the gears is an idler gear; the other is attached to a driver and is referred to as the power gear. External gear pumps consist of two mating gears that rotate inside a casing. The rotation of the driver gear turns the idler, or follower gear, rapping fluid and displacing it. Typically, in this type of operation, the driver gear is mounted on top. The discharge of an external gear pump remains constant unless the shaft speed is changed. The suction and discharge ports of an external gear pump are located on the opposite ends of the casing. When the pump is first started, air is forced out and into the discharge line. This process creates low-level vacuum on the suction side. This vacuum causes fluid to enter the pump and be trapped between the gears. As the gears rotate, the fluid is swept around the housing and out of the discharge port.
5. Internal Gear Pumps
Internal gear pumps operate with only two moving parts: a power gear driving an internal idler gear. When the power gear rotates, liquid enters the pump through the suction line. Since the pump is self-priming, the voids between the teeth of the power gear and the off-center idler gear fill with liquid. During rotation, liquid is separated by a crescent-shaped spacer. Liquid is pressed into the spaces above and below the crescent. As the gears rotate around the circular pump casing, the liquid is discharged out of the pump. The main components of an internal gear pump are a power gear or rotor; an idler gear; an idler pin; a driveshaft; a circular casing; the crescent, axial, and radial bearings; seals; and a relief valve. The idler gear rotates freely on a cylindrical idler pin. A main bearing and a second bearing on the free end of the shaft support the driveshaft. Soft packing or mechanical seals can be used on the pump. Internal gear pumps require very little maintenance. These pumps can be constructed of stainless steel for corrosive environments or of carbon steel or cast iron where applicable. The chemical processing industry uses internal gear pumps for chemicals such as acetone, acids, alcohol, alkalis, ammonium hydroxide, butadiene, polymers, resins, solvents, waxes, and xylenes. Internal gear pumps can also be magnetically coupled; magnetic coupling eliminates the need for shaft seals.
6. Sliding Vane Pumps
Sliding vane pumps consist of spring-loaded or nonspring-loaded vanes attached to a rotor, or impeller, that rotates inside an oversized circular casing. As the offset impeller rotates by the inlet port, liquid is swept into the vane slots. A small crescent-shaped cavity is formed inside the pumping chamber that the vanes extend into. As the liquid nears the discharge port, it is compressed as the clearances narrow. The compressed liquid is released at the discharge port. The vanes on the pump are made of a softer material than the rotor and casing. A bevel on each vane closely matches the rounded edges of the chamber, and the softer vane material tends to wear evenly during the life of the vane. Sliding vane pumps typically are used with process liquids that have good lubricating qualities. Vane pumps are used in hydraulic systems, vacuum systems, and low pressure oil systems.
7. Flexible Vane Pumps
In a flexible pump system the rotor is composed of a soft elastomer impeller, keyed to fit over the driveshaft that penetrates the pumping chamber. The pumping chamber is designed to provide good contact between the impeller and the inner chamber. Speeds are typically low since the rubbing velocity between the flexible vanes and chamber wall is significant. The impeller is centered in the pumping chamber. Flexible vane pumps are frequently used in vacuum service.
8. Lobe Pumps
Lobe pumps have two rotating lobe-shaped screws that mesh during operation. As the lobes turn, voids are created that compress liquids around the outside of the pumping chamber. In a lobe pump, a set of external timing gears and bearings allows the lobes to turn in unison without making contact with each other. This feature allows the pump to transfer a wide variety of fluids. Because the lobes do not touch, the pump can run empty without damaging the system. A lobe pump has a driving and a driven shaft, two lobes, an external set of timing gears, and bearings. As fluid enters the pump, it is divided into two equal streams. The pumping action of the lobes moves the process fluid in two streams around the lobes in the close space between the casing and the lobes. The streams combine at the discharge port. The direction of rotation of the driver will determine the locations of the inlet and outlet ports on the pump. Lobe pumps are designed to provide high flow rates at low pressures; they have excellent suction and pump a variety of fluids.
9. Piston Pumps
Reciprocating piston pumps use a piston and a back-and forth motion to displace fluid. During normal operation, the piston pump has a suction stroke and a discharge stroke. The suction stroke occurs when the piston pulls out of the cylinder. This motion creates a low pressure vacuum in the cylinder, causing the discharge valve to close and the suction line to open, filling the cylinder. On the return stroke or discharge stroke, the suction valve slams shut, and the fluid is forced out the discharge valve. This type of pump continues to operate no matter how high the discharge head is. The typical piston pump has a piston, piston ring, connecting rod, suction and discharge valves, casing and cylinder, and relief valve. Piston pumps are sealed internally and externally. Internally, the piston rings form a seal on the piston that can be seen only by tearing down the pump. The external packing seal is located where the piston rod enters the casing.
10. Plunger Pumps
Reciprocating plunger pumps operate with a back-and-forth motion and a device called a plunger to displace controlled amounts of liquid. The primary difference between a plunger pump and a piston pump is in the shape of the piston or plunger element and the way they seal. A piston pump has rings mounted on the piston that form a seal. The plunger on a plunger pump does not have moving rings. The plunger moves in and out of an O-ring or packing medium to form its own stationary seal. A major advantage of this type of sealing system is that the pump seals easily and can be replaced without major breakdown of the equipment. The basic components of a plunger pump are a plunger, crankshaft, connecting rod, pumping chamber, suction inlet valve, and discharge outlet valve.
