What are Boiler Mountings?
Boiler mountings are a set of safety devices installed for the safe operation of a boiler. There are seven main mountings on a boiler shell; safety valve, steam stop valve, vent valve, pressure gauge, water level indicator, feed check valve, and fusible plug.
This equipment saves the boiler from damage due to extreme pressure, steam backflow, shell collapse due to vacuum, unregulated steam pressure, low water level, backflow of feed water to the pump, and dry running respectively. Today we will discuss boiler mountings and their types used in steam boilers with further explanation of their construction and working.
Regular accidents and boiler break down can happen due to improper maintenance and cleaning. Without boiler mounting, one can damage the boiler and its surrounding with one mistake followed by a series of events.
Without a high lift safety valve, the shell could explode resulting in great loss of life and machinery; similarly, a shell could collapse under vacuum if an air vent is not present during cooling down.
During cargo operation, there is an increased demand for steam and so the mountings help operate the boiler with more and ease and safety.
Boiler mountings are as follows:
- Safety Valve
- Main Steam Stop Valve
- Auxiliary Steam Stop Valve
- Water level gauge glass
- Air Vent valve
- Feed check Valve
- Pressure gauge connection
- Blow Down Valve
- Scum Blow Down Valve
- Sampling Connection
- Whistle Valve
- Low Level Alarm
- Soot Blower
- Automatic Feed valve Regulator
- Mud box
- TDS Sensor and Sample
1. Safety Valves
Safety valves are fitted on every boiler to avoid over-pressurizing. Normally three safety valves are fitted on the boiler with one on the superheater and the rest two on the steam drum. In no condition these valves, be less than two in number and must lift at pressure 3% above boiler working pressure irrespective of boiler types.
A steam valve is made up of a cast iron body with two independent valves fitted on the valve seat. These valves are connected to a lever by means of a pivot held tight to its position by spring.
The spring force keeps the valve sits shut on the valve seat under normal conditions. When the upward pressure exceeds the downward spring force; the valve is lifted and excess steam is released into the atmosphere.
2. Steam Stop valve
A steam stop valve is connected to the boiler to stop and regulate steam flow from the boiler to the distribution lines. The main steam stop valve on the boiler is kept shut to avoid back-flow of steam to the boiler. The flange of the steam stop valve is bolted on top of the steam drum.
The valve main body is made of cast iron while the valve seat is made from gunmetal. The spindle on one end is connected to the valve while the other end is to the handle wheel passing through (yoke/Gland nut) and gland packing.
The valve is operated by rotating the handwheel. The rotating handwheel, in turn, rotates the spindle which moves the valve up allowing a path for steam to flow.
Auxiliary steam Stop Valve: A separate steam line provided by the boiler for the small auxiliary system is supplied via this valve in most of the steam systems on vessels. The valve is smaller in size and is generally a non-return type.
3. Vent Valve
A vent valve is installed on the boiler shell to vent air from the steam drum during starting of the boiler. These vent valves also come in handy during boiler shutdown as they let fresh air enter the boiler drum avoiding its collapse under pressure. A vent valve can also be used to release/dump moist steam at the start.
4. Pressure Gauge
Pressure gauges are fitted to the steam drum and superheater to indicate steam pressure inside. These gauges are fitted on the front top of the boiler shell and represent pressure in the bar. A bourdon tube of a closed cross-section is attached to the steam space on one end through a siphon tube.
The tube itself contains water to avoid steam entering the pressure gauge. The pointer is connected to the threaded gear attached to the spindle. When pressure is applied to the bourdon tube it becomes circular turning the spindle. This causes the pointer to move along with the gear; representing the boiler pressure.
5. Water Level Indicator
A pair of water level indicators are installed directly to the boiler shell with an additional remote reading gauge installed at a convenient position. They are installed directly on the front end of all boiler types; showing water level in the boiler drum. It consists of a glass tube with three independent valves (Steam valve, water valves, and drain valve).
