Description
Structural Configuration
Series double valves: According to ASME PG-60 and ship conventions, two valves must be connected in series on the bottom drain pipe - a slow opening valve (globe valve/gate valve) on the boiler side and a quick opening valve (plug/ball valve/dedicated quick drain valve) on the outer side. The quick opening valve only fully opens and closes, and throttling is the responsibility of the slow opening valve to avoid the sealing surface being cut out of the groove by high-speed gas-liquid two-phase flow.
Valve body material: heat-resistant cast steel/forged steel, valve seat and disc are often welded with Stellite hard alloy to resist flash steam corrosion; Ship side shut-off valves are mostly made of bronze, with a lower pressure resistance level than furnace side valves.
Nominal diameter: DN20-50, horizontal shell boiler bottom not less than DN40.
Automation options: Modern marine boilers can be equipped with timers and pneumatic actuators (such as DBV-10 type) for regular automatic quick discharge, or connected to TDS control for continuous surface discharge (continuous discharge is another route, not at the bottom).
Operating Sequence
1. Select low load/standby or shutdown cooling before proceeding; First, raise the water level to the normal high level (low water level after anti drainage), and discharge the floating slag first and then the bottom.
2. Fully open the furnace side slow opening valve (fully open at once to prevent the medium from throttling and flushing at the valve seat).
3. Quickly open the quick open valve, controlled by the quick open valve on/off, and discharge for 20-60 seconds each time. The process can be repeated 2-3 times using water flow oscillation to stir up sediment.
4. When closing, first close the fast opening valve, then close the slow opening valve (in reverse order).
5. Close it tightly and touch the downstream pipe temperature - if it is still hot or steaming, it indicates that the quick opening valve or check valve is leaking.
NOTE: Propulsion/auxiliary boilers usually do not allow bottom discharge during high load navigation, and can only be operated when the boiler is shut down or at low load.
Function
Sedimentation removal (primary function)
The calcium and magnesium salts, rust, welding oxides, and mud and sand in the boiler water will precipitate to the lowest point of the drum/header after heating, forming water slag. These slag have a higher specific gravity than water and will not be discharged along with the surface. They can only be quickly opened and closed by the bottom valve during shutdown or low load, using high pressure difference to "flush" the slag out.
Control the concentration of boiler water
During the evaporation process, the boiler water continuously concentrates, and the alkalinity, chloride ions, and TDS (total dissolved solids) increase. Surface continuous discharge (CBD) discharges upper layer water, while bottom discharge helps reduce overall salt content and alkalinity by removing high concentration turbid water from the bottom layer, avoiding steam carrying water, steam water co distillation, and metal caustic embrittlement.
Prevent local scaling and pipe bursting
If the slag is not discharged, it will form hard scale on the water-cooled wall pipe mouth and the corner of the header, leading to deterioration of heat transfer, overheating of the pipe wall, expansion and even explosion. Regular practice of the bottom discharge valve is equivalent to clearing the intestines and stomach of the boiler.
Auxiliary emptying during shutdown maintenance
Although a drain valve is specifically used to stop the boiler and drain the boiler water, the bottom drain valve can also be used to drain the remaining water when it is cold, making it convenient for internal inspection and dredging.
