9 replies to this topic

[stu]

Dear Sir,
can we provide the isolation valve immediate on the PSV outlet or otherwise we have to provide any piping on the PSV outlet and then we have to provide the valve.
Please explain the reasons too.
thanks in Advance.
regards,
stu

[fallah]

Dear Sir,
can we provide the isolation valve immediate on the PSV outlet or otherwise we have to provide any piping on the PSV outlet and then we have to provide the valve.
Please explain the reasons too.
thanks in Advance.
regards,
stu

Because the tail pipe after PSV should normally be expanded with respect to size of PSV outlet flange,the isolation valve to be provided immediately at the PSV outlet and then after the isolation valve tail pipe to be expanded.It has also economical benefit regarding ignorance of installing larger size of isolation valve after expander.

[Qalander (Chem)]

Dear Sir,
can we provide the isolation valve immediate on the PSV outlet or otherwise we have to provide any piping on the PSV outlet and then we have to provide the valve.
Please explain the reasons too.
thanks in Advance.
regards,
stu

Dear
I fear that It is Almost "Criminal" to install any isolating valve at the exit of PSV since the pressure safety concept lands in jeopardy unless and until

• you have multiple redundancies(parallel equal capacity PSV's) in place for alternative usage.
• The valve in question if at all mandatorily be kept "car-seal open" and/or "Chain locked in open condition"

Hopefully I have been able to convey the seriousness

• since Mechanical integrity and
• Containment Capability of Upstream System(Be it any equipment;vessel or tank or any pipeline system)
• will be at extreme risk of unpreditable failure.

Hope this helps in envisaging the correct way forward if I am not mistaken,here!

[chemsac2]

I presume your question is on whether flange-to-flange connection between PSV and isolation valve is acceptable.

Section 6.3.2 of API 520, part-II probably does not recommend flange-to-flange connection of PSV and outlet isolation valve.

I would not have such an installation on following grounds:

- Increased pressure drop in tail-pipe as equivalent length increases and that too in small diameter portion of the tailpipe (however, if full bore valves are used, I am not sure if valve equivalent length is to be considered in DP calculations. But we do use it for conservativism)
- Isolation valve vendor confirmation would be required that isolation valve flow area is greater than PSV orifice area
- If pipe spool is provided between PSV and isolation valve is provided with vent valve, a check can be made if valve or PSV is leaking.

Regards

[fallah]

I presume your question is on whether flange-to-flange connection between PSV and isolation valve is acceptable.

Section 6.3.2 of API 520, part-II probably does not recommend flange-to-flange connection of PSV and outlet isolation valve.

I would not have such an installation on following grounds:

- Increased pressure drop in tail-pipe as equivalent length increases and that too in small diameter portion of the tailpipe (however, if full bore valves are used, I am not sure if valve equivalent length is to be considered in DP calculations. But we do use it for conservativism)
- Isolation valve vendor confirmation would be required that isolation valve flow area is greater than PSV orifice area
- If pipe spool is provided between PSV and isolation valve is provided with vent valve, a check can be made if valve or PSV is leaking.

Regards

As i investigated,section 6.3.2 of API 520, part-II doesn't include any limitation for fit to fit connection between PSV and isolation valve.If you found any statement indicates the issue,please submit.

On the other hand,there is no considerable difference between pressure drop (as equivalent lenght) in two cases,while applying smaller size valve would be better in operational/economical point of view.

[chemsac2]

This is what API says in that section (section 4.2 in API 520 part 2, 1994 edition. Probably section 6.3.2 of API 520 which I referred to in my previous post was the latest API 520 part 2 edition. I do not have latest edition at home, but in office. But I remember statement is same as below)

"When isolation valves are installed in pressure relief valve discharge piping, a means to prevent pressure buildup between the pressure relief valve and the isolation valve should be provided (for example, a bleeder valve). Also, the installation of bleed valves should be considered to enable the system to be depressured prior to performing maintenance on the system as shown in Figures 9 through 12"

When I re-read that section in API 520, part 2, I came across figure 9 in API 520, part 2, 1994 edition (attached with this post). As can be seen from figure, API allows flange to flange connection but needs to have a bleed connection on isolation valve. Alternatively as stated in note 3, it allows use of spool piece with bleeder valve as well.

Thus, it seems to be open to individual's choice. In all the projects I have worked on till date (mostly in refinery and petrochemical sector), I have not seen flange to flange connection.

Pressure drop in small diameter portion would be a problem for low set pressure systems. In one particular case of feed surge drum, we had to have tailpipe size almost two sizes higher than PSV orifice flange (12" tailpipe for 6R8 orifice).

Regards

 Figure 9_API 520_P2_1994.jpg   57.42KB   76 downloads

[fallah]

Thus, it seems to be open to individual's choice. In all the projects I have worked on till date (mostly in refinery and petrochemical sector), I have not seen flange to flange connection.

Agreed with you statement as:seems to be open to individual's choice,because contrary to your experience almost all configurations i have seen for PSV discharge piping shown fit to fit connection.

Edited by fallah, 22 May 2010 - 02:58 AM.

[Qalander (Chem)]

This is what API says in that section (section 4.2 in API 520 part 2, 1994 edition. Probably section 6.3.2 of API 520 which I referred to in my previous post was the latest API 520 part 2 edition. I do not have latest edition at home, but in office. But I remember statement is same as below)

"When isolation valves are installed in pressure relief valve discharge piping, a means to prevent pressure buildup between the pressure relief valve and the isolation valve should be provided (for example, a bleeder valve). Also, the installation of bleed valves should be considered to enable the system to be depressured prior to performing maintenance on the system as shown in Figures 9 through 12"

When I re-read that section in API 520, part 2, I came across figure 9 in API 520, part 2, 1994 edition (attached with this post). As can be seen from figure, API allows flange to flange connection but needs to have a bleed connection on isolation valve. Alternatively as stated in note 3, it allows use of spool piece with bleeder valve as well.

Thus, it seems to be open to individual's choice. In all the projects I have worked on till date (mostly in refinery and petrochemical sector), I have not seen flange to flange connection.

Pressure drop in small diameter portion would be a problem for low set pressure systems. In one particular case of feed surge drum, we had to have tailpipe size almost two sizes higher than PSV orifice flange (12" tailpipe for 6R8 orifice).

Regards

 Figure 9_API 520_P2_1994.jpg   57.42KB   76 downloads

The image indeed Confirms that the Block(Gate) Isolating valve at discharge(venting) side of Pressure relief valve is conditionally provided with "Car-seal opening" or 'Locked Open Chaining'.
Thanks chemsac2

[cmp74]

Dear Sir,
can we provide the isolation valve immediate on the PSV outlet or otherwise we have to provide any piping on the PSV outlet and then we have to provide the valve.
Please explain the reasons too.
thanks in Advance.
regards,
stu

We use (for the most cases) spectacle blinds at PSV discharge.Since its a full bore opening, no impact on pressure drop.

[fallah]

We use (for the most cases) spectacle blinds at PSV discharge.Since its a full bore opening, no impact on pressure drop.

I don't think safety limitations allowed you to use spectacle blind in PSV inlet/discharge lines.

Even for isolation of vessels in maintenance case,spool pipe to be used in PSV coneection to vessel instead spectacle blind.