Reported by:
Eng. Khaled Al-Oufi
Submitted:
19 / Aug / 2006
25 / Ragab / 1427
Function:
Oil separator function is to purify water from oil before drain it to sea. This contamination of oil mixed with water will pollute the sea water. This contamination occurs in the plant normally. It comes because of draining the condensation of oil tank moisture. Also the second source is oil heater steam.
Design the System:
The system simply contents of two tanks. One of them is to receive the mixed of oil and water and separate the oil depending on density theory so that oil float on the water and form oil layer at the top. The water drains from down filter to next tank. The second tank is to purify the water from small parts of oil could escape from the first tank. The water will drain to the sea through two floating valves. These valves ensure that drain water pure and free of oil contamination. The system is shown in figure 2-1.
Figure 2-1
Problem definition:
After design the oil separator system, the system should examinant before putting on the service. After examination the system, the system failed because of two reasons. First civil fault, the water leaks through the wall of the tanks. Second is a mechanical fault, floating valve not opening even the float covered by water.
Mechanical Fault Reason:
Why floating valve not opening? This is a problem and expected reasons are the float can't take the weight of the valve and this wasn't the reason because when putting the float valve out of the drain pipe it's floated. To find the reason, filling the tank again by water, and try to lift the valve manually, suddenly it's fluctuating and hammering the seat and close again. This phenomena guide to the reason which is the water flow-rate is too high and this pulls the valve disc by making a vacuum under the disc. The reason not clears yet. Why water flow with high rate? The main reason for all this is different in water level between water level in the tank and discharge pipe flange. Also difference between discharge flange and drain level effect the problem. Figure 4-1. This different cause difference on the pressure and this difference on pressure cause the water to flow faster.
Figure 4-1
Suggested Solutions and Ideas:
To solve the floating valve problem there are many solutions.
First idea is to cancel all discharge pipes and floating valves too. Substitute this drain system by making discharge hole at the wall and add floating filter.
Advantages:
1- simple
2- ensure to maintain water level
Disadvantages:
1- takes time
2- fear to damage the tank wall while breaking the hole
3- escaping oil possible
Second idea is to increase the height of drain hole.
Advantages:
1- ensure to maintain water level
2- ensure oil from escaping
Disadvantages:
1- takes time
2- hard to change the drain hole level.
3- maintenance of floating valve periodically
Third idea is changing discharge pipes level by connecting pipes. These pipes valves extend the discharge level to tank water level.
Advantages:
1- simple
2- quick
3- easy to produce
Disadvantages:
1- special pipes material needed
2- maintenance of floating valve periodically
Select the Solution:
To avoid random selection and choose the best solution from previous ideas, Must and Want method should be used. Must criteria are purity of the drain water and level reliability of the system. For Want criteria, time takes 5 impac points, simplicity to fabricate takes 10 points, cheapest takes 7 points and maintenance takes 7 points. Table 6-1 and 6-2 show the method.
From table 6-1 second and third idea pass to Want criteria।
From table 6-2 third idea of extend the pipe level scores highest point। So that, the best solution is to fabricate extending pipe which rise the discharge level to the level of tank water। Figure 6-1 shows the solution।
Figure 6-1: Extended pipe solution
Conclusion:
Mechanical fault was floating valve not open and at time open it manually it was fluctuating and close again। The reason was difference in level between discharge level and water level in the tank. The water flow rate was high due to difference on pressure so that create vacuum under the valve disc. This vacuum causes valve to fluctuating and close. The best solution is to raise the discharge level up to water level in the tank by extended pipes. After applying this solution and test it, it passes and the problem is solved.
