Hello all, we are replacing an e

[Iradah]

Hello all,

we are replacing an existing turboexpander-compressor with new smaller size one due to lower plant inlet flow.
the capacity of the compressor has changed from 345 mmscfd at normal operations to 115 mmscfd. with this new expander-compressor, the need for recycle flow on the compressor has changed. My question is what is the effect of the smaller compressor capacity on the current size of the anti-surge valve? We are planning to change only the expander-compressor with no other chagnes. Do we need to reduce the Cv of the anti-surge valve? I found that the current Cv valve requires 1% opening for 5 mmscfd recycle flow. What are the risks of using this big size anti-surge valve in controlling such small flow? at 115 mmscfd the opening will be 22%.
Please advise how I can analyze the situation of surge control with the smaller compressor capacity?
I appreciate your inputs.

Iradah

 

[TD2K]

My concern would be how well the valve will control in this portion of its range, nearly 75% isn't being used. Recycle valves always have to operate at some point just off their seat depending on the required recycle rate for your throughput but you are just making it harder for this valve to perform well.

I would generate new sizing data and discuss it with your Instrumentation engineer how they feel the control will operate with the existing trim versus a new valve or trim (you can very likely get a custom trim made for your existing valve if you want). A new valve trim is a minor cost.

 

[BenThayer]

typically anti-surge valves are sized to provide minimum flow to avoid the surge per the system curves.

i have never seen an anti-surge valve sized for 100% of the flow. So if you only need to pass ~33% of the flow, your valve would be about 7% open if the system is dead-headed.

 

[turb1]

It is in your best interest to contact the vendor supplying the new turbo-expander and ask them for the recommended surge valve sizing. They will typically give you a maximum Cv for the surge valve which you can then double check against your own sizing. I imagine that you are going to want to re-trim or replace the valve for the new service. An oversize surge valve can be a particular problem for a turbo-expander since the speed of the machine will decrease when the valve opens. If this valve is oversized (ie poor control) then the speed of the machine will fluctuate significatly which can feedback into the surge valve position depending on the type of surge control logic that is used (flow/speed for example). If the logic is just a straight flow then this won't be as much of a problem.