This is from my opinion,
- For Isolation valves even in case of Hydrocarbon applications, Gate valves have been used instead of ball valves.
For me, your system is big. the choose of valve is depends so many factor such as fluid type (slurry, dirty, clean service, hazardous and flammable, corrosive etc.), fluid characteristic (gas, steam, liquid etc.), pressure and temperature limitation, operation and maintenance requirement and etc. Based on my experience, we cannot say it is fault to use gate valve for gas service. just that, I’ve found for the gas service, normally ball valve is being used and some standard says, ball valve is more compact and reliable against leakage than gate valve.
- No positive isolation (Removable spool / spectacle blind) is used.
Positive isolation is a procedure whereby physical separation between systems is achieved. Closing valve does not achieve positive isolation. Normally positive isolation shall be provided during hot work to be done, equipment need to hydrostatically tested or inspected, or equipment to be opened whilst other remainder of the unit is still in operation. My suggestion just use valve with flange. Whenever need positive isolation at any point, just close the valve first, drain and put spade for positive isolation.
- No PSV sparing is provided.
For me, it’s good to have one spare IF when the situation to service the current online relief valve, we can temporarily isolate that one and run the PSV spare one. Let say you have any alternative method to protect equipment from overpressure during PSV maintenance, spare might be no need. For thermal expansion, for me no need spare.
It is good to sit together with the design engineer and do the design review plus challenge the design. They might have their intention why they use gate valve, don’t put the spare and etc2.