Gas Compressor Anti-surge line

[Pro409]

Hi dear members,
I want to know at what condition anti-surge line control valve shall be equipped with the bypass line?
any comment is appreciated.

 

[JLSeagull]

I don't recall ever seeing a manifold with block valves and a bypass valve on an anti-surge valve. Service for the anti-surge valve normally requires isolation of the compressor module. Block valves around the surge valve could be accidentally left closed.

 

[rneill]

Not sure on your question ... are you asking when you should have independent anti-surge vs. recycle lines?

This is somewhat a philosophical question and my thoughts are as follows:

- A recycle valve is a regular process control valve which may be required to operate as part of normal operation but an anti-surge valve is a safety valve which is necessary to protect the machine from damage. While the recycle valve can in many cases provide this protection, that is not necessarily what it is designed to do and the sizing you apply to optimize the performance as a recycle valve (provide good process control) will not allows be the sizing you would want to provide the response speed necessary for the valve to protect the machine.
- Some people believe (myself included) that safety devices such as anti-surge valves and ESD valves should not be used for normal process control or operating functions as they may be less likely to be "available" when they are needed to operate as the "safety". In addition, having a recycle valve and an antisurge valve provides two levels of protection instead of just one. If recycle was a regular part of operation, I myself would opt for separate devices but if recycle was an "off case", I might be more inclined to consider using only a single device.

Ultimately, a decision on whether to provide both a recycle valve and an antisurge valve will be based on a range of factors and can really only be made after a careful review of the specific operating condition, necessary sizing, and corporate philosophy. In some cases a dynamic simulation of the system will help determine the operating characteristics of the system and may indicate the need for two independent devices.

 

[JLSeagull]

I agree with rneill about the difference between a recycle valve and an anti-surge valve.

For parallel compressor modules or a series of compression stages a recycle valve would likely be outside the individual compressor modules.

 

[rneill]

There is really no point in a manual bypass valve around a compressor anti-surge valve (as is typical for normal control valves) since unlike a normal process control valve, you can not operate the compressor without an active automated anti-surge valve in place. For a typical control valve, you could usually position a manual bypass and limp along while you serviced the control valve but this is not the case with anti-surge since you could not achieve the response time necessary to prevent surge with a manually operated valve (even if an operator was standing right there).

I suppose you could install duplicate parallel anti-surge valves so that when one was taken off line, the other was placed into service ... but I have not seen this done and as pointed out, there is a great risk that the block valves could be erroneously positioned. Typically, if the anti-surge valve needs serviced, you have to shut down the compressor as mentioned by JLSeagull.

 

[Pro409]

Dear members, thanks for your reply.
You are absolutely right. I also agree with your explanations about the function and importance of the anti-surge control valve. Its name includes “control” but in fact it is a safety device like a pressure safety valve that acts SUDDENLY. So, it can not be replaced by the by pass block valve( like other process control valves).. But, origin( or reason) of my query is that I have seen some design works in which the manual block valve is used as by pass of the ani-surge control valve that I can not found any reason( or justification) for it.. My imagination was that it may have a specific reason I am not aware of it. Perhaps designer have made a mistake.

Jlseagul says: Service for the anti-surge valve normally requires isolation of the compressor module. Block valves around the surge valve could be accidentally left closed.

Similar to around the pressure safety valves, there will be no anxiety about the blockage of the line If the LOCK OPEN block valve be installed on the down stream & upstream of the anti-surge control valve. These isolation valves are also useful for maintenance the anti-surge valve without the isolation of the compressor module.

 

[rneill]

Ultimately, it's up to the end user to decide if they will accept locked open block valves upstream and downstream of the anti-surge valve. Locking does reduce the risk but does not eliminate it ...

Regarding the bypass, there is no functional benefit to a manual bypass at all and I would assume that this is a mistake made by someone who did not really understand the specialized function of an anti-surge valve and was just doing what they do for all control valves.