Design a Flow Orifice to take down DP to Closed Drain 4

[Vivaldi M. Smith]

Good morning.
I am trying to design two flow orifices to be able to reduce the DP to protect downstream pressure ( Closed Drain). The pressure in the line is around 1100 PSIG, one line is gas, and the other line is liquid.
The Gas line ( natural gas ) is 12" and the distance from valve to valve is about 41 feet. The liquid line ( Condensate) is 6" about the same distance, 41 feet. If I calculate the volume trap in the pipe, how do I find the time to depressurize through a 2-inch completely? it is that flow that I want to use to calculate the flow orifice. will that be correct?
Please help.

 

[LittleInch]

I don't think so.

normally what you do is calculate what the back pressure in the closed drain system is for given flow rates. Once you get to a back pressure at the drain point which is more than the design pressure of the drain system then that becomes your flowrate. Now if that flowrate doesn't allow you to de pressurise your section within 15 minutes down to 7 bar / 100 psig then you are not in accordance with API521?. But 41 ft of pipe is a small volume, but if just dumpied into the drain line could have very high pressure for a short period of time.

Having a combined liquid and gas drain is, IMHO, a VERY bad idea as if the drain line has liquid in and then you dump the gas line , you can get v high liquid velocities which will break your drain line.

The gas line IMHO, should not be being connected to this line and should have its own vent line.

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

 

[Vivaldi M. Smith]

Thanks littleInch, but I cannot redesign the whole Plant right now, I just want to do what they ask me to do.

 

[LittleInch]

No but you can't close your eyes to it either.

So in this case the worst case condition is that the line is full of liquid and you probably don't want to do more than say 5 m/sec, at probably not more than 0.5 barg is your gas volume flow to limit the flow to something which won't cause a a hazard further downstream. If you're only "draining" 41 ft of pipe, it will be done in <15 minutes.

The other thing to look at which might restrict gas flow is what is the design pressure of the drain tank and how much flow can you get up the drain tank vent. That's often the bottleneck and you don't want to blow your tank up or have liquid jetting out of the top of the drain tank vent....

Condensate you might need to look at how much it flashes off d/s the orifice and do the same thing.

How did this get through a HAZOP??

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

 

[mk3223]

Hi, Smith
It's okay for the first statement as

I cannot redesign the whole Plant right now

But, sorry to say that there is an issue for the 2nd statement as

I just want to do what they ask me to do.

 

[georgeverghese]

Draining blocked in inventories is only to be done after having depressured the section to the extent possible with the equipment downstream / upstream. Pressurised closed drains are NOT meant for dealing with high pressure inventories.
You've got to get your lead engineer to give you the full bottle on the operating philosophy for closed drains, which may be in one of the design reports he's got stashed away. Your line of questions is some what alarming.

 

[Snickster]

I would never put natural gas into a closed drain system, especially at 1100 psig in 12" main line - that's a lot of gas. It should go to a vent system which then goes to flare. If not to a flare then to a safe atmospheric vent if it is allowed by environmental codes. Now if you think there are liquids in the gas line that need to be drained then you should vent the line first, then drain.

I agree - for the condensate I would depressurize first before opening to an atmospheric drain system. I assume this is condensate removed from the natural gas flowstream so it could also contain some natural gas. There should be a procedure in place to do this.

 

[1503-44]

I gave each response a star, because they all are right on, straight best advice and I've got nothing to add, other than
Take the hint!

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[casflo]

I agree with the previous answers. It is not a good idea to drain a liquid and a gas through the same pipe, even if done separately.
In any case, the orifice design should be of multi-stage type, each stage consisting of a multi-hole plate.
This design will prevent cavitation in the case of the liquid and reduce noise in the case of the gas.
As the liquid is at 65F, its vapor pressure is 0.305 psia, much lower than the atmospheric ambient pressure, so there will be no flashing.

 

[1503-44]

On second thought, maybe I will add my comments.
How is it that you are even thinking of "draining" a GAS? It wants to go UP.
You just ran it through seperators for that very reason, so you don't have to deal with 2 phase flow, now you want to mix them again?
Run your gas to a vent/flare or somewhere that gas should go; ie. not to a drain. That's like trying to shovel waves back to the ocean.

Plus ... An orifice plate is also NOT pressure protection for downstream piping. Your downstream drain piping needs to be designed for 1100 psig, or have relief valves.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."