LPG Storage vessel loadout to tanker

[anthracen]

Gentlefolk!

I am currently designing an LPG loadout for propane. The system comprises a 150T storage vessel, a centrifugal pump, a throttle valve and the loadout arm. The storage vessel operates at a pressure of 7-10 bar(G)(vapor pressure, depending on ambient temperature), pressure drop in the pipeline is negligible" and the tanker is filled against vapor pressure of propane.
This means all the pump needs to do is overcome static head and pressure loss.
There is a throttle valve downstream of the centrifugal pump to prevent pump dead head and fluid flashing.
Is there any well proven or standard control to regulate the valve downstream of the pump (PDIC, PIC)at LPG loadouts?

 

[skoutso]

anthracen,

all depends on what you need to control given the system congiguration and the way it is operated. There are quite a few pumps operated with no means of control at all, operations being manual. There also fully automated systems with regards to pump startup/normal operation/shutdown. The primary canditate variable to control is the flow. Exceeding the rated flow will most probably create a safety hazard. Without knowing the details of your system i would propose a flow control with pressure control override. The override pressure controller will fully close the discharge valve when the pump is stopped (level inside the truck is reached). At startup, the pump will operate against closed discharge as it is required for almost all the medium duty pumps. As the pressure builds up, the flow controller will take over untill the end of the loadout.

Good Luck.

 

[Montemayor]

anthracen (Chemical):

As always, when there is communication between different countries, there are differences in language and its usage. By "loadout for propane" I assume you mean unloading (or off-loading) of saturated, liquid propane. By "the tanker is filled against vapor pressure of propane", I assume you mean that you have a vapor balance line from the delivery tanker to the storage vessel 100% open. If so, then the transfer operation is as you describe: the transfer only has to "overcome static head and pressure loss". This is a conventional and proven method of liquid transfer.

Such a liquid transfer system needs NO THROTTLING VALVE or device. The throttling valve actually does what you state you want to prevent: "to prevent pump dead head and fluid flashing". I would strongly recommend you remove such a valve or any other device (other than the required block valve) from the discharge of the transfer centrifugal pump. I hope I am making myself very clear on this because you may not recognize that throttling a centrifugal pump allows you to dead-head it!

Do not interpret my recommendation to infer that the use of a throttle valve is dangerous; it isn't. It just simply is not needed, makes the operation more complex than it needs to be, and slows down the transfer operation while creating undesired flash vapors and pressure drops. There is no logical reason for its use unless the pump's capacity is beyond that allowed for unloading and you have to control its capacity (which you haven't stated). Normally, this is never the case. I normally have off-loaded saturated liquids at 150 - 200 gpm (or higher, if legally allowed to).

Of course, without the discharge throttle valve the whole system simplifies and you have no need to consider how to regulate it. This is the normal way the operation is carried out.

Contrary to what skoutso recommends, I never start my transfer pumps against a closed discharge. I have never found any need for this and find that this is detrimental when handling a saturated liquid. I equalize both vessels on the liquid and vapor sides and simply allow the pump to do the simple transfer starting up totally inundated with liquid. I have never seen or found any reason to throttle or start up dead-headed for this application. Agreeably, there is a sudden rush of electric current to the pump's motor; but this is designed for in transfer pumps in order to simplify the operation and allow automatic stop/start.


Art Montemayor
Spring, TX

 

[anthracen]

Dear skoutso and Montemayor!
Thank you, I appreciate your competent replies!
Actually, I have described a simplified loadout above. This new Propane vessel loadout ties into an existing LPG system with three other bullets storing Butane and Propane, which are off-loaded as pure LPG's or mixtures of each (60/40, 80/20)using positive displacement pumps.
It was decided to avoid complicated and expensive cross-linked controls between the various vessels during i.e. loading mixtures. This means there are no vapor equalisation lines.
This in turn necessitates a fixed duty point (system curve) for the centrifugal pump at the new vessel, which the PDIC should take care of.
What is your take on a system like that?
Cheers