PRV Installation

Your application is so highly technical i belive you are knocking at the wrong door.

You should get in touch with people like Crosby/Dressers/Spirax Sarco etc.

Why not, unless there is some further blockage downstream?

(I am not a pipeline engineer, so there may be some detail beyond this simple idea that I do not know.)

Theoretically, any device that allows excess pressure in the form of fluid movement from the upstream to the downstream side of this valve will help. I assume the design engineer has determined that the 4" line will pass sufficient flow to relieve the necessary pressure? The area ratio between the 16" and 4" lines is roughly 16:1 which means that velocity in the 4" line will be 16 times higher than that in the mainline just prior to the valve closure.....if the 4" line is intended to pass the full mainline flow (which physics will make it attempt to do). Once again, what does the design engineer intend? What do the installation drawings say?

Yes, you may connect the discharge of a properly sized PRV and associated piping to the downstream side of the blocked valve. I have seen this done for thermal relief in the US in refineries. Thermal relief is a small, usually a 1" relief valve, in a line that may be blocked in and subjected to thermal heating from the sun, causing excessive pressure builup, and its discharge is across one of the blocked in valves. I understand that this is not a thermal relief scenario, but the connection scheme is the same, so, I suppose what I am saying is that someone has done it before.

I would encourage you to seek advice on the valve sizing and type from one of the larger manufacturers, especially with the operating pressure and MASP being so close. Of course, with only 20 psig between MASP and surge pressure, and the operating pressure being so close to MASP, there is little margin of safety. There are also alternatives to a relief valve, such as rupture disk, surge tank and rupture pins. You will also need to run simulations to determine the correct set pressure and type of technology best suited to prevent a MASP violation.

One other point, how credible is an "instantaneous" closure of the valve, you stated that it takes 17 seconds for the valve to close? What is the pressure profile with regard to a 17 second valve closure rate? If the scenario never happens, and you are required to mitigate this scenario, then it would be possible to put in a more economical system than PRV's. On the other hand, what happens when the pipeline is further de-rated and the system has to be changed or modified?

Good luck.