5 replies to this topic

[andybud]

Dear Sirs,

 

I am designing an offshore platform seawater cooling system with a design rated flow of approximately 10,000 m3/h. This is used to cool a secondary cooling water loop consisting of demineralised water, before going overboard to a disposal caisson. The seawater level rises and falls by approximately 2 m in the Caisson due to waves and tides.

 

The Seawater system consists of  3 x 50% seawater lift pumps located in a Caisson, which pump to seawater Coarse Filters (3 x 50%) and then to a set of Seawater / Cooling Water Heat Exchangers ( 4 x 33%).

 

As the line sizes on the common seawater headers are huge, approximately 40 inches, a control valve would be difficult to buy as well as expensive. My question is, where should i place the restriction orifice to control the flow in the system?

 

Is it better to have a restriction orifice downstream of each heat exchanger to throttle the flow?

 

Or is it better to have a restriction orifice downstream of each seawater lift pump?

 

What are the advantages to either design, if any?

 

Any pointers would be greatly appreciated, many thanks

 

Andrew

 

 

Attached Files

•  Seawater Sketch.jpg   39.87KB   3 downloads

[samayaraj]

Hi Andrew,

 

For controlling flow, I may suggest you the followings:

 

1. Add VFD to the pump electric motor to vary the overall flow if the flow change is frequent

2. Add butterfly valve in the pump discharge and throttle it according to required flow (Manual operated is sufficient If the flow change is not frequent, else go for motor operated from remote)

3. Add butterfly valve in each heat exchanger to throttle individual flow across it.

 

RO is meant for fixed flow reduction. As you are pumping 10,000 m3/hr water, flow restriction will have huge impact in pump operating cost. For that, you can buy a pump with specific flow condition. Generally, cooling water pumps will handle huge flow hence it will be sized according to the required flow only.

Edited by samayaraj, 10 April 2019 - 05:49 AM.

[thorium90]

I think the most economical option would be to have an electric or hydraulic operated butterfly valve at the discharge of each pump. Based on the flow rate, about 36in should suffice. Anyway, with this size of pump, you must throttle the pump discharge during startup anyway. If these pumps are DOL, it could burst the piping if the discharge valve is full open.

The video below is quite informative.

 

https://www.youtube....xZ-lMWs&t=0m15s

 

The start procedure would be to open the valve to about say 20%, then start the pump, wait for awhile, then slowly open the pump discharge to full. Then start the second pump with the same procedure.

Alternatively, put a VFD on all pumps. Considering the size of the pumps, this will probably be something like 400kW, high voltage pumps. Which would make the VFD's quite expensive. However, the VFD would remove the need for the throttled startup.

 

The third option for throttling the inlet to each HX still has to be complemented with either option 1 or 2 since option 3 alone does not address the pump startup issues. Only the VFD or pump discharge valve will solve the problem. Anyhow, I would think each HX would already have a valve for isolation, and it would probably be a butterfly valve anyway..

 

Anyhow, i don't think an electric butterfly valve is that expensive. For this size, I would say its about 15k USD per valve. That's much less than the price of the VFD or the pump.

Edited by thorium90, 11 April 2019 - 04:33 AM.

[andybud]

Many thanks for your answers. I shall do this.

[breizh]

hi,

lift pumps' vendors should help you too. You should guarantee that you HX is full of sea  water all the time to get a good heat transfer .

 

Good luck

Breizh

[thorium90]

Yes, as per Breizh recommendation, do take note of the piping design to include the possibility for startup venting of the heat exchangers. Otherwise the air pockets can get stuck in there for a long time... 

Edited by thorium90, 11 April 2019 - 04:43 AM.