Change of Loading/Unloading Flow rates

[RCJoseph]

Hi guys,

Im new here! Just joined the forums.
I would like to have some tank designer's expert opinion on the below work. Would be of great help if anyone could advise me as im not tank designer.

We are currently working on project for crude oil storage tank farm facility. In the terminal there are total of 10 number of tanks used to store crude oil product.

Product specification :
Product Name – Crude Oil
Viscosity = 50 cSt
Density = 860-891 Kg/ m3.
Vapor Pressure = 71 Kpa.
Volatility = Highly Volatile.

The existing tank information is (refer attached tank data sheets)
Design: API 650
TK - 101 to TK to 106 tanks (72m Dia x 22.3m high) and TK - 107 to TK - 110 (26m Dia x 20m high)
All mentioned tanks are having external floating roofs

End user now requires these tanks to be used for large cargo loading / unloading operation. Flow rate for ship loading & ship unloading operation is being changed to 12000 m3 / hr from the current 5000 m3 / hr. Because of this change in flow rate, we anticipate there will be change in tank venting rate (in-breathing / out-breathing), number & size of vents, inlet diffuser etc.

My query is regarding how i can size the inlet diffuser for this new requirement? What sort of modification is required in existing inlet diffuser, tank PVRV valves sizes in order to comply with end user new requirement?

Also, will there be any other modifications needed to be done to these storage tanks?
Please note any other alternative solution (other than the above modification) to cater the end user’s new requirement is also welcome. This is to avoid the pre-requisite works like tank emptying, gas freeing, removal sludge which will be a time consuming process during the execution stages.

Your advises or suggestion will be highly helpful.

 

[LittleInch]

As a tank with an external floating roof tank, I have no idea what that flowrate highlighted actually refers to and maybe is a hang over from an fixed roof tank data sheet so someone just filled it in because it was there.

Floating roofs have a fill rate and empty rate when the roof is sitting on its legs, but this is usually very low because the vents in the roof are very small so the flowrates noted really aren't valid for that occasional issue.. The whole point of a floating roof tank is that it doesn't allow free surface to emit vapours, providing the vapour pressure is < 0.7 bara so I really don't understand the issue about vents.

For the inlet diffuser I would simply work out the current exit velocity and then up size your diffuser to match it or add / use another nozzle.

You don't have to worry about roof speed - I asked that question a few months ago and no one could say what the max roof speed was or if it actually made any difference - turns out it didn't.

If you don't want to enter the tank to do mods on the inlet system, you can sometimes get away with using the manhole as a nozzle and allowing for a removable spool piece when you do need access...

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

 

[RCJoseph]

Hi,

Thanks for the response. i had forwarded you response to my process engineer who responded below.

The basic concept of a floating roof storage tank is that the stored liquid inventory is kept covered and protected by applying a floating roof that sits directly on the liquid and seals the liquid from the atmosphere by a seal between itself and the internal tank walls. The roof can be directly touching the top liquid level or pontoons can be used to allow the roof to float.

If any vapors or inerts exist in the space between the bottom of the floating roof and the top of the liquid level, they rise and fall with the liquid level. These vapors/inerts are not subjected to any "pressure". The pump-in or pump-out of liquid inventory also does nothing to increase any vapor pressure in the tank. The only pressure that varies is the hydrostatic head of the level height in the tank. So increase in pumping rate will not have any effect on auto bleeder vent or PV vent provided on roof. More over as tank is EFRT (external floating roof type) so API 2000 is not applicable here. As such there is no vapor space in tank.

Automatic Bleeder vent & rim room vent with PV valve is already provided on tank roof.

To my knowledge, there is no direct restriction on the roof velocity of floating roof tanks, at least we do not consider any in the design of API 650 tanks. But following may affect this parameter indirectly:
1. Ingoing and outgoing pipe lines have liquid velocity limits, as recommended by practices.
2. Floating roof is "sitting" on its legs during last phase of tank emptying or first phase of tank filling, so flow has restrictions similar to cone roof tanks in this case. E.g. cannot exceed capacity of (rim) vents located on the roof, fluid velocity cannot be > 1 m/s if air can contact flowing liquid. But these depend on the policy established for tank emptying or filling.
3. Similarly filling time between HLL and HHLL should be about 20 min (depending on distance, assessed critical conditions, etc), so that the operator could react.
So in this case only concern is regarding accumulation or generation of static electricity inside the tanks due to increase in pumping flow rate. This can be compensated by limit exit velocity of product from diffuser .

Please give feedback on the above points & advise suitability of tank diffuser to suit new operating conditions.

 

[LittleInch]

Looks like a similar and good response to me. I'd forgotten about the change in HLL to HHLL to take account of the higher flowrate and hence you need a bigger distance between these set points to still have the same time to do something.

Like I said before, either increase the size (diameter) of the diffuser to make the exit velocity the same as you have now or add more nozzles to do the same thing. the lower the inlet velocity into the tank the better, but I wouldn't exceed 2 m/sec if you can help it.

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