Marine Engineering Study Materials

What is cavitation, causes and remedies ?

• Cavitation can be occurred when the pump suction contains air or when the pump is running faster than the designed speed.
• Low pressure regions occur in the flow at points where high local velocities exist.
• If vaporisation occurs due to these low pressure areas then bubbles occur, these expand as they move with the flow and collapse when they reach a high pressure region.
• Such formation and collapse of bubbles is very rapid and collapse near a surface can generate very high pressure hammer blows which results in pitting, noise, vibration, and fall off in the pump efficiency.
• So the pump should be run at designed speed and throttling of the suction valve should be avoided.

What are the reasons for ballast pump not delivering water ?

1. Incomplete priming
2. Too high suction lift
3. Low net positive suction head
4. Air leaks in suction line
5. Gas or air lock
6. Suction filter choke

Screw pump overhaul procedure ?

1. Put off breaker and remove fuse from motor starter box
2. Close suction and discharge valve
3. Remove suction and discharge pipe flange bolt & nut, coupling bolt of motor coupling
4. Remove pump foundation bolts & take out pump from motor
5. Remove cover both side & remove driver screw & driven screw with bearing and mechanical seal
6. Inspect mechanical seal, bearing and renew if required.
7. Clean all parts and assemble back.

What are the check points in screw pumps ?

• Mechanical seal
• Bearing
• Driver screw and driven screw wear & tear
• Pump casing internal surface wear
• Clearance between driver and driven screw
• Clearance between driven and casing.
• Clearance between driver and casing.

What is the centrifugal pump overhaul procedure ?

1. Put off breaker and remove fuse from motor starter box
2. Close suction and discharge valve
3. Remove suction and discharge pipe flange bolt & nut, coupling bolt of motor coupling
4. Remove pump foundation bolts & take out pump from motor
5. Remove impeller lock & nut & take out the impeller
6. Remove mechanical seal.
7. Remove bearing cover from other side & take out shaft with ball bearing
8. Renew mechanical seal and bearings if required.
9. Clean and inspect all parts and assemble.

What are the check points in centrifugal pumps ?

• Impeller
• Wear ring
• Neck bush
• Ball bearing
• Shaft.
• Mechanical seal.

Reference:

Oral Guide by – MIN ZAR TAR