Fluids are moved by pumps, fans, blowers, and compressors. These use work to increase the mechanical energy of a fluid, which in turn can increase the flow rate (velocity), pressure, or elevation of the fluid. Definitions overlap, but broad categories can be defined -- the characterization is based on the phase of the fluid, the flow capacity, and the required pressure change (head).
Liquids are typically moved by pumps. Gases are moved by fans (large volume, small pressure difference), blowers (large volume, moderate pressure difference), or compressors (large pressure differences). Specialized equipment is also used to produce vacuums in process systems.
There are two main categories of pumps -- positive displacement and centrifugal. The choice is based on the liquid to be pumped and the desired head and capacity.
Centrifugal pumps are probably most common in industrial applications. They may be built in a very large number of materials. Capacity ranges up to 6000 gpm are common, as are heads to 600 feet, all without special drivers. Performance drops off significantly when handling viscous fluids or when air or vapor are present in the liquid.
For a given head and capacity, centrifugal pumps tend to be smaller and lighter than other types, hence costs are lower.
Positive displacement pumps operate by trapping a fixed volume of liquid then releasing it to a higher pressure by means of a piston or rotary gear.
Reciprocating pumps use a piston, plunger, or diaphragm to raise the pressure of a liquid. The pumping chambers are surrounded by one-way valves so that liquid can only move in from the low pressure side and out from the high pressure side. They are classed as "single acting" if fluid is moved only on the downstroke, or "double acting" if fluid is moved by both sides of the piston.
Because of the mechanism, these pumps produce a pulsating flow; but since flow is independent of head, they can be used to produce large pressure changes.
Reciprocating pumps are no longer common in most industrial installations. They are best for low volume, high head applications (up to 50000 psi). They cannot be used when pulsating flow is a problem.
Diaphragm pumps are a sub-class of reciprocating pump. The pumping chamber is separated from the moving parts by a flexible diaphragm. Their chief advantage is that the fluid being pumped never comes in contact with the mechanism and eliminates leakage; thus they are good for toxic or very expensive liquids. They cannot produce large head differences.
Rotary pumps use a gear, lobe, screw, cam, or vane to compress liquid. Liquid enters through a gap between the rotating element and pump wall at a low pressure where it is trapped. Then, as the element rotates, it squeezes the liquid out through a one-way valve on the opposite side of the casing.
Typically, rotary pumps are used in high head, low flow applications. They are good for high viscosity and low vapor pressure fluids. The fluid pumped must be "lubricating"; solids cannot be present. A key difference from centrifugal pumps is that discharge pressure variation has little effect on capacity.
Rotary pumps are common in laboratory settings because they have constant displacement at a set speed, and so can be used as metering pumps. Rotary pumps are also extremely common in fluid-power applications.
The two key quantities in every pump design are the capacity (flow rate, typically in gpm) and total head developed by the pump. This information must be provided by the process engineer.
"Head" is just a way of expressing pressure -- specifically, in terms of the height of a column of liquid that would produce the same pressure (want to review the concepts from ChE 307?). Pressures measured in "mmHg" should be familiar to anyone who has had a chemistry course, but there is no reason why mercury should be the only fluid used. In practice, pressure measurements in inches or feet of water are also very common. Many (most?) pump manufacturers use head units in their information, so make sure you know how to use them.
Consider a mechanical energy balance written on a pump, standing alone. The only frictional losses are those within the pump itself. These can be accounted for using an efficiency term applied to the work.
All pumps must have a driver to supply power. Typically, drivers are electric motors or steam turbines; gas engines may be used in remote locations. As a general rule, motors are single speed devices -- variable speed motor drivers are expensive -- while turbines can be operated at variable speed by the addition of a governor.
The power requirement of a pump depends on the total head developed and the mas to be pumped per unit time. It is calculated by multiplying the shaft work term, Ws, by the mass flowrate.
When considering power requirements, be sure you know what you are trying to find. Do you want the power delivered to the fluid (Pf), sometimes called the "work", "water", or "liquid" horsepower? If so, the pump efficiency isn't needed. Do you want the power supplied to the pump (PB), usually called the "brake horsepower" (bhp)? This will be a smaller number since it accounts for leakage and friction losses. To get it, just include the efficiency (defined as LHP/BHP). If you want the power supplied to the driver, you need to include the driver and coupling efficiencies as well.
Pump efficiencies typically range between about 65 and 80%. Driver efficiencies are higher, at 80 to 90%.
As liquid moves into a pump, there is a pressure drop due to the effects of the entrance, friction in the suction piping, etc. Although the pressure is soon increased, if the pressure drops below the vapor pressure of the fluid being moved, the liquid may vaporize. The bubbles that form cause a volume increase and "choke" the pump. Then, as the pressure is increased by the pumping action, the bubbles implode, creating shockwaves that can pit and erode the equipment. This phenomena is called cavitation and can severely damage the pump. Cavitation also causes serious noise and vibration problems.
