2" sensing line placement

[avgguyfrom85]

I'm not sure if this is the appropriate place to post this, so please direct as needed.

I'm curious about the sensing line placement in regards to a low pressure pressure relief valve. I was on a job site today while we were servicing equipment and noticed the 2" sensing line plumbed into a flange adapter spool accommodating a 10" pressure relief valve. My question is, what is the potential effect of having a pressure sensing line so close to a unit designed to relieve pressure? I presume that normally the VRU would come on before the PV operates. Thoughts? Thanks,

 

[don1980]

If this sensing line is for a pilot vacuum device, then there's no reason for concern. The pressure sensing line is reading static pressure. When there's flow through this nozzle, due to high internal pressure, there will be a difference between the static pressure in the tank itself and the static pressure in the pipe/nozzle where flow is taking place. In other words, the vac device will be reading a slightly lower pressure than the actual static pressure in the tank. The flowrate would have to be extremely high for the static pressure to be < atm pressure. The vacuum device isn't seeing the true static pressure in the tank, but that vac device isn't needed when the tank is relieving due to high internal pressure.

Hypothetically, if the flowrate was high enought to create a vacuum where the sensor line is connected, and the vac device opened, then it would experience reverse flow. Effectively, it will supplement the pressure relief device. That's very unlikely to happen, but if it did there's no harm - certainly no safety hazard.

 

[avgguyfrom85]

Don, thank you for the reply.

The 10" was a weight loaded unit, set at 1.0 Oz pressure and 0.5 Oz vacuum. Generally for the scope of work we deal with, sensing line will connect to Varec 180/181 (weight loaded as well) or S&J Oil regulators, set at between 0.8" WC and 0.2" WC. Other times its a switch device connected to some computer logic that will either tell the compressor or make up gas to come on depending on the pressure readings. Again, most times the operation range is measured in 0.1"WC. I guess from a design stand point, you could have have issues reading tank pressure if the valve is leaking or failed open, sending false readings. Every day I seem to have more questions, so I appreciate all the time taken to address them.

Avg