INFORMATION SHEET

RESERVE FEED AND TRANSFER SYSTEMS, MAKE UP AND EXCESS FEED

Information Sheet Number 62B-222

INTRODUCTION

In theory the steam cycle is a closed loop system where the quantity of feedwater is endlessly recycled. In practice however, losses occur throughout the cycle due to such things as leaking packing glands, pump leak-off, steam atomization, boiler surface and bottom blowdowns, etc. These losses must be replaced by making feedwater with the evaporators and reintroducing this make up feedwater into the system. It would be difficult to connect the evaporators to the main feed system and maintain a continuous make up source while retaining the ability to isolate the distilling unit in the case of an evaporator malfunction. For this reason, multiple tanks are installed, the number and volume dependent upon the type of ship. Generally one tank will be designated as the make up feed (MUF) tank and the other as the emergency or reserve feed tank in each plant. Collectively, these tanks are referred to as reserve feed tanks. All feed tanks are connected through the reserve feed transfer manifold or the cross connect line on multi-plant ships.

REFERENCES

(a) Feedwater systems NSTM 255 Vol. 1

INFORMATION

A System Components: To begin, we need to briefly review the process and purpose of feedwater. The ship’s boilers are of primary importance for maintaining a ship at sea or in an at-sea ready condition. In order to provide for this make up of feedwater throughout the basic steam cycle, the feedwater must begin at a central generation and storage location. In previous lessons the evaporators were discussed in detail. A reminder is the distribution of water from the evaporators to the reserve feed tanks. These are designated as Feedwater Reserve tanks. (Refer to Figure 1)

Figure 1

a) The reserve feed tanks are equipped with suction and fill connections and a overflow/vent pipe. All tanks have sounding tubes for the watchstander to take soundings and for the Oil King to draw a sample for chemical testing. All of these tanks are connected to the reserve feed transfer manifold which is used to distribute the water generated from the evaporators.

1. Reserve feed transfer manifold

a. This manifold is normally located in the fireroom. The internal valve arrangement of the manifold allows feedwater to be received and transferred from tank to tank. In multi-plant ships, feedwater can be transferred from plant to plant. It also connects the evaporators and the topside fill riser into the feed system and allows filling of the feed tanks from either source.

2. Reserve feed transfer system

a. In order to move water from the feed tanks, we must have the capability to transfer the feedwater where it is required. Reserve feed transfer pumps (RFTP), emergency feed pumps (EFP), or air is introduced into the tanks to accomplish this task.

b. Tank level indicating systems (TLI's) can monitor feed water tanks remotely by reading the Gem gages installed. These will be discussed in a later lesson.

Figure 2

B. Reserve feed is water in storage awaiting usage in the plant in the event of water losses by use of catapults, soot blowing or surface blowing. These losses must be made up by introducing water back into the steam cycle. Although each plant is designed to operate independently, some occasions may arise where water would need to be transferred from one plant to the other. (Refer to figure 2).
Reserve Feed System

1. The reserve feed tanks (RFT) are filled from one of three sources through the reserve feed transfer manifold (refer to figure 1 and 2):

a. First, the topside riser is used to bring feed from a pier service (tanker truck or demineralizer) to initially fill the RFT's, Second, the ship’s evaporators will be aligned once in operation and Third, you can transfer water from one tank to the other using the RFTP/EFP. Prior to aligning feedwater to the tanks, it is tested for proper chemistry (discussed in the BW/FW course).

b. While operating, the RFT's are normally filled from the evaporators. You cannot distill to the tank that is aligned for make up.

c. At any time the water can be transferred by gravity drain, air pressure or by the RFTP/EFP.

d. Once filled it shall be tested prior to use as MUF to ensure satisfactory water chemistry.

e. From the RFT/MUF tank, water is sent to the main or auxiliary condensers to maintain the inventory of feedwater in the system required for plant operations.

2. Make up/Excess feed valves (refer to figure 3)

Figure 3

a. The purpose of the make up and excess feed valves is to maintain the water inventory of the feed system by controlling DFT level. The majority of today's steam ships have make up and excess feed valves. However, there are some that do not use vacuum drag (discussed in previous lessons) and manual rundown valves.

