Surface condensers steam-turbine

Specimen Location Steam turbine surface condenser... 

I did not understand thermodynamics at university. It seemed to be an exotic subject with no relevance to my future as a refinery process engineer. It wasn t until I had spent many years working with steam turbines, surface condensers, steam distribution systems, and most especially vacuum ejectors that I realized that without a firm understanding of thermodynamics, I could not operate or troubleshoot such facilities with any degree of confidence. 

On the vapor outlet of steam turbine surface condensers, there is a two-stage ejector system and a two-stage condenser. Referring to Fig. 25.5, this two-stage condenser is actually a small condenser divided into two portions on the shell side by a partition baffle. 

Figure 25.5 Steam turbine surface condenser vacuum is bad due to a leak in the ejector condenser shell-side partition plate.

Figure A1.5 Typical scheme of coal-fired thermal power plant (AuthorAJser BillC https // commons.wikimedia.org/wiki/File PowerStation2.svg website approached January 26, 2016) (1) Cooling tower (2) cooling-water pump (3) transmission line (3-phase) (4) step-up transformer (3-phase) (5) electrical generator (3-phase) (6) low-pressure (LP) steam turbine (7) condensate pump (8) surface condenser (9) intermediate-pressure steam turbine (10) steam control valve (11) high-pressure (HP) steam turbine (12) deaerator (13) feedwater heater (14) coal conveyor (15) coal hopper (16) coal pulverizer (17) boiler steam drum (18) bottom ash hopper (19) superheater (20) forced draught (draft) fan (21) reheater (22) combustion air intake (23) economizer (24) air preheater (25) precipitator (26) induced-draught fan and (27) flue gas stack.

Space needs to be provided for the auxiliaries, including the lube oil and seal systems, lube oil cooler, intercoolers, and pulsation dampeners. A control panel or console is usually provided as part of the local console. This panel contains instmments that provide the necessary information for start-up and shutdown, and should also include warning and trouble lights. Access must be provided for motor repair and ultimate replacement needs to be considered. If a steam turbine is used, a surface condenser is probably required with a vacuum system to increase the efficiency. AH these additional systems need to be considered in the layout and spacing. In addition, room for pulsation dampeners required between stages has to be included. Aftercoolers may also be required with knockout dmms. Reference 8 describes the requirements of compressor layouts and provides many useful piping hints. 

The required relieving rates for these PR valves are based upon the steam rate to the turbine As a rough guide, the appropriate size can be checked against the following tabulation, which are recommendations of the "Standards of the Heat Exchanger Institute, Surface Condenser Section."... 

There are three potential types of OTEC power plants opcii-cyclc, closed-cycle, and hybrid systems. Open-cycle OTEC systems exploit the fact that water boils at temperatures below its normal boiling point when it is under lower than normal pressures. Open-cycle systems convert warm surface water into steam in a partial vacuum, and then use this steam to drive a large turbine connected to an electrical generator. Cold water piped up from deep below the oceans surface condenses the steam. Unlike the initial ocean water, the condensed steam is desalinated (free of salt) and may be collected and used for drinking or irrigation. 

A surface condenser condensing the steam from a process turbine drive operates at 1.0 in. Hg abs. The condensing load is 85,000 Ibs/hr steam. WTat is the capacity of the ejector ... 

When the steam leaves a condensing turbine, it passes to a surface-type condenser for recovery of the condensate. Vacuum equipment (jets or pumps) are necessary to achieve high vacuums on the condenser. 

Steam-turbine lubricants Lubricants in steam turbines are not exposed to such arduous conditions as those in engines. The main requirement is for high oxidation stability. However, they may be exposed to aqueous condensate or, in the case of marine installations, to sea water contamination, so they have to be able to separate from water easily and to form a rustpreventing film on ferrous surfaces, and it is usual to employ rust inhibitors. The problem of tin oxide formation on white-metal bearings is associated with the presence of electrically conducting water in lubricants and can be over-come by keeping the lubricant dry . 

Utility plants primarily employ reheat condensing turbines, whereas cogeneration plants and larger process industries that produce their own electrical power tend to employ extraction turbines. Both types of turbine rely on a surface condenser to receive exhaust steam from the LP turbine stage and condense it to liquid for reuse. 

Where extraction turbines are employed, it is most efficient to operate at the lowest level of condensing possible (the highest level of extraction). This occurs when steam from the turbine is extracted and directed by various takeoffs, either to air or water heaters or to industrial processes requiring steam. This practice occurs because no heat is lost from the steam flowing from the throttle to the extraction takeoff (whence it can be delivered for beneficial use by the industrial process), whereas over two-thirds of the heat flowing from the throttle to the surface condenser is ultimately rejected, so efficiency suffers. 

These turbine units finally exhaust the steam at considerably less than atmospheric pressure to a condenser (in most circumstances a surface condenser is employed). The condenser is designed to raise turbine operating efficiency by reducing the turbine back-pressure to an absolute minimum. This is achieved by condensing the exhaust steam into a smaller volume of condensate, thus creating a substantial vacuum. 

Most boiler plants with electrical power generating facilities employ surface condensers. These are shell-and-tube heat exchangers in either one-, two-, or four-pass configurations. Surface condensers typically receive cooling water on the tube-side and steam on the shell-side of the heat exchanger. The LP turbine steam generally is received at the top of the condenser and proceeds through the condenser in a downward flow, while the FW turbine exhaust steam enters at the side. 

A design of turbine whereby the driving force is provided by exhaust steam condensing in a surface condenser. 

