Location of Relief Valve on Crude Stabilizer Column 2

[Fulan]

Greeting Gents,
Is it possible to lower the relief valve located in the top of crude stabilizer column. (Set Press is 50 psig, Size is 16”, Type in Conventional)

Due the high elevation, we encountered many problem in removing or installing the relief valves specially during windy weather

Please help if you have any suggestions

Thanks
Fulan

 

[TD2K]

Maybe.

The PSV likely goes to atmosphere given your 50 psig set point and type, this is not unusual for a crude column.

You'll need to look at the possible location(s) and analyze them whether they provide the necessary required relief capacity, don't introduce any new risks or identify how those risks will be managed (such as additional block valves that would have to be car sealed or locked open) and check that the inlet line losses with the new location are still acceptable as a result of additional interconnecting piping or equipment such as air coolers.

Atmospheric relief valves are typically located on the top of columns to disperse the vapors into the atmosphere where they mix with atmospheric air and are diluted. If you relocate the PSVs you will need to look at the new discharge locations and whether there are any unacceptable risks created as a result of the new location. I would especially consider any surrounding equipment if they are at a similar or higher elevation.

 

[pennpiper]

Fulan,
You did not share with us your location or where this Crude Stabilizer facility is. No matter where it is I would be very surprised if the relief valve(s) was venting to atmosphere as suggested by the previous poster.

Is this facility already built and now you find that you must rethink your options?

I have seen where the circumstances allowed PSV's have been located on the Crude overhead line at a lower elevation just before it enters the condensers (shell & tube or air cooler). The PSV outlets were then tied in to the Flare Header with a shorter and less complicated routing.

This location it self does not in it self lessen the complexity for removal and handling during maintenance turn-around. I feel that very large PSV's are best handled by dedicated monorail structures on top of these tall vessels.



prognosis: Lead or Lag

 

[don1980]

Yes, it can be lowered. The only constraint is that vapor valves must be located in the vapor section of the vessel, and liquid valves located in the liquid zone. Obviously, you can't move this to a nozzle on a downcomer, but it can be done if you have a spare nozzle in a clear vapor location.

However, in most cases that's neither necessary or advisable. It's not advisable because it can cause tray damage to the trays above the relief valve, if the relief valve ever opens. It's not necessary because a better location is at the condenser platform. Normally the elevation of the condenser is very accessible - well below the top of the column. Install the PSV on the vapor line, just upstream of where it connect to the condenser. The relief design must account for the pressure drop in the overheads line, but seldom a problem.

 

[TD2K]

Lots of the Refineries I've been in have atmospheric relief valves. There's a drive since Texas City to get away from atmospheric reliefs but there are issues with just sending the stream to the flare:


1. A lot of older columns are designed for relatively low pressures, 50 to 75 psig. Even with a balanced bellows PSV you are restricted to about 1/2 of that pressure for many, but not all, bellow PSVs as flare backpressure. You can go to higher backpressures and accept a derating of the PSV's capacity but you typically quickly run out of area.
2. A lot of refineries have increased capacity over the years increasing the load on their flares. In sizing cases such as plant wide power failure, loss of cooling, etc the pressures in the flare system may not support relieving these low pressure vessels to the flare.
3. These columns typically have high relief loads. That makes a loaded flare system even worse off for backpressures.

One project I was involved with routed many of their atmospheric relief valves designed as high risk to the Refinery flare systems (LP and HP) but we couldn't route all of the atmospheric PSVs to the flares and even with the scope we had, a dynamic simulation of the flare was needed to convince ourselves we had sufficient capacity. A traditional steady state flare analysis for the loads even before the additional loads we wanted to send to the flare showed the flares to be fully loaded. While this analysis is much cheaper than simply installling a new flare, it's not a quick process.

 

[Fulan]

Thank you Gents for the replay

However, the discharge piping of the relief is connected to flare header to protect any excessive gas pressure . The suggestion is to extend the suction piping from the top of the column to proper elevation to ease the removing/ installation of the PSV. IS it possible?

Note: We have other relief valves in the reboiler system (Heat Exchanger)
Also, this is existing system in GOSP not refinery

 

[TD2K]

I hope your flare or vent system is large enough to keep the pressure at the outlet of the PSV during relief to less than 5 psig if the PSV is actually a conventional relief valve.

Can you provide a sketch what you are looking at doing with some elevations?

 

[SDwijanarko]

Fulan,

That is possible and you need to check and verify the pressure drop along the extended line. It is recommended to be less than 3% of set pressure. You may also want to consider and check for potential of condensing vapor on the line.

SD