Why molybdenum steel valve seats are used?

The valve lead and valve seat work under most demanding conditions. These must remain tight when the valve is seated. When the exhaust valve just starts to open the gases at high pressure and temperature escape at great velocity that may cause erosion of the valve and seat material. Such erosion also take place if some dirt or grit from ash gets lodged between the valve and seat and there is leakage of gas.

If the fuel contains elements such as sodium and vanadium, at high temp a viscous and sticky compound is deposited that is very corrosive. At low temperature there may be dew formation and sulphur oxides produce during combustion coming in contact with water may form acid causing corrosion of parts.

The valve seat therefore require periodic servicing. If the seat were an integral part of exhaust valve cage or the cylinder cover, then the entire cage or cover may need to be replaced, which would be very expensive. For this reason detachable valve seats made of high quality heat resistant material is fitted.

Valve Seats main functions:

Compression gas sealing – Prevents compressed gaseous bodies and combustion gas from leaking into the manifold.
Heat transfer – Releases heat in the valve to the cylinder head
Strength – Holds tight when the valve is mounted.
Wear-resistance – Hard to wear down under high heat and high load.

Properties of molybdenum

Molybdenum is hard slivery metal with very high melting point. It is primarily use to make alloys with other metals. The most common alloy of molybdenum are those with steel. Molybdenum improve strength, toughness, resistance to wear and corrosion and ability to harden steel.

Comments

Bilge Injection Valve

Bilge Injection is a valve that enables the engine room bilges to be pumped out directly overboard in the event of an emergency such as flooding. The valve is normally fitted to the end of a branch connection with the main sea water suction line. This enables large main seawater cooling pumps to be used as a bilge pump in an emergency. Emergencies like fire and flooding involve the use of seawater. If there is a fire, seawater is the biggest resource of water available in the sea. Similarly, if it involves flooding of the engine room, cargo spaces or any other place on the ship for that matter; you would again require pumping the sea water out of the ship. In both these cases, you require pumps. There are two valves in close proximity namely main injection valve and bilge injection valve. Both of them have their own independent controls. The diameter of the bilge injection valve is kept nearly 66% of the main valve diameter which draws water directly from the sea th...

Difference Between A, B & C-Class Divisions?

IMO Symbol A Class Division IMO Symbol B Class Division SOLAS has tables for structural fire protection requirement of bulkheads and decks. The requirements depend on the spaces in question and are different for passenger ships and cargo ships. The Administration has required a test of a prototype bulkhead or deck in accordance with the Fire Test Procedures Code to ensure that it meets the above requirements for integrity and temperature rise. Types of Divisions: "A" Class "B" Class "C" Class "A" Class: "A" class divisions are those divisions formed by bulkheads and decks which comply with the following criteria: They are constructed of steel or equivalent material They are suitably stiffened They are constructed as to be capable of preventing the passage of smoke and flame to the end of the one-hour standard fire test. they are insulated with approved non-combustible materials such that the average tempera...

Why do we blow through engine before starting ?

The engine is blown through on air before starting it : a) To Blow out through indicator cock any residual exhaust gas or other products of combustion trapped inside the cylinder after shutting the engine. b) To check if any jacket cooling water from cylinder head or turbocharger or any other source, has leaked while the engine was shut and collected on top of piston. If while blowing through, water comes out of indicator cock or relief valve, we need to investigate and rectify the fault before starting the engine. It is imperative that the engine is blown through before starting. There have been cases where the generator engine has been started from control room without bothering to blow th...

Marine Engineering

Search This Blog