INFORMATION SHEET

 

                                HP/LP/FW DRAINS (DRAIN COLLECTING SYSTEMS)

                                                   Information Sheet Number 62P-107

INTRODUCTION

 

            Water in steam systems result in impingement, erosion and corrosion which can cause major damage to equipment and piping.  Steam propulsion plants have various drain collecting systems which collect and gather this water for reuse.  These drain collecting systems are vital in keeping the steam propulsion plant as efficient as possible.  The basic steam cycle assumes nearly all steam is ultimately converted back to water and reused.  When water is lost from the cycle, additional water is added to compensate for the loss.  Contamination of any steam drain collecting system can prevent the reuse of those steam drains, causing a decrease in overall plant efficiency.

 

REFERENCES

 

            (a)  Principles of Naval Engineering  NAVPERS 10788-B

            (b)  Piping Systems  NSTM Chapter 505

            (c)  Boiler Water/Feedwater  NSTM Chapter 220 Vol II

 

INFORMATION

 

A.        Drains are divided into different systems depending on the source and the drain water's destination.  By having separate systems we can isolate any contaminated system and still recover drain water from the other drain systems.

 

Except for main propulsion turbine steam and low pressure drain systems, each of the drain systems consists of a separate drain main piped throughout the machinery spaces.  Each system ultimately discharges its drain water to the feed/condensate system, a waste tank, or overboard.  This lesson will discuss the following drain systems:

 

1.  High pressure drains (HP)

2.  Freshwater drains (FW)

 

3.  Low pressure drains (LP)

 

4.  Main propulsion turbine steam drains

 

5.  Oil heating coil steam drains

 

6.  Oily water drains, waste water drains

 

B.         High Pressure Drain Systems collect drains through various branch connections from various steam systems and equipment.  The HP drain system has the following major components:  piping,  valves, gages and relief valves.

 

                                                    TYPICAL HP DRAIN SYSTEM

 

 

Figure 1

 

1.  All equipment and piping going to the HP drain system have an inlet cut out valve, a strainer, an orifice and a stop-check valve on the discharge side of the orifice to permit continuous drainage during operation (see Figure 2).  The stop-check valve prevents back flow from the HP drain system in the event that system pressure is greater than the pressure thru the orifice.  A constant flow drain orifice is used in the HP drain system because condensate forms at a fairly constant rate during light-off and warm up.  The opening in the orifice plate effectively removes the condensate formed during these warm up periods and yet restricts the escape of live steam to under 20 psi.  Much of the condensate removed by the orifice flashes to steam as it passes from the high pressure (inlet) side to the low pressure (outlet) side of the orifice. The most common problems associated with the orifice are a clogged  inlet strainer or a clogged orifice hole.  If this happens, the orifice is no longer allowing condensate to drain from the piping system.  Without drainage, the condensate continues to back up, leading to possible piping ruptures, personnel hazards and equipment failure. If the improper size of orifice is inadvertently installed or created through erosion, excessive pressure can occur on the low pressure side of the orifice causing overpressurization of the drain system.This over pressurization can rupture piping and severely injure personnel.  A spring-loaded relief valve is installed to prevent this overpressurization.

 

 

TYPICAL HP DRAIN ARRANGEMENT

 

 

 

 

Figure 2

 

           

 

2.  The branch lines of this system tie together into a common drain main which discharges into the DFT.  HP drains, along with Auxiliary Exhaust, are the heating source for, and assist in the proper operation of the deaeration process in the DFT.  HP drains for a particular system or piece of equipment can be shifted to the freshwater drain system  if a problem such as erosion of an orifice occurs.

 

C.        The Freshwater Drain System (See Figure 3) collect drains from low pressure (less than 150 psig) steam, exhaust steam piping systems and from equipment in the main spaces that require continuous drainage while in operation at pressures at or below 150 psig (i.e. main and auxiliary air ejector condensers).  The system also collects the drains from steam and exhaust warm up connections and serves as an alternate means of drainage from the steam whistle and seawater heater of the distilling plant.  The FW drain system consists of piping, valves, gages, freshwater drain collecting tank (FWDCT), pumps and controllers.

 

 

                                             TYPICAL FW DRAIN ARRANGEMENT

Figure 3

 

            1.  The piping system is made of lower grade steel due to lower pressure and temperatures.  The branch lines contain various globe valves and swing check valves.  The branch lines tie into the FW drain main.  A freshwater drain collecting tank receives the drains from the drain main.  There is usually one pump per tank and on some ships two.  These pumps are vertically mounted, centrifugal, single stage, motor driven pumps.  While some ship's do not have a FWDCT Pump, all ship's have a piping arrangement to allow the vacuum from the Main Condenser or Auxiliary Condenser's to draw the drains from the FWDCT into the respective hotwell.  This line is called the Vacuum Drag.  Using the FWDCT Pump, the contents of the FWDCT are discharged to the condensate system prior to the DFT (this prevents drains from being cooled, thus losing enthalpy)             (see Figure 4).

 

2.  The freshwater drain system has various features which enable the operator to control when the pumps start and stop.  The FWDCT can have one pump that runs continuously with float valves that regulate the flow or two pumps that cycle on and off (by use of pressure switches) as needed.

 

            3.  The vacuum drag valve is generally float controlled and is used as an alternate method or when the drain pumps are incapable of maintaining the proper level.  The valve starts to open when the water level is slightly above low and is fully open when the water level is high for the tank.  A manually operated cut out valve in the vacuum drag line must be opened before the method can be used.

