4 replies to this topic

[omideng]

Hi

We have a problem with our diesel backup system for turbine generators and I hope I can get your valuable inputs. I really appreciate for the time you put to go through this long description.

 

Recently we have found that there are a lot of free water in our diesel system. Strainers were clogged and water samples show diesel bug development in the system. Centrifuges are not also able to remove water effectively. A root cause analysis has been done by the operation team members and the air ingress through the vents into the system and moisture condensation was considered as a main suspect. (An estimate amount of 30 to 50 liter per day of moisture condensation). During the previous 2 weeks we have drained about 20 m3 free water from the system. I tried to red mark the main components of the diesel system contributing to this problem in the attached file.

 

We have two raw diesel tanks and two treated diesel tanks on south and north part of the platform. The raw diesel tanks are located on the main deck and the treated diesel tanks are located on the higher deck (upper mezzanine deck). The wind direction in our area is from south to north. So, we will have a little higher barometric pressure on the south treated tank and a little lower barometric pressure on the north. We checked the vents on the treated tanks and there is a considerable air breath-in through the vent on south tank and air breath-out through the vents on north treated diesel tank which confirms this pressure differential.This air can communicate with the untreated tanks on the main deck through the overflow lines (red line).

 

There is also a common header with NO valves (assume that these valves should be always open due to some operational limitations) that is used for periodic filling of raw diesel tanks from supply boats. I should say that the filling lines in the raw diesel tanks have no standpipe inside the tanks. So, the interconnection between these two tanks let the already ingressed air to communicate to both raw diesel tanks and the circulation of air exists between the vapor space of these 4 tanks.

 

It’s worth mentioning that several years ago, we did some modification on the system. At that time the lift pumps inside the raw diesel tanks were screw pumps with floating suction. But after the modification the screw pumps were replaced with centrifugal pumps (2*100%) without floating suction.

 

Some alternatives have been already generated to avoid water ingress and condensation and now we are looking for effective and cheapest solution:

   1- Installing a manual instrument air blanketing valve instead of normal open vents.

Questions:

a-  By manual blanketing, is there still the risk for over-pressurisation of the tank? Do we still need a pressure vent to avoid it?

 

   2- Installing a PVV instead of open vents

Questions:

a- By using a PVV, do we still need to have flame arrestor?

b- How can we manage barometric pressure changes on the PVV?

c- The design pressure for the tanks is 70 mbarg. Is there any procedure or industry practice to determine set pressure and overpressure of the PVV? Should we proceed with set pressure 2.1 mbarg and over-pressure of 4.2 mbarg? How would be the effect of barometric pressure changes on the set pressure?

d- Is there any special operational issue with this solution?

 

   3- Installing standpipe in the filling nozzle of the raw diesel tank and use it as a liquid seal to stop air ingress. This alternative involves difficulties and shutdown requirement for installation.

​Questions:

a-  Is hazard of static ignition is valid here (due to splashing of the diesel into the tank without standpipe)?  If the answer is yes then we can justify the installation of the standpipe.

 

And one last question:

 

Can the centrifugal pump affect the centrifuge performance by making smaller water droplets? (changing the pump is not the main purpose of our evaluation because it is very costly)

 

 

Attached Files

•  Diesel system.pdf   144.32KB   18 downloads

Edited by omideng, 28 October 2021 - 07:41 AM.

[PingPong]

I am not an expert on storage matters but I will give you my opinion anyway:

 

Recently we have found that there are a lot of free water in our diesel system............A root cause analysis has been done by the operation team members and the air ingress through the vents into the system and moisture condensation was considered as a main suspect.  (An estimate amount of 30 to 50 liter per day of moisture condensation). During the previous 2 weeks we have drained about 20 m3 free water from the system.

20 m3 in 2 weeks would be 1400 liter per day on average, not 50 liter per day.

 

Lots of water condensation from ambient air in diesel tanks seems highly unlikely to me.
You indicate that the problem is recent, so the ambient air is unlikely to be the problem. Climate change does not occur that fast.

You should however check for any rain water leakage.

 

For water to condense from air it requires contact with a surface that is colder than its dewpoint.
According to the drawing each diesel tank has an electric heater on TC. What is the setpoint of this TC ? What is the dewpoint of the ambient air?

 

As the problem is recent: what changed just before the problem started? That's what you should focus on.

If nothing changed inside your unit then something must have changed outside your control.

The most likely suspect is then the supplier of the diesel, or the refinery where the diesel was produced.