Steam used to drive a turbine can be extracted at an intermediate pressure, for further use of the low-pressure steam. Rarely is the steam vented to the atmosphere, as this wastes steam, and the condensate is also lost. Many turbines exhaust steam, under vacuum, to a surface condenser. The lower the pressure in the surface condenser, the greater the amount of work that can be extracted from each pound of steam (see Chap. 17). 

Discounting the presence of air leaks, the temperature inside the surface condenser determines the pressure of the steam exhausting from the turbine. This pressure is the vapor pressure of water at the surface condenser outlet temperature. 

Figure 8.10 shows the type of surface condenser widely used on older steam turbines. Note that it has both a vapor and a liquid outlet. The... 

Air-cooled surface condensers. Figure 8.11 shows a surface condenser elevated above the steam turbine. This creates an additional problem, in that moisture from the turbine exhaust steam will accumulate in the bottom of the turbine case. A special drain line from the turbine s case is needed to prevent condensate backup from damaging the spinning wheels. 

The turbine case pressure was increased by raising the pressure in the air-cooled surface condenser. This was accomplished by shutting off several of the air fans, which, in turn, increased the condensing temperature of the exhaust steam. But why would raising the turbine case pressure drain the turbine, anyway After all, increasing the surface condenser pressure also increased the pressure in the drum that the turbine case drained to. 

The condensing turbine does not produce exhaust steam. All the turbine exhaust steam is turned into water in a surface condenser. We will study surface condensers in Chap. 18. The surface condenser is just like the sort of vacuum condensers we discussed in Chap. 16, sections on steam jets. The exhaust-steam condenses under a deep vacuum— typically 76 mm Hg, or 0.1 atm. Basically, then, a condensing steam... 

In Chap. 16 we reviewed several problems pertaining to steam jet precondenser and intercondenser problems. The surface condensers, which serve condensing steam turbines, are subject to all the same... 

Having replaced the loop seal piping, (some units use a steam trap instead of this loop seal), I started steam flow to the turbine. But the vacuum in the surface condenser, which had started out at an excellent 27 in Hg, slipped down to 14 in Hg. This loss in vacuum increased the backpressure in the turbine case. The higher pressure in the turbine case reduced the velocity of the steam striking the buckets on the turbine wheel, which reduced the amount of work that could be extracted from each pound of steam. 

The gas that accumulates inside the surface condenser is called the noncondensable load to the steam jets. Some of the noncondensable load consists of C02 accidentally produced when the boiler feedwater is vaporized into steam. Air leaks through piping flanges and valves are other sources of noncondensable vapors. But the largest source of noncondensable vapors is often air drawn into the turbine case, through the shaft s mechanical seals. To minimize this source of leaks, 2 or 3 psig of steam pressure is ordinarily maintained around the seals. However, as the turbine s shaft seals deteriorate, air in-leakage problems can overwhelm the jet capacity. This will cause a loss of vacuum in the surface condenser. 

Under certain conditions, such as high water temperatures, insufficient water supplies and problems of blowdown disposal, systems that depend on convection and use air as the transport medium may be preferable. The two types of dry cooling towers are the direct and indirect systems. Figures 4.21 and 4.22 show these systems in operation for nuclear station cooling. Indirect units use a surface or jet condenser at the turbine to condense exhaust steam. Water from the condenser is pumped to the dry tower for cooling and recirculation back to the condenser. In the direct system, steam is condensed in cooling coils without interfacing with a condenser. 

In direct condensing, turbine exhaust steam is conveyed through a trunk to the air-cooled coils, where cooling air passing over the finned coil surfaces condenses the steam. Steam enters the coil section and condenses as it travels downward, with steam and condensate flowing in the same direction, minimizing pressure loss and increasing the heat transfer coefficient. 

In the 1950s Hickman developed a centrifugal vapor compression evaporator for seawater desalination (53). This device consisted of multiple spinning discs. Seawater sprayed on one side of the disc evaporated, while the centrifugal force removed the residue from the plate surface. The vapor was compressed and returned to the opposite side of the plate, where condensation provided the heat for evaporation and the desired freshwater for recovery. Overall heat transfer coefficients of 18 kW/m2-K are about three times higher than those achieved in steam turbine condensers. 

Figures 8.10 and 8.11 are based on the driver FOB fabrication shop cost only. For the completed driver package, additional items, such as a surface condenser for the condensing steam turbine or a starting motor or air piston starter for the gas engine, are required. Tables 8.29 to 8.32 are each dedicated to providing factoring for these specific costs. Such costs will again be based on the compressor horsepower, BHP.

Figure 106. Steam system o( a modern steam reforming a) Steam drum. 125 bar b) NH, loop c) Turbine for syngas compressor d) Turbine for process air compressor and alternator e) Surface condenser f) Condensate treatment g) BFW pump...

Often, there exists spare capacity in a surface condenser, one that condenses exhaust steam from a centrifugal compressor or from a steam turbine under vacuum. This spare capacity may often be over 10% of the normal operating capacity and can be attributed to one or more of the following ... 

The surface condenser is generally used with turbines, both because it usually permits of a better vacuum and because the condensed steam, free from oil, is thereby immixed with the (usually inferior) circulating water. In engine plants, if circulating water is salt or otherwise unsuitable for boiler feed, the condensed steam may be used for boiler feed if a surface condenser is used, but care must be taken that cylinder oil does not reach the boilers. 

A condenser used to condense turbine exhaust steam. Both direct contact and surface condensers are used in direct steam geothermal power plants. 

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