INFORMATION SHEET

STEAM DRAIN COLLECTING SYSTEMS

Information Sheet Number 62B-220

INTRODUCTION

Steam propulsion plants have various systems which collect and gather liquid drainage 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. If water was lost from the cycle, additional water would be added to compensate for the loss. Contamination of any steam drain collecting system can prevent the reuse of steam drains, causing a decrease in overall plant efficiency. Steam drains are reused to make the cycle as efficient as possible.

REFERENCES

(a) Ship's Information Book for LHD-1 S9-LHA-AA-SIB-010/LHA-1

(b) Propulsion Operating Guides for LHD-1 0910-LP-244-7600, LHA-1 S9-LHA -AA-POG-010/LHA-1

(d) NAVOSH Afloat OPNAVINST 5100.19B

INFORMATION

A. Steam drain systems-As steam is used throughout the ship and the propulsion plant, some condensation or residual water builds up in piping and low points in the systems, creating a potential for damage to equipment and piping systems. The drain collecting systems are designed to handle these conditions. They collect and dispose of this condensation from various steam driven equipment and piping systems for reuse.

1. The drain collecting system is composed of three different systems: high pressure drains (HP), low pressure drains (LP), and fresh water drains (FW).

2. Each of the steam drain systems consists of a separate drain main which runs through all main machinery spaces. FW and LP drains empty into the fresh water drain collecting tank (FWDCT). HP drains empty into the DFT. LP drains may also be aligned overboard.

B. The High Pressure Drain System collects drains from steam systems and equipment that operate at pressures at or above 150 psig. The HP drain system has the following major components: piping and valves, controls, instrumentation and protective devices.

1. The piping system is composed of steel piping joined by raised flanges. The system was originally designed with steam traps but most have been converted to orifices for greater reliability.

a. The system valves are manually operated. The exception is spring loaded relief valves (usually one in each space served by the system) which all relieve to the bilge.

b. Pressure gages are provided throughout the system to monitor the system.

2. All equipment and piping going to the HP drain system have cut-out valves, an orifice, and a stop-check valve on the discharge side of the orifice to permit continuous drainage during operation (see Figure 1).

a. A constant flow orifice is used in the HP drain system because condensate forms at a fairly constant rate before maximum superheat conditions are attained in the system.

b. The opening in the orifice plate effectively removes the condensate formed during these warm up periods and reduces the pressure of steam to about 20 psi on the outlet side of the orifice plate.

c. The condensate removed by the flow drain orifice flashes to steam as it passes from the high pressure inlet to the low pressure outlet of the orifice.

3. The branch lines of the HP drain system tie together into a common drain main which discharges into the DFT. These drains act as a heating source for, and assist in, the proper operation of the DFT.

C. The Low Pressure Steam Drain System (see Figure 2) collects the drains from the constant and intermittent steam systems and from steam equipment outside the machinery spaces which operate at pressures below 150 psig. These systems include the galley, scullery, laundry, and heating systems. The LP drain system terminates in the fresh water drain collecting tank (FWDCT). In most steam ships there are no systems outside the main machinery spaces which use steam above 150 psi.

D. The Fresh Water Drain System (see Figure 3) collects drains from steam piping and equipment operating at pressures below 150 psig within the main machinery spaces that require continuous drainage and is used to drain systems when lighting off from a cold plant status.

1. The FW drain system consists of piping and valves, a fresh water drain collecting tank, FW drain collecting tank pumps, controls, instrumentation and protective devices.

2. The piping system is made of lower grade materials due to lower pressures and temperatures. The branch lines contain various globe valves and swing check valves. The branch lines tie into the FW drain main and the fresh water drain collecting tank receives the drains from the drain main (see Figure 3).

a. The FW drain collecting tank (FWDCT) has one or two pumps depending on the ship class. The pumps are vertical, centrifugal, single stage, motor driven pumps with interlocked power sources.

b. The contents of the FWDCT are discharged to the condensate system prior to the DFT. This prevents drains from being cooled and reducing plant efficiency.

c. In an emergency, vacuum drag to the main condenser can be used in place of the pumps. A few ships are constructed with vacuum drag as the only method for drain return to the condensate system.

3. The FWDCT has various features which control when the pump(s) will start and stop. Each FWDCT is fitted with two pressure switches controlled by floats, enabling the pump(s) to start or stop.

4. Vacuum drag is used as an alternate to the FWDCT pumps. A manually operated cut-out valve must be opened in the vacuum drag line before this can be used and then a float valve regulates the flow.

a. Vacuum drag uses the high vacuum in the main or auxiliary condensers to draw the water out of the FWDCT, similar to a straw sucking water out of glass. Caution must be used when using vacuum drag for a few reasons:

(1) Rapid removal of water from the drain tank can cause loss of vacuum in the condenser being used. When the water is removed from the tank, air can be drawn into the condenser if the float valve doesn't operate correctly.

(2) If the drain tank has contaminated water in it, rapid contamination of the entire condensate and feed systems can occur. All FW drains eventually end in the condensate system. If only the FWDCT is contaminated, the main engine could be operated while the contaminated drains would go to the bilge. If on vacuum drag, the main condenser would also become contaminated.

5. The fresh water drain system is protected from excessive pressure by spring loaded relief valves. There is usually a relief valve in each machinery space served by this system.

6. The fresh water drain tank has two methods to monitor the tank level. A gage glass is used for local monitoring and a tank level indicating system is used for remote reading of the tank level. The remote reading is usually taken at the control station in the space.

E. Contamination-Sources of possible contamination include shore or sea water contaminants and solid particulate from piping systems.

1. 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.

2. Sea water 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 sea water contamination through tube leakage or brine carryover. Because of this, these drains are tested prior to aligning them to the condensate or fresh water drain system and monitored continuously. Any sea water heat exchanger possesses the potential to seriously contaminate the plant.

3. Solid particulate contamination generally comes from piping systems. As the material used in the piping system degrades over time, small 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.

4. Drain funnels and covers are used to protect the open drain systems from the entrance of stray contaminants. All funnels are required to be covered with a metal tabbed cover. These covers prevent sea water, lagging, paint chips, or rust from inadvertently contaminating the system.

5. Testing and isolation are vital to controlling contamination. It is very important to isolate any contamination as soon as it is suspected. Any contamination left unchecked or not minimized by isolation can cause a cascading effect. For example, since LP drains can be aligned to the fresh water drain collecting tank and be discharged to the condensate system, contamination of the LP drains can enter the condensate system. The condensate system discharges to the DFT, and becomes FEED WATER, for use in the boiler. If the contamination is left unchecked, all of these systems can be affected and troubleshooting the problem becomes more difficult. Strict testing in accordance with NSTM Chapter 220 Vol II is essential.

F. Safety

1. All steam drains are dangerous to personnel due to the temperature and pressures of the systems. Caution should be taken while operating these systems.

2. Because the F.W. Drain System is an open system (open to atmosphere), steam using this type of drain, presents a hazard to personnel (see Figure 5). A loud, verbal warning should always be given before using these drains (WATCH OUT FOR DRAINS!!).

3. Do not work on any system unless all associated equipment is secured, properly tagged out and verified. Securing drains to equipment in operation can drastically affect proper operation of the equipment.

4. Various drain systems have piping that is made of various materials. Ensure all materials are of the proper rating.

5. Test and take corrective actions on suspect drain systems in accordance with NSTM Chapter 220 Vol II.