INFORMATION SHEET

 

                                                  MAIN CONDENSATE SYSTEMS

                                                   Information Sheet Number 62P-110

 

 

INTRODUCTION

 

            Main propulsion turbines exhaust large quantities of steam.  The steam cycle is a closed loop and requires the steam to be "converted" back to condensate to complete the loop.  Steam is condensed in the main condenser and collected in the hotwell.  The condensate system also receives drains and the necessary makeup feed needed for the steam cycle to be maintained.  The condensate along with drains and make up feed is pumped to the DFT and starts the next phase of the steam cycle.

 

 

REFERENCES

 

            (a)  Condensers, Heat Exchangers and Air Ejectors  NSTM Chapter 254

            (b)  Pumps  NSTM Chapter 503

            (c)  Main Condensers  NAVSHIPS 0946-LP-010-9010

            (d)  Main Air Ejectors  NAVSHIPS 0946-LP-010-7010

            (e)  Main Condensate Pump  NAVSHIPS 0947-LP-090-6010           

 

INFORMATION

 

A.        MAIN CONDENSATE SYSTEM

 

            1.  The main condensate system starts at the main condenser.  The main condenser is a single pass shell and tube type heat exchanger (figure 1).  Cooling water flows through the tubes removing the latent heat of vaporization from the steam coming in contact with the outer surface of the tubes.  The condensate now goes to the hotwell.   The hotwell is a collection area of condensed exhaust steam from the low pressure turbine and the auxiliary exhaust system.  The collected condensate is routed through piping to the condensate pumps.  Rubber impregnated canvas expansion joints are installed to compensate for thermal expansion and provide sound isolation in the piping on some ships. 

 

            2.  Main condensate pumps (MCP's) are either two or three stage centrifugal pumps that operate on the principle of submergence control.  They pump the condensate from the hotwell through the air ejector condenser into the DFT (figure 2).

 

                                               END VIEW OF MAIN CONDENSER

 

                                                           Figure 1

 

            3.  Recirculation lines ensure a constant flow of condensate through the air ejector condenser during reduced flow situations to prevent overheating of the air ejector condenser.   Examples of low flow situations include start-up, cool down, casualties and low speeds.  There is usually a thermostatic recirculation valve with a manual bypass valve to regulate the amount of condensate being recirculated.  In the line after these valves are high and low recirculation valves directing the condensate to the top or bottom of the condenser tube area.  The more tube area of the condenser that the recirculating condensate is in contact with the more the condensate is cooled.  Therefore, the high recirculation valve is used in low flow situations to ensure adequate cooling, such as a Stop bell or start-up and the low recirculation valve is used during normal operations when there should be adequate flow.  These valves are  commonly called recircs.

 

            4.  Thermometers are installed to monitor the condensate temperature entering and leaving the air ejector condenser.  The temperature of the condensate leaving the air ejector condenser is the temperature sensed by the thermostatic recirculation valve.

 

            5.  Salinity indicators monitor the degree of conductivity of the condensate leaving the main condenser.  These indicators allow for detection and monitoring of possible contaminated condensate after it is in contact with a seawater heat exchanger (the condenser).

 

                                                TYPICAL CONDENSATE SYSTEM

 

Figure 2

 

 

            6.  The condensate that is pumped through the air ejector condenser is the cooling medium for the air ejector, condensing their steam.  Air ejectors are two stage twin elements that operate on 150 psig steam, removing air and non-condensable gases from the main condenser.

 

 

 

 

B.         SUBMERGENCE CONTROL

 

            1.  The most common way of maintaining a hotwell level in a non-throttled condensate system is through submergence control.  Submergence control requires no operator action.  The condensate pump uses submergence control to maintain flow through the pump.  The term submergence control implies controlled pump cavitation due to system configuration.  Usually, pump cavitation is to be avoided, however, the condensate pump operating in a non-throttled system is especially designed for operation with some cavitation.  The amount of pump cavitation determines the capacity of water it pumps.

 

            2.  The amount of cavitation is determined by the amount of NPSH at the pump suction.  NPSH is the amount of suction provided to a pump that is in excess of Psat and represents the amount of energy available at the pump suction to get liquid to flow into the pump.  Available NPSH is the NPSH which exists at a pump's suction at any given time.  The NPSH at a condensate pump suction is determined by the pressure in the condenser (Pambient) and the height of water in the hotwell above the pump suction.  Since the condensate pump suction is maintained at a vacuum, a small contribution is made by existing pressure in the condenser and can be ignored.  The major effect on condensate pump NPSH (the margin to pump cavitation) is made by the level of water above the pump suction or the amount the pump is submerged.  Since this amount of submergence determines pump NPSH which governs the capacity of the pump (in gpm), this method of control of pump capacity is called submergence control.  As the hotwell level rises and falls during transient conditions (changes in speed), the pressure at the suction side of the condensate pump rises and falls and the pump characteristics will vary.  A review of a system operating curve would show the differences in pump head capacities and could be used as a system troubleshooting tool.

 

                        a.  If there is a smaller load on the turbine there will be less steam entering the condenser and the hotwell will be pumped down.  As the level of submergence is reduced, the NPSH will be reduced and partial cavitation may take place.  Since the cavitating pump does not remove condensate as efficiently as before the transient, the hotwell level starts to rise.  Eventually the level rises enough so that the suction pressure rises.  Eventually the pump stops cavitating and the process repeats itself.

