Centrifugal pumps steam turbines

Figure 3-8. Refinery oil and boiler feed high pressure centrifugal pump. Courtesy Delaval Steam Turbine Co., currently Transamerica Delaval, Inc.)...

No matter the duty (utilities excepted), most boiler plant systems employ electric motor-driven, single-stage or multistage centrifugal pumps to supply FW. These may be of conventional design or derivations such as a turbine pump. However, steam-driven reciprocating FW pumps (steam pumps) are still employed in some countries and for certain smaller applications. 

A steam turbine is a machine with an ancient genealogy. It is a direct descendant of the overshot water wheel, used to kick off the industrial revolution in England, and the windmill still used in Portugal. Turbines are widely used in process plants to drive everything from 2-hp pumps to 20,000-hp centrifugal compressors. They are versatile machines, in that they are intrinsically variable-speed devices. Electric motors are intrinsically fixed-speed machines. It is true that there are a variety of ways to convert AC (alternating-current) motors to variable speed, but they are all expensive and complex. 

There are two general types of steam turbines extraction and condensing. The most common turbine with which the process operator comes into contact is used to spare an electric-motor-driven centrifugal pump. The three-phase, AC motors used in the United States and South America are either 1800 or 3600 rpm. The motors used in Europe are 1500 or 3000 rpm. Small steam turbines (20 to 500 hp) used to drive centrifugal pumps, are rated for the same speed as the electric motors that are used in that particular service. 

Step 6. Multiply the horsepower per pound of steam value calculated in step 5 by the turbine steam flow, in pounds per hour. This is the total shaft work that appears at the turbine s coupling. This is the amount of horsepower that is available to spin a centrifugal pump. 

Figure 17.4 shows a centrifugal pump, driven by a steam turbine. The correct operating speed for the pump and turbine is that speed that puts the process-control valve in a mostly open, but still controllable, position. As we slow the turbine to force open the process-control valve, the turbine s governor valve will close. Steam flow to the turbine will decline in accordance with fan laws ... 

Steam turbines. In modern plants the centrifugal synthesis gas compressors, including recycle, are almost exclusively driven by a steam turbines. These are generally extraction turbines with a condensing section. Steam is extracted at suitable pressure levels (e.g. 45-55 bar) to provide, for example, the process steam in steam reforming plants, and for other drivers, e.g., air compressor, ammonia compressor, boiler feed water pumps, and blowers. 

Pumps.—Boiler-feed pumps are of the plunger (direct-steam or power-driven) or rotary (centrifugal) type the latter being driven by a steam turbine or electric motor. Steam-driven pumps are used when there is need of their exhaust for feed-water heating. Centrifugal pumps are generally more compact, and easier to maintain than plunger pumps. 

Surface condensers are used in conjunction with condensing steam turbines that drive large centrifugal compressors. As depicted in Exhibit 4-25 the exhaust steam enters the top of the condenser and passes through the shell, which is filled with tubes. Cold water is pumped through the tubes while hot exhaust steam passes around the outside. Hot water, called condensate, results and collects in the hot well at the bottom of die condenser. 

Power pumps typically have high efficiency and are capable of developing very high pressures. Either electric motors or turbines can drive them. They are relatively expensive pumps and can rarely be justified on the basis of efficiency over centrifugal pumps. However, they are firequently justified over steam reciprocating pumps where continuous duty service is needed due to the high steam requirements of direct acting steam pumps. 

This is an example of Bernoulli s equation in action. A steam vacuum ejector (jet) works in the same way. Centrifugal pumps and centrifugal compressors also work by converting velocity to pressure. Steam turbines convert the steam s pressure and enthalpy to velocity, and then the high velocity steam is converted into work, or electricity. The pressure drop we measure across a flow orifice plate is caused by the increase of the kinetic energy of the flowing fluid as it rushes (or accelerates) through the hole in the orifice plate. 

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