Answer EKM Question 1

Q1. With reference to 2-Stroke Slow Speed Engine:
A. Sketch and describe Main Engine Exhaust Valve.

B. List out a procedure for test of Main Engine Exhaust Valve after overhaul.

Answer: A. Sketch and describe Main Engine Exhaust Valve.

Hydraulic valve actuator:

(a) Exhaust valves actuated by means of a Push Rod and Rocker Arm Unit face a problem of absence of true axial force on valve stem during operation. The operation results excess wear between valve stem and its bushing because of side thrust exerted by push rod on the valve stem during opening.

(b) Hydraulic actuator overcomes the above problem. A piston is fitted on the top of the valve stem inside a hydraulic cylinder, Piston operates by means of an actuator pump unit through high pressure sheathed pipe,

(c) A pair of piston rings, provides sealing in the open bottomed hydraulic cylinder. An oil bleed valve located at the top of hydraulic cylinder allows a controlled amount of oil to pass to air cylinder located below the hydraulic cylinder for lubrication.

(d) The camshaft driven actuator pump delivers oil to the valve to open, so that on the download stroke oil returns to the actuator pump cylinder as there is no delivery valve in the pump discharge line allows the valve to close under the action of closing mechanism.

(e) A relief valve is located at the top of the pump cylinder.

(f) A makeup oil connection is located just above the top of the pump piston to ensure full oil discharge during downward stroke.

(g) A puncture valve on the actuator pump may be actuated to release some pressure to delay exhaust valve opening when starting the engine in Astern direction to ensure full starting air affect.

Valve Assembly:

(a) Cage type valves comprises water cooled body, detachable seat, valve, spring unit and valve drive.

(b) The single valve is located in the centre of the cylinder cover and connected to it by means of four or more studs and hydraulically tightened nuts. The valve cage form a gas tight seal with cylinder cover with a mild steel or soft iron ring.

(c) Valve cage is made of cast iron as it is not subjected to mechanical/thermal loading.

(d) Detachable seat of molybdenum steel is able to resist thermal/corrosive environment and can easily be machined.

(e) Seat area generally incorporates an insert of stellite material (hard alloy) is expensive but having good corrosion and damage resistance at high temperature.

(f) Valve spindle guide (bush) of bronze fitted in the valve body.

(g) Spindle can be made up of the following combinations:

-Austenitic material with stellite facing on the seat and a coated stem [chrome coated (phasing out due to environmental reasons) or cermet].

- Two part Austenitic stem as above but with a Nimonic head and no insert on the seat [Nimonic -Nickel based alloy are wear and corrosion resistance at high temperature].

-A one piece Nimonic valve with no stellite insert.

Air Spring Cylinder:

Some positive force is needed to ensure valve closes after the rocker or hydraulic opening force is removed.

(a) Coil springs strength had to be sufficient to ensure that the rocker push rod follower remained in contact with its cam at all times to avoid impact damage. coil springs are subjected to fatigue failure. Cause rubbing wear at the contact face at the valve body and spring cover. Provide a frictional contact between valve body and valve steam which prevents valve rotation.

(b) Air Spring: A piston is fitted to the valve stem below the hydraulic cylinder and this piston reciprocates in a cylinder compressing the air in the cylinder when the valve opens. This compressed air acts on the piston to close the valve, when the upcoming hydraulic force is removed. Air leakage from the cylinder is made up from an sir supply at a pressure of 5 bar through non-return valve.

(c) The area of the piston is calculated to ensure the correct closing force. The space above the piston is vented to ensure no opening force.

(d) Lubrication of the cylinder wall by controlled leakage of hydraulic oil from hydraulic cylinder.

(e) Sealing air to the gland contains oil mist in order to limit wear at the seal and valve stem.

(f) At the top of the casing a spring loaded check rod passes through a bush and has contact with the groove in air piston. It indicates valve opening and also valve rotation as it re-seats.

Valve Rotation:

(a) When residual fuel is burned it produces number of damaging products including Vandium pentaoxide slag which is corrosive and solidified, can cause damage to the valve face and seat. If the valve is allowed to rotate slightly as it closes a light grinding effect is produced at the faces and these deposits are removed.

(b) By valve rotation local over heating is avoided (which may be because of defective fuel injector).

(c) A Rotocap device is not practical for large low speed engine as it produces set angle of rotation when valve is opening.

(d) The "Spinning Rotation" effect is brought by the action of exhaust gas on vanes or spinners fitted on the valve spindle. When exhaust valve opens the escaping exhaust gas acts on the spinners produces rotation if no frictional contract exists. If coil springs are used, so replaced by "Air Spring".

B. List out a procedure for test of Main Engine Exhaust Valve after overhaul.

Before fitting:-

1. Lift up the valve with the engine room crane (Whereby the valve may open) so that drop down check can be performed. Connect compressed air to the air cylinder to close the valve: Check with feeler gauge that there is a clearance between the outer parts of the seating faces of valve housing and spindle. 2.  After shutting-off the compressed air supply and releasing the air through the non-return valve on the control unit, the exhaust valve should open.  Repeat it a couple of times. 3. Leave the valve closed, and shut-off the compressed air. If the exhaust valve opens within 15 minutes, check for air leakages around the air spring.

4. Before mounting the valve on the engine, connect 7 bar working air to the air cylinder to keep the exhaust valve closed during the mounting process. 

5. Mount a new O-ring on the bottom piece and on the bottom of the exhaust valve. Lubricate the O-rings with Vaseline and the threads of the studs with anti-seizure paste. 

6. Position the valve in the cylinder cover bore, guiding it in accordance with the exhaust flanges and the guide pin on the exhaust side of the exhaust valve.

Checks after fitting:-

1. Check all the connections are made properly,  cooling water is vented, no water leakage to be found. sealing between bottom piece and valve housing is checked by opening air supply.

2. Indicator is engaged on the top of exhaust valve and valve is always closed by supplying spring air before opening the hydraulic oil for actuator.

3. Hydraulic oil to be supplied to the actuator and system is purged. Connections to be checked for any leakage. 

4. Engine to be tried out on air and operation of valve to be observed with the motion of indicator.

5. unit performance to be taken during the next voyage, recorded and compared.