Suction Piping Consideration

On ship, transfer fluida is important system during operation ship. Like on main engine, engine operation can not run without transfer fuel from fuel oil daily tank to engine, similarly cooling line, on sea water cooling line, Main engine will black out or over heating if there is no transfer sea water from sea chest to main engine.

This transfer fluida need equipment, that equipment wee call pump. On pump have two important position, suction side and discharge side. Suction pump is important position on pump operation, because of that we need some consideration to make suction design. That's consideration are :

1. The suction line size should always be one size larger than the pump suction connection size.
2. The optimum lenght of full size straight pipe, devoid of any obstruction to flow, immedietly up-stream of the pump suction connection, should be 10x suction pipe diameter.
3. The number valve and fitting in the suction line should be minimized because can increase pipe looses.
4. Suction piping should be designed and installed to minimize entrapped air. This consist of, avoidance of horisontal plane concentric reducer and proper orientation of eccentric reducer and straight position install on the top, and positive slop line.
5. Directly connected elbows should be avoided and when absolutly necessary, should be of the long radius style oriented to minimized un-balanced flow to impeller.
6. Suction line pipe joint design should be air tight. This is particularly true in long suction line and in situations in wich the situation lift is large.
7. Pump install on position as low as posible with liquid source to get NPSH requirement.
8. On suction line, velocity of fluida is designed on 5 ft/sec or 1.5 m/s.
9. If on suction line connect to manifold, the branch manifold line should be have velocity range on 5-8 ft/ sec or 1.5-2.5 m/s.

All above consideration is designed to get some advantage as followed :

1. To avoid hydraulic noise and associated vibration.
2. To avoid cavitation damage and its detrimental effects on vobration, bearing life, and seal life.
3. To avoid noisy operation, random axial load oscillations.
4. To avoid collecting air or gas at hight point.

Pressure Test

On shipyard normally have 2 time pressure test. Like on ABS Steel Vessel Rule 4.6.2 / 7.3 Hydrostatic Test. 

1. On shop pressure test, this test doing after workshop fabrication complete and accepted by owner. On this test to check welding quality, any leaking or not on shop weld position. This is refer to ABS Rule 7.3.1 "Hydrostatic Test of Pipes Before Installation Onboard" 

2. On site pressure tested, this teste doing after on site pipe erection complete and accepted by class and owner.  On this test to check flange spool joint and field weld position. This refer to ABS rule 7.3.3 "Test After Installation"

On pressure test there is some rule must be followed :

1. Pressure tust must 1.5 working pressure.
2. Pressure gauge must have active calibration certivicate.
3. Pipe line must accepted for final pipe line inspection by owner and class.
4. Pressure test must aware with safety oriented, like using HP square ball valve etc.

On Shipyard industry, normally have 2 methode pressure test.

1. Hydrotest thats mean pressure test using water pressure
2. Pneumatic test pressure thats mean pressure test using air pressure.

For item no 2 or pneumatic test, following ABS Steel rule 2012 no 4.6.2 / 7.3.4 "Pneumatic Test in Lieu of Hydrostatic Test" on this rule mention that's In general, Pneumatic test in lieu of hydrostatic test are not permitted. Only on special case, where it is impracticable to carry out the required hydrostatic tests, pneumatic tests may be considered. In such cases, the procedure for carrying out pneumatic test, having regard to safety of personnel, is to be submitted to the surveyor review.

Why pneumatic not permitted for pressure test, may be this is about safety reason. On pneumatic test we use compress air, the nature of the air is easy to expands. this is dangerous if we doing on open area, the weather will give dirrect effect to air, if we start test on morning and then we keep pressure untill noon, the air pressure will increase fastly and some time air pressure will be over presure requirement and will be explosion.

Other reason is, pneumatic difficult to check leaking position. This is not like on hydrotest, on hydrotest at least we will found seepage water on pipe joint when pipe leaking but on pneumatic test for small leaking difficult to identify althought we use shoap water to spry around pipe joint but this one not evective action.

Last reason, to avoid cheating trick, If we compare between pneumatic test and hydrotest, hydrotest is more difficult to cheat, because if any little leaking will show or identify with water drops, and for that case very easy to looking for. I f we use pneumatic test, we need good water foam to make air bubbles for leaking identification. And sometime, we can not see this air bubbles.