Thermal expansion protection 2

[jalvarez]

Just to present a case, maybe this is an extension of the one titled “Thermal Relief Protection for Short Sections of Pipe”:
A pipeline, approximately 10000 ft long, between a tank and a port, was provided with several block valves, targeting to limit the consequences of a possible release.
As the pipeline handled a liquid, the possible thermal expansion problem of each segment between valves was solved with the installation of pressure relief valves.
As the discharge of the PRV’s to the atmosphere was not possible, and not having another independent line, each PRV’s discharge was connected to the next upstream pipeline segment.
PRV’s were conventional, not balanced bellow.
As the end of the pipeline, in the port, is normally closed, this arrangement permits to close only one of the valves. If a second block valve is closed, the pressure increase in the segment closer to the tank produces an increase in the superimposed back pressure for the precedent PRV, and this last one won’t open when required.
Solutions:
Use of balanced bellow PRV’s. It was disregarded, as the use of bellows isn’t recommended taking into account that the PRV’s are outside the plant fence.
Collection of the possible liquid being discharged from the PRV’s in an independent and vessel: The amount of liquid calculated was so high that this was also discarded. It happens with liquids of high expansion coefficients, close to its boiling point. And the same occurs if the vent to the atmosphere of the bellow
Solution adopted: Let the PRV’s as they are, provide instructions to operation people to sweep with N2 after use of the pipeline, and installation of legends on each valve recommending not to close more than one valve, in case of an emergency.
Comments?
I apologize if this is complicate , but without a drawing…
Have a safe day
jalvarez

 

[25362]

Sr Alvarez, it is not clear to me how a series of out-of-the-fence hand-operated valves could be used to limit unwanted releases. Please explain.

If this is a pressurized liquid hydrocarbon line, the multiplicity of gate valves along the pipe wouldn't add to the overall safety. Would it ?

Could you expand a bit on the pipe size, the liquid moved therein, and the procedures used to sweep the line with nitrogen and its ulterior re-commissioning ? Thanks.

 

[Guidoo]

In such a case I would look at the following:

- All thermal relief valves, except for relief valve at starting location (tank), relief towards upstream pipe section (cascade), use conventional relief valve with spring setting of say 0.5 bar. This means that the valve will open when downstream pressure is 0.5 bar higher than upstream pressure.

- Relief valve at starting location has a set pressure equal to system design pressure - (number of downstream relief valves x 0.5). This relief valve reliefs towards the tank and will probably be conventional as well (provided backpressure/setpressure < 0.10, otherwise it will be balanced bellows).

So if you have a 150# system with design pressure of 19 barg and total 7 sections that can be blocked in, the relief valve nearest tank has a set pressure of 16 barg (19
- 6x0.5 = 16)

Of course, this will only work if the number of sections is limited, otherwise the first relief valve will open during normal offloading as well.

 

[Guidoo]

Just a note that in my story above I assumed that there will be no elevation changes between tank and port.

 

[jalvarez]

Guidoo:
You've got it! The case is practically what you described, with some elevations to take into account for some of the valves. So, now we only need to adjust the numbers.

25362: The valves are out of the fence but not accesible to anyone except for operators. If there is a release, they may close the proper valves minimizing the problem. If the failure is a catastrophic one, obviously, nothing to do. The next step will be to upgrade the system with remote operated valves. The pipeline handles organic chlorine products.
Sweeping with nitrogen is not necessary anymore following Guidoo solution, except when we change the product.
Thanks again, gentlemen.
Have a safe day