Tank in a Closed loop How big?

[StoneCold]

I have a hot oil closed loop process. The process flow is like this.

Tank-->Pump-->Exchanger1-->Exchanger2-->Back to Tank.

The current tank is big enough to accomodate fluid expansion and provides a high enough liquid level to feed the pump but I am wondering if there is a rule of thumb for sizing tanks in closed loop systems. Like the normal liquid level in the tank should represent one minute of flow. (The current tank only has about 10 seconds worth of flow)
Has anyone ever seen a guideline on this. I have a vortex breaker in the bottom of the tank and nitrogen inerting on the tank provides more than enough NPHS for the pump. The system seems to work fine right now but I wonder if it is really on the ragged edge and I should have had a bigger liquid buffer?
Any ideas or comments?

Thanks

 

[Montemayor]

Stone:

The "rule" - or basis - of design that I've always followed in the process design (especially the size) of a tank within a closed loop (such as a refrigeration loop, or a hot oil loop such as yours) is that the tank should primarily serve the practical purpose of being a reservoir for all the fluid employed in the closed loop.

What I mean by this is that although the tank serves a process useful purpose as a surge and inventory control chamber, it also represents your ability to do the obvious and necessary work of temporarily storing all your circuit's fluid while you have a need to do inspection or maintenance on the closed loop's various components. This part of the tank's work function turns out to be the one that usually yields the largest size and, therefore, is the controlling design factor for this type of vessel. You have to have a means of temporarily storing your heating fluid when the time comes to do repairs, inspections, or maintenance and this tank fulfills that function well since it normally is fully insulated for heat conservation and poses no burn hazard for your plant personnel.

I hope this helps in giving you a process design guideline.

 

[abeltio]

there are exceptions to the statement by Montemayor, e.g.: closed loops on-board ships.

the rule i've used is:
1. the expansion tank must be located high enough to provide the minimum pressure required at the inlet of the different services, e.g. for cooling of a diesel motor

and the height selected must be enough to avoid having a "fountain" when the system is stopped.

in a closed loop system the pressure developed at the pump discharge is the pressure drop along the system, static head = 0ft

pump seals require pressure between certain values to operate correctly.

2. so far, the only systems where i've found a requirement to have a min. capacity in terms of duration of operation was in liquid fuel systems.

3. the expansion tank is a good place to locate the tie-in to replace lost fluid.

4. in case that the hot-oil needs to be drained for maintenance, special precautions need to be in place as the oil will not return to the tank: drain points with double block should be fitted at each low point in the system, with provisions to collect the drained oil.






saludos.
a.

 

[25362]

Is Exchanger1 the hot oil heater ?

The designs I have seen have two drums (pressure vessels), the smaller one (on top) serves as a hot oil expansion vessel, the other as hot oil storage.

Both receiving inert gas under pressure control and connected to flare, as well, also under pressure control.

As for the volume of the storage tank one should follow Montemayor's advice, both vessels designed for the highest foreseen temperature. Other control features depend on the complexity of the system.

 

[StoneCold]

Montemayor, abeltio, 25362
Thanks for your replies.
I will try the storage tank idea to size the next one. I have encountered problems already where it was necessary to drain out the oil as there was no way to isolate it in one vessel. That would probably make startup easier as well.

25362 I have seen the dual tank designs as well. I believe that they were running their main tank all the way full and relied on the second tank to handle any expansion volume. I am just running my tank about half full cold and about 75% full when it is hot, but my system is very small.

Thanks guys.

StoneCold.

 

[liberoSimulation]

You can check the design basis of the lube oil system that is a closed loop with a reservoir + exchanger + filter and similar to your case.
I think that it can give you some information and ideas might be applicable to your system.

Hope this would help you.

Regards

 

[itdepends]

RE your initial post. Why do you have a vortex breaker in the tank? Does the oil actually return to the tank as part of the closed loop?

To avoid this issue (and still have a resevour) you can pipe the return from the system directly to the suction of the circulation pump. The tank then justs tees off from the pump suction (providing your expansion volume).

There shouldn't be any need to allow the flow to go through the tank unless the stagnant oil can cause problems (solidify/high viscosity if cold -e.g heavy fuel oils).

 

[25362]

Another reason to go through the tank would be to liberate any light ends, gases that may have been produced by thermal degradation of the hot oil before it reenters the pump.

 

[MJCronin]

To all,

To the best of my knowledge, there are different purposes for these tanks and different designs. (ie. surge only, expansion and surge, etc)

There is an explanation on this link:

http://www.radcoind.com/TechTips4.htm

Some additional sizing information is:

http://www.budgetmolders.com/PDF03/140.pdf

My opinion only....

-MJC

 

[25362]

MJCronin, it appears the first link doesn't consider the possible use of an inert gas blanket. I couldn't raise the second link, probably a difficulty with my PC.