To prevent cavitation, it is important that the pressure within the pump suction be compared to the vapor pressure of the liquid. The difference between the total suction head at the suction flange and the vapor pressure of the liquid is called the Net Positive Suction Head, or NPSH. NPSH is typically stated in feet of liquid.
As a process engineer, then, one must determine the "available NPSH" and compare it to the "required NPSH" of the pump. If available NPSH is greater than or equal to required NPSH, the pump should not cavitate. Required NPSH is calculated by the pump manufacturer and is often plotted on the performance curve. McCabe, Smith, and Harriott (p. 191) report that required NPSH ranges from about 5 ft for small centrifugal pumps to 50 ft for very large pumps. For large, high energy pumps (i.e. boiler feed pumps), Welch (p. 849) recommends NPSHA be 1.5 to 2.0 times the NPSHR.
You should always calculate the available NPSH if:
In most cases, the calculation isn't too complicated; however, special caution must be taken in cases where the suction liquid is saturated with dissolved gases. These will start to come out almost immediately and can cause cavitation problems.
EXAMPLE?
A centrifugal pump increases liquid pressure by increasing its velocity by means of a rotating impeller. Liquid enters at the center of the impeller, is accelerated by the impeller vanes, and leaves through the side of the pump casing.
There is a trade-off relationship between the capacity (flow rate) of a centrifugal pump and the head it can add to a fluid. Commonly the relationship is represented by a performance curve. These plot head vs. flow rate as function of impeller speed (one curve for each speed measured).
NEED FIGURESince efficiency and power requirements are also functions of capacity, many manufacturers include them on performance curve plots. Power requirements depend on the density of the fluid being pumped, and so typically must be multiplied by the specific gravity of the fluid.
Pump efficiency is the ratio of the "energy imparted by the pump" to the "energy required by the pump". This tells us how much energy is lost by the pump mechanism. If flow were completely frictionless, a centrifugal pump would be 100% efficient; however, the world isn't that ideal.
The total head generated by a centrifugal pump is limited by the attainable rotational speed. Multistage pumps, using several impellers in series, can be used to obtain larger total heads.
The actual performance of an installed centrifugal pump is determined by its characteristic performance curve (obtained from the manufacturer) and the resistance vs. flow curve for the piping network (which must be calculated). The intersection of the two curves is the operating point, so if both can be expressed mathematically, the problem can be stated as one of simultaneous equations.
Sometimes it is useful to consider operating a pump with a different impeller or at a new speed. Often, when such a possibility is being evaluated, the pump curve does not show the required configuration. In these cases, it is possible to estimate the new requirements using appropriate ratios, called the affinity laws or fan laws:
NPSH also typically varies as the cube.
All centrifugal pumps are subject to minimum flow requirements to prevent mechanical problems due to temperature rise, etc. These are of particular concern for installations where the pumps are liable to be operated intermittently or "closed in". For small pumps, typical minimum flow values are 30% of the flow at BEP (best efficiency point). Larger and multistage pumps are likely to have minimum values closer to 50% of flow at BEP.
Another class of minimum flow problems occurs at the left side of the performance curve. This is suction side recirculation. It effectively increases the NPSH required to prevent cavitation.
If you anticipate large turndowns in your pumping system, you probably should consider including a spillback or recirculation loop as low flow protection for your pump.
The hydraulic performance of a centrifugal pump depends on the shape and proportions of the impeller. This relationship can be expressed in terms of a dimensionless quantity -- the specific speed.
A similar dimensionless number, the suction specific speed, is used to rank a pumps ability to operate under low NPSH:
Leakage from pumps can be very dangerous. Seals and gaskets are thus very important. The seal between the rotating stationary parts is a complicated problem -- a stuffing box, so called because it is typically packed with loose sealing material, is located where the rotating shaft enters the pump case.
Standards?
Viscosity Corrections?
Priming (MSH p. 204)?
Fans and blowers move large volumes of gas, typically through fairly large ducts. Fans produce very small pressure differences (inches of water), while blowers produce differences up to about 2 atm. Both are rotary devices.
Most fan and blower calculations use standard cubic feet or standard cubic meters. You should remember this idea from your material balance course (refresh your memory).
Large fans are usually centrifugal, and basically work the same as a centrifugal pump. Since fans produce very little pressure change, it is typically safe to use incompressible fluid equations when modeling the system.
Levenspiel (p.8) reports blower efficiencies between 0.55 and 0.90.
Pumps
Fans & Blowers
R.M. Price
Original: 11/99
Modified: 12/21/99