2 replies to this topic

[Delano]

Hi Guys

I was recently given this problem. Could someone please point me in the right direction of how to calculate the required capacity (flow) of the following PRV:

PRV located on a blocked in section of pipe 150 mm diameter and 30 m length of pipeline in an LPG depot.

This is the only info that I have been given by my professor.

Any help would be greatly appreciated.

Thanks,
Delano

[Art Montemayor]

Delano:

This thread, in my opinion, belongs in the Relief Devices Forum. However, due to the basic nature of the query, I’ll leave it here in the Student Forum.

Based on the information you give (calculate the required flow capacity of a PRV located on a blocked-in 30 m length section of 150 mm diameter pipeline in an LPG depot), I have to assume that is exactly ALL that you have been told. Based on that – and the fact that you are probably a student – I sense that your instructor is trying to test your basic engineering ability in analyzing a problem before attacking it.

In other words start by looking at the BASIC facts: you have a saturated liquid filling 100% of the blocked-in pipe. Rely on the basic fact that the words LPG mean Liquefied Petroleum Gas and that it is handled, stored, and transported in the SATURATED liquid phase. Knowing that, you should be aware that if you block-in a liquid and allow it to remain there, filling 100% of the pipe, any temperature increase in the liquid will cause it to try to expand and will generate a hydraulic pressure of humongous size by expanding only a very small amount. What is immediately needed is what is called a “thermal relief” valve – a relatively small relief valve that opens to relieve hydraulic expansion pressure. This type of relief valve is used in these typical cases of liquid expansion in pipes and since the amount (flow capacity) of expansion fluid that is created by the increase in temperature is rather small, the size and capacity of these valves are frequently never calculated. Therefore the practical and correct response to the question is that a conventional, un-calculated thermal relief valve (either ½ or ¾ inch size) will protect the pipeline against thermal expansion. HOWEVER, in the event of a fire case, then you should calculate the exposed area of the blocked-in pipe, refer to API 521 for an estimate of the amount of expected LPG vaporization due to the fire and this will set the size of the PRV for the fire case. I would recommend that both a thermal relief and a PRV for the fire case be installed due to the possibility of these cases occuring individually or together.

API 521 explains what I have described in detail and sets the necessary steps and calculations required to resolve this problem. I highly recommend you obtain a copy of API 521, read, and study it carefully. With this preparation done, you should be able to address and resolve this problem with high marks.

[Delano]

Sorry for the late reply

Thank you very much for the help