safety valve

Ajeet,
You did not give us very much information to work with. It would be helpful to know the following:
- Is the exhaust going to atmosphere or to a closed collection system
- The commodity
- The pressure and temperature
- Is this a single PSV installation or is this a double (i.e.: one and a spare)
- The PSV size
- The general geographic location (arctic or desert)

However, I will give you some basics.

A. For small threaded thermal relief valves.
These relief valves are sometimes referred to as TSV's. These safety valves are normally 3/4" x 1" NPS and have a male thread inlet and a female thread outlet. These valves are normally used on water cooled shell and tube heat exchangers. They prevent over pressure due to heat build-up in the cooling water when the exchanger is blocked in and the hot process side is still flowing. A TSV may be installed with the stem in the vertical or horizontal plane. The TSV may be installed into a threaded "boss" furnished on the bottom exchanger cooling water nozzle or on a threaded connection added to the lower cooling water piping. The installation does not normally need an inlet block valve and will discharge (down) to the ground or paving. No outlet piping is required.

B. For small to large flanged PSV
1. A single PSV to atmosphere ( non-toxic gas or liquids and steam). Select a location that is at a high point, the top of a vessel or at a highpoint of the piping for the system to be protected. The PSV inlet piping should be the full size of the PSV inlet and vertical with a line class block valve (such as a gate valve). Then the PSV, bolted to the block valve. The PSV is normally in the bottom and out the side so outlet will exit in the horizontal. Because the commodity is non-toxic and we are discharging to atmosphere no outlet block valve is required. The outlet piping will normally be a flange connected to the PSV outlet, then a 90 degree elbow turning up and one to two meters of pipe to reach a safe discharge elevation. The top end of the vertical pipe will be left open and may be straight cut or angle cut at a 45 degrees. The low point of the elbow, close to the weld that joins the ell to the outlet flange will have a 1/2" (25mm) hole to drain condensate and rain.

2. A single PSV to a closed flare system.
Select a location that is at a high point, the top of a vessel or at a highpoint of the piping for the system to be protected. The PSV inlet piping should be the full size of the PSV inlet and vertical with a line class block valve (such as a gate valve). Then the PSV, bolted to the block valve. The PSV is normally in the bottom and out the side so outlet will exit in the horizontal. The outlet piping will normally be a flanged block valve bolted to the PSV and then a flange turning down followed by whatever piping is required to connect the line to the flare header.

If gate valves are used in a PSV installation the inlet and outlet block valves should be installed with the stem orientation in the horizontal plane. This is to prevent the possibility of the "gate" coming loose from the stem and falling into the "closed" position.

I hope this helps

Ajeet,
I guess I did not ask part of my question correctly. I will try again.

Commodity is what is in the line, not what the line is made of.

What is the commodity?

With that pressure and that tempurature and exhausting to atmosphere I hope it is steam.

Ajak,
It seems I did a bad job of explaining my recommendation. I will try again.
You wrote: "I believe the optimum design will be the block valve connect straight to the PSV outlet, but in some cases where there is a constraint in space, is it allowed to have a spool in between the PSV outlet and the block valve?"
- Yes, a spool piece is allowed between the PSV outlet and the Block Valve.

"This means the valve will be in vertical position after the spool and elbow going down."
- Not necessarily. We need to consider the orientation of the two items: Item #1, the discharge pipe and item #2, the discharge block valve.
- Item #1: (a) The Discharge pipe can exit the PSV and proceed to the Flare Header in one single horizontal run. Or (b) the Discharge pipe can exit the PSV and turn down for some vertical distance then turn horizontal and proceed to the Flare Header.
- With Item #1 (a) the Gate Valve would be positioned in a horizontal pipe run but the Item #2 (Valve) needs to be rotated down so the Valve Stem is in the horizontal.
- With Item #1 (b) the Gate Valve could be placed in either of the two horizontal portions. If the valve is placed in one of the Horizontal runs than the previous positioning advice applies. if the Gate Valve is located in the Vertical pipe run than the Valve stem will automatically be in the horizontal configuration, no other action is required.