Steam and water valve separates the glass tube with boiler steam and water respectively. Drain valve on other hand used to drain water from a glass tube. A metal ball is provided on the waterside of the gauge glass to avoid subsequent accidents and water loss; by water flashing off steam in event of glass rapture/failure.
Under normal conditions both steam and water, valves are open allowing water and steam pressure to balance. In event of incorrect reading, we need to blow through; by closing the water valve and opening the drain valve.
A strong blow will indicate the steam valve is clear; now repeat the process with the steam valve closed and the water valve opened. Strong blow-off steam with hissing sound indicates the water valve is clear. Now close the drain valve and let the water fill in; slowly open the steam valve equalizing the pressure.
6. Feed Check Valve
A feed check valve is a non-return valve type fitted on the boiler shell just below the normal water level. It regulates the flow of feed water restricting possible backflow to the feed pump. These valves are usually fitted with an extended spindle for remote operation.
The inlet and outlet points of these valves are exposed to different pressure. When the feed pump is operational; sufficient pressure is built at the inlet point of the valve. When the inlet pressure is more than the outlet point the valve lift allowing water to pass to the boiler.
7. Fusible Plug
A fusible plug is the threaded gunmetal cylinder with a conical plug and tappet hole drilled into it. This hole is then filled with an alloy of a low melting point such as tin. The plug can be of either fire actuated or steam actuated type fitted over the combustion chamber.
Under normal conditions, one side of the plug is exposed to extreme temperature while the other is submerged underwater; keeping it cool. This low melting point alloy cannot melt away till submerged and so remain intact even under extreme condition.
In event of the water level reach a safe limit and the plug tip is exposed to steam; the tin alloy will melt exposing the combustion chamber to steam. As steam is not an effective coolant and convection medium the tin alloy can’t transfer heat to the steam leading it to melt away. This sudden injection of steam into the furnace will stop the combustion protecting the boiler from any damage.
8. Blow-down valve
Location: Blow Down is an important Boiler Mountings, which is located at the bottom of the boiler.
Function: To empty the boiler whenever required & to remove the mud, scale or sediments accumulated at the bottom of the boiler.
It is used when chloride levels become very high to empty a boiler entirely for cleaning purposes or to treat the boiler’s water.
9. Scum blows down valve
Location: This Boiler mounting is fitted to the crown plate of the furnace or the firebox.
Function: To put off the fire in the furnace of the boiler when the level of water in the boiler falls to an unsafe limit and thus avoids the explosion which may place due to overheating of the furnace plate.
A shallow dish type system equipped at ordinary water level that enables floating impurities to blow down, oil foaming, etc. from the surface of the water.
10. Sampling connection /Salinometer Valve
Location: A sampling water valve arrangement is generally equipped with a cooler in sequence so that water samples can be collected at any moment for the testing of feed water.
Function: For salinity tests of water
11. Whistle valve
If a steam whistle is provided in the ship, steam is supplied directly from the boiler, via a small non-return valve form known as a whistle valve.
12. Low-Level alarm
Function: an apparatus used to trigger low water level audible conditions.
13. Soot blowers
Function: Required for soot blowing and pipe surface combustion products. It is operated by steam or compressed air.
14. Automatic feed water regulator
Location: A device that is vital and fitted into the feed line
Function: Automatic feed water regulator is one of the important boiler mountings to ensure the appropriate level of water in all load circumstances. In boilers with high evaporation rates, a multiple feed element water control system is used.
Function: The boiler is fitted with multiple manhole gates enabling inspection, cleaning, and maintenance of boiler tubes and inner components by the crew.
Location: Usually one gate is fitted in the steam drum and one in the water drum.
16. Mud box
Function: The mud box in the boiler is used to collect mud (muddy impurities) from the water drum
Location: Fitted at the bottom of the water drum.
17. TDS Sensor and Sample
Function: Most modern boilers are now equipped with this system for continuous monitoring of total dissolved solids in the boiler water. The sensor compares the true value to the setpoint, and an audio-visual alarm is given if the value is on the higher side.
A manual blowdown to introduce fresh feedwater into the system to reduce the total dissolved solids.