Parallel Pump operation with different head for LNG 1

[FluidME]

For a LNGC to FSRU conversion project, we are installing Feed pumps in each LNG Cargo Tank, 4 in total.

The new Feed pump will feed LNG to the regasification skid via a new Feed Main.

For flexibility, it is required to have the provision of feeding the regas skid via the existing Cargo pumps as well, by either cargo pump, by either feed pump or combined (parallel) use of feed and cargo pumps. The cargo pump is therefore also connected to the Feed main.

The shut off head of the feed pump is greater than the cargo pump.

How do I ensure that the cargo pump does not reach its shut off pressure due to the back pressure created by feed pump. What kind of control mechanism can be used here?

 

[LittleInch]

Speed control of the new pump or flow control of the new feed pumps before it reaches the header.

Normal operation xylene on the intersection point of the pump curves. If they are not quite steep at that point you could get hunting off flow between the pumps.

Can you post the pump curves of the two pumps?

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[TugboatEng]

You must have variable speed. You can slow the high pressure feed pumps down so that they have the same discharge pressure as the low pressure cargo pumps.

Under what circumstances do you expect to run types of pumps simultaneously?

 

[LittleInch]

VFD is probably best but depends on the pump curves which we don't have.

And type of pump

And whether flow control is at the moment

In fact any real data to work with....

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Also: If you get a response it's polite to respond to it.

 

[FluidME]

Thank you LittleInch and TugboatEng

Pump characteristics for different speeds are not provided, please find the respective pump curves.

Cargo pump:

Feed pump:

Operating conditions:
Feed pump and cargo pump located in different tanks will be operated simultaneously.
We have to design for different scenarios such as: One tank can be at 90% capacity while the other can be at 10% capacity.

Regarding flow control, do we provide a desired pressure set point and throttle the Feed pump flow to meet the set point.
OR provide pressure sensors to measure the cargo pump pressure and throttle the flow accordingly.

 

[LittleInch]

Fluid ME,

Impossible to tell what will happen here as you really need to start with a system curve for your new Feed main and also work out how you will control flow and what your operating scenrios are wrt flow.

SO what is the desired flow rate in the feed main in total when both pumps are operating?

Given that the cargo pump is so much higher capacity than the new feed pump it might actually just work ok, but then how many new feed pumps will be operating at the same time as the old cargo pump?

You've only got a fairly narrow range of flow in your feed pump to allow the cargo pump to do anything (470 to 570 m3/hr).

I would be tempted to go for flow control, start the cargo pumps first and then the feed pumps controlled mainly on flow higher than the single feed pump capacity, but also monitor / control on pressure/ head to keep the header pressure below abut 175 to 180m head. You can do both using one valve or one speed controller.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LittleInch]

This isn't made easy if the tanks are tall and they are at different liquid levels. LNG might be fairly low density, but difference of 20m = 10m head from the pumps will mean things change over time.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Snickster]

It appears you are trying to use different combination of pumps to achieve a range of flowrates without using VFD's? For the cargo pump single operation, if you already get a system curve of about 187 feet at 500 gpm based on Feed pump operation, wouldn't a single operating cargo pump already be operating at minimum flow dotted line area or very close to it?

 

[Snickster]

Not knowing much about your system and what you are trying to accomplish, maybe you can possibly install recirculation orifice lines in cargo pump or feed pump or both so that the operating point will move further to the right on the curves when pumping a net flow to the system and recirculating some flow back to the suction. This will keep cargo pump operating far to the right of minimum flow and keep feed pumps from riding too high on the curve. The recycle line if put on the cargo pump may be needed in any case when pumping to the storage, whether in parallel or single operation. If a recycle line is installed on the feed pumps it may only be needed when running with the cargo pump in parallel.

 

[georgeverghese]

With a fixed speed operation on the feed pumps, looks like min flow recycle will have to be 475m3/hr, rather close to the design case operating flow of 500m3/hr.

Can you persuade the team to go for a 2speed motor on the cargo pump, if VFD is somehow objectionable ?