(1) Both valves are air operated diaphragm valves controlled by the pneumatic output of the pilot controllers. In the event of a loss of control air, a spring opposing the air input will close the excess feed valve. For the make up feed valve the spring will open the valve upon a loss of control air. It is better to have to much water in the system than not enough upon a loss of control air pressure.

(2) The excess feed valve is located in a branch line off of the main condensate line prior to entering the DFT. When open, it diverts condensate from entering the DFT into the tank that is aligned for make up. This can be a problem on ships equipped with a demineralizer as the heat from the water from the condensate system can break down the resin bed causing gross contamination of the feed system.

(3) The make up feed valve is located on the inlet side between the reserve feed tank and the main or auxiliary condensers. It opens when DFT level is low to allow MUF to be drawn into the condensers by the vacuum maintained inside the condensers. On most ships, make up feed is introduced by vacuum drag into the main condenser. The main condenser, while in operation will run under a sub-atmospheric pressure commonly know as a vacuum. When more water is introduced into the condensers, the principle of submergence control will begin to take affect. The higher the water level the more volume the condensate pumps will produce. This will ultimately send more water to the DFT and the water level will return to the normal operating level.

(4) This system does have it’s disadvantage. Using vacuum drag, you stand a better chance of gross contamination of the entire feed water system during a feedwater chemistry casualty.

b. Deaerating feed tank (DFT) water level is the monitored parameter in the steam cycle to determine the need for make-up feed (MUF) or the need to excess feedwater from the system back to the tank on make-up. (Refer to Figure 2).

(1) The levelmatic control system of the DFT senses both actual water level and a reference level pressure input through hydrostatic pressure legs to control and monitor the make up and excess feed requirements.

(2) Hydrostatic legs

(a) Reference leg - A vertical length of pipe inside the DFT which is kept full of water. This provides a static reference pressure input signal to the level control system.

(b) Variable leg - Connected to the underside of the DFT to provide a pressure signal to the level control system which varies with DFT level.

(c) Both legs are required so a change in shell pressure (due to Auxiliary Exhaust pressure fluctuations) affects both legs equally. This ensures the difference in leg pressures will be maintained independent of the DFT shell pressure.

(d) Pilot controllers sense a difference in hydrostatic head pressure and develop a pneumatic output relative to actual water level. These pneumatic output signals go to the MUF and excess feed valves to control the feed inventory of the DFT.

3. Ion exchanger (demineralizer)

a. Located in the MUF line between the MUF tank and the main or auxiliary condenser.

b. The function of the demineralizer is to remove scale forming contaminants and suspended solids from the MUF.

c. Can be bypassed if the resin bed is exhausted and make up feed is required.

d. Most ship classes have a demineralizer installed.

NOTE: Resin beds generally last six (6) months, depending upon the MUF usage. Special conductivity cells are installed to monitor the inlet and outlet readings of the demineralizer.

4. Salinity cells

a. One is located in the MUF line prior to the main or auxiliary condensers.

b. The salinity cell monitors the MUF prior to entering the main or auxiliary condenser.

5. Make up/Excess feed system operation (refer to figure 3)

6. As DFT level lowers, the MUF valve begins to open to allow water into the condensers. Lets look at an example of this with a DFT that has a normal water level of 1000 gallons, the MUF valve starts to open at 895 gallons and would be fully open at 800 gallons.

a. When vacuum drag is aligned, the make up feed is introduced directly into the main condenser. The theory of submergence control will take affect on the condensate pump and more volume of water will be pumped to the DFT.

b. When the level in the DFT rises, the make-up feed valve will shut as long as the water level remains above this set point of water level in the DFT.

c. When the level in the DFT rises, the excess valve starts to open and dumps condensate back to the tank that is aligned for make up before it enters the DFT. An example of this is a DFT water level of 1105 gallon starts to open the excess valve and at 1200 gallons this valve will be fully open for a DFT with a normal water level of 1000 gallons.

d. The water level in the DFT continues to remain at normal, the excess valve will fully close.

e.Remember, the DFT level is the monitoring and controlling location for excess and make-up feed. This level in the DFT is maintained to provide sufficient water in storage in the event the boiler should have an extreme low water level casualty.