11. Diaphragm Pumps
A diaphragm pump uses a flexible sheet (diaphragm) to displace fluid. This type of pump has a crankshaft or eccentric wheel attached to a connecting rod. The connecting rod is anchored firmly to the center of the diaphragm. The outer edge of the diaphragm is bolted or secured to the exterior casing. As the eccentric wheel (referred to simply as the eccentric) starts its rotation, the diaphragm connecting rod goes up and down. This reciprocating motion creates a pumping action that displaces fluid. The pumping chamber below the diaphragm is connected to suction and discharge lines. Spring-loaded valves open or close, depending on the pressure in the chamber. Diaphragm pumps have several advantages compared with most other types of pumps. They completely seal off the area between the diaphragm and the pumping cavity; they can be used to pump a variety of chemicals; and they can be used with low or negative suction head.
The energy conservation equation for pump or hydraulic turbine systems comes from Bernoulli’s Theorem and relates the total head in two points of the system, the friction losses between these points and the equipment total head. Elevations are measured from the equipment datum.
The total head at any system point is:
Centrifugal pumps are designed to work in specific services at specific rates. The best operating condition for a pump usually is indicated on the pump’s efficiency curve. The efficiency curve includes several values: flow rate (in GPM), total head in feet (discharge head minus suction head; sometimes called differential head), pump efficiency, required pump horsepower, and pump NPSH. Because pumps are not always operating under their optimal conditions, they are designed to work across a range of rates and liquid properties. The operating parameters (driver horsepower, impeller size, liquid properties, pump efficiency, pump capacity and head, NPSH requirements, and so on) are all shown on a graph known as the pump curve.
When there is a problem with a pump’s performance or a change in the service, the pump curve is one of the first documents consulted. Manufacturers typically include pump curves with their products so that engineers and operators can refer to them before changing or redesigning a pump system. If the pump is operated at higher rates, efficiency decreases and the pump could be damaged.
The relationships between rotational speeds, impeller diameter, capacity, head, power, and NPSHR for any particular pump are defined by the affinity laws. These equations are to predict new curves for changes in impeller diameter and speed. The capacity of a centrifugal pump is directly proportional to its speed of rotation and its impeller diameter. The total pump head developed is proportional to the square of its speed and its impeller diameter. The power consumed is proportional to the cube of its speed and its impeller diameter. The NPSHR is proportional to the square of its speed. These equations apply in any consistent set of units but only apply exactly if there is no change of efficiency when the rotational speed is changed. This is usually a good approximation if the change in rotational speed is small.
A different impeller may be installed or the existing modified. The modified impeller may not be geometrically similar to the original. An approximation may be found if it is assumed that the change in diameter changes the discharge peripheral velocity without affecting the efficiency. Therefore, at equal efficiencies and rotational speed, for small variations in impeller diameter, changes may be calculated using the affinity laws. These equations do not apply to geometrically similar but different size pumps. In that case dimensional analysis should be applied.
The affinity equations apply to pumps with radial flow impellers, that is, in the centrifugal range of specific speeds, below 4200. For axial or mixed flow pumps, consult the manufacturer.
Often pumps are installed in series or in parallel with other pumps. In parallel, the capacities at any given head are added; in series, the heads at any given capacity are added. A multistage pump is in effect a series of single stage units. Below figures show series and parallel pumps curves, a system curve, and the effect of operating one, two or three pumps in a system. In both figures, the operating points for both pumps "A" and "B" are the same only when one pump is operating.
For 2 or 3 pumps operating, the points are not the same because of the pump curve shapes. Hence, due consideration should be given to the pump curve shape when selecting pumps for series or parallel operation. Parallel operation is most effective with identical pumps; however, they do not have to be identical, nor have the same shut-off head or capacity to be paralleled. When pumps are operating
in parallel it is imperative that their performance curves rise steadily to shut-off. A drooping curve gives two possible points of operation, and the pump load may oscillate between the two causing surging.
Unlike a positive displacement pump, a centrifugal pump can, and should, be controlled by throttling the discharge to obtain the desired operating point. The operating point must always fall on the pump curve to prevent damage to the pump, such as that resulting from cavitation. However, operating too far to the left or right of the performance curve is less than optimal for a centrifugal pump as it can cause shaft deflection, bearing and seal wear, and if too far to the right, cavitation. When a centrifugal pump is operated very close to dead-head (also called “back” or “to the left” of the curve), the discharge flow is throttled and reduced. With nowhere else to go, this causes most of the fluid inside the pump to recirculate. This can cause noise and erosion at the eye of the impeller and can also raise the temperature of the fluid.
Cavitation occurs when suction pressure drops below NPSH. At this point, the liquid vaporizes and forms gas pockets inside the pump casing. As these pockets form and collapse, the pump can be severely damaged. The sound of a cavitating pump closely resembles the noise you would hear if steel ball bearings were dumped into a pump’s suction line. Cavitation sends slugs of liquid and vapor through the pump. Each slug has an impact on the internal components of the pump. Serious damage will occur if this problem is not resolved quickly.
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