 

            3.  A pump operating with submergence control may be noisy and may show signs of damage to the pump impeller or casing due to cavitation.  Since a pump with submergence control is operated very close to cavitation, pump impeller must be made of erosion resistant materials in order to survive the conditions created by cavitation.  Condensate pump impellers are usually made of monel which is known for its resistance to erosion.  This type of condensate system is one of the few cases in which a constant speed centrifugal pump can provide an increase in both flow rate and pressure.  Recall that in most cases, as flow rate increases, pump head drops.

 

C.        PRINCIPLES OF OPERATION

 

            1.  Exhaust steam from the LP turbine is directed over the tubes in the main condenser.  Seawater flowing through the tubes removes the latent heat of vaporization, changing the steam into condensate.  The condensate drains into the main condenser hotwell which acts as a storage reservoir.

 

            2.  The MCP takes a suction on the hotwell and discharges into the condensate system.  Condensate flowing out of the MCP passes through the main air ejector condenser, where the first preheating occurs.  The condensate carries the heat away from the 150 psig steam used to operate the air ejectors.  The steam is condensed and drained to the freshwater drain system.

 

            3.  Downstream of the air ejector condenser, a recirculation line off the main condensate system returns condensate to the main condenser through a thermostatically controlled valve that controls the flow of condensate during low load conditions.  It is provided with a manual bypass valve for start-up, cool down or casualties.  Condensate is passed through either the low recirc during normal operation (enters at the hotwell) or the high recirc (entry above the condenser tubes), during start-up, cool down and casualties.

 

            4.  The first stage of the air ejector takes a suction on the main condenser and discharges air and motive steam to the suction of the second stage.  The second stage takes a suction on the discharge of the first stage and discharges air and motive steam to the air ejector inter and after portion of the condenser.  The motive steam is condensed and drained via a loop seal to the FW drain system leaving the air behind.  A gland exhauster fan is typically used to take a suction on the air ejector condenser inter and after sections and on the gland exhaust condenser.  The gland exhauster fan maintains a vacuum of 1" to 4 " in the inter, after sections of the main air ejector condenser and the gland exhaust condenser is maintained at 5" to 7" of vacuum, as it is closer to the gland exhauster fan.  Air and non-condensable gases are drawn off  from the main air ejector condenser inter and after sections to the gland exhaust condenser by the gland exhauster fan.  Any steam that is condensed in the gland exhaust condenser is condensed and returned to the FW drain system.  The remaining air and non-condensable gases are discharged from the fan to the atmosphere through a ventilation duct (figure 3).

 

 

 

 

 

END VIEW OF AIR EJECTOR ASSEMBLY

 

 

 

Figure 3

 

D.        SYSTEM INTERRELATIONSHIP

 

            1.  The condensing of the steam from the turbines assists in the maintaining of vacuum in the condenser in normal operation, as steam occupies a greater area than the condensate.  If excessive condensate collects in the hotwell and is not removed by the condensate pumps, the condenser loses vacuum.  The condensate is taking up area normally occupied by the steam being condensed.  The less area for the steam to occupy means less condensing of the steam and less assistance in maintaining condenser vacuum.

 

            2.  Condensate can also affect the DFT through quenching and flashing.  Quenching and flashing are caused by the rapid addition of relatively cold condensate to the hot DFT.  This rapid addition of condensate causes the shell temperature of the DFT to lower quickly which in turn lowers the DFT shell pressure.  The result is quenching and flashing which can cause the loss of the DFT and ultimately the boiler.  The slow opening of valves from additional pumps as they are placed in operation usually prevents this problem.

           

            3.  Condensate provides cooling water to condense exhaust steam from the main air ejectors in the main air ejector condenser (150 psig steam/FW drains).  Without the condensate to cool and condense the air ejector steam discharge, the main air ejectors would "back up" and the removal of air and non condensable gases from the condenser decreases.  This causes the condenser to lose vacuum.  The recirculation system ensures a cooling medium for the air ejector condenser.

 

            4.  Condensate is used as a desuperheating medium for the distilling plant salt water heater during start up.  Also, the condensate main receives drains from the saltwater heater during normal operation.

 

            5.  A line from the DFT ties into the condensate recirculation system for initially filling the main condenser hotwell.

 

E.         MISCELLANEOUS BRANCH LINES OFF AND INPUTS TO THE CONDENSATE MAIN

 

            1.  Since the condensate system is the only way for water to enter the DFT and continue the steam cycle, any other water must be tied into the condensate piping main to make it back to the DFT.  Also, some of the branch lines from the condensate main are used in certain equipment for cooling and lubrication.  Examples of these branch lines include: the evaporator seawater heater drain pump can be aligned to discharge to the condensate main or to freshwater drains which are discharged by pump into the condensate system.  Main feed pump gland cooling is provided by condensate which prevents any contamination of the feed.  For treating the boiler, the chemical injection tank topping off line and the static leg of the DFT level control has a fill connection from the condensate system.

 

 

 

 

 

 

F.         LAY-UP FOR MAIN CONDENSER STEAM SIDE  (NSTM CHAPTER 254)

 

            1.  Lay-up procedure for the steam side of condensers is divided into two time requirements.

 

                        a.  Idle period of up to one month: drain the hotwell and keep drained.

 

                        b.  Extended idle condition in excess of one month:  drain as soon as possible and dry out using an electrically heated air blower.  The blower shall be discharging into a hotwell opening.  The moist air shall be allowed to vent to the atmosphere from an upper shell of the turbine exhaust casing opening.  After the drying process, secure all condenser openings, and check the condenser weekly and repeat the drying process if necessary.