4 replies to this topic

[linda_pro]

Hi all,
I have an issue about safety as following. Pls advice me if you have experience in similar situation.
the current LNG plant send out at 40 barg and design pressure at 76 barg. the owner want to expand the plant by installing another system which send out LNG to network at 80 barg.they also want can use the 80barg system to send out at 40barg in peak time. and then They tie-in the 80barg system to 40 barg system and using isolation valves to isolate between two system.

what i have in my is using double block and bleedvalves with motor to control these isolation valves open and close. but for me, i think it is quite risk if the operator have wrong operation to open this valves when the 80barg system in operating. this cause to damage all 40barg system which have design pressure less than 80barg.

there is any suggestion about safeguard from your opinion?

and there is any risk it might have except wrong operation?

thanks in advance for any sharing

cheers,

[Art Montemayor]

Linda:

This sounds like an application similar to what is used in "pigging" operations.

In order to ensure that the sequence and operation of the correct valves is done correctly and sytematically EVERY time, a "keyed-valve" operation is installed. This involves using valves that require a "key". Upon being activated, the first valve yields a key that only works on the next, sequential valve and the same is applied to all other valves that follow in the sequence of operation. In that way, the operator is protected from committing a fatal error in the sequence of operation - which is a possibility.

Additionally, a double check valve installation is used in the low pressure LNG production line upstream of the junction with the high pressure LNG.

[paulhorth]

Linda,
If you connect a high pressure source to a system with lower design pressure, there will be a relief case to be considered, because the lower pressure system could be overpressured if it is blocked in. You are right to identify this risk. The section which could be blocked in could be quite short, or it could be the entire 40 bar pipeline through to the final block valve. You need to study the P&IDs for the system. The blocked-in case could possibly be prevented with a keyed valve system as Art has described, but this soon becomes impractical if there are more than one or two valves downstream and if some of them are remote from the inlet. Even if the isolations are open, you should consider if more gas can flow in than can flow out to the downstream users. Then a relief valve (or a pair) will be required for the total possible inflow or the difference between inflow and outflow.

Paul

[linda_pro]

Thanks Art et Paul for your reply.
@Paul, this is an option of my feasibility study project and hence there is no PID available. I am requesting to do safety analyse for this option . in case high risk is found, this option shall be cancel. (i am trying to kill this option )
@Art: Two check valve can't be applied here because the double block valves will work in 2 ways. The owner wants 80 barg system is designed to operate at 40 barg as well, so it means when 80 bargs system runs at 40 barg, the current 40 barg system will be shared by opening these double block valves.

Nice weekend!
cheers,

[Bobby Strain]

If my client requested this, I would be sure to give her a written response recommending that this connection should not be made. You can protect the low pressure system with a relief valve or a HIPPS system as long as you can restrict flow from the high pressure to the low pressure system. But it is difficult to provide protection when there is an unrestricted connection between the systems with only manually operated valves. Key lock valves are the worst idea ever offered for any kind of protection, because there are always duplicate keys. Maybe you can consider a physical disconnect using a spool, and only install the spool when appropriate.

Bobby

Edited by Bobby Strain, 24 November 2012 - 10:57 AM.