A PV is installed on pump discharge line to regulate the discharge fluid pressure from 526 psig to 360 psig with a throughput of 70,000 bopd. If for any reason, the PV fails, it is expected that excessive pressure would be experienced in the downstream pipeline. To avoid this, a PSV would be installed.
What condition/scenario should be considered to determine the relief flow rate to size the PSV? The purpose of this is to protect the downstream pipeline from excessive pressure because it has been derated to 360 psig.
Someone suggested that the relieving flowrate should be the total throughput from the valve. Is the condition right?
The solution is easy to visualize if you look at your pump curve. Specifically, look at the relationship between pressure (head) and flow:
- Draw a horizontal line on the pump curve to represent the max head allowable before the pipeline is overpressured.
- Mark the point where this horizontal line intersects your impeller curve.
- Draw a vertical line downward, to the x-axis (flow).
- If we ignore friction loss in the pipe, that’s you’re answer.
As long as you have that amount of flow, or more, the pressure will be acceptable.
That’s the technical answer, but now let’s look at this from a the perspectives of safety and practicality. If you use a PSV, where are you going to send the discharge? This is going to be a big flowrate and you can’t just go to the ground with it unless we’re talking about water. Also, you’re probably looking at installing several big parallel PSVs. Consider alternate methods for limiting the pressure during this emergency. One option is to use a HIPS to trip the pump when the system pressure exceeds the max value. Or, use a HIPS to open a recirculation valve. You might also be able to greatly reduce the required relief flowrate by cutting down the pump impeller. If you can live with a lower dP across the control valve you’ll be able to reduce the impeller size. That will also save energy.
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