9 replies to this topic

[R NESAMANI]

Hi all,

What is the best type of valve for positive isolation of LPG pipeline system. Pressure of LPG is 80 kg/cm2 (Max). ?

Can we use inline blind...?

[S.AHMAD]

Common practice for positive isolation is double block and bleed. Blind can also be installed in-between the block valves.

[fallah]

Hi all,

What is the best type of valve for positive isolation of LPG pipeline system. Pressure of LPG is 80 kg/cm2 (Max). ?

Can we use inline blind...?

NEZAMANI,

In general, positive isolation just would be achieved by blinding or spool removal. Hence, no valve could be able to create positive isolation.
You can use a ball valve followed by a globe valve with a spectacle blind in its upstream. If the fluid is to be routed to closed drain a check valve should be considered inbetween to avoid back flow occurence.

Fallah

[paulhorth]

Nesamani,
A double block and bleed group of valves should be used with a blind or removable spool on the DOWNSTREAM side, not in between the valves. See the attached sketch. This DBB plus blind arrangement is always used for positive isolation on every job I have worked on - for system pressures above a certain threshold, usually 600# flange rating but sometimes 300#.

Putting the blind between the valves is no use, since when the flange is broken to swing the blind, the only isolation against upstream pressure is a single block valve which may leak.

Paul

[paulhorth]

Sorry, here is the DBB sketch.
Paul

Attached Files

•  DBB.pdf   50.91KB   96 downloads

[fallah]

Putting the blind between the valves is no use, since when the flange is broken to swing the blind, the only isolation against upstream pressure is a single block valve which may leak.

Paul,

For physical separation of a part of a process or pipeline by double block and bleed valving, spectacle blind should be installed out of the valves and at that side of process or pipeline to be isolated, but in double block for connection to closed drain (as what i mean in my previous post) the spectacle blind should be installed between two valves or upstream of second one, because at this situation swing the blind would easily being done without any effect from downstream and also because the point to be isolated is in upstream of the first valve.

Fallah

Edited by fallah, 18 October 2011 - 08:12 AM.

[R NESAMANI]

Thank you all.
Can we use inline blinds...?

[paulhorth]

Fallah,
I agree with you about the closed drains arrangement. I understood Nesamani to be referring to isolation of a pipeline from the plant, not a closed drain. For this isolation, we use a DBB and downstream blind, as you say.

Nesamani,

Can we use inline blinds...?

Yes.
Did you look at my sketch? It shows an inline spectacle blind on the downstream side of the double block and bleed valves.

Paul

[S.AHMAD]

1. Is there any specific reason to install inline-blind?
2. Yes for large pipeline where the blind are heavy and maintenance are quite frequent
3. One purpose of bleed point is also to check whether any leaks.
4. The probability of two valves leak at the same time is very remote.
5. Blinds can be installed also on the valve-flange. Especially for small pipeline. This will eliminate to install additional flanges specific for blind.
6. Maintenance can be on either side. So you need two inline-blinds. One one either side.

Edited by S.AHMAD, 18 October 2011 - 07:44 PM.

[R NESAMANI]

Thank you all for your reply.
Actual purpose of isolation is for Pump overhauling job. We have 3 pumps connected with common suction & disch. header. Suction & disch. valve is Ball valve. But if we want to job in one pump, we close the ball vale. But its passing. So we have planned for installing one more valve ahead of the ball valve. My seniors are advising abt gate valve. But i feel that gate valve also will pass after some time. So i need permenant solution.