 

            4.  The freshwater drain system is protected from excessive pressure by spring loaded relief valves.

 

 

D.        The Low Pressure Drain System (LP drains) collect the drains from the constant and intermittent steam systems and from steam equipment outside the machinery spaces which operate at pressures below 150 psig, such as galley, laundry, heating coils, topside equipment, etc.  The LP drain system aligns drains into the freshwater drain tank.  The LP Drain system consists of a low grade steel piping, various valves and a spring loaded relief valve.

 

                                            TYPICAL LP AND FW DRAIN SYSTEM

 

Figure 4

 

E.         The Main Engine Turbine Drain System is a piping system used to return condensate formed in the main engine turbine unit to the feedwater system via the main condenser.  When the drain piping isolation valves are opened, the higher pressure steam and water mixture from the turbine flows through the piping to the lower pressure area of the main condenser.  Main propulsion turbine drains are piped directly to the main condenser thru cut-out valves which enable the watchstander to align the drains for warm-up, maneuvering, underway or securing based on EOP procedure CP METD.  There are no controls or monitoring gages in the system.

 

F.         The Oil Heating Coil Drain System collects drains from steam piping and equipment that may come in contact with oil.  These drains are led to the inspection tank drain main through bimetallic traps.  This main in turn discharges to an inspection tank located usually near the DFT.  If the drains to the inspection tank are not contaminated they may be returned directly to the DFT.  If oil contamination is detected in the inspection tank, the cut-out valves in the discharge to the DFT and freshwater drain system from the inspection tank must be closed immediately.  Furthermore, all inspection drains discharged to the oily water drain main must be closed until the condition is corrected.  Contaminated drains entering the DFT or the FW drain system can cause contamination of the boiler and feed systems, due to the flow path of the cycle.  If the inspection tank bypass is used, the drains are diverted to the oily water drain main to avoid entry of contaminated water into the feedwater system.  The inspection tank and drain main are protected against excessive pressures by relief valves installed in the system which relieve to the oily water drain main.

 

G.        The Oily Water and Waste Drain System consists of a main that runs through the fireroom and engineroom, terminating in a bilge sump tank.  Except for some miscellaneous drains that end in the bilge, separate systems are installed in the main machinery spaces to maintain the bilges generally clean and dry under all operating conditions.    Ships with main machinery rooms have complete separate mains and tanks for each space.  The main collects water leakage and drainage from all equipment that is in contact with or serving the oil system.  The main is fitted with isolation valves for the spaces served.  All drains are routed to the main via funnels or drip pans from the various equipment.  The waste water drain system is routed in the same manner as the oily water drain system.  This main collects other drains which are not suitable for return to the freshwater system.  From the bilge sump, the waste and oily water drains are drawn into the oily waste system and through the oily water separator.  The oily water separator is used to separate the oil from the water in the oily waste and has specific oil discharge limit requirements in accordance with Navy environmental regulations.  The oil can then be discharged off the ship by the bilge and stripping pump through a topside connection to an oil waste offload barge or in an emergency an eductor can be used to discharge the oil overboard.  This is done only IAW current Hazmat and environmental regulations.

 

 

H.        IMPACT OF DRAINS ON PLANT OPERATIONS

 

1.  The importance of reclaiming drains and preventing contamination cannot be overemphasized.  Since the steam cycle is designed to be a closed loop, losses of water have to be replaced and cause a decrease in overall plant efficiency.  Failure to reclaim the drains can cause a substantial increase in usage of feedwater, requiring more distillate to be used as feedwater.  All steam drain collecting systems are designed to return the condensate back to the system to try and maintain plant efficiency.  Improper alignment of drain systems can cause drains to back up, or equipment in operation not to drain correctly causing equipment failure, and can cause catastrophic failure of piping systems injuring personnel.

 

2.  Preventing drains from becoming contaminated or contaminating other systems is also a major plant concern.  Sources of possible contamination include shore or seawater type contaminants and solid particulate from piping systems.

 

            a.  Shore contamination generally comes from shore source feedwater or steam.  All ships must test this water and the drains from this steam prior to retaining them on board.  Not all shore stations provide shore steam or water which is certified for retention.  NSTM Chapter 220 Vol II contains specific testing criteria for the specific retention of shore source steam drains or water.

 

            b.  Seawater contamination usually comes from a component in the propulsion plant.  The saltwater feed heater or air ejector condenser drains of the distilling plant are frequent causes of seawater contamination through tube leakage or brine carryover.  Because of this, these drains are tested prior to aligning them to the condensate or freshwater drain system and monitored continuously.  Any seawater heat exchanger possesses the potential to seriously contaminate the plant.

 

            c.  Solid particulate contamination generally comes from piping systems.  As the material used in the piping system degrades over time, small pieces of rust or particulate can be deposited into the drains contaminating them.  Usually, flushing the system clears the problem.  If not, the section of piping should be replaced.  This problem manifests itself gradually and is detected by slightly higher test indications over time.

 

            d.  Drain funnels and covers are used to protect the open drain systems from stray contaminants.  All funnels are required to be covered with a hinged metal cover that prevent seawater, lagging, paint chips, or rust from inadvertently contaminating the system.

 

e.  Testing and isolation are keys to controlling contamination.  It is very important to isolate any contamination as soon as suspected.  Any contamination left unchecked or not minimized by isolation can cause a cascading effect.  For example LP drains normally are aligned to the freshwater drain collecting tank which is discharged to the condensate system.  The condensate system is discharged into the DFT, which becomes feedwater, and ultimately is discharged into the boiler.  If the contamination is left unchecked, all of these systems can be affected and troubleshooting becomes more difficult.  Strict testing and monitoring in accordance with NSTM Chapter 220 Vol II is essential.