Without a detailed piping diagram, the results of the Hazop that must have preceded this “by-pass” operation, and a complete, accurate explanation of what the operation entails, it is not possible to risk giving advice on this query. This is a serious query about a serious operation.
Just review what you have written - without a detailed sketch or piping diagram of any kind:
“It is planned to provide temporary cooling”
You don’t provide “temporary” cooling to any pipeline being prepared to receive LNG at a temperature of -260 oF. You must first pre-cool the pipeline to at least -260 oF, before attempting to use it as a transport line. If you don’t totally pre-cool the line, 2-phase flow will ensue and you will have to handle the resulting mixture either downstream or in the pipeline itself. You make no mention of this procedural requirement and what it entails. It isn’t just about using the by-pass valve; it’s about what will result downstream when you do open the by-pass and allow saturated LNG to come into contact with a relatively “hot” pipeline.
“Upstream P of the bypass: 130psig
Upstream P of the bypass: 30psigLNG T: 250F”
Come on J_Leo, explain how both upstream pressures can be different. I probably can guess the response: it’s a typo. My point here is that there shouldn’t be any “typos” with such a critical and serious query. Is the temperature of “250F” also a typo? Please be accurate in your basic data. That is a minimum requirement of any process design engineer. Are we supposed to respond to typos? You really meant -250 oF, right?
An LNG flow velocity of 60 ft/sec is ultra high in stainless steel in anyone’s design. I normally would not design for any velocity higher than 10 ft/sec.
If the downstream initial pressure is really 30 psig I presume it is all gaseous natural gas and what happens to this gas volume when a subsequent 2-phase flow follows it is not explained in detail - nor how the downstream pressure is kept constant. All these items and how they are controlled and secured as safe should be documented in the customary and required HazOp. And I wouldn't refer the local query to an operator (who possibly was not present at the required HazOp).
One thing I can positively tell you, based on my LNG experience: you are going to generate 2-phase flow and you will have to handle the resulting vapor. How you do it is what determines the flow rate that you can tolerate into the un-cooled pipeline. But your haven't told us that. First find out how much vapor you can safely generate and that will set the max initial LNG injected into the downstream pipeline.