Answer MEP Question 1

Q. With respect to the operation of the two-stage air compressor. Explain 

A. The causes of the collapse of the discharge valve spring. 

B. Breakage of valve plates.

C. Overheating of the discharged air with unrestricted airflow.

D. Inoperative piston rings

E. Occasional lifting of 2nd stage relief valve. 

F. Puncture of bursting disc of 1st stage inter-cooler. 

G. Noticeable reduction in the capacity of the compressors over a period of time. 

A. The collapse of discharge valve spring.

(i) Overheated air or valve assembly. 

(ii) Obstruction in valve operation due to dirt, dust or carbon deposition. 

(iii) Excessive moisture in the air. 

(iv) Lubrication is inadequate. 

(v) Fatigue fracture. 

(vi) Corrosion. 

Overheating of the valve can be due to the following reasons:- 

- Improper cooling of the air in the intercooler. intercooler may be dirty or have air trapped. Airside have oil/carbon deposition or waterside have scale adhered. Cooling water supply is not sufficient which can be due to partial opening of the valves or supply pump has lost efficiency. 

- Air can be overheated on repeated compression of the same volume. This is because of chocked suction filter or leakage discharge valves. 

Valve operation may be obstructed because of the following reasons:- 

- Improper or excessive lubrication of the valve or the oil may get deteriorated. 

- Dirt or dust carried by air. 

- Scraper rings are worn out. 

B. Breakage of valve plates. 

(i) Running hours exceeded the maximum manufacturer recommended limit, causes the material of the valve plate to become unfit for the service and result in breakage of the same. If the compressor valves are not overhauled in the right frequency, the valve plates shall not run more than 5000 operating hours even if they are not leaking.

(ii) When not fitted properly with adequate torque or with an improper copper gasket the valve will tend to flutter. Also, excessive worn out of plate may cause the same effect, this will lead to breakage of the valve plate. The valves run  with worn or without worn copper gasket or a proper locking arrangement starts to flutter in the space and breaks.

(iii) When the valve is assembled incorrectly the valve plate will break. This involves interchanging of suction to discharge and inadequate tightening torque. The  assembly  nut  shall  be  tightened   to  the correct torque, improper torque can lead to valve breakage.

(iv) Deposits on the valve will make its operation sluggish which results in loss of efficiency and make the compressor running hotter, which will result in breakage of valve plates. Accumulation Of oil and carbon leads to creating sticky valves which do not operate,  collection of carbon and vapour also leads to Combustion and detonation  due to heat

(v) Valve plates break because of the impact caused by breakage of valve springs or loss of their resilience. 

C. Overheating of the discharged air with unrestricted airflow.

Since the airflow is unrestricted it can be assured that the valves and filters are in a healthy state and are not causing the overheating of air. Other reasons for the temperature rise of the air discharged can be fouling of inter-stage coolers. The following reasons may reduce the efficiency of an intercooler:- 

- Improper cooling of the air in the intercooler. 

- Intercooler may be dirty or have air trapped. 

- Airside have oil/carbon deposition or waterside have scale adhered. 

- Cooling water supply is not sufficient which can be due to the partial opening of the valves or the supply pump has lost efficiency. 

D. Inoperative piston rings.

Excessively worn out: because of use after the running hours recommended by the manufacturer. 

Improper lubrication: This can be because of a choked lube oil strainer or filter, because of a loose belt of the lube oil pump, because of the low level of lube oil in the sump. Or lube oil may get deteriorated. 

Poor size or material of the Piston ring used during the last overhauling. 

Airflow is restricted because of the leaky valves, choked filters or partially closed valves in the discharge line, causing the air to get overheated and rendering the lubrication of the liner ineffective. compressor running hot. 

Excessive worn out of the liner and thus the rings are not sealing against the liner and air is leaking past the rings. 

Improper maintenance, carbon deposition in the ring grooves may be caused by the seizure of the rings. 

E. Occasional lifting of 2nd stage relief valve. 

(i) Non-return valve in the discharge line of the compressor is stuck or clogged. Which providing a restriction in the flow of compressed air causing the relief valve in the second stage to lift occasionally. 

(ii) H.P trip of the air compressor is set above the relief valve setting. This should activate the air receiver relief valve, but if for any reason the relief valve of the air receiver is not operating, it will cause the 2nd stage relief valve to lift.

(iii) Main discharge valve of the air compressor is partially closed or being clogged. Or the filling valve of the air receiver is closed or being clogged. Clogging can be due to carbon deposition or broken valve plate pieces or foreign particles stuck in the valve.

(iv) Relief valve is malfunctioning or the pressure setting of the valve is inappropriate. Check and calibrate the second stage relief valve.  

(F). Puncture of Bursting Disc. 

Leakage of high-pressure air from the tubes in the waterside of the inter-cooler can damage the body of the inter-cooler, bursting disc is fitted on the inter-cooler which upon rapture releases the high pressure to atmosphere and act as a safeguard for the body of inter-cooler and compressor. Puncturing of the bursting disc indicates a high pressure in the cooling water space, caused by leaky air tube, high pressure of the water inside, high temperature of water and cooling water pressure fluctuation. 

(G). Noticeable reduction in the capacity of the compressor over a period of time. 

(i) Clogged air suction filter will cause less fresh air intake and re-expansion of the air in the bumping clearance. This will result in loss of compressor efficiency and will make the compressor running hot. 

(ii) Valve plates are dirty, damaged or worn out thus not sealing properly to achieve the required compression. A leaky suction valve will reduce the volume of air in the stage.

(iii) Inadequate cooling of the intercooler, this is due to dirty tubes, scale formation in the cooling surface on waterside, oil or carbon deposition on airside, the inadequate flow of the cooling water, or improper temperature of the cooling water. 

(iv) Air is leaking from various joints or gaskets, leakage from valve glands, un-loader drain valve sticky or leakage from the air bottle drain. Major air leak in the system may also add to the compressor performance hence to assess the performance the compressor should be tested on the standby air bottle with valves ensure closed and not leaking.

(v) Blow past, which is caused due to worn liner or piston rings considerably contribute in the reduction in capacity of the air compressor.

(vi) Higher value of bumping clearance will cause an ample volume of air un discharged in the same stage, that will expand in the next suction stroke, this cause a less volume of fresh air intake and reduce the volumetric efficiency of the air compressor. Also the compressor will run hotter.

(vii) Lower value of bumping clearance will cause the a reduced volume of fresh air intake and thus reduce the volumetric efficiency. Bumping clearance is altered by wrong thickness of cylinder head gasket, main bearing and big end bearing worn out.