Suction pressure depends upon the tank level. When tanks level are high , suction pressure is high and vice versa. The height of cargo divided by 10 shall be your approx suction pressure.
Sometimes when you prepare for cargo discharging and fill separators, you get less suction pressure because of vapour lock in cargo pump separators. The solution is simple.
1. Open discharge valve of pump.
2. Open line master on deck.
3. Open goose neck.
4. Now open bulkead masters.
5. Open tank valve upto 10%. Let the separators fill properly. Any vapour will escape through cargo discharge line to goose neck into the tank.
Let's take an example of VLCC.
When tank ullage reaches 20-22 mtrs ( max 28 mtrs ) , suction pressure normally comes close to 0, that means the amount of cargo required to discharge at same pressure is equal to the amount of available cargo at suction side.
Now in VLCC, the cargo line going inside the tank is approximate 2 mtrs above tank top. This additional creates a negative suction pressure of about 0.2 kg / cm2.
That means while discharging you can safely upto -0.2 kg / cm2 suction pressure.
When suction pressure goes below this level, that is the time to reduce pump rpm to maintain suction pressure in between 0 - -0.2 kg/cm2
Discharge pressure.
Discharge pressure depends upon various factors.
1. Shore pipe line length
2. Shore pipe line diameter
3. Shore tank height
4. Shore tank level.
5. Suction pressure at vessel end.
6. Pump RPM
Discharge pressure is maintained as per charter party requirements or as per company SMS. The pressure is always given at ship's manifold. Eg. My vessel can discharge cargo with a maximum manifold pressure of 10.0 kg/cm2.When you are discharging and tank levels get low, the first indication that tanks are not getting enough oil is fluctuating discharging pressure. As soon as discharging pressure fluctuates, pump RPM can be reduced, if required further, pump discharge valve can be throttled.
How to use the pressure to take tanks to stripping level.
1. As soon as discharging pressure fluctuates, reduce pump RPM
2. Keep reducing RPM as required.
3. When pumps are at minimum RPM and discharge pressure is fluctuating, start throttling pump discharge valves.
4. If required, stop 1 pump out of 3 or further 2 pumps out of 3 aa required. This depends upon vessel to vessel and line groups