 

Sample the diesel when it is supplied and analyze its water content, not only free but also dissolved and emulsified water.

Also check how long it takes for any water in the diesel to settle completely. Leave it standing in a glass cylinder for weeks if necessary.

 

Can the centrifugal pump affect the centrifuge performance by making smaller water droplets?

Yes, and what is worse: a centrifugal pump can cause an emulsion if the diesel contains components that act as surfactants. The original pumps where screws, probably to avoid that problem.

 

And that's where the supplier of the diesel comes in again: maybe there was something changed in the production or handling of the diesel before it reaches your unit, or maybe the diesel now comes from another refinery than until recently.

Edited by PingPong, 31 October 2021 - 08:00 AM.

[omideng]

Thanks a lot for your recommendations. I just want to add that the last time the system was completely overhauled was at 2019. So, we can assume that this amount of water (as well as the water that we have already drained during this period) has accumulated on the tanks gradually. As I described our system has air draft and re circulation through the vapour space of the tanks and return lines. So, condensation rate will be much higher than normal.

 

 

I am not an expert on storage matters but I will give you my opinion anyway:

 

Recently we have found that there are a lot of free water in our diesel system............A root cause analysis has been done by the operation team members and the air ingress through the vents into the system and moisture condensation was considered as a main suspect.  (An estimate amount of 30 to 50 liter per day of moisture condensation). During the previous 2 weeks we have drained about 20 m3 free water from the system.

20 m3 in 2 weeks would be 1400 liter per day on average, not 50 liter per day.

 

Lots of water condensation from ambient air in diesel tanks seems highly unlikely to me.
You indicate that the problem is recent, so the ambient air is unlikely to be the problem. Climate change does not occur that fast.

You should however check for any rain water leakage.

 

For water to condense from air it requires contact with a surface that is colder than its dewpoint.
According to the drawing each diesel tank has an electric heater on TC. What is the setpoint of this TC ? What is the dewpoint of the ambient air?

 

As the problem is recent: what changed just before the problem started? That's what you should focus on.

If nothing changed inside your unit then something must have changed outside your control.

The most likely suspect is then the supplier of the diesel, or the refinery where the diesel was produced.

 

Sample the diesel when it is supplied and analyze its water content, not only free but also dissolved and emulsified water.

Also check how long it takes for any water in the diesel to settle completely. Leave it standing in a glass cylinder for weeks if necessary.

 

Can the centrifugal pump affect the centrifuge performance by making smaller water droplets?

Yes, and what is worse: a centrifugal pump can cause an emulsion if the diesel contains components that act as surfactants. The original pumps where screws, probably to avoid that problem.

 

And that's where the supplier of the diesel comes in again: maybe there was something changed in the production or handling of the diesel before it reaches your unit, or maybe the diesel now comes from another refinery than until recently.

[PingPong]

So, condensation rate will be much higher than normal.

I still doubt that there will be condensation of water from the air.

Once again: for water to condense from air it requires contact with a surface that is colder than its dewpoint.

Measure dry and wet bulb temperature of the air at different times over the day and night, and determine each time the dewpoint using psychrometric chart.

Compare that with the temperature of the diesel.

If diesel temperature is higher than the dewpoint of the air then there can not be condensation.

 

If you want to avoid air flowing through the tanks via the common feed lines then install checkvalves in those lines.

Make sure the checkvalve is installed such that it falls closed if no liquid flow.

 

I notice on the drawing that there is a water line coming in on the left side: Stream 10 coming from Fresh Water System and connected with both centrifuges. No valves shown. What is the purpose of that? And are you absolutely sure that no water leaks into the diesel system via that?

[omideng]

Hi again

the worst case scenario for water condensation can occur during summer. The average relative humidity is 70-80% and day temperature can go up to 25C and night temperature can go to 15C. Diesel temp is around 13-15C. Based on psychrometric chart condensation will happen. 

The fresh water stream is a small amount of water that acts as seal water for operation of the centrifuge.There is no leak due to that stream.

 

Regards

 

 

So, condensation rate will be much higher than normal.

I still doubt that there will be condensation of water from the air.

Once again: for water to condense from air it requires contact with a surface that is colder than its dewpoint.

Measure dry and wet bulb temperature of the air at different times over the day and night, and determine each time the dewpoint using psychrometric chart.

I notice on the drawing that there is a water line coming in on the left side: Stream 10 coming from Fresh Water System and connected with both centrifuges. No valves shown. What is the purpose of that? And are you absolutely sure that no water leaks into the diesel system via that?