2 replies to this topic

[GS81Process]

I've been asked whether a thermal relief PSV can be installed on an existing vent valve connection.  I have attached a sketch of the system for reference.

 

A little background information. The existing system consists of a shell and tube heat exchanger, with glycol/water on the shell side (cool side) and pressurized water on the tubes (hot side).  The exchanger is fitted with a rupture disk sized for the tube failure case (set at 10 kg/cm2).  There are piston actuated isolation valves on the inlet and outlet of the cool (glycol) side of the exchanger which are used to isolate the glycol to/from the exchanger when high pressure is detected in the shell.  This is done to prevent from losing substantial glycol inventory from both the supply and return headers in the event that the rupture disk bursts. 

 

One problem with the existing arrangement is as follows.  The piston actuated isolation valves on the glycol supply/return fail closed in the event of power failure.  The glycol then gets blocked in the shell, and gets heated up by the warmer pressurized water.  This results in the rupture disk bursting.  The problem is that it takes a lot of time to change a rupture disk and some of the glycol which does not reach the downstream containment system must be drained and disposed. 

 

The proposed solution is to install a smaller 3/4"x1" thermal relief valve on the glycol outlet piping, set at a slightly lower than the burst disc (9 kg/cm2).  This valve will lift for the thermal relief case and the re-seat, thus preventing the rupture disk from bursting.  

 

There is an existing 3/4" vent valve on a high point in the outlet piping.  Someone has proposed to use the existing vent valve connection for installing the new PSV, by having a tee connection after the existing valve and adding an additional valve to serve as the new vent as per the attached sketch.  I would personally prefer to tap a new location for the PSV and keep the existing 3/4" vent valve seperate.  I don't believe it's good practice to combine a high point vent with a PSV on the same connection.

 

Thoughts?

 

 

 

 

  

 

 

 

Attached Files

•  System Sketch.pdf   41.57KB   14 downloads

[Dacs]

Is this pressurized water part of a network? Would it be possible just to add a SOV at the heating water upstream that's tied in to the PAH interlock?

 

Another thing, can you lose IA without losing the heating water?

 

That said, is it possible just to utilize the inlet piping leading to the rupture disc and make a tapoff going to your proposed PSV? Its location may be tricky considering the static heads involved.

[fallah]

GS81Process,

 

You can use existing valve provided that it would be in CSO position. You can install TSV directly (via a short spool piece) on the existing valve and take a branch from spool piece terminated to a new 3/4" valve to do not only as a maintenance drain between existing vent valve and PSV but also as a high point vent. 

 

One more point: The PSV (here TSV) set point (9 kg/cm2) should be lower than the minimum burst pressure (including burst tolerance) of the rupture disk to avoid possibility of disk bursting sooner than PSV opening in the case we need to just PSV opening.

 

Be noted, in fact and obviously tapping a new connection on the shell for PSV is preferred but the exchanger's shell after tapping might be subject to new hydrotest, PWHT, new